REFRIGERATION ENGINEERING AND COLD CHAIN

Transcription

1 For Class use only ACHARYA N. G. RANGA AGRICULTURAL UNIVERSITY B. Tech (Food Technology) Course No.: FDEN 222 Credit Hours: 3 (2+1) REFRIGERATION ENGINEERING AND COLD CHAIN STUDY MATERIAL Prepared by Dr. S.Kaleemullah College of Food Science and Technology Pulivendula

2 Department of Food Engineering Course No : FDEN-222 Title : Refrigeration Engineering and Cold Chain Credit hours : 3 (2+1) Theory Lecture Outlines L.No. Lecture outline P.No. 1 Refrigeration and air conditioning- Definitions - Necessity of refrigeration and air 4 conditioning Factors affecting comfort air conditioning 2 Definition of Refrigerant - History of refrigerants - Classification of Refrigerants 6 Primary refrigerants Secondary Refrigerants Halo carbon refrigerants 3 Azeotrope refrigerants - Inorganic refrigerants - Ammomia - Air - Carbon dioxide 11 - Sulphur dioxide - Water 4 Hydro carbon refrigerant Designation system for refrigerants Designation 15 sytem for Dichloro-tetrafluoro-ethane 5 Designation system for Dichloro-difluoro-methane - Selection of a refrigerant 17 6 Thermodynamic properties of refrigerants 19 7 Chemical requirements of refrigerants Physical properties of refrigerants 21 8 Secondary refrigerants Brines- Applications of various brines 25 9 Types of Refrigerators Air Refrigerator Vapour refrigerator Advantages and 27 Disadvantage of vapour compression refrigeration system over air refrigeration system 10 Mechanism of a simple vapour compression refrigeration systems Evaporators Capacity of an evaporator Factors affecting heat transfer 31 capacity of an evaporator Types of evaporator 12 Bare tube coil evaporator Finned evaporator Plate evaporator Shell and 34 tube evaporator 13 Shell and coil evaporator Tube in tube Flooded evaporator Dry expansion 38 evaporator 14 Natural convection evaporator Forced convection evaporator Frosting evaporator Non Frosting evaporators Defrosting evaporators 42

3 3 15 Compressor Classification Suction pressure, Discharge pressure, 45 Compression ratio, Suction volume, Stroke volume, Clearance factor, Compressor capacity, Volumetric efficiency 16 Reciprocating compressor - Parts of a reciprocating compressor - Cycle of a 48 reciprocating compressor 17 Rotary compressor Centrifugal compressor Advantages and disadvantages 52 of a centrifugal compressor 18 Condensers Working of a condenser Factors affecting the condenser 57 capacity Heat rejection factor 19 Classification of condensers Air cooled condensers Water cooled condensers 61 Tube in condenser Shell and coil condenser 20 Fouling factor Difference between air cooled and water cooled condensers 66 Evaporative condenser 21 Expansion Devices Types of expansion devices Capillary tube Hand operated expansion valve Low side float valve High side float valve Ice manufacturing principle of ice production Application of refrigeration in different food products Examples of Food processing by refrigeration and storage Fruits and 79 vegetables 26 Examples of Food processing by refrigeration and storage - Meat products fish 80 poultry products dairy products 27 Food Freezing Freezing systems Indirect contact systems Plate Freezers air blast Freezers Freezers for liquid foods Direct contact systems Air blast - Immersion Frozen food properties Density Thermal conductivity Enthalpy Apparent 91 specific heat Apparent thermal diffusivity 31 Freezing time Factors influencing freezing time Freezing rate Thawing time Quality Changes in Foods during Frozen Storage 99

4 4 Lecture No. 1 Refrigeration and air conditioning- Definitions - Necessity of refrigeration and air conditioning Factors affecting comfort air conditioning Introduction: Refrigeration is the process of removing heat from an enclosed space, or from a substance and moving it to a place where it is Unobjectionable. It also includes the process of reducing and maintaining the temperature of a body below the general temperature of its surroundings. For example, if some space (say in cold storage) is to be kept at - 2 C (271K), we must continuously extract heat which flows into it due to leakage through the walls and also the heat which is brought into it with the articles stored after the temperature is once reduced to - 2 C (271K). Thus in a refrigerator, heat is virtually being pumped from a lower temperature to a higher temperature. According to Second Law of Thermodynamics, this process can only be performed with the aid of some external work. It is thus obvious that supply of power (say electric motor) is regularly required to drive a refrigerator. Theoretically, a refrigerator is a reversed heat engine or a heat pump which pumps heat from a cold body and delivers it to a hot body. The substance which works in a heat pump to extract heat from a cold body and to deliver it to a hot body is called a refrigerant. Necessity of Refrigeration and Air Conditioning: Refrigeration system is used for the manufacture of ice and similar products. It is also widely used for the cooling of storage chambers in which perishable foods, drinks and medicines are stored. The refrigeration has also wide applications in submarine ships, aircraft and rockets. Air conditioning refers to the treatment of air so as to simultaneously control its temperature, moisture content, cleanliness, odour and circulation, as required by occupants, a process or products in the space. The subject of refrigeration and air conditioning has evolved out of human need for food and comfort. It also deals with the

5 5 conditioning of air for industrial purposes, food processing, storage of food and other materials. Factors Affecting Comfort Air Conditioning: 1. Temperature of air: In air conditioning, the control of temperature means maintenance of any desired temperature within an enclosed space even though the temperature of the outside air is above or below the desired room temperature. This is accomplished either, addition or removal of heat from the enclosed space as and when demanded. It may be noted' a human being feels comfortable when the air is at 21 C with 56% relative humidity. 2. Humidity of air: The control of humidity of air means the decreasing or increasing moisture contents of air during summer or winter respectively in order to produce comfortable healthy conditions. The control of humidity is not only necessary for human comfort but it increases the efficiency of the workers. In general, for summer air conditioning, the relative humidity should not be less than 60% whereas for winter air conditioning it should not be than 40%. 3. Purity of air: It is an important factor for the comfort of a human body. It has noticed that people do not feel comfortable when breathing contaminated air, even if it is within acceptable temperature and humidity ranges. It is thus obvious that proper filtration, cleaning, purification of air is essential to keep it free from dust and other impurities. 4. Motion of air: The motion or circulation of air should be controlled, in order to keep constant temperature throughout the conditioned space. Hence, there should be equi-distribution of air throughout the space to be air conditioned.

6 6 Lecture No. 2 Definition of Refrigerant - History of refrigerants - Classification of Refrigerants Primary refrigerants Secondary Refrigerants Halo carbon refrigerants Introduction: The development of refrigeration and air conditioning industry depends to a large extent on the development of refrigerants to suit various applications and the development of various system components. At present the industry is dominated by the vapour compression refrigeration systems, even though the vapour absorption systems have also been developed commercially. The success of vapour compression refrigeration systems owes a lot to the development of suitable refrigerants and compressors. The theoretical thermodynamic efficiency of a vapour compression system depends mainly on the operating costs, safety, reliability and serviceability etc. depend very much on the type of refrigerant and compressor selected for a given application. The refrigerant is a heat carrying medium which during their cycle (i.e. Compression, condensation, expansion and evaporation) in the refrigeration system absorbs heat from a low temperature system and discards the heat so absorbed to a higher temperature system. History of Refrigerants: The first refrigerant used was ether employed by Perkins in hand operated vapour compression machine. In the earlier days, ethyl chloride (C2H5Cl) was used as a refrigerant which soon gave way to ammonia as early as in At about the same time, sulphur dioxide (SO2) in 1874, methyl chloride (CH3Cl) in 1878 and carbon dioxide (CO2) in 1881, formed application as refrigerants. During many new refrigerants, such as N2O3, CH4, C2H6, C2H4, C3H8 were employed for low temperature refrigeration. Hydrocarbons were, however, found extremely inflammable, dichloro methane (CH2Cl2), dichloro ethylene (C2H2Cl2) and monobromomethane (CH3Br) were also used as refrigerants for centrifugal machines. A great break through occurred in the field of refrigeration with the development of freons by E. I. du pont de Nemours and Co. Freons are a series of fluorinated

7 7 hydrocarbons, generally know as fluorocarbons derived from methane, ethane etc. as bases. Although Freon is a trade name, it has come to be commonly used for these compounds. With fluorine, chlorine and sometimes bromine in their molecule, Freons form a series of refrigerants with wide range of normal boiling points (boiling point or saturation temperatures at one atmosphere pressure) to satisfy the varied requirements of different refrigerating machines. The presence of fluorine in the molecule makes the compound non toxic and imparts other desirable physical and physiological characteristics. Desirable Properties of an Ideal Refrigerant: There is no ideal refrigerant which can be used for all type of applications i.e. there is no ideal refrigerant. A refrigerant is said to be ideal if it has all of the following properties. 1. Low boiling point 2. High critical temperature 3. High latent heat of vaporisation 4. Low specific heat of liquid 5. Low specific volume of vapour 6. Non-corrosive to metal 7. Non-flammable and non-explosive 8. Non-toxic 9. Low cost, 10. Easy to liquify at moderate pressure and temperature 11. Easy of locating leaks by odour or suitable indicator, and 12. Mixes well with oil The standard comparison of refrigerants, as used in the refrigeration industry is based on evaporating temperature of - 15 C and a condensing temperature of + 30 C. Classification of Refrigerants: The refrigerants may, broadly, be classified into the following two groups. 1. Primary refrigerants 2. Secondary refrigerants.

8 8 The refrigerants which directly take part in the refrigeration system are called primary refrigerants whereas the refrigerants which are first cooled by primary refrigerants and then used for cooling purposes, are known as secondary refrigerants. The primary refrigerants are further classified into the following four groups. 1. Halo-carbon refrigerants, 2. Azeotrope refrigerants, 3. Inorganic refrigerants, 4. Hydro-carbon refrigerants. Halo-carbon Refrigerants: The American Society of Heating, Refrigeration and Air-conditioning Engineers (ASHRAE) identify 42 halo-carbon compounds as refrigerants, but only a few of them are commonly used. The following table 1 gives some of the commonly used halo-carbon refrigerants. Table. 1. Commonly used halo-carbon refrigernats Refrigerant Number R 11 R 12 Chemical name Chemical formula Trichloromono fluoromethane Dichlorodifluoromethane CCl3F CCl2 F2 R 13 Monochlorotrifluoromethane CCl F3 R 14 Carbonatetrafluoromethane CF4 R 21 R 22 R 30 R 40 R 100 R 113 R 114 R Dichloromonofluoromethane Monochlorodifluoromethane Methylene chloride Methyl chloride Ethyl chloride Trichlorofluroethane Dichlorotetrafluoroethane Monochloropentafluoroethane CHCl2F CHClF2 CH2Cl2 CH3Cl C2H5Cl CCl2FCClF2 or C2Cl3F3 CClF2CClF2 or C2Cl2F4 CClF2CF3 The halo-carbon compounds are all synthetically produced and were developed as Freon family of refrigerants. Freon is a registered trade mark of E. I. Du Pont de Nemours and Co., America. Most of the halo-carbon refrigerants, are now available from

9 9 other manufactures under various trade names such as Genetron, Isotron etc. The first of the halo-carbon refrigerant i.e. R 12 was developed in 1930 by Thomas Midgley. 1. R-11, Trichloromonofluoromethane (CCl3F): The R-11 is a synthetic chemical product which can be used as a refrigerant. It is stable, non-flammable and non-toxic. It is considered be a low-pressure refrigerant. It has a low side pressure of bar at -15 C and high side pressure of bar at 30 C. The latent heat at -15 C is 195 kj/kg. The boiling point at atmospheric pressure is C. Due to its low operating pressures, the refrigerant is exclusively used in large centrifugal compressor systems of 200 TR and above. The leaks may be detected using a soap solution, a halide torch or by using an electronic detector. R-11 is often used by service technician as a flushing agent for cleaning the internal parts of a refrigerator compressor when overhauling systems. It is useful after a system had a motor burn out or after it has a great deal of moisture in the system. By flushing moisture from the system with R-11, evacuation time is shortened. R-11 is one of the safest cleaning solvent that can be used for this purpose. The cylinder colour code for R-l1 is orange. 2. R-l2, Dichlorodifluoromethane (CCl2F2): The R-12 is a very popular refrigerant. It is colourless, almost odourless liquid with boiling point of -29 C at atmospheric pressure. It is non toxic, non-corrosive, non-irritating and non-flammable. It has a relatively low latent heat value which is an advantage in small refrigerating machines. The large amount of refrigerant circulated will permit the use of less sensitive and more positive operating and regulating mechanisms. The refrigerant is used in many different types of industrial and commercial applications such as refrigerators, freezers, water coolers, room and window air conditioning units etc. Its principal use is found in reciprocating and rotary compressors, but its use in centrifugal compressors for large commercial air conditioning is increasing. R-12 has a pressure of 0.82 bar at -15 C and a pressure of 6.4 bar at 30 C. The latent heat of R-12 at - 15 C is 159 kj/kg. The leak may be detected by soap solution, halide torch or an electric leak detector. Water is only slighted soluble in R-12. The Solution formed is very slightly corrosive to any of the common metals used in refrigerator construction. The addition of mineral oil to the refrigerant has no effect upon

10 10 the corrosive action. The refrigerant is available in a variety of cylinder sizes and the cylinder colour code is white. 6. R-22, Monochlorodifluoromethane (CHClF2): The R-22 is a man-made refrigerant developed for refrigeration installations that need a low evaporating temperature, as in fast freezing units which maintain a temperature of - 29 C to - 40 C. It has also been successfully used in air conditioning units and in household refrigerators. It is used with reciprocating and centrifugal compressors. It is also used for low temperatures refrigeration applications, cold storage, food freezing and storage etc. The boiling point of R-22 is - 41 C at atmospheric pressure. It has a latent heat of kj/kg at -15 C. The refrigerant is stable and is non-toxic, non-corrosive, nonirritating and non-flammable. The evaporator pressure of this refrigerant at -15 C is 1.92 bar. This refrigerant has good solubility in oil down to -9 C. The leaks may be detected with an electronic leak detector. The cylinder colour code for R-22 is green.

