CHAPTER 3 REFRIGERANTS AND MIXTURE PREPARATION

Transcription

1 CHAPTER 3 REFRIGERANTS AND MIXTURE PREPARATION Refrigeration is the process of removing heat from the space to be cooled and transferring it to a place where it is unobjectionable. The primary purpose of refrigeration is producing and maintaining the temperature which is lower than that of the surroundings. In prehistoric times, man found that his game would last during times when food was not available unless it is stored in snow or in the coolness of a cave for use during the seasons of unavailability. In China, before the first millennium, ice was harvested and stored in insulated houses. Greeks, Hebrews, and Romans placed large amounts of snow into storage pits dug into the ground and insulated with straw and wood. In India, evaporative cooling was employed in early days. For the vaporization of the liquid latent heat is exhausted the surroundings so that surroundings get cooled. The intermediate stage in the history of cooling foods was to add chemicals like potassium nitrate or sodium nitrate to water causing the temperature to fall. Cooling wine by this method was recorded in 1550, as were the words "to refrigerate. Commercial refrigeration is believed to have been initiated by an American business person, Alexander C. Twinning, in Shortly afterwards, an Australian, James Harrison, examined the refrigerators used by Twinning and Gorrie and introduced vapour-compression refrigeration to the meatpacking and brewing industries.

2 Ferdinand Carre of France developed a somewhat more complex system in Unlike earlier compression-compression machines, which use air as a coolant agent, Carre s equipment contains rapidly expanding ammonia because boiling point of ammonia is at much lower temperature than water and is thus can absorb more heat. Carre's refrigerators have been widely used, and vapour compression refrigeration has become, the most widely used method of cooling. Methods of refrigeration can be classified as non-cyclic, cyclic and non-conventional methods. 3.1 NON-CYCLIC REFRIGERATION In these methods, refrigeration can be accomplished by melting ice or by subliming dry ice. These methods are used for small-scale refrigeration such as in laboratories and portable coolers, or in workshops. Foodstuffs preserved at this temperature or slightly above have an increased storage life. Solid carbon dioxide, known as dry ice, can also used as a refrigerant. 3.2 CYCLIC REFRIGERATION This consists of a refrigeration cycle, where heat is removed from a low-temperature space or source and rejected to a high-temperature sink with the help of external work. The most common types of refrigeration systems use the vapourcompression refrigeration cycle. Absorption heat pumps are used in a minority of applications.

3 Cyclic refrigeration can be classified as: Vapour Cycle Gas Cycle 3.2.1Vapour Cycle Refrigeration Vapour cycle refrigeration can further be classified as: 1. Vapour Compression Refrigeration 2. Vapour Absorption Refrigeration 1. Vapour Compression Refrigeration Capillary Tube P Condenser 4 5 Evaporator Q Con Q Eva P Con 4 Q Con P Eva 5 Q Eva 1 W h 5 = h 4 h1 h2 h Compressor W Figure 3.1 Single stage vapour compression refrigeration system A) Schematic diagram B) p-h diagram 1 Outlet of Evaporator/ Inlet to the Compressor 2 Outlet of Compressor/ Inlet to Condenser 3 Point in the Condenser where phase change starts 4 Outlet of Condenser/ Inlet to Throttling Device 5 Outlet of Throttling Device/ Inlet to Evaporator

4 1 2 : Isentropic Compression 2 4 : Isobaric Heat Rejection 4 5 : Isenthalpic Expansion 5 1 : Isobaric Heat Absorption Figure 3.1(a) shows a schematic diagram of a vapour compression refrigerator, which consists essentially of a hermetic reciprocating compressor, an evaporator, an air cooled condenser and a capillary tube [66]. These components are connected by pipelines in which a refrigerant with suitable thermodynamic properties circulates. The corresponding pressure-enthalpy (p h) diagram is shown in Figure 3.1(b). In order to simulate the vapour compression refrigerator, a number of assumptions are made. They are (a) steady state operation (b) no frictional pressure drop through pipelines, i.e. pressure changes only through the capillary tube and the compressor (c) heat gains or heat losses from or to the system are neglected (d) no superheating or sub cooling takes place and (e) the compressor has isentropic efficiency of 75% [13], It must be noted that the above diagrams are either for single component refrigerants or azeotropic mixtures. The T-s diagram for zeotropic mixtures will be discussed at a later stage in the calculations part. However the expressions for various parameters would remain the same. The pressure ratio (PR) is defined as the ratio of the condensing pressure (P2) to the evaporating pressure (P1), i.e. PR = P P