11 11 Lecture No. 3 Azeotrope refrigerants - Inorganic refrigerants - Ammomia - Air - Carbon dioxide - Sulphur dioxide - Water Azeotrope Refrigerants: The term Azeotrope refers to a stable mixture of refrigerants whose vapour and liquid phases retain identical compositions over a wide range of temperatures. However, these mixtures, usually, have properties that differ from either of their components. Table. 2. Azeotrope refrigerants Refrigerant No. R 500 R 502 R 503 R Azeotropic mixing Refrigerants. 73.8% R-12 & 26.2% R % R-22 & 51.2% R % R-23 & 59.9% R % R-32 & 51.8% R-115 Chemical formula CCl F2 / CH3CH F2 CHClF2 / CClF2CF3 CHF3 / CClF3 CH2F2 / CClF2CF3 R-500: It is an Azeotropic mixture of 73.8% R-12 (CCl2F2) and 26.2% of R- 152 (CH3CHF2) it is non flammable, low in toxicity and non-corrosive. It is used in both commercial applications but only in systems with reciprocating compressors. It has a fairly constant vapour pressure temperature curve which is different from the vaporizing curves for either R- 152a or R- 12. This refrigerant offers about 20% greater refrigerating capacity than R-12 for the same size of motor when used for the same purpose. It can be used whenever a higher capacity than that obtained with R-12 is needed. The solubility of water in R-500 is highly critical. It has fairly high solubility with oil. The leakage may be detected by using soap solution, a halide torch, an electronic leak detector or a colour tracing agent. Water is quite soluble in this refrigerant. It is necessary to keep moisture out of the system by careful dehydration and by using driers. The cylinder colour code for this refrigerant is yellow. The evaporator pressure of this refrigerant is 1.37 bar at -15 C and its condensing pressure is 7.78 bar at 30 C. It has a boiling point of - 33 C at atmospheric pressure. Its latent heat at -15 C is 192 kj/kg.

12 12 R-502: It is an Azeotropic mixture of 48.8% R-22 (CHClF 2) and 51.2% of R-115 (CClF2CF3) it is non flammable, non-corrosive, practically non-toxic liquid. It is a good refrigerant for obtaining medium and low temperatures. It is suitable where temperatures from -18 C to -51 C are needed. It is often used in frozen food lockers, frozen food processing plants, frozen food display cases and in storage units for frozen foods and ice-cream. It is only used with reciprocating compressors. The boiling point of this refrigerant at atmospheric pressure is -46 C. Its evaporating pressure at -15 C is 2.48 bar and the condensing pressure at 30 C is bar. Its latent heat at - 29 C is kj/kg. This refrigerant has all the qualities found in other halogenated (fluorocarbon) refrigerants. It is non-toxic, non-flammable, non- irritating, stable and non-corrosive. The leaks may be detected by soap solution, halide torch or electronic leak detector the cylinder colour code for this refrigerant is orchid. Presently the most commonly used organic refrigerants are the chloro fluoro derivatives of CH4 and C2H6. The fully halogenated ones with chlorine in their molecule are chloro fluorocarbons, referred to as CFC s. Those containing H atoms in the molecule along with Cl and F atoms are referred to as hydro chloro fluoro carbons or HCFC s. Those having no chlorine atoms in the molecule are hydro fluorocarbons or HFC s. Simple hydrocarbons are HC s. We have HC s, HFC s, HCFC s and CFC s. CFCs and HCFCs were identified as high C.O.P refrigerants, with many favourable properties e.g. non - flammability, low toxicity and material compatibility that led to their widespread use as refrigerants. The F atom in the molecule of a refrigerant makes the substances physiologically more favourable. The Cl atom in the molecule is considered responsible for the depletion of Ozone layer in the upper atmosphere (stratosphere), thus allowing harmful ultraviolet radiation to penetrate through the atmosphere and reach the earth s surface. The H atoms in the molecule imports degree of flammability to the substance depending upon the number of these atoms. Inorganic Refrigerants:

13 13 The inorganic refrigerants were exclusively used before the introduction of halocarbon, refrigerants. These refrigerants are still in use due to their inherent thermodynamic and physical properties. The various inorganic refrigerants are given in the following table 3. Table. 3. Inorganic refrigerants Refrigerant No: R 717 R 729 R 744 R 764 R Chemical name Ammonia Air Carbon dioxide Sulphur dioxide water Chemical formula NH3 CO2 SO2 H2O 1. R-717 (Ammonia): Ammonia (NH3) is one of the oldest and most widely of all the refrigerants. Its greatest application is found in large and commercial reciprocating compression systems where high toxicity is secondary. It is also widely used in absorption systems. It is a chemical compound of nitrogen and hydrogen and under ordinary conditions, it is a colourless gas. Its boiling point at atmospheric pressure is 33.3 C and its melting point from the solid is -78 C. The low boiling point makes it possible to have refrigeration at temperatures considerably below 0 C without using pressures below atmospheric in the evaporator. Its latent heat of vaporisation at -15 C is 1,315 kj/kg. Thus, large refrigerating effects are possible with relatively small sized machinery. The condenser pressure at 30 C is bar. The condensers for R-717 are usually of water cooled type. Ammonia It is a poisonous gas if inhaled in large quantities. In lesser quantities, it is irritating to the, nose and throat. This refrigerant is somewhat flammable and when mixed with air in the ratio of 16% to 25% of gas by volume, will form an explosive mixture. The leaks of this refrigerant may quickly and easily detected by the use of burning sulphur candle which in the presence of ammonia forms white fumes of ammonium sulphite. This refrigerant attacks copper and bronze in the presence of a little moisture but does not corrode iron or steel. Since ammonia is lighter than oil, its separation does not create any problem. The excess oil in the evaporator may be removed by opening a value in the bottom of the evaporator.

14 14 This refrigerant is used in large compression machines using reciprocating compressors and in many absorption type systems. The use of this refrigerant is extensively found in cold storage, warehouse plants, ice cream manufacture, ice manufacture, beer manufacture, food freezing plants etc. 2. R-729 (Air): The dry air is used as a gaseous refrigerant in some compression systems, particularly in air-craft air conditioning. 3. R-744 (Carbon dioxide): The principal refrigeration use of carbon dioxide is same as that of dry ice. It is non-toxic, non- irritating and non-flammable. The boiling point of this refrigerant is extremely low ( C) that at -15 C, a pressure of well over 20.7 bar is required to prevent its evaporation. At a condenser temperature of +30 C, a pressure of approximately 70 bar required to liquify the gas. Its critical temperature is 31 C and triple point is C. Due to its high operating pressure, the compressor of a carbon dioxide refrigerator unit is very small even for a comparatively large refrigerating capacity. However, because of its low efficiency as, to other common refrigerants, it is seldom used in household units, but is used in some industrial applications and aboard ships. 4. R-764 (Sulphur dioxide): This refrigerant is produced by the combustion of sulphur in air. It is very stable refrigerant with a high critical temperature and it is non flammable and non explosive. It has a very unpleasant and irritating odour. SO2 is not injurious to food and is used commercially as a ripener and preservative of foods. It is extremely injurious to flowers, plants and shrubbery. SO2 in its pure state is not corrosive, but when there is moisture in the refrigerating system be held to a minimum. SO2 does not mix readily with oil. R 118 (water): The principal refrigeration use of water is as ice. The high freezing temperature of water limits its use in vapour compression systems. It is used as the refrigerant vapour in some absorption systems and in systems with steam jet compressors.

15 15 Lecture No. 4 Hydro carbon refrigerant Designation system for refrigerants Designation sytem for Dichloro-tetrafluoro-ethane Hydro-carbon Refrigerants: Most of the hydro-carbon refrigerants are successfully used in industrial and commercial installations. They possess satisfactory thermodynamic properties but are highly flammable and explosive. The various hydro-carbon refrigerants are given in the following table 4. Table. 4. Hydrocarbon refrigerants Refrigerant No: R 170 R 290 R 600 R 600a R 1120 R 1130 R 1150 R 1270 Chemical name Ethane Propane Butane Isobutene Tri chloroethylene Dichloroethylene Ethylene Propylene Chemical formula C2 H6 C3 H8 C4 H10 C4 H10 C2 H4 Cl3 C2 H4 Cl2 C2 H4 C3 H6 Hydro carbon refrigerants are not commonly used now a days. Designation System for Refrigerants: The refrigerants are internationally designated as 'R' followed by certain numbers such as R-11, R-12, R-1l4 etc. A refrigerant followed by a two digit number indicates that a refrigerant is derived from methane base while three digit number represents ethane base. The numbers assigned to hydro-carbon and halo-carbon refrigerants have a special meaning. The first digit on the right is the number of fluorine (F) atoms in the refrigerant. The second digit from the right side is more than the number of hydrogen (H) atoms present. The third digit from the right is one less than the number of carbon (C) atoms, but when this digit is zero, it is omitted. The general chemical formula for the refrigerant, either for methane or ethane base, is given as Cm Hn Clp Fq, in which n+p+q = 2m+2 where m = Number of carbon atoms,

16 16 n = Number of hydrogen atoms, p = Number of chlorine atoms, and q = Number of fluorine atoms. The number of the refrigerant is given by R (m - 1)(n + 1) (q). Let us consider the following refrigerants to find its chemical formula and the number. 1. Dichloro-tetrafluoro-ethane: In this refrigerant, Number of chlorine atoms, p = 2 Number of fluorine atoms, q = 4 Number of hydrogen atoms, n = 0 We know that n + p + q = 2m = 2m+2 m=2 i.e. Number of carbon atoms = 2 Thus the chemical formula for dichloro-tetrafluoro-ethane becomes C2Cl2F4 and the number of refrigerant becomes R (2-1) (0+1) (4) or R-114.

17 17 Lecture No. 5 Designation system for Dichloro-difluoro-methane - Selection of a refrigerant 2. Dichloro-difluoro-methane: In this refrigerant, Number of chlorine atoms, p = 2 Number of fluorine atoms, q = 2 Number of hydrogen atoms, n = 0 We know that n + p + q = 2m = 2 m+2 m=1 i.e. Number of carbon atoms = 1 Thus the chemical formula for dichloro-difluoro-methane becomes CCl2F2 and the number refrigerant becomes R(1-1)(0+1)(2) or R-012 i.e. R-12. The brominated refrigerants are denoted by putting an additional B and a number to denote as to how many chlorine atoms are replaced by bromine atoms. Thus R13 B1 is derived from R13 with the replacement of one chlorine atom by a bromine atom. Its chemical formula is there fore CF3 Br. In the case of isomers i.e. compounds with the same chemical formula but different molecular structure, subscripts a, b etc. are used after the designations. In this manner, there are 15 fluoro chloro derivatives of methane, 55 derivatives of ethane, 332 of propane and so on. Besides this, there are brominated ones. There are also azeotropes, which are mixtures of refrigerants but behave like pure substances. They are given arbitrary designations e.g. R 502 for a mixture of 48.8% R 22 and 51.2% R 115. Unsaturated compounds for which n+p+q=2m, are distinguished by putting the digit 1 before (m 1). Thus ethylene is R 1150.

18 18 In the case of common inorganic refrigerants, numerical designations have been given according to their molecular weight added to 700. Thus ammonia whose molecular weight is 17 is designated as R717. Similarly water is designated as R718 and carbon dioxide as R 744. Selection of a Refrigerant: Refrigerants have to be physically non toxic and non flammable. Thermodynamically there is no working substance which could be called an ideal refrigerant. In general, a refrigerant may be required to satisfy requirements which may be classified as thermodynamic, chemical and physical. The choice of a refrigerant for a given application is governed mainly by the refrigerating capacity (very small, small, medium or large) and refrigerating temperature required, such as for air conditioning (5 C), cold storage (-10 to 2 C), refrigerator (-25 C), food freezing (-40 C) etc.