5 The condensing and evaporating pressures are determined corresponding to the condensation and evaporation temperatures. The condensation temperature is decided by the temperature of the ambient air, whereas the evaporation temperature is determined by the load temperature based on the required freezer air temperature. The performance parameters of the VCR system are Co-efficient of Performance (COP), is the ratio of refrigeration effect (RE) to the work done (W) COP = W RE 3.2 The energy balance of the evaporator and compressor give refrigeration effect and work done. Refrigeration effect = (h1 h5) kj/kg 3.3 Work done = (h2 h1) kj/kg Vapour Absorption Refrigeration (VAR) In the early years of the twentieth century, the vapour absorption cycle using ammonia-water systems was popular and widely used. After the development of the vapour compression cycle, the vapour absorption cycle lost much of its importance because of its low coefficient of performance (about one fifth of that of the VCR cycle). Today, the vapour absorption cycle is used mainly where fuel for heating is available but electricity is not, such as in recreational vehicles that carry LP gas. It is also used in industrial environments where plentiful waste heat overcomes its inefficiency.

6 The absorption cycle is similar to the VCR cycle, except for the method of raising the pressure of the refrigerant vapour. In the absorption system, the compressor is replaced by an absorber which dissolves the refrigerant vapour in a suitable liquid, a liquid pump is used which raises the pressure and a generator which, on heat addition, drives off the refrigerant vapour from the high-pressure solution. Some work is required by the liquid pump but, for a given quantity of refrigerant, it is much smaller than needed by the compressor in the vapour compression cycle. In a VAR system, a suitable combination of refrigerant and absorbent is used. The most common combinations are NH3-H2O and H2O -lithium bromide Gas Cycle When the working fluid is a gas that is compressed and expanded but does not change its phase, the refrigeration cycle is called a gas cycle. Air is most often used as the working fluid in gas cycle refrigeration. The gas cycle is less efficient than the vapour compression cycle because in the gas cycle the heat is carried in the form of sensible heat only. A gas cycle refrigeration system will require a large mass flow rate and it would be bulky. Because of their lower efficiency and larger dimensions, air cycle coolers are not used now-a-days in global cooling devices. Air cycle refrigeration finds its application in air craft refrigeration because the compressed air is already available and there

7 is no need of a separate compressor for refrigeration process which reduces the weight per ton of refrigeration. 3.3 NON-CONVENTIONAL METHODS There are some special methods to produce low temperatures which are thermoelectric refrigeration, vortex tube and steam jet refrigeration. 1. Thermoelectric Refrigeration Thermoelectric cooling uses the Peltier effect to create a heat flux between the junctions of two different types of materials. This effect is commonly used in camping and portable coolers and for cooling small instruments and electronic components. 2. Vortex Tube The vortex tube used for spot cooling, when compressed air is available and thermo-acoustic refrigeration using sound waves in a pressurized gas to drive heat transfer and heat exchange. 3. Steam Jet Refrigeration It is quite similar to more conventional refrigeration cycles, with an evaporator, a compression device, a condenser and a refrigerant as the basic components of the system. Instead of mechanical compression device, the system characteristically employs a steam ejector or booster to compress the refrigerant to the condenser pressure level.

8 3.4 REFRIGERANTS Any substance that absorbs heat through expansion or vaporization may be called a refrigerant [67]. Examples are ammonia, R12, R134a, R22 and hydro carbons etc. A broader definition may include secondary cooling mediums such as brine solutions and cold water Requirements of a Good Refrigerant: 1) It should be non poisonous and non toxic. 2) It should be non explosive and non-inflammable. 3) It should be non corrosive. 4) One should be able to detect the leak easily. 5) It should have low boiling point 6) Parts moving in the fluid should be easy to lubricate. 7) It should have a well balanced enthalpy of evaporation per unit mass. 8) It should have a small relative displacement to obtain a certain refrigerating effect. 10)Minimum pressure difference between the vaporizing and condensing pressures is desirable Classification of Refrigerants: Refrigerants have been classified by three groups. They are: 1) Group One: These are the safest refrigerants. They do not show flame propagation when tested in air at 21 C and bar. (Example: R113, R11, R21, R114, R12, R30, R22, R744, R502, R13, R14, R500, R134a, etc.)