19 19 Lecture No. 6 Thermodynamic properties of refrigerants Thermodynamic Properties of Refrigerants: 1. Boiling temperature: The boiling temperature of the refrigerant at atmospheric pressure should be low. If the boiling temperature of the refrigerant is high at atmospheric pressure, the compressor should be operated at high vacuum. The high boiling temperature reduces the capacity and operating cost of the system. 2. Freezing temperature: The freezing temperature of a refrigerant should be well below the operating evaporator temperature. Since the freezing temperatures of most of the refrigerants are below -35 C, this property is taken into consideration only in low temperature operation. 3. Evaporator and condenser pressure: Both the evaporating (low side) and condensing (high side) pressures should be positive (i.e. above atmospheric) and it should be as near to the atmospheric pressure as possible. The positive pressures are necessary in order to prevent leakage of air and moisture into the refrigerating system. It also permits easier detection of leaks. Too high evaporating and condensing pressures (above atmospheric) would require stronger refrigerating equipment (i.e. compressor, evaporator and condenser) resulting in higher initial cost. The reciprocating compressors are used with refrigerants having low specific volumes, high operating pressures and high pressure ratios. The centrifugal compressors are used with refrigerants having high specific volumes, low operating pressures and low pressure ratios. 4. Critical temperature and pressure: The critical temperature of a refrigerant is the highest temperature at which it can be condensed to a liquid, regardless of a higher pressure. It should be above the highest condensing temperature that might be encountered. If the critical temperature of refrigerant is too near the desired condensing temperature, the excessive power consumption results. 5. Coefficient of performance and power requirements: For an ideal refrigerant operating between - 15 C evaporator temperature and 30 C condenser temperature, the theoretical coefficient of performance for the reversed Carnot cycle is

20 R-II has the coefficient of performance equal to Which is closest to the Carnot value of Practically, all common refrigerants have approximately the same coefficient of performance and power requirement. 6. Latent heat of vaporisation: A refrigerant should have a high latent heat of vaporisation at the evaporator temperature. The high latent heat results in high refrigerating effect per kg of refrigerant circulated which reduces the mass of refrigerant to be circulated per tonne of refrigeration. 7. Specific volume: The specific volume of the refrigerant vapour at evaporator temperature (i.e. volume of suction vapour to the compressor) indicates the theoretical displacement of the compressor. The reciprocating compressors are used with refrigerants having high pressures and low volumes of the suction vapour. The centrifugal or turbo compressors are used with refrigerants having low pressures and high volumes of the suction vapour.

21 21 Lecture No. 7 Chemical requirements of refrigerants Physical properties of refrigerants Chemical Requirements of Refrigerants: 1. Flammability: Hydro-carbon refrigerants such as methane, ethane, propane butane are highly explosive and flammable. Ammonia is also explosive in a mixture with air in concentrations of 16 to 25 per cent of gas by volume of ammonia. None of the CFCs is explosive or flammable. Some of the HFCs which are being considered as alternatives to CFCs because of the ozone layer depletion problem, such as R 152, R152a, R143 etc, are flammable. However, R134a is neither flammable nor explosive. 2. Toxicity: From the consideration of comparative hazard to life from gases and vapours, compounds have been divided into six groups by under writers laboratories. Group six contains compounds with a very low degree of toxicity. It includes CFCs. Group one at the end of the scale, includes the most toxic substances such as sulphur dioxide. Because of flammability and toxicity, ammonia is not used in domestic refrigeration and air conditioning. The use of toxic refrigerants its only limited to cold storages. 3. Solubility of water: Water is only slightly soluble in R-12. At - 18 C, it will hold six parts, per million by weight. The solution formed is very slightly corrosive to any of the common metal. The solubility of water with R-22 is more than R-12 by a ratio of 3 to 1. If more water is present than can be dissolved by the refrigerant, the ice will be formed which chokes the expansion valve capillary tube used for throttling in the system. This may be avoided by the proper dehydration the refrigerating unit before charging and by the use of silica gel drier of the liquid line. Ammonia is highly soluble in water. Due to this reason, a wetted cloth is put at the point of leak to avoid, harm to the persons working in ammonia refrigerating plants. 4. Miscibility: The ability of a refrigerant to mix with oil is called miscibility. This property of refrigerant is considered to be a secondary factor in the selection of a refrigerant. The degree of miscibility depends upon the temperature of the oil and pressure of the refrigerating vapour. The Freon groups of refrigerants are highly miscible

22 22 refrigerants while ammonia, carbon dioxide, sulphur dioxide and methyl chloride are relatively non-miscible. In compressor, some oil is carried by the high temperature refrigerant vapour to the condenser and ultimately to the expansion value and evaporator. In the evaporator, as the refrigerant evaporates, a distillation process occurs and the oil separates will result in a reduced heat transfer coefficient, oil choking in the evaporator due to restriction caused to refrigerant flow and even blockage and ultimately to oil starvation in the compressor. The miscible refrigerants are advantageous from the heat transfer point of view. They give better lubrication as the refrigerant acts as a carrier of oil to the moving parts. The miscible refrigerants also eliminate oil-separation problems and aid in the return of from the evaporator. 5. Effect on perishable materials: The refrigerants used in cold storage plant and domestic refrigerators should be such that in case of leakage, it should have no effect on perishable materials. The freon group of refrigerants have no effect upon dairy products, vegetables, meat, flowers and furs. There will be no change in colour, taste or texture of the material when exposed to freon. Methyl chloride vapours have no effect upon furs, flowers, eating foods or drinking beverages. Sulphur dioxide destroys flowers, plants and furs, but it does not effect foods. Ammonia dissolves easily in water and becomes alkaline in nature. Since most fruits and vegetables are acidic in nature, therefore ammonia reacts with these products and spoils the taste. 6. Action with material of construction: Ammonia attacks copper and copper bearing materials. Iron and steel are however, found suitable for use with ammonia. The recommended material for use piping with halocarbons is copper. Copper continues to be preferred for construction of heat exchangers in Freon systems. Physical Properties of Refrigerants: 1. Stability and inertness: An ideal refrigerant should not decompose at any temperature normally encountered in the refrigerating system. It should not form higher

23 23 boiling point liquids solid substances through polymerization. Some refrigerants disintegrate forming non condensable gases which causes high condensing pressure and vapour lock. The disintegration of refrigerant may be due to reaction with metals. In order to avoid this, a refrigerant should be inert with respect to all materials used in refrigerating system. The freon group of refrigerants are stable upto a temperature of 535 C. Above this temperature, it decomposes and forms corrosive and poisonous products. The freon refrigerants are not used with rubber gaskets as it acts as a solvent with rubber. Since sulphur dioxide does not decompose below 1645 C, therefore it is one of the most stable refrigerants. 2. Corrosive property: The freon group of refrigerants are non-corrosive with practically all metals. Ammonia is used only with iron or steel. Sulphur dioxide is non corrosive to all metals in the absence of water because sulphur dioxide reacts with water and forms sulphuric acid. 3. Viscosity: The refrigerant in the liquid and vapour states should have low viscosity. The low viscosity of the refrigerant is desirable because the pressure drops in passing through liquid and suction lines are small. The heat transfer through condenser and evaporator is improved at low viscosities 4. Thermal conductivity: The refrigerant in the liquid and vapour states should have high thermal conductivity. 5. Dielectric strength: The dielectric strength of a refrigerant is important in hermetically 'sealed units in which the electric motor is exposed to the refrigerant. As the windiness are cooled by suction vapour. The relative dielectric strength of the refrigerant is the ratio of the dielectric strength of refrigerant ratio of the dielectric strength of the refrigerant is the ratio of the dielectric strength of nitrogen and the refrigerant. vapour mixture to the dielectric strength of nitrogen at atmospheric pressure and room temperature. The dielectric strengths of R22 is This shows that fluorocarbons are good electrical. 6. Leakage tendency: The leakage tendency of a refrigerant should be low. If there is a leakage of refrigerant, it should be easily detectable. The leakage occurs due to opening in the joints or flaws in material used for construction. Since the fluorocarbon

24 24 refrigerants are colourless, therefore, their leakage will increase the operating cost. The ammonia leakage is easily detected due to its pungent odour. The leakage of fluorocarbon refrigerants may be detected by soap solution, a halide torch or an electronic leak detector. The latter is generally used in big refrigerating plants. The ammonia leakage is detected by using burning sulphur candle which in the presence of ammonia forms white fumes of ammonium sulphite. 7. Cost: The cost of refrigerant is not so important in small refrigerating units but it is very important in high capacity refrigerating systems like industrial and commercial. The ammonia being the cheapest is widely used in large industrial plants such as cold storages and ice plants. The refrigerant R-22 is costlier than refrigerant R-12. The cost of losses due to leakage is also important.

25 25 Lecture No. 8 Secondary refrigerants Brines- Applications of various brines Secondary refrigerants - brines: Brines are secondary refrigerants and are generally used where temperatures are required to be mainted below the freezing point of water i.e. 0 C. In case the temperature involved is above the freezing point of water (0 C), then water is commonly used as a secondary refrigerant. Brine is a solution of salt in water. It may be noted that when salt is mixed in water, then the freezing temperature of the solution becomes lower than that of the water. This is due to the fact that the salt while dissolving in water takes off its latent heat from the solution and cools it below the freezing point of water. The mass of the salt in the solution expressed as the percentage of the mass of solution is known as concentration of the solution. As the concentration of the solution increases, its freezing point decreases. But if the concentration of the salt is increased beyond a certain point, the freezing point increases instead of decreasing. The point, at which the freezing temperature is minimum, is known as eutectic temperature and the concentration at this point is known as eutectic concentration. The brine used in a particular application should have a concentration for which the freezing point of the brine is at least 5 C to 8 C lower than the brine temperature required. The brines commonly used are calcium chloride (CaCl2), Sodium chloride (Na Cl ) i.e. common salt and glycols such as ethylene glycol, propylene glycol etc. The calcium chloride brine has the eutectic temperature of -55 C at salt concentration of 30% by mass. This brine is primarily used where temperatures below 18 C are required. It is generally used in industrial process cooling and product freezing. The chief disadvantages of calcium chloride brine are its dehydrating effect and its tendency to impart a bitter taste to food products. The sodium chloride brine has the eutectic temperature of C at salt concentration of 23% by mass. This brine is used in chilling and freezing of meat and fish.

26 26 Both of the above two brines are corrosive in nature for metallic containers which put limitation on their use. Also the thermal properties of the above two brines are less satisfactory. Other water soluble compounds known as antifreeze are also used for decreasing the freezing point of water for certain refrigeration uses. Ethylene and propylene glycol have number good properties. Since they are non-corrosive and nonelectrolytic even in the presence of water, these brines are most extensively used as antifreeze elements. Table.5. Applications of various brines Applications 1. Breweries 2. Diaries 3. Food process 4. Ice creams 5. Ice plant 6. Meat packaging Brine used Propylene glycol Sodium chloride, Calcium chloride, Propylene glycol Sodium chloride, Calcium chloride, Propylene glycol Calcium chloride, Propylene glycol Sodium chloride Sodium chloride, Calcium chloride

27 27 Lecture No. 9 Types of Refrigerators Air Refrigerator Vapour refrigerator Advantages and Disadvantage of vapour compression refrigeration system over air refrigeration system Types of Refrigerators: Based on working medium, they are classified as 1. Air Refrigerator 2. Vapour Refrigerator 1. Air Refrigerator: In this working agent is air. 2. Vapour Refrigerator: In this working agent are vapours like ammonia, CO2, SO2, Freon etc. The main difference between these two classes of refrigeration is the non condensability of air and condensability & evaporativeness of the above vapours within the working range of temperatures of refrigerators. This means that for using refrigerator, air can not be subjected to the change of state (from liquid to vapour and vice versa). In other words, air behaves as a perfect gas and heat changes are brought about by changes in temperature only i.e. only sensible heat changes are taking place. Unlike this, in the case of vapours, heat changes are in the latent heat form and fluid alternatively changes from liquid to vapour and back to liquid. There are two types of machine under vapour refrigerators. They are 1. Vapour Compression 2. Vapour Absorption The main difference between the two is the manner in which the external heat is added to the vaporised refrigerants. Simple vapour compression Refrigeration system: A vapour compression refrigeration system is an improved type of air refrigeration system in which a suitable working substance, termed as refrigerant, is used. It condenses and evaporates at temperatures and pressures close to the atmospheric conditions. The refrigerants usually used for this purpose are ammonia (NH3), Carbon dioxide (CO2) and Sulphur dioxide (SO2).

28 28 The refrigerant used does not leave the system but is circulated throughout the system alternately condensing and evaporating. In evaporating, the refrigerant absorbs its latent heat from the brine (salt water It is used as it has a very low freezing temperature) which is used for circulating it around the cold chamber. While condensing, it gives out its latent heat to the circulating water of the cooler.the vapour compression refrigeration system is therefore a latent heat pump as it pumps its latent heat from the brine and delivers it to the cooler. Now a days the vapour compression refrigeration system is used for all purpose of refrigeration. It is generally used for all industrial purposes from a small domestic refrigerator to a big air conditioning plant. Advantages and disadvantages of vapour compression refrigeration system over air refrigeration system: Advantages: 1. It has smaller size for the given capacity of refrigeration. 2. It has less running cost. 3. It can be employed over a large range of temperatures. 4. The coefficient of performance is quite high. Disadvantages: 1. The initial cost is high. 2. The prevention of leakage of the refrigerant is the major problem in vapour compression system.

29 29 Lecture No. 10 Mechanism of a simple vapour compression refrigeration systems Mechanism of simple vapour compression refrigeration: Simple vapour compression refrigeration system (Fig. 1) consists of the following five essential parts. Figure 1. Simple vapour compression refrigeration system 1. Compressor: The low pressure and low temperature vapour refrigerant from evaporator is drawn into the compressor through the inlet or suction valve A, where it is compressed to a high pressure and temperature. The high pressure and temperature vapour refrigerant is discharged into the condenser through the delivery or discharged into the condenser through the delivery or discharge value B. this high pressure must be below the critical pressure of the refrigerant.

30 30 2. Condenser: The condenser or cooler consists of coils of pipe in which the high pressure and high temperature vapour refrigerant is cooled and condensed. The refrigerants, while passing through the condenser, gives up its latent heat to the surrounding condensing medium which is normally air or water. 3. Receiver: The condensed liquid refrigerant from the condenser is stored in a vessel known as receiver from where it is supplied to the evaporator through the expansion valve or refrigerant control valve. 4. Expansion valve: It is also called throttle valve or refrigerant control valve. The function of the expansion valve is to allow the liquid refrigerant under high pressure and temperature to pass at a controlled rate after reducing its pressure and temperature. Some of the liquid refrigerant evaporates as it passes through the expansion valve, but the greater portion is vaporised in the evaporator at the low pressure and temperature. The liquid / gas mixture leaving the expansion valve is termed flash gas. 5. Evaporator: An evaporator consists of coils of pipe in which the liquid vapour refrigerant at low pressure and temperature is evaporated and changed into vapour refrigerant at low pressure and temperature. In evaporating, the liquid vapour refrigerant absorbs its latent heat of vaporisation from the medium (air, water or brine) which is to be cooled. Note: In any compression refrigeration system, there are two different pressure conditions. One is called the high pressure side and the other is known as low pressure side. The high pressure side includes the discharge line (i.e. piping from delivery value B to the condenser), Condenser, receiver and expansion valve. The low pressure side includes the piping from the expansion valve to the evaporator and the suction line (i.e. piping from the evaporator to the suction valve A). The expansion valve separates the high pressure region from the low pressure region.

31 31 Lecture No. 11 Evaporators Capacity of an evaporator Factors affecting heat transfer capacity of an evaporator Types of evaporator Introduction:The evaporator is an important device used in the low pressure side of a refrigeration system. The liquid refrigerant from the expansion valve enters into the evaporator where it boils and changes into vapour. The function of an evaporator is to absorb heat from the surrounding location or medium which is to be cooled, by means of a refrigerant. The temperature of the boiling refrigerant in the evaporator must always less than that of the surrounding medium that the heat flows to the refrigerant. The evaporator becomes cold and remains cold d to the following two reasons. 1. The temperature of the evaporator coil is low due to the low temperature of the refrigerant inside the coil. 2. The low temperature of the refrigerant remains unchanged because any heat it absorbs is converted to latent heat as boiling proceeds. Note: The evaporator is also known as a cooling coil, a chilling coil or a freezing coil. The evaporator cools by using the refrigerant's latent heat of vaporisation to absorb heat from the medium being cooled. Capacity of an Evaporator: The capacity of an evaporator is defined as the amount of heat absorbed by it over a given period of time. The heat absorbed or heat transfer capacity of an evaporator is given by Q = UA (T2 - T1) W or J/s where U = Overall heat transfer coefficient in W/m2 C, A = Area of evaporator surface in m2, T2 = Temperature of medium to be cooled (or temperature outside the evaporator)

32 32 C, and T1 = Saturation temperature of refrigerant at evaporator pressure (or temperature inside the evaporator) in C. Factors Affecting the Heat Transfer Capacity of an Evaporator: 1. Material: In order to have rapid heat transfer in an evaporator, the material used for the construction of an evaporator coil should be a good conductor of heat. The material which is not affected by the refrigerant must also be selected. Since metals are best conductors of heat, therefore they' are always used for evaporators. Iron and steel can be used with all common refrigerants. Brass and copper are used with all refrigerants except ammonia. Aluminium should not be used with Freon. 2. Temperature difference: The temperature difference between the refrigerant within the evaporator and the product to be cooled plays an important role in the heat transfer capacity of an evaporator. The following table shows the suggested temperature difference for some of the products to be cooled. It may be noted that a too low temperature difference (below g0c) may cause slime on certain products such as meat or poultry. On the other hand, too high a temperature difference causes excessive dehydration. 3. Velocity of refrigerant: The velocity of refrigerant also affects the heat transfer capacity of an evaporator. If the velocity of refrigerant flowing through the evaporator increases, the overall heat transfer coefficient also increases. But this increased velocity will cause greater pressure loss in the evaporator. Thus the only recommended velocities for different refrigerants which give high heat transfer rates and allowable pressure loss should be used. 4. Thickness of the evaporator coil wall: The thickness of the evaporator coil wall also affects the heat transfer capacity of the evaporator. In general, the thicker the wall, the slower is the rate of heat transfer. Since the refrigerant in the evaporator coils is under pressure, therefore the evaporator walls must be thick enough to withstand the effects of that pressure. It may be noted that the thickness has only a slight effect on total heat transfer capacity because the evaporators are usually made from highly conductive materials.

33 33 5. Contact surface area: An important factor affecting the evaporator capacity is the contact surface available between the walls of evaporator coil and the medium being cooled. The amount of contact surface, in turn, depends basically on the physical size and shape of the evaporator coil. Types of Evaporators 1. According to the type of construction (a) Bare tube coil evaporator, (b) Finned tube evaporator, (c) Plate evaporator, (d) Shell and tube evaporator, (e) Shell and coil evaporator, (f) Tube-in-tube evaporator, 2. According to the manner in which liquid refrigerant is fed (a) Flooded evaporator, and (b) Dry expansion evaporator 3. According to the mode of heat transfer (a) Natural convection evaporator, (b) Forced convection evaporator 4. According to operating conditions (a) Frosting evaporator, (b) Non-frosting evaporator, (c) Defrosting evaporator

34 34 Lecture No. 12 Bare tube coil evaporator Finned evaporator Plate evaporator Shell and tube evaporator 1.Bare Tube Coil Evaporators: The simplest type of evaporator is the bare tube coil evaporator, as shown in Fig. 2, and is also known as prime-surface evaporators. Because of its simple construction, the bare tube coil is easy to clean and defrost. Fig. 2. Bare tube coil evaporator. A little consideration will show that this type of evaporator offers relatively little surface. Contact area as compared to other types of coils. The amount of surface area may be increased by simply extending the length of the tube, but there are disadvantages of excessive tube length. The effective length of the tube is limited by the capacity of expansion valve. If the tube is too long for the valve's capacity, the liquid refrigerant will tend to completely vaporise early in its progress through the tube, thus leading to excessive superheating at the outlet. The long tubes will also cause considerably greater pressure drop between the inlet and outlet of the evaporator. This results in a reduced suction line pressure. The diameter of the tube in relation to tube length may also be critical. If the tube diameter is too large, the refrigerant velocity will be too low and the volume of refrigerant will be too great in relation to the surface area of the tube to allow complete vaporisation. This, in turn, may allow liquid refrigerant to enter the suction line with possible damage to the compressor (i.e. slugging). On the other hand, if the diameter is too small, the pressure drop due to friction may be too high, and will reduce the system efficiency.

35 35 The bare tube coil evaporators may be used for any type of refrigeration requirement. Its use is, however limited to applications where the box temperatures are under 0 C and in liquid cooling, because the accumulation of ice or frost on these evaporators has less effect on the heat transfer than on those equipped with fins. The bare tube coil evaporators are also extensively used in house-hold refrigerators because they are easier to keep clean. 2. Finned Evaporators: The finned evaporator, as shown in Fig. 3, consists of bare tubes or coils over which the metal plates or fins are fastened. Fig. 3. Finned evaporator. The metal fins are constructed of thin sheets of metal having good thermal conductivity. The shape, size or spacing of the fins can be adapted to provide best rate of heat transfer for a given application. Since the fins greatly increases the contact surfaces for heat transfer, therefore the finned evaporators are also called extended surface evaporators. The finned evaporators are primarily designed for air conditioning applications where the refrigerator temperature is above 0 C. Because of the rapid heat transfer of the finned evaporator, it will defrost itself on the off cycle when the temperature of the coil is near 0 C. A finned coil should never be allowed to frost because the accumulation of frost between the fins reduces the capacity. The air conditioning coils which operate at suction temperatures which are high enough so that frosting never occurs, have fin spacing as small as 3 mm. The finned coils which frost on the on cycle and defrost on the off cycle have wider fin spacing.

36 36 3. Plate Evaporators: A common type of plate evaporator is shown in Fig. 4. In this type of evaporator, the coils are either welded on one side of a plate or between the two plates which are welded together at the edges. The plate evaporators are generally used in house-hold refrigerators, home freezers, beverage coolers, ice cream cabinets, locker plants etc. Fig. 4. Plate evaporator. 4. Shell and Tube Evaporators: The shell and tube evaporator, as shown in Fig. 5, is similar to a shell and tube condenser. It consists of a number of horizontal tubes enclosed in a cylindrical shell. The inlet and outlet headers with perforated metal tube sheets are connected at each end of the tubes. Fig. 5. Shell and tube evaporator.

37 37 These evaporators are generally used to chill water or brine solutions. When it is operated as a dry expansion evaporator, the refrigerant circulates through the tubes and the liquid to be cooled fills the space around the tubes within the shell. The dry expansion shell and tube evaporators are used for refrigerating units of 2 to 250 TR capacity. When it is operated as a flooded evaporator, the water or brine flows through the tubes and the refrigerant circulates around the tubes. The flooded shell and tube evaporators are used for refrigerating units of 10 to 5000 TR capacity.

38 38 Lecture No. 13 Shell and coil evaporator Tube in tube Flooded evaporator Dry expansion evaporator 1. Shell and coil evaporator: The shell and coil evaporator, as shown in Fig. 6, are generally dry expansion evaporator to chill water. The cooling coil is a continuous tube that can be in the form of a single or double spiral. The Shell may be sealed or open. The sealed shells are usually found in shell and coil evaporators used to cool drinking water. The evaporators having flanged shells are often used to chill water in secondary refrigeration system. Fig. 6. Shell and coil evaporator. They are used where small capacity (2 to 10 TR) liquid cooling is required. It may be noted that the shell and coil evaporator is restricted to operation above 5 C in order to prevent the freezing problems. 2. Tube In Tube or Double Tube Evaporator: The tube-in-tube evaporator (or double tube evaporator) as shown in Fig. 7, consists of one tube inside another tube. The liquid to be cooled flows through the inner tube while the primary refrigerant or secondary refrigerant (i.e. water, air or brine) circulates in the space.between the two tubes. The tube- in-tube evaporator provides high heat transfer rates. However, they require more space than shell and tube evaporators of the same capacity. These evaporators are used for wine cooling and in petroleum industry for chilling of oil.

39 39 Fig. 7. Tube-In-tube or double tube evaporator. 3. Flooded Evaporator: In a flooded evaporator, as shown in Fig. 8, a constant liquid refrigerant level is always maintained. Fig. 8. Flooded evaporator. A float control valve is used as an expansion device which maintains constant liquid level in the evaporator. The liquid refrigerant from the receiver passes through a low side float control valve and accumulator before entering the evaporator coil. The accumulator (also called a surge drum or surge tank) serves as a storage tank for the

40 40 liquid refrigerant. It maintains a constant liquid level in the evaporator and helps to separate the liquid refrigerant from the vapour returning to the compressor. Due to the heat supplied by the substance to be cooled, the liquid refrigerant in the evaporator coil vaporises and thus the liquid level falls down. The accumulator supplies more liquid to the evaporator in order to keep the liquid refrigerant in the evaporator at proper level. In this way, the level of liquid refrigerant in the accumulator also falls down. Since the float within the float chamber rests on liquid refrigerant at the same level as that in the accumulator, the float also falls down and opens the float valve. Now the liquid refrigerant from the receiver is admitted into the accumulator. As the liquid level in the accumulator rises and reaches to the constant level, the float also rises with it until the float control valve closes. Since the evaporator is almost completely filled with liquid refrigerant, therefore the vapour refrigerant from the evaporator is not superheated but it is in a saturated condition. In order to prevent liquid refrigerant to enter into the compressor, an accumulator is generally used with the flooded evaporators. The liquid refrigerant trapped in the accumulator is re-circulated through the evaporator. The evaporator coil is connected to the accumulator and the liquid flow from the accumulator to the evaporator coil is generally by gravity. The vapour formed by vaporising the liquid in the coil being lighter, rises up and passes on to the top of the accumulator from where it is supplied to the suction side of the compressor. The baffle plate arrests any liquid present in the vapour. The advantage of the flooded evaporator is that the whole surface of the evaporator coil is in contact with the liquid refrigerant under all the load conditions. Thus, it gives high heat transfer rates (i.e. more efficient cooling) than a dry expansion evaporator of the same size. However, the flooded evaporator is more expensive to operate because it requires more refrigerant charge. The flooded evaporators have many industrial applications, especially in the chemical and food processing industries. 4. Dry Expansion Evaporator: The dry expansion evaporators are not really dry at all. They simply use relatively little refrigerant as compared to flooded evaporators having the same coil volume. They

41 41 are usually only one-fourth to one-third filled with liquid refrigerant. A simple bare-tube dry expansion evaporator is shown in Fig. 9. Fig. 9. Dry expansion evaporator. In dry expansion evaporators, the liquid refrigerant from the receiver is fed by the expansion valve to the evaporator coil. The expansion valve controls the rate of flow of liquid refrigerant in a way that all the liquid refrigerant is vaporised by the time it reaches at the end of the evaporator coils or the suction line to the compressor. The vapour is also superheated to a limited extent. In a dry expansion system, the refrigerant does not recirculate within the evaporator as in flooded type evaporator.