9 2) Group Two: These are toxic and somewhat inflammable refrigerants. These refrigerants have a lower flammability limit of more than 0.10kg/m 3 at 21 0 C and bar and a heat of combustion of less than 19kJ/kg. (Example: R1130, R611, R160, R764, R40, R717 etc.) 3) Group Three: These are highly inflammable refrigerants and this group is defined by a lower flammability limit of less than or equal to 0.10 kg/m 3 at 21 0 C and bar or a heat of combustion more than or equal to 19 kj/kg. (Example: R600a, R290, R600, R1270, etc.) Desirable Thermo physical Properties of Refrigerants 1) Evaporator and Condenser Pressures: In order to avoid any leakage of air and moisture from outside and to be able to detect leakage of refrigerant from the system, it is preferable that both evaporator and condenser pressures should be above the atmospheric pressure; but then these pressures should not be very high because the construction of compressor, condenser and evaporator will have to be heavy and consequently initial cost will increase. The compression ratio should be as small as possible to avoid leakage across the piston. 2) Critical Temperature and Pressure: If the critical temperature of a refrigerant is very near to the condensing temperature, the power requirements are large. 3) Freezing Temperature: A refrigerant is required to have its freezing temperature much below the operation temperature in the plant.

10 4) Latent Heat of Vaporization: The more is the latent heat of vaporization, the more is the refrigeration effect. Thus mass of refrigerant required for per ton of refrigeration will be reduced. The area under reduction due to throttling and area under the super heat horn becomes negligibly small as compared to enthalpy of vaporization. The COP in such a situation will be close to that of Carnot value. 5) Specific Volume: The theoretical compressor displacement depends on the specific volume of the refrigerant vapour at evaporator temperature, i.e. at suction to compressor and the refrigerating effect per kg of refrigerant. Small volume of displacement per ton of refrigeration allows reciprocating compressor to be used, whereas centrifugal compressors are preferred when volume displacement per ton of refrigeration is high. 6) Stability and Inertness: An ideal refrigerant should not decompose at temperature of operation in the cycle and should not get polymerized. Some refrigerants decompose into gases which do not condense in the condenser and cause high condenser pressures and vapour lock. 7) Viscosity: It is desirable that both the liquid and vapour refrigerants should have low viscosity so that the pressure drops during flow are small. Heat transfer is also improved in the evaporator and the condenser due to low viscosity.

11 8) Thermal Conductivity: High thermal conductivity is desirable for efficient heat transfer in evaporator and condenser. Moreover, the surface wetting characteristics also improve heat transfer. 9) Oil Effect: With non oil miscible refrigerants, due to poor heat conduction properties of oil, large heat transfer surfaces are required. Thus miscibility is an advantage both from points of view of heat transfer and that refrigerant acts as carrier of oil to moving parts. The choice of a refrigerant for a VCR is limited by 1) Economy (2) Equipment type and size and (3) Application Alternative Refrigerants One of the major challenges posed to the Montreal Protocol is to protect the stratospheric ozone layer and also global warming while ensuring that developing countries are not economically disadvantaged during their transition to new technologies that do not rely on ozone depleting substances (ODS). This is particularly applicable to the refrigeration sector, which accounts for the largest share of ODS consumption in developing countries and touches virtually every person s life, directly or indirectly. HFC refrigerants have no Ozone Depletion Potential, but they do have a high Global Warming Potential [40]. The GWP of HFCs is not as high as CFCs, but they are significantly higher than the natural refrigerants such as hydrocarbons and ammonia. The international agreement, Kyoto Protocol, between developed nations seeks to reduce