42 42 Lecture No. 14 Natural convection evaporator Forced convection evaporator Frosting evaporator Non Frosting evaporators Defrosting evaporators 1. Natural Convection Evaporator: The natural convection evaporators are used where low air velocity and minimum hydration of the product is desired. The domestic refrigerators, water coolers and small freezers have natural convection evaporators. The circulation of air in a domestic refrigerator by natural convection is shown in Fig. 10. The evaporator coil should be placed as high as possible in the refrigerator because the cold air falls down as it leaves the evaporator. Fig. 10. Air circulation with correct shape of coil. In Natural Convection, the velocity of air depends upon the temperature difference between the evaporator and the space to be cooled. When the temperature difference, using a natural convection evaporator, is low (less than 8 C), the velocity of air is too low for satisfactory circulation. A low temperature difference than 8 C may even cause slime on certain products such as meat or poultry. On the other hand, too great a temperature difference causes excessive dehydration. The coil should occupy at least 2/3rd of the width of the path of the air circulation and 3/4th the length of box. The natural convection can be improved by the use of baffles. These are simply Sheet metal plates which guide the ascending and descending air currents in their proper channels. 2. Forced Convection Evaporators: In this the air is forced over the refrigerant cooled coils and fins. s is done by a fan driven by an electric motor. The fins are provided to increase the heat transfer rate. The forced convection evaporators are more efficient than natural convection evaporators. They are suited for cooling units as well as for refrigerator cabinets used to store bottled beverages or foods in sealed containers.

43 43 They may be divided into three groups according to the velocity of air leaving the unit. a. Low velocity cooling units: These units have a discharge air rate from 60 m/min to 90 m/min. They are used in comfort air conditioning where low noise and low air velocity rates are needed. Both centrifugal and propeller type fans are used with low velocity cooling units. b. Medium velocity cooling units: They have an exit velocity of air from 150 m/min to 240 m/min. They are frequently used in refrigerators and freezers where drafts and noise are not problem. The propeller fans are usually the source of air circulation in these units. c. High velocity cooling units: They have a discharge air rate above 240 m/min. They re used principally in blast freezers in special product refrigerators requiring quick pull-down of temperature. The high velocity cooling units usually use centrifugal fans as the source of air circulation. 3. Frosting Evaporators: They always operate at temperatures below 0 C. This means that the coil frosts continually when in use and it must be removed at regular intervals for most efficient operation. The frost which forms on the evaporator comes from the moisture in the air. as the frost grows in thickness, the coil or cooling efficiency decreases until the ice and frost is removed. The evaporators in household refrigerators, bare pipe coils in storage boxes and low temperature evaporators fall under the frosting evaporators. 4. Non-frosting Evaporators: The non-frosting evaporators operate at a temperature above 0 C at all times. So, the frost does not form on the evaporator. Occasionally, the evaporator may build up a light coat of frost just before the compressor shuts off. This frost immediately melts on the off cycle. The big advantage of a non-frosting evaporator is its operation at a temperature close to freezing (0.6 C to 1 C). At this temperature, it does not draw the moisture out of the air rapidly.

44 44 This permits to maintain a relative humidity from 75 to 85% in the cabinet. This helps to keep the food fresh and stops shrinking in weight. They are used to high temperature work such as air conditioning, bakery refrigeration and candy storage. 5. Defrosting Evaporators:A defrosting evaporator is one in which frost accumulates on the coils when the compressor is running and melts after the compressor shuts off. During the running of the compressor shut off. During the running of the compressor, the evaporator remains at a temperature of about -7 C to - 6 C. At these temperatures, the frost forms on the coil surface. But after the compressor stops, the coil warms upto a temperature slightly above 0 C and the frost melts before the compressor starts again. A defrosting evaporator is usually a finned coil because of the necessity of rapid heat transfer during the off cycle. It also keeps a high relative humidity of about 90 to 95 %.

45 45 Lecture No. 15 Compressor Classification Suction pressure, Discharge pressure, Compression ratio, Suction volume, Stroke volume, Clearance factor, Compressor capacity, Volumetric efficiency Introduction: A refrigerant compressor, as the name indicates, is a machine used to compress vapour refrigerant from the evaporator and raise its pressure so that the corresponding saturation temperature is higher than that of cooling medium. It also continually circulates the refrigerant through the refrigerating system Since the compression of refrigerant requires some work to be done on it, therefore, compressor must be driven by some prime over. Note: Since the compressor virtually takes the heat at a low temperature from the evaporator and pumps it at the high temperature to the condenser, it is often referred to as a heat pump. Classification of Compressors: 1. According to the method of compressors (a) Reciprocating compressors (b) Rotary compressors (c) Centrifugal compressors 2. According to the number of working strokes (a) Single acting compressors (b) Double acting compressors 3. According to the number of stages (a) Single stage (or single cylinder) compressors (b) Multi-stage (or multi-cylinder) compressors 4. According to the method of drive employed (a) Direct drive compressors

46 46 (b) Belt drives compressors 5. According to the location of the prime mover (a) Semi-hermetic compressors (direct drive, motor and compressor in separate housings), (b) Hermetic compressors (direct drive, motor and compressor in same housings). Important Terms: The important terms used in reciprocating compressor (Fig. 11) are given below. Fig. 11. Reciprocating compressor 1. Suction pressure: It is the absolute pressure of refrigerant at the inlet of a compressor. 2. Discharge pressure: It is the absolute pressure of refrigerant at the outlet of a compressor. 3. Compression ratio: It is the ratio of absolute discharge pressure to the absolute suction pressure. Since the absolute discharge pressure is always more than the absolute suction pressure, the value of compression ratio is more than unity. The compression ratio may also be defined as the ratio of total cylinder volume to the clearance volume. 4. Suction volume: It is the volume of refrigerant sucked by the compressor during its suction stroke. It is the volume swept by the piston when it moves from its inner dead position to outer dead centre.

47 47 Mathematically, Vρ = π 4 D2L Where D = Diameter of cylinder, L = Length of piston stroke. 6. Clearance factor: It is the ratio of clearance volume (Vc) to the piston displacement volume (Vp). Mathematically, clearance factor, C= Vc Vp 7. Compressor capacity: It is the volume of the actual amount of refrigerant passing through the compressor in a unit time. It is equal to the suction volume (V s). It is expressed in m3/s. 8. Volumetric efficiency: It is the ratio of the compressor capacity or the suction volume (Vs) to the piston displacement volume (Vp) Mathematically, ηv = vs vp A good compressor has a volumetric efficiency of 70 to 80 %.

48 48 Lecture No. 16 Reciprocating compressor - Parts of a reciprocating compressor - Cycle of a reciprocating compressor Reciprocating Compressors: The compressors in which the vapour refrigerant is compressed by the reciprocating (i.e. back and forth) motion of the piston, are called reciprocating compressors. These compressors used for refrigerants which have comparatively low volume per kg and a large differential pressure, such as ammonia (R-717), R-12, R-22, and methyl chloride (R-40). The reciprocating compressors are available in sizes from a fraction of a ton which are used in domestic refrigerators and upto about 100 tons for large capacity installations. The two types of reciprocating compressors in general use are single acting vertical compressors and double acting horizontal compressors. The single acting compressors (Fig. 12) usual have their cylinders arranged vertically, radially or in a V or W form. The double acting compressors usually have their cylinders arranged horizontally. The compressor may be operated with an electric motor or an internal combnstion engine. Fig. 12. Parts of reciprocating compressor. An important parameter that influences the performance of a compressor is the compressor capacity. The compressor capacity is affected by several factors that are inherent to the design of equipment include

49 49 a) Piston displacement b) Clearance between the piston head and the end of cylinder when the piston is at the top of its stroke and c) Size of the suction and discharge valves. Other factors that influence the compressor capacity are associated with the operating conditions a) Revolutions per minute b) Type of refrigerant c) Suction pressure d) Discharge pressure. Fig. 13. A complete cycle of a reciprocating compressor. The piston displacement can be examined by considering the above Fig. 13 Point A refers to the cylinder volume of 100% with the vapour at the suction pressure. The vapours are compressed to about 15% of cylinder volume shown by point B. during the compression cycle, both the suction valve and discharge value remain closed. During the discharge cycle B C, the discharge valve opens and the gas is released. This value represents the volume is decreased to 5%. This value represents the volume between the head of the piston and the end of the cylinder.

50 50 As the piston starts travelling in the opposite direction, during the expansion cycle CD, the high pressure gas remaining in the clearance expands. Attempts are always made to keep the clearance as small as practically possible, since the clearance space represents a loss in capacity process DA represents suction; the suction value remains open during this process and the vapour are admitted to the cylinder. The piston retracts to the location indicated by point A the cycle is then repeated. In Actual practice the suction and discharge pressure lines (BC and DA) are not straight lines due to the pressure drop across the values. The discharge lines above and the suction line lies below the theoretical straight lines indicated in figure. This difference can be explained by the fact that at the beginning of discharge, the cylinder pressure must be higher than the discharge pressure to force the value open against the spring pressure. Similarly, at the beginning of the suction process, the pressure must be lower than the suction pressure to open the suction value. Compressor capacity control: The speed can be controlled by the use of a variable speed electric motor. Bypassing gas is the most commonly used method of capacity control. In one bypass system, the discharge side of the compressor is connected to the suction side. A solenoid value is used to bypass the discharge gas directly to the suction side of the compressor. Thus the refrigerant delivered from a bypassed cylinder of the compressor is stopped, and the compressor capacity is reduced. The bypass system does not result in any significant savings in power requirement. A more desirable bypass system involves keeping the suction value open, allowing the gas to simply surge back and forth in the cylinder. Thus, the bypassed cylinder delivers no refrigerant. Hermetically sealed solenoids are used to control the opening of the suction values. In mulitcylinder compressors, it is possible to inactivate desired cylinders when the load is low. For a four cylinder compressor with three cylinders inoperative, the compressor capacity can be reduced by as much as 75%. In a double acting reciprocating compressor, the suction and compression takes place on both sides of the piston. It is thus obvions, that such a compressor will supply double the volume of refrigerant than a single acting reciprocating compressor.

51 51 The low capacity compressors are air cooled. The cylinders of these compressors usually have fins to provide better air cooling. The high capacity compressors are cooled by providing water jackets around the cylinder. Hermetic Sealed Compressors : When the compressor and motor operate on the same shaft and are enclosed in a common casing, they are known as hermetic sealed compressors. These types of compressors eliminate the use of crank shaft seal which is necessary in ordinary compressors in order to prevent leakage of refrigerant. These compressors may operate on either reciprocating or rotary principle and may be mounted with the shaft in either the vertical or horizontal position. The hermetic units are widely used for small capacity refrigerating systems such as in domestic refrigerators, home freezers and window air conditioners. The hermetic sealed compressors have the following advantages and disadvantages Advantages:1. The leakage of refrigerant is completely prevented. 2. It is less noisy. 3. It requires small space because of compactness. 4. The lubrication is simple as the motor and compressor operate in a sealed space with the lubricating oil. Disadvantages: 1. The maintenance is not easy because the moving parts are inaccessible. 2. A separate pump is required for evacuation and charging of refrigerant.

52 52 Lecture No. 17 Rotary compressor Centrifugal compressor Advantages and disadvantages of a centrifugal compressor Rotary Compressors:In rotary compressors, the vapour refrigerant from the evaporator is compressed due to the movement of blades. The rotary compressors are positive displacement type compressors. Since the clearance in rotary compressors is negligible, they have high volumetric efficiency. These compressors may be used with refrigerants R-12, R-22, R-114 and ammonia. There are two basic types of rotary compressors: 1. Single stationary blade type rotary compressor 2. Rotating blade type rotary compressor 1. Single stationary blade type rotary compressor: It consists of a single stationary cylinder, a roller (or impeller) and a shaft (Fig. 14). The shaft has an eccentric on which the roller is mounted. A blade is set into the slot of a cylinder in such a manner that it always maintains contacts with the roller by means of a spring. The blade moves in and out of the slot to follow the rotor when it rotates. Since the blade separates the suction and discharge ports, it is often called a sealing blade. When the shaft rotates, the roller also rotates so that it always touches the cylinder wall. Fig.a) shows the completion of intake stroke (i.e. the cylinder is full of low pressure and temperature vapour refrigerant) and the beginning of compression stroke. When the roller rotates, the vapour refrigerant ahead of the roller is being compressed and the new intake from the evaporator is drawn into the cylinder, as shown in Fig. b). As the roller turns towards mid position as shown in Fig. b) more vapour refrigerant is drawn into the cylinder while the compressed refrigerant is discharged to the condenser. At the end of compression stroke, as shown in Fig. d), most of the compressed vapour refrigerant is passed through the discharge port to the condenser. A new charge of refrigerant is passed through the discharged port to the condenser. A new charge of refrigerant is drawn into the cylinder. This in turn, is compressed and discharged to the

53 53 condenser. In this way, the low pressure and temperature vapour refrigerant is compressed gradually to a high pressure and temperature. Fig. 14. Stationary single blade rotary compressor. 2. Rotating blade type rotary compressor: It consists of a cylinder and a slotted rotor containing a number of blades (Fig. 15). The centre of the rotor is eccentric with the centre of the cylinder. The blades are forced against the rotation of the motor.

54 54 Fig. 15. Rotating blade type rotary compressor. The low pressure and temperature vapour refrigerant from the evaporator is drawn through suction port. As the rotor turns, the suction vapour refrigerant entrapped between the two adjacent blades is compressed. The compressed refrigerant at high pressure and temperature is discharged through the discharge port to the condenser. 3. Centrifugal compressor:the centrifugal compressor (Fig. 16) for refrigeration systems was designed and developed by Dr. Willis H. Carrier in This compressor increases the pressure of vapour refrigerant to a high pressure by centrifugal force. The centrifugal compressor is generally used for refrigerants that require large displacement and low condensing pressure, such as R -11 and It 113. However, the refrigerant R-12 is also employed for large capacity applications an lowtemperature applications. It consists of an impeller to which a number of curved vanes are fitted symmetrically, the impeller rotates in an air tight volute casing with inlet and outlet points. The impeller draws in low pressure vapour refrigerant from the evaporator. When the impeller rotates, it pushes the vapour refrigerant from the centre of the impeller to its periphery by centrifugal force. The high speed of the impeller leaves the vapour refrigerant at a high velocity at the vane tips of the impeller. The kinetic energy thus attained at the impeller outlet is converted into pressure energy when the high velocity vapour refrigerant passes over the diffuser. The diffuser is normally a vaneless type as it permits more efficient part load operation which is quite usual in any air- conditioning plant. The volute casing collects the refrigerant from the diffuser and it further converts

55 55 the kinetic energy into pressure energy before it leaves the refrigerant to the condenser. The centrifugal compressors have no valves, pistons and cylinders. The only wearing parts are the main bearings. Fig. 16. Centrifugal compressor. Advantages and Disadvantages of Centrifugal Compressors over Reciprocating Compressors Advantages: 1. Since the centrifugal compressors have no valves, pistons, cylinders, connecting rod etc., the working life of these compressors is more as compared to reciprocating compressors. 2. These compressors operate with little or no vibration as there are no unbalanced masses. 3. The operation of centrifugal compressors is quiet and calm. 4. The centrifugal compressors run at high speeds (3000 r.p.m. and above), these can be directly connected to electric motors or steam turbines. 5. Because of the high speed, these compressors can handle large volume of vapour refrigerant, as compared to reciprocating compressors.

56 56 6. The centrifugal compressors are especially adapted for systems ranging from 50 to 5000 tonnes. They are also used for temperature ranges between 90 C and + l0 C. 7. The efficiency of these compressors is considerably high. 8. The large sizes centrifugal compressors require less floor area as compared to reciprocating compressors. Disadvantages: 1. The main disadvantage in centrifugal compressors is * surging. It occurs when the refrigeration load decreases to below 35 percent of the rated capacity and causes severe stress conditions in the compressor. 2. The centrifugal compressors are not practical below 50 tonnes capacity load. 3. The refrigerants used with these compressors should have high specific volume.

57 57 Lecture No. 18 Condensers Working of a condenser Factors affecting the condenser capacity Heat rejection factor Introduction: The condenser is an important device used in the high pressure side of a refrigeration system. Its function is to remove heat of the vapour refrigerant discharged from compressor. The hot vapour refrigerant consists of the heat absorbed by the evaporator and heat of compression added by the mechanical energy of the compressor motor. The heat from the hot vapour refrigerant in a condenser is removed first by transferring it to the walls of the condenser tubes and then from the tubes to the condensing or cooling medium. The cooling medium may be air or water or a combination of the two. The selection of a condenser depends upon the capacity of the refrigerating system, the type. refrigerant used and the type of cooling medium available. Working of a Condenser: The working of a condenser may be best understood by considering a simple refrigerating system as shown in Fig 17. The corresponding p-h diagram showing three stages of a refrigerant cooling is shown in Fig 18. Fig. 17. Simple refrigerating system.

58 58 Fig. 18. p-h diagram of a simple refrigerating system. The compressor draws in the superheated vapour that the refrigerant contains the heat it absorbed in the evaporator. The compressor adds more heat (i.e the heat of compression) to the superheated vapour. This highly superheated vapour from the compressor is pumped to the condenser through the discharge line. The condenser cools the refrigerant in the following three stages. 1. First of all, the superheated vapour is cooled to saturation temperature (desuperheating) corresponding to the pressure of the refrigerant. This is shown by the line 2-3 in Fig 18.The de superheating occurs in the discharge line and in the first few coils of the condenser. 2. The saturated vapour refrigerant gives up its latent heat and is condensed to a saturated liquid refrigerant. This process, called condensation, is shown by the line The temperature of the liquid refrigerant is reduced below its saturation temperature (i.e. sub cooled) in order to increase the refrigeration effect. This process is shown by the line 4-5.

59 59 Factors Affecting the Condenser Capacity: The condenser capacity is the ability of the condenser to transfer heat from the hot vapour refrigerant to the condensing medium. The heat transfer capacity of a condenser depends upon following factors. 1. Material: Since the different materials have different abilities of heat transfer, th ' the size of a condenser of a given capacity can be varied by selecting the right material. It may be noted that higher the ability of a material to transfer heat, the smaller will be the size of condenser. 2. Amount of contact: The condenser capacity may be varied by controlling the amount of contact between the condenser surface and the condensing medium. This can be done by varying the surface area of the condenser and the rate of flow of the condensing medium over condenser surface. The amount of liquid refrigerant level in the condenser also affects the amount of contact between the vapour refrigerant and the condensing medium. The portion of the condenser used for liquid sub-cooling can not condense any vapour refrigerant. I 3. Temperature difference: The heat transfer capacity of a condenser greatly depends upon the temperature difference between the condensing medium and the vapour refrigerant. As temperature difference increases, the heat transfers rate increases and therefore the condenser capacity increases. Generally, this temperature difference cannot be controlled. But when temperature difference becomes so great that it becomes a problem, devices are available that will change the amount of condensing surface and the air flow rate to control condenser capacity. Most air-cooled condensers are designed to operate with a temperature difference of 14 C. Heat Rejection Factor: The load on the condenser per unit of refrigeration capacity is known heat rejection factor. The load on the condenser (Qc) is given by Qc = Refrigeration capacity + Work done by the compressor

60 60 =RE+W Heat rejection factor, HRF = Qc R +W = E RE RE =1 + W 1 R =1 + ( COP = E ) RE COP W Heat rejection factor depends upon the coefficient performance (COP) which in turn depends upon the evaporator and condenser temperatures. In actual air-conditioning applications for R-12 and R-22, and operating at a condenser temperature of 40 C and an evaporator temperature of 5 C, the heat rejection factor is about 1.25.

61 61 Lecture No. 19 Classification of condensers Air cooled condensers Water cooled condensers Tube in condenser Shell and coil condenser Classification of Condensers: According to the condensing medium used, the condensers are classified into the following three groups: 1. Air cooled condensers, 2. Water cooled condensers, and 3. Evaporative condensers. 1. Air Cooled Condensers: An air-cooled condenser (Fig. 19) is one in which the removal of heat is done by air. It consists of steel or copper tubing through which the refrigerant flows. The size of tube usually ranges from 6 to 18 mm outside diameter, depending upon the size of condenser. Fig. 19. Air cooled condenser. Generally copper tubes used because of its excellent heat transfer ability. The condensers with steel tubes are used in, ammonia refrigerating systems. The tubes are usually provided with plates or fins to increase the surface area for heat transfer, The fins are usually made from aluminium because of its light weight. The fin spacing is quite wide to reduce dust clogging. The plate condensers have no fins, so they require

62 62 considerably large surface areas. However, they are cheaper to construct and require little maintenance. Both these types of condensers can be founds in house hold refrigerators. The condensers with single row of tubing provide the most efficient heat transfer. This is because the air temperature rises at it passes through each row of tubing. The temperature difference between the air and the vapour. refrigerant decreases in each row becomes effective. However, single row condensers require more space than multirow condensers. The air cooled condensers may have two or more rows of tubing, but the condensers with up to six rows of tubing are common. Some condensers have seven or eight rows. However more than eight rows of tubing are usually not efficient. This is because the air temperature will be too close to the condenser temperature to absorb any more heat after passing through eight rows of tubing. There are two types of air-cooled condensers: 1. Natural convection air-cooled condensers: In natural convection air-cooled condenser the heat transfer from the condenser coils to the air is by natural convection. As the air comes in contact with the warm condenser tubes, it absorbs heat from the refrigerant and thus the temperature of air increases. The warm air being lighter, rises up and the cold air from below rises to take away the heat from the condenser. As the rate of heat transfer in natural connection condenser, they require a larger surface area as compared to forced convection condensers. These are used only in small capacity applications such as domestic refrigerant freezers, water coolers and air-conditioners. 2. Forced convection air-cooled condensers: In the fan (either propeller or centrifugal) is used to force the air over the condenser coils to increase its heat transfer capacity. 2. Water Cooled Condensers: A water cooled condenser is one in which water is used as the condensing medium. They, are always preferred where an adequate supply of clear inexpensive water and means of water disposal are available. These condensers are commonly used in commercial and industrial refrigerating units.

63 63 The water cooled condensers operate at a lower condensing temperature then an air cooled condenser this is because the supply water temperature is normally lower than the ambient air temperature, but the difference between the condensing and cooling medium temperatures is normally the same (i.e. 14 C) Thus, the compressor for a water cooled condenser requires less power for the same capacity. The water cooled condensers are classified into three groups. 1. Tube in Tube or double tube condensers 2. Shell and coil condensers 3. Shell and tube condensers 1.Tube-in-tube or double tube condensers: It consists of a water tube inside a large refrigerant tube (Fig. 20). The hot vapour refrigerant enters at the top of the condenser. The water absorbs the heat from the refrigerant and the condensed liquid refrigerant flows at the bottom. Since the refrigerant tubes are exposed to ambient air, therefore some of the heat is also absorbed by ambient air natural convection. Fig. 20. Tube-in-tube condenser. The cold water in the inner tubes may flow in either direction. When the water enters at bottom and flows in the direction opposite to the refrigerant, it is said to be a

64 64 counter flow system. On the other hand, when the water enters at the top and flows in the same direction as refrigerant, it is said to be a parallel flow system. The counter-flow system is preferred in all types of water cool condensers because it gives high rate of heat transfer. Since the coldest water is used for final cooling of the liquid refrigerant and the warmest water absorbs heat from the hottest vapour refrigerant, the temperature difference between the water and refrigerant remains fairly constant throughout the condenser. In case of parallel flow system, as the water and refrigerant flow in the same direction, the temperature difference between them increases. Thus the ability of water to absorb heat decreases at it passes through the condenser. 2. Shell and coil condensers: A shell and coil condenser consists of one or more water coils enclosed in a welded steel shell (Fig. 21). Both the finned and bare coil types are available. It may be either vertical or horizontal. In vertical type of condenser, the hot vapour refrigerant enters at the top of the shell and surrounds the water coils. As the vapour condenses, it drops to the bottom of the shell which often serves as a receiver. Most vertical type shell and coil condensers use counter flow water system as it is more efficient than parallel flow water system. Fig. 21. Shell-coil condenser. In the shell and coil condensers, coiled tubing is free to expand and contract with temperature changes because of its spring action and can withstand any strain caused by temperature changes. Since the water coils are enclosed in a welded steel shell, the mechanical cleaning of these coils is not possible. The coils are cleaned with chemicals. They are used for units upto 50 tonnes capacity.

65 65 3. Shell and tube condensers: It consists a cylindrical steel shell containing a number of straight water tubes (Fig. 22). The tubes are expanded into grooves in the tube sheet holes to form a vapour- tight fit. The tube sheets are welded to the shell at both the ends. The removable water boxes are bolted to the tube sheet at each end to facilitate cleaning of the condenser. Fig. 22. Shell-tube condenser. The condenser tubes are made either from steel or copper, with or without fins. The steel tubes without fins are usually used in ammonia refrigerating systems because ammonia corrodes copper tubing. In this type of condenser, the hot vapour refrigerant enters at the top of the shell and condenses as it comes in contact with water tubes. The condensed liquid refrigerant drops to the bottom of the shell which often serves as a receives

66 66 Lecture No. 20 Fouling factor Difference between air cooled and water cooled condensers Evaporative condenser Fouling Factor: The water used in water cooled condensers always contain a certain amount of minerals and other foreign materials, depending upon its source. These materials form deposits inside the condenser water tubes. This is called water fouling. The deposits insulate the tubes, reduce their heat transfer rate and restricts the water flow. The fouling factor is the reciprocal of heat transfer coefficient for the material of scale. 1. For copper tubes used for R-12 and R-22 condensers, the fouling factor is m2s K/ J. 2. For steel tubes used in ammonia condensers, the fouling factor is m2s K/J. Table 6. Comparison of air cooled and water cooled condensers Air cooled condenser Water cooled condenser 1. As the construction of air cooled 1. As the construction of water cooled Condenser is very simple, the initial condenser is complicated, the initial cost cost is less. The maintenance cost is is high. The maintenance cost is also also low. high. 2. There is no handling problem with air cooled condensers. 2. The water cooled condensers are difficult to handle. 3. The air cooled condensers do not 3. The pipes are required to take water to require piping arrangement for carrying the air. 4. There is no problem in disposing of used air. take water to and from the condenser. 4. There is a problem of disposing the used water unless a recirculation system is provided. 5. Since there is no corrosion, The fouling 5. Since corrosion occurs inside the tubes effect is low. carrying the water, the fouling effects are high. 6. They have low heat transfer capacity 6. They have high heat transfer capacity due to low thermal conductivity of air. due to high thermal conductivity of

67 67 water. 7. They are used for low capacity plants 7. They are used for large capacity plants. (< 5 TR ) 8. The fan noise becomes objectionable. 8. There is no fan noise. 9. The distribution of air on condenser 9. There is even distribution of water on surface is not uniform. the condensing surface. 10. They have high flexibility. 10. They have low flexibility. Evaporating condensers: It uses both air and water as condensing mediums to condense the hot vapour refrigerant to liquid refrigerant. These condensers (Fig. 23) perform the combined functions of a water cooled condenser and a cooling tower. Fig. 23. Evaporative condenser. During operation, the water is pumped from the sump to a spray header and sprayed through nozzles over the condenser coils through which the hot vapour refrigerant from the compressor is passing. The heat transfers from the refrigerant through the condensing tube walls and into the water that is wetting the outside surface of the tubes. At the same time, a fan draws air from the bottom side of the condenser and discharged out at the top of the condenser. The air causes the water from the surface of the condenser coils to evaporate and absorb the latent heat of evaporation from the remaining water to cool it. Though most of the cooling takes place by evaporation, the air can also absorb some sensible

68 68 heat from water. Since the heat for vaporising the water is taken from the refrigerant the vapour refrigerant condenses into a liquid refrigerant. The cold water that drops down into a sump is recirculated. In order to make up the deficiency caused by the evaporated water, additional water is supplied to the sump. A float valve in the sump controls the make-up supply. The eliminator is provided above the spray header to stop particles of water escaping along with the discharge air.