12 emissions of carbon dioxide and five other Green House Gases (GHGs), of which HFCs are one [8). HC refrigerants are simple compounds containing carbon and hydrogen and do not contain any halogens like chlorine, fluorine etc. These refrigerants are non -toxic but highly flammable and have zero ODP and negligible GWP. HC refrigerants are completely miscible with commonly used mineral oils as well as PAG and POE [26]. R134a is becoming widely accepted as the replacement for R12 in domestic refrigerator/freezer and automotive air-conditioning applications and also as the replacement for medium pressure chillers. First, R134a is not compatible with the mineral oils commonly used for compressor lubrication [30]. There are a number of synthetic candidates being evaluated for use with R134a, but none have been proven totally useful. The refrigeration capacity and coefficient of performance (COP) of alternative refrigerants must also be established. Several investigations have been conducted to determine the capacity and performance of alternatives relative to their CFC counter-parts. A comparison of R134a with R12 in a residential heat pump showed that approximately the same heating output was achieved with R134a. But the COP of the system was approximately 15% less with R134a than with R12 [6]. In another series tests were conducted at ARI (Airconditioning and Refrigeration Institute). Heat Pump Rating Conditions showed that R134a exhibits a 6 to 11% increase in COP for moderate and warm rating conditions [41], while R134a has a nearly identical COP to that of R12 for a cold rating condition. In a test conducted for a

13 household refrigerator/freezer, R134a was shown to consume approximately 8% more power than R12 and require more run-time, resulting in energy consumption 45% greater than R12. Even though R134a has proven as an alternative refrigerant to the CFCs and has lower GWP of 1300 which is less than R12 (8500) is considerably high and has to be controlled as per the Kyoto Protocol. Since last couple of years hydrocarbons are being used as alternative refrigerants to R12 and R134a due to their excellent thermo physical properties. But pure hydrocarbons are not suitable for drop in replacement for existing systems due to mismatch of its saturation properties. It demands changes in the design, especially compressor [36]. The use of hydrocarbons was restricted due to its flammable properties. The above discussion indicates that a lot of work had been done already and is still continuing to develop and test CFC pure-refrigerant alternatives. These alternatives have a lot of potential, but their total acceptance has not been found out Synthetic Mixtures The synthetic mixtures may be broadly classified as azeotropic, near-azeotropic, or zeotropic [62]. 1. Azeotropes An azeotrope is defined as a point at which the concentration of the liquid and the vapour phase is the same for a given temperature and pressure. An azeotrope, a mixture behaves like a single-constituent system. Almost all azeotropic refrigerants have a boiling point lower

14 than either of the constituents (which are known as a minimum temperature or maximum pressure azeotrope). 2. Near Azeotropes For a near-azeotropic mixture, the vapour and liquid concentrations at a given temperature and pressure differ slightly. Most azeotropic refrigerant mixtures become near-azeotropic when the pressure or temperature is varied from the azeotrope point. R410A is a near-azeotropic mixture of 50%R32/50%R125. For standard condenser pressures and temperatures, the bubble and dew points for this concentration vary by less than C. 3. Zeotropes or Non Azeotropes For a zeotropic mixture, the concentrations of the liquid and the vapour phase are never equal. This creates a temperature glide during the phase change. Zeotropic mixtures are the most common type of refrigerant blend. Use of non azeotropic refrigerant mixture reduces the irreversibility and increases the COP [50]. 3.5 MONTREAL PROTOCOL The Montreal Protocol on substances that deplete the ozone layer is an international treaty designed to protect the ozone layer by a scheduled phasing out of the ozone depleting substances. It is believed that if the international agreement is adhered to, the ozone layer is anticipated to recover by Due to its extensive adoption and implementation it has been hailed as an example of outstanding international cooperation. Kofi Annan says that "perhaps the single most successful international agreement to date has been the Montreal Protocol".

15 Impact of Montreal Protocol Since the Montreal Protocol came into effect, the atmospheric concentrations of the most important CFCs and related chlorinated hydrocarbons have either leveled off or decreased. Also, the concentration of the HCFCs increased drastically atleast partly because HCFCs have been substituted by CFCs in most cases. On a molecule-for-molecule basis HFC compounds are upto 10,000 times more potent greenhouse gases than carbon dioxide. The Montreal Protocol currently calls for a complete phase out of HCFCs by 2040 in developing countries like India, but does not place any restriction on HFCs. Since the HCFCs themselves are as powerful as greenhouse gases, the mere substitution of HFCs for CFCs does not significantly increase the rate of anthropogenic global warming, but over a period of time a steady increase in their use could increase the danger that human activities may change the climate. 3.6 GLOBAL WARMING POTENTIAL AND KYOTO PROTOCOL GWP is a measure of how much a given mass of greenhouse gas is estimated to contribute to global warming. It is a relative scale which compares the gas in question to that of the same mass of CO2. A global warming potential is calculated over a specific time interval and the value of this must be stated whenever a global warming potential is quoted other wise the value becomes meaningless. Its usage is being governed by the Kyoto Protocol. In 1997 the world nations came together in Kyoto in Japan to discuss Global Warming, the Kyoto Protocol finally came into force. The