69 69 Lecture No. 21 Expansion Devices Types of expansion devices Capillary tube Introduction: The expansion device (also known as metering device or throttling device) is an important device that divides the high pressure side and the low pressure side of a refrigerating system. It is connected between the receiver (containing liquid refrigerant at high pressure) and the evaporator (containing liquid refrigerant at low pressure). The expansion device performs the following functions. 1. It reduces the high pressure liquid refrigerant to low pressure liquid refrigerant before being fed to the evaporator. 2. It maintains the desired pressure difference between the high and low pressure sides of the system, so that the liquid refrigerant vaporises at the designed pressure in the evaporator. 3. It controls the flow of refrigerant according to the load on the evaporator. Note: The expansion devices used with dry expansion evaporators are usually called expansion valves where as the expansion devices used with flooded evaporators are known as float valves. Types of Expansion Devices: Following are the main types of expansion devices used in industrial and commercial refrigeration and air conditioning system. 1. Capillary tube 2. Hand-operated expansion valve 3. Automatic or constant pressure expansion valve 4. Thermostatic expansion valve 5. Low side float valve and 6. High side float valve

70 70 Capillary Tube: The capillary tube (Fig. 24) is used as an expansion device in small capacity hermetic sealed refrigeration units such as in domestic refrigerators, water coolers, room air conditioners and freezers. It is a copper tube of small internal diameter and of varying length depending upon the application. The inside diameter of the tube used in refrigeration work is generally about 0.5 mm to 2.25 mm and the length varies from 0.5 m to 5 m. It is installed in the liquid line between the condenser and the evaporator. A fine mesh screen is provided at the inlet of the tube in order to protect it from contaminants. In its operation, the liquid refrigerant from the condenser enters the capillary tube. Due to frictional resistance offered by a small diameter tube, the pressure drops. Since the frictional resistance is directly proportional to the length and inversely proportional to the diameter, more pressure drop will be created for the long capillary tube once selected for a given set of conditions and load can not operate efficiently at other conditions. Fig. 24. Capillary tube. The refrigeration system using capillary tube has the following advantages. 1. The cost of capillary tube is less than all other forms of expansion devices. 2. When the compressor stops, the refrigerant continues to flow into the evaporator and equalizes the pressure between the high side and low side of the system. This considerably decreases the starting load.on the compressor. Thus a low starting torque motor (low cost motor) can be used to drive the compressor. 3. Since the refrigerant charge in a capillary tube system is critical, no receiver is necessary.

71 71 Lecture No. 22 Hand operated expansion valve Low side float valve High side float valve Hand-operated Expansion Valve: The hand-operated expansion valve (Fig. 25) is the most simple type of expansion valve but it requires an operator to regulate the flow of refrigerant to the evaporator manually. The conical-shaped needle valve extends down into the valve port and restricts the flow area through the port. When closed, the valve rests on its conical seat. This valve is limited to systems operating under nearly constant loads for long periods of time, such as in ice making plants and cold storages. Fig. 25. Hand operated expansion valve. Low-side Float Valve: Low side float valve (Fig. 26) is located in the low pressure side (i.e. between the evaporator and compressor suction line) of the refrigeration system. It maintains a constant level of liquid refrigerant in the evaporator and float chamber by opening and closing a needle valve.

72 72 Fig. 26. Low-side float Valve. The float valve is a hollow ball attached to one end of a float arm is connected to a needle valve. The movement of the float valve is transmitted to the needle valve by the float arm which closes or opens the flow of liquid refrigerant. When the liquid refrigerant in the evaporator vaporises, its level falls down. This causes the float to drop and thus opens the needle valve, thereby allowing liquid refrigerant from the liquid line to the float chamber and then to the evaporator to make up for the amount of vaporisation. When the desired liquid level is reached, the float rises and closes the needle valve. The major advantage of the low-side float valve is that it maintains a constant liquid level in the evaporator under all loading conditions regardless of the evaporator pressure and temperature. High-side Float Valve: The high side float valve (Fig. 27) located on the high pressure side (i.e. between the condenser and evaporator) of the refrigeration system. It controls the flow of liquid refrigerant to the evaporator according to the load and maintains a constant liquid level in the evaporator and float chamber by opening or closing of a needle valve. The liquid refrigerant from the condenser flows to the float chamber. As the level of liquid refrigerant in the float chamber rises, the float ball also rises, thereby opening the needle valve. This allows the liquid refrigerant to flow into the evaporator. when the liquid level in the float chamber falls down, the float ball also drops, thereby closing the needle valve.

73 73 Fig. 27. High-side float Valve. The condenser supplies the liquid refrigerant at the same rate as it evaporates in the evaporator. Since the rate of vaporisation depends upon the load on the evaporator, the high side floats valve functions according to the load. The high side float valve may be used with dry expansion evaporators.

74 74 Lecture No. 23 Ice manufacturing principle of ice production Ice Manufacture: The commercial ice is produced by freezing potable water in standard cans placed in rectangular tanks. The tanks are filled with chilled brine. For increasing the heat transfer from the water in the can to the chilled brine, the brine solution is kept in constant motion by agitators. The agitators can be either horizontal or vertical and are operated by means of electric motors. The brine temperature is maintained by the refrigeration plant at l0 C to -11 C. The ammonia gas is used as the refrigerant because of its excellent thermal properties. It also produces very high refrigerating effect per kg of refrigerant and low specific volume of the refrigerant in vapour state. The high temperature, high pressure ammonia vapours are condensed in a condenser which may be of shell and tube type or evaporative type. The condensed liquid ammonia is collected in the receiver and then expanded through the expansion valve. Due to the expansion, the pressure of the liquid ammonia is considerably reduced. It then passes through the evaporator coils surrounding a brine tank in which brine solution is filled. The low pressure liquid ammonia absorbs heat from the brine solution, equivalent to its latent heat of vaporisation, gets converted to vapour state and is once again fed to compressor to complete the cycle. The layout of an ice manufacturing system is shown in Fig. 28. The brine tanks are usually fabricated of 6 mm thick mild steel plates with tie rods welded end to end. The depth of tank is such that the brine level is around 25 mm higher than the water level in the cans. The tank is insulated on all the four sides and from the bottom. The insulated wooden lids are provided to cover the top insegments, to facilitate the removal or replacing of ice-cans. The ice-cans are fabricated from galvanized steel sheets and are given chromium treatment to prevent corrosion due to chemical reactions. It also prevents corrosion from reaction with ammonia (NH3) in case it leaks from the system.

75 75 Fig. 28. Ice plant. In order to get clear transparent ice, water in the can is agitated by the use of low pressure air, through the tubes suspended from the top. Due to agitation, the dissolved impurities such as salts, even colours get collected in the unfrozen water core. It is desirable that it should be taken out and replaced with fresh water. It may be noted that the ice of potable water (treated or untreated) frozen at a temperature lower than -12 C can crack. Therefore, brine temperature is kept at a higher level, say _11 C to _10 C. Water in the ice cans placed in the brine cools rapidly up to a temperature of about 3 C to 4 C. Thereafter, it takes more time for water to touch 0 C. The rate of freezing decreases substantially as the thickness of the ice layer increases. This is because of the reason that the ice layer offers more thermal resistance to the heat flow from the water to the brine. Notes: 1. Generally sodium chloride (NaCl) or calcium chloride (CaCl2) is used as brine. Due to its low cost and being less injurious, sodium chloride brine is preferably used. 2. The freezing time is dependent upon the brine temperature and the extent of brine and water agitation.

76 76 Cooling of Milk (Milk Processing): The milk is known to be one of the most perishable foods and if it is not maintained at sufficient low temperature, it gets spoiled due to the growth of bacteria and other organisms. Some kind of bacteria rapidly multiplies in number at temperatures of 21 C to 38 C (294 K to 311 K) and the milk gets spoilt and sour. As the temperature of the milk is reduced, the bacterial growth decreases and practically ceases at 0 C to 5 C (273 K to 278 K), though the bacilli are not killed even at very low temperatures. The bacterial content can be eliminated to a great extent by heating the milk to 62 C (335 K) and holding it at that temperature for about 30 minutes. Thereafter to minimize the bacterial growth and preservation, the milk is cooled to 4 C to 5 C (277 K to 278 K). This process of heating and immediately cooling the milk for controlling the bacterial growth is known as pasteurization. The pasteurization is generally done in a batch type process. In this process, the raw milk is heated by hot water or steam to 62 C. In case the heating is done by hot water, it is sprayed around the outside lining of the vat by a distributor, which gets collected in a sump at the bottom of the vat, reheated and once again sprayed. The steam heating is carried out by flowing steam in the space provided between lining and the casing of the vat. The heated milk is then cooled, first by the cooling tower water and then by the chilled water or brine to 4 C to 5 C, which is the desired temperature for filling the milk in bottles. The heating and cooling is done by passing the milk (by milk pumps) through the heat exchange plates. The milk flows between the two plates and the hot water, cooling tower water or chilled water (brine) is circulated through alternate pairs of plates. The direction of flow of heating or cooling fluids is opposite to that of milk to obtain better heat transfer. In order to control the fat content of milk, it is desired to churn the milk. Such milk is retailed as 'toned milk'. The fat thus removed is processed as butter and stored at 4 C to 5 C. The cheese is another product from milk and is stored at about 4 C. The ice-cream is another product, which is manufactured by using milk fat along with sugar and other ingredients of ice-cream. The mixture is pasteurized to a temperature of 70 C to 80 C (343 K to 353 K) and homogenized. The homogenization is the process of breaking down the fat and solid content of the mixture to a very finely divided state to avoid any lump formation while freezing the mixture. The plate type heat

77 77 exchangers are used for heating and cooling the liquid mix to 5 C. Then the mixture is frozen in ice-cream freezers. During freezing, the ice-cream is whipped to mix certain amount of air. Generally the ice-cream is frozen to about C to - 5 C (270.5 K to 268 K), depending upon the type of mixture used. Thereafter, it is hardened in the hardening rooms to a temperature of -18 C (255 K). The hardening rooms are maintained at about -30 C to -35 C (243 K to 238 K) for rapid freezing of the balance water content in the ice-cream. The rapid freezing ensures small size of the ice crystals.

78 78 Lecture No. 24 Application of refrigeration in different food products Applications of refrigeration in different food products: The most significant application of refrigeration is in food preservation, whether it is by way of processing or for storage. Processing is done by heating, heat drying, etc., and by refrigeration such as in chilling, freezing or freeze-drying. Storage may be of either chilled or frozen product. Some of the important products involved in processing are candy, beverages, meat, poultry, fish, bakery and dairy products, fruits and vegetables, fruit-juice concentrates, precooked foods, etc. The common products preserved by storage after chilling are fruits such as apples, pears, grapes, citrus fruits etc. vegetables such as onions, potatoes, tomatoes, etc. and dry fruits, candies, milk, eggs and their products. Storage under frozen conditions is resorted to for preserving the food value as well as to store perishable products over a long period. The common items of frozen food are fish, meat, poultry, and some vegetables such as peas, beans, carrots, cauliflower etc. It may be noted that all products are not amenable to freezing. An interesting feature of the chilled and frozen food industry is the cold chain that must be maintained from the form to the consumer. An important link in this chain is that of transport refrigeration. It must be noted that processing by simple heat drying is a more economical way of food preservation. However, with many products, it may involve loss of quality in which case it is not considered suitable.

79 79 Lecture No. 25 Examples of Food processing by refrigeration and storage Fruits and vegetables Examples of Food processing by refrigeration and storage: Fruits and vegetables: The rapid pre cooling after harvesting increases the storage life considerably. Futher, controlled atmosphear (CA) storage greatly reduces the respiration rate and retards the deterioration of the product, thus increasing its storage temperature. A controlled atmosphere aims at maintaining the oxygen level at a low value of 3-5 percent. A few vegetables such as peas, beans, carrots and cauliflower are also found suitable for freezing. They are partially cooked before freezing. Quick freezing is not so critical in the case of vegetables. Never less, air blast freezers are commonly used for individual freezing is also being resorted to on a small scale packaged products. De hydro freezing is another process that has been found successful in the preservation of some products such as apples, apricots, peas etc. this involves partial dehydration, and then freezing. The process retains the advantages of both, without the disadvantages of either. One clear advantage is the reduced refrigeration load during freezing. Another is the reduced weight of the product for transport. In simple de hydration, there is a marked deterioration in quality during the last stages of drying. This is eliminated in de hydro freezing.