16 very phrase Kyoto Protocol has become synonymous with the idea of saving the earth from global meltdown. The Kyoto Protocol aims to tackle global warming by setting target levels for nations to reduce green house emissions worldwide. These targets vary between countries and regions, but globally the initial target is to reduce greenhouse gas emissions to 5.2% below 1990 levels (base levels) during the commitment period, i.e., [56]. The focus of the Kyoto Protocol however, is on the reduction in the levels of the following six gases: Carbon dioxide (CO2), Methane (CH4), Nitrous Oxide (N2O), Hydroflurocarbons (HFCs), Perflurocarbons (PFCs) and Sulphur Hexafluoride (SF6). 3.7 REASONS FOR SELECTION OF R134a AND HYDROCARBON REFRIGERANTS R134a has the following advantageous properties. 1) As compared to R12 which has an ODP of 1, R134a has an ODP of 0 due to complete absence of chlorine atoms. 2) When compared to R12, which has a GWP of 8500, R134a has a GWP which is approximately one tenth of that of R12 i.e (Here GWP of CO2 = 1; time base = 100 years). 3) Unlike the hydrocarbon blends, R134a is non-flammable. 4) R134a has an excellent material compatibility. 5) It is non toxic as compared to refrigerants like ammonia. However it does require some ventilation to avoid displacement of oxygen. 6) It has a COP pattern and magnitude which is near to that of R12.

17 Hydrocarbons (HCs) have similar properties to CFCs and HCFCs. The HCs with better properties as refrigerant are isobutane (R600a), propane (R290) and their mixtures. These substances fulfil thermo physics, ecological, physiologic and economic requirements to be located among the best options to substitute CFC or HFCs. The most important characteristics are: 1) The thermodynamic properties of hydrocarbons are similar to that of R134a. HCs have low viscosity and high thermal conductivity that guarantee a good performance of the system. These superior transport properties are believed to contribute to the higher energy efficiency of hydrocarbons. 2) As shown in Table 3.1 the global warming potential (GWP) of hydrocarbons is much lower than that of synthetic refrigerants. 3) The Table 3.1 also shows that the ozone depletion potential (ODP) of hydrocarbons is zero. 4) Another advantage of hydrocarbons is their solubility in most of oils like mineral oil and POE which is traditionally used as lubricants in the hermetic compressors. 5) They are compatible with the materials used with traditional refrigerant, metal components and oils. 6) High latent heat in the boiling process 7) As the density is less than lower than CFCs/HFCs, inspite of its flammability, the refrigerant mass requirements are low. 8) The ecological advantages include zero ozone depletion potential, non toxic substances and negligible global warming potential.

18 Table 3.1 Properties of various refrigerants Refrig erant Code Molecu lar weight Boiling Point 0C ( bar) Critical Tempera ture 0 C Latent heat kj/kg Explosive Limits in air, % by volume ODP GWP R R134a Nonflammable Nonflammable R R600a CRITERIA FOR SELECTION OF CHOSEN MIXTURES The present experiment was mainly performed to test a ternary mixture of R134a/R600a/R290. Though, less than R12 s GWP; R134a still has a pretty high GWP. This value could be reduced if it is mixed with hydrocarbons which have very low GWP values. Mixing of HFC and HC refrigerants allows the adjustment of the undesirable properties of the individual components such as flammability of HC refrigerants which can be reduced by adding non-flammable components like HFCs [19, 62]. Unless major changes in the compressor are made, the saturation properties of the alternative refrigerant should match closely with the base refrigerant [10]. Safe