80 80 Lecture No. 26 Examples of Food processing by refrigeration and storage - Meat products fish poultry products dairy products Examples of Food processing by refrigeration and storage: Meat Products: Ammonia-refrigeration systems are commonly used for the chilling of carcass and holding them in rooms. The cooling is by chilled-brine spray or sprayed coil. In the case of brine, a 20 percent solution of NaCI is chilled to a temperature -7 to -9 C. The brine absorbs heat and moisture from the air. The diluted brine leaving the room must be strengthened regularly. The majority of installations these days use a sprayed-coil system with air being cooled with brine sprayed over a direct expansion coil. The average specific heat of carcass is 3 kj/kg.k. The carcass should be reduced to a temperature of 1.5 C in a short time after slaughter. The room temperature could be as low as C. Refrigeration is also required for chilling water for washing the product. More and more meat products are, however, being preserved by freezing. The processing steps before freezing involve cutting, chipping, grinding, forming, pressing and slicing. The recommended storage temperature is -29 C for a storage life of 12 months or more for beef, lamb, pork, etc. At -23 C, a reasonable length of storage between 8 to 18 months is possible. The product quality is seriously impaired if the temperature goes up to - 9 C at any stage. Fishery Products: Refrigeration is required in many ways in the processing, preservation and transport of fresh and frozen fish and their products. Care starts from the stage of catching in fishing boats and trawlers. Icing of fish is first done to chill it to a temperature of 0 C. The melt-water also helps to wash off the slime and bacteria. Often the fish are stored in tanks using refrigerated sea water at -1 C. It is found that shrimp when stored with refrigerated sea

81 81 water is superior to that stored with ice. The storage life of chilled fish is normally days. The valued varieties of fish that are frozen include salmon, shrimp, shell fish, etc. Individual Quick Freezing (IQF) techniques are preferred for these products. Quick freezing has many advantages. It prevents bacterial spoilage and ensures rapid handling, packaging and good appearance of the product, and also makes optimum utilization of the freezer space. Quick-freezing is achieved by using low refrigeration temperatures. Contact plate freezers are suitable with large size products. Blocks of fish are frozen in multilayered plates in contact with primary or secondary refrigerant. For obtaining individual freezing of small-sized fish, such as shrimp, two methods are employed. Air-blast freezers use air at -29 to -40 C flowing at 2.5 m/s or more. Immersion freezers make use of liquid Freon or even liquid nitrogen at atmospheric pressure. The freezing time depends on the water content and thermo physical properties of fish. It also depends on the refrigeration temperature. Frozen fish is normally stored at a temperature of -23 C or lower. Poultry Products: Poultry meat is preserved both by chilling and freezing. Chilling requires a lower capital investment where as freezing offers quality and flexibility of operation. A major fraction of poultry meat is processed and transported by liquid chilling by the use of flake ice. Air chilling results in a considerable amount of dehydration at the surface. The approximate time required for chilling to 7 C is about 115 minutes for dressed birds, 50 minutes for eviscerated birds and 25 minutes for cut-up ones. As for freezing, the use of air-blast tunnels operating at air temperatures of -29 to -40 C and air velocities of 2.5 m/s or more is recommended. This ensures rapid freezing which is essential to obtain good quality and appearance. Cooling is also required in poultry farms. A temperature higher than 29 C results in reduced egg production and weight and thinner shells. A temperature of 24 C is recommended for poultry-keeping.

82 82 Dairy Products: Most dairy plants pre-cool milk to 2.5 C before processing for the separation of cream and blending for producing standardized quality. There are hot as well as cold, milk separators. In the latter, the separation process takes place at a temperature of 4 C. After separation and blending, the milk is sent for pasteurization which is accomplished by heating followed by the chilling of milk. Heating is done either by hot water or steam and holding the milk at 62 C for about 30 minutes. The milk is then cooled first by water from a cooling tower and then by chilled water. The rate of, flow of chilled water should be such that its rise in temperature is not more than 5.5 C. When a direct-expansion system is used for cooling, the refrigerant flow is controlled by an automatic expansion valve so that the temperature does not drop below -2 C lest the milk may freeze. Water-chilling equipment normally consists of an ice-bank type water chiller consisting of an insulated tank with water, direct expansion coil and pump. This system ensures a constant temperature of chilled water of 0-1 C. Also, it takes care of load fluctuations by allowing excess refrigeration to be stored in the form of ice on immersed coils. Refrigeration is also required for cooling the cream before churning and the mechanical separation of butter. For the storage of butter, a temperature of 0 to 4 C is satisfactory. But for storage for 6 months or more, a temperature of -23 to -18 C must be used. This is the same as for some frozen products. Thus butter can be kept in freezer compartment of refrigerators for storing beyond a period of six months or more. Ice-cream manufacture is another industry that makes great use of refrigeration. The ice-cream freezer has to whip in a certain amount of air in the mix, simultaneously with freezing. The product is 0 to 100% frozen between the temperature ranges of -2.5 to - 55 C approximately.

83 83 Lecture No. 27 Food Freezing Freezing systems Indirect contact systems Food Freezing:Preservation of a food by freezing occurs by several mechanisms. At temperatures below 0 C there is a significant reduction in growth rates for micro organisms and in the corresponding deterioration of the product due to microbial activity. The same temperature influence applies to most other reactions that might normally occur in the product, such as enzymatic and oxidative reactions. In addition, the formation of ice crystals within the product changes the availability of water to participate in reactions. As the temperature is reduced and more water is converted to a solid state, less water is available to support deteriorative reactions. Although freezing as a preservation process generally results in a high-quality product for consumption, the quality is influenced by the freezing process and frozenstorage conditions. The freezing rate or time allowed for the product temperature to be reduced from above to below the initial freezing temperature will influence product quality, but in a variable manner depending on the food commodity. For some products, rapid freezing (short freezing time) is required to ensure formation of small ice crystals within the product structure and minimal damage to the product texture. Other products are not influenced by structural changes and do not justify the added costs associated with rapid freezing. Still other products have geometric configurations and sizes that do not allow rapid freezing. The storagetemperature conditions influence frozen food quality in a significant manner. Any elevation in storage temperature tends to reduce the preservation quality of the process, and fluctuations in storage temperature tend to be even more detrimental to product quality. It is obvious that the optimum freezing process depends on the product characteristics. The importance of residence time in the freezing system must be emphasized, as well as the need for accurate freezing time prediction.

84 84 Freezing system: To achieve freezing of a food product, the product must be exposed to a lowtemperature medium for sufficient time to remove sensible heat and latent heat of fusion from the product. Removal of the sensible heat and latent heat results in a reduction in the product temperature as well as a conversion of the water from liquid to solid state (ice). In most cases, approximately 10% of the water remains in the liquid state at the storage temperature of the frozen food. To accomplish the freezing process in desired short times, the low-temperature medium is at much lower temperature than the desired final temperature of the product, and large convective heat-transfer coefficients are created. The freezing process can be accomplished by using either indirect or direct contact systems. Most often, the type of system used will depend on the product characteristics, both before and after freezing is completed. There are a variety of circumstances where direct contact between the product and refrigerant is not possible. Indirect Contact Systems: In numerous food-product freezing systems, the product and refrigerant are separated by a barrier throughout the freezing process. This type of system is illustrated schematically in Fig. 29. Although many systems use a nonpermeable barrier between product and refrigerant, indirect freezing systems include any system without direct contact, including those where the package material becomes the barrier. Fig. 29. Indirect contact freezing system.

85 85 Lecture No. 28 Plate Freezers air blast Freezers Freezers for liquid foods Plate Freezers: The product is frozen while held between two refrigerated plates. In most cases, the barrier between product and refrigerant will include both the plate and package material. The heat transfer through the barrier (plate and package) can be enhanced by using pressure to reduce resistance to heat transfer across the barrier (Fig. 30). In some cases, plate systems may use single plates in contact with the product and accomplish freezing with heat transfer across a single package surface. As would be expected, these systems are less efficient, and they are costly to acquire and operate. Fig. 30. Plate freezing system. Plate-freezing systems can be operated as a batch system with the product placed on the plates for a specified residence time before being removed. In this situation, the freezing time is the residence time and represents the total time required to reduce the product from the initial temperature to some desired final temperature. In general, the batch plate-freezing system has significant flexibility in terms of handling diverse product types and product sizes. The plate-freezing system operates in a continuous mode by moving the plates holding the product through an enclosure in some prescribed manner. the product is held between two refrigerated plates throughout the freezing process. The movement of

86 86 plates (and product) occurs as the plates index upward or across within the compartment. At the entrance and exit to the freezing system, the plates are opened to allow the product to be conveyed to or from the system. In a continuous plate-freezing system, the freezing time is the total time required for the product to move from entrance to exit. During the residence time, the desired amounts of sensible and latent heat are removed to achieve the desired frozen product temperature. Air-Blast Freezers: In many situations, the product size and/or shape may not accommodate plate freezing. For these situations, air-blast freezing systems become the best alternative. In some cases, the package film is the barrier for the indirect freezing, with cold air being the source of refrigeration. Air-blast freezers can be a simple design, as in the case of a refrigerated room. In this situation, the product is placed in the room, and the low-temperature air is allowed to circulate around the product for the desire residence of freezing time. This approach represents a batch mode, and the refrigerated room may act as a storage space in addition to the freezing compartment. In most cases, freezing times will be long because of lower air speeds over the product, inability to achieve intimate contact between product and cold air, and the smaller temperature gradients between product and air. Most air-blast freezers are continuous, In these systems, the product is carried on a conveyor that moves through a stream of high-velocity air. The length and speed of, the conveyor establish the residence of freezing time. These times can be relatively small based on the use of very low-temperature air, high air velocities, and good contact between individual product packages and the cold air. Continuous air-blast freezing systems use a variety of different conveying arrangements such as tray conveyors, spiral conveyors, and roller conveyors. Most often, the system used will depend on product characteristics. Freezers for Liquid Foods: The third general type of indirect freezing systems includes those designed primarily for liquid foods. In many situations, the most efficient removal of thermal energy from a liquid food can be accepted, the most common type is a scraped-surface system (Fig. 31).

87 87 Fig. 31.Scrapped-surface heat exchanger The food contact areas of a scraped surface cylinder are fabricated from stainless steel, pure nickel, hard chromium plated nickel, or other corrosion resistant material. The inside rotor contains blades that are covered with plastic laminate or molded plastic. The rotor speed varies between 150 and 500 rpm. Although higher rotation speed allows better heat transfer, it may affect the quality of the process product by possible maceration, thus we must carefully select the rotor speed and the and the annular space between the rotor and the cylinder for the product being processed. The cylinder containing the product and the rotor is enclosed in an outside jacket. The cooling medium is supplied to the out side jacket. Commonly used media include brine or a refrigerant such as Freon. The constant blending action accomplished the scraped surface heat exchanger is often desirable to enhance the uniformity of product flavour colour, aroma and textual characteristics. The heat exchangers for freezing liquid foods are designed specifically for freezing, with the heat-exchange shell surrounding the product compartment becoming an evaporator for a vapour compression refrigeration system. For freezing liquid foods, the residence time in the freezing compartment is sufficient to decrease the product temperature by several degrees below the temperature of initial ice-crystal formation. At these temperatures, between 60 and 80% of the latent heat has been removed from the product, and the product is in the form of a

88 88 frozen slurry. In this condition, the product flows quite readily and can be placed in a package for final freezing in a low-temperature refrigerated space. The scraped-surface heat exchanger ensures efficient heat exchange between the slurry and the cold surface. Freezing systems for liquid foods can be batch or continuous. The batch system places a given amount of unfrozen liquid in the compartment and allows the freezing process to continue until the desired final temperature is reached. The product compartment is a scraped-surface heat exchanger but is operated as a batch system. In the case of ice-cream freezing, the system is designed with facility for injection of air into the frozen slurry to achieve the desired product consistency. In a continuous freezing system for liquid foods, the basic system is a scraped -surface heat exchanger using refrigerant during phase change as the cooling medium. The rotor acts as a mixing device, and the scraper blades enhance heat transfer at the heat-exchange surface.

89 89 Lecture No. 29 Direct contact systems Air blast - Immersion Direct-Contact Systems: Several freezing systems for food operate with direct contact between the refrigerant and the product as shown in Fig. 32. In most situations, these systems will operate more efficiently since there are no barriers to heat transfer between the refrigerant and the product. the refrigerants used in these systems may be lowtemperature air at high speeds or liquid refrigerants with phase change while in contact with the product surface. In all cases, the systems are designed to achieve rapid freezing, and the term individual quick freezing (IQF) will apply. Fig. 32. Direct contact freezing system. Air Blast: The use of low-temperature air at high speeds in direct contact with small product objects is a form of IQF. The combination of low temperature air, high convective heattransfer coefficient (high air speed), and small product shape leads to short freezing time or rapid freezing. In these systems, the product is moved through the high-speed-air region on a conveyor in a manner that controls the residence time. The types of product that can be frozen in these systems are limited to those that have the appropriate geometries and that require rapid freezing for maximum quality.