19 limit of hydrocarbons in refrigerators is 150g as specified by Prof. R.S.Agarwal. In any refrigerator, if the charge of the refrigerant is less than or equal to 150 grams no need to take any safety precautions with flammability concern [60]. In the proposed ternary mixture of R134a/R600a/R290, the total quantity of the hydrocarbons is less than 150 grams so the mixture is a non-flammable refrigerant. 3.9 THEORETICAL ANALYSIS FOR THE SELECTION OF OPTIMUM COMPOSITION OF REFRIGERANT MIXTURES A majority of refrigeration systems in the India are using R134a as their refrigerant. In a refrigeration system, the most expensive component is the compressor. Thus if a surrogate to R134a is achieved which could be used without the replacement of the compressor, the substitute would be highly economical. Thus the most important performance parameter that is considered for selecting a specific composition from a large number of compositions was the matching of the saturation properties. The saturation properties of the HC mixture (50%R600a/50%R290) match closely the saturation properties of R134a [10]. Therefore, any mixture of R134a/R600a/R290 at any mole fractions can have saturation properties very close to R134a. The proposed alternative ternary mixture can be considered as drop in replacement for R134a refrigerators. COP was considered as a secondary performance parameter and calculations were done for each composition. As a tertiary performance character, we considered the pressure ratio, to ensure that the

20 operating pressures were in attainable limits for the compressor of the domestic refrigerator. For the selection of the best alternative mixture, all the calculations were performed at C and 40 0 C of evaporator and condenser temperatures respectively. All percentages are in terms of percentage by mass. In order to compare various refrigerants as working fluids in domestic refrigerator the thermodynamic properties were taken from the software REFPROP 6.0 [58]. Ternary Mixture: In the ternary mixture, the compounds selected for simulative testing were R134a, Isobutane (R600a) and Propane (R290) the calculation procedure explained by the Philippe F.Launay [49] was followed (a) The percentage of R134a was varied in steps of 5% by keeping the remnant percentage of mass shared equally between isobutane and propane. The results have been given tabulated in Table 3.2. The following were the inferences drawn from results. As the percentage of R134a in the ternary mixture was increased, corresponding saturation pressures are matching closely with the pure R134a as shown in Figure 3.2 which reflects the proposed alternative mixture can be used as drop in replacement for R134a. The COP started increasing with the increasing percentage R134a and reach a maximum value at 25% of R134a (Mixture-3) and then started decreasing with a further increase in R134a mass fraction.

21 Table 3.2 Performance comparison of selected alternative refrigerants with the base refrigerants with same operating conditions Refrigerant Refrigeration Effect(kJ/kg) Specific Work (kj/kg) COP Specific Volume (m 3 /kg) R134a HC mixture (50%R290/50%R600a) Mixture-1 (5%R134a/47.5%R290/ 47.5%R600a) Mixture-2 (15%R134a/42.5%R290 / 42.5%R600a) Mixture-3 (25%R134a/37.5%R290 / 37.5%R600a) Mixture-4 (35%R134a/32.5%R290 / 32.5%R600a) Mixture-5 (45%R134a/27.5%R290 / 27.5%R600a) THEORETICAL ANALYSIS OF PROPERTIES OF THE SELECTED ALTERNATIVE REFRIGERANT MIXTURES Pure R290 or R600a is not suitable as a direct drop in replacement for R134a. However, the saturation properties of the mixture, developed by Rauolt's rule, of HC mixture (50% R600a and 50% R290), matches closely the saturation properties of R134a. Therefore, any mixture of R134a/R600a/R290 at any mole fraction can have saturation properties very close to R134a. The deviation of vapour pressure with saturation temperature of the chosen alternative

22 refrigerants, R134a and HC mixture are plotted in Figure 3.2. It shows that the alternative mixtures, viz, mixture-1, mixture-2, mixture-3, mixture-4 and mixture-5 have close values of vapour pressure with R134a. In this work, different masses of R134a/R600a/R290 mixtures were studied to find the best mass of this mixture to replace R134a in domestic refrigerators. To decide the charge quantity density of the liquid refrigerant is an important parameter. Refrigerant charge is a key parameter in a Vapour compression refrigeration system that influences the performance of the system. The deviation of liquid density with saturation temperature of the chosen alternative refrigerants and R134a are plotted in Figure 3.3. It shows that the alternative mixtures mixture-1, mixture-2, mixture-3, mixture-4 and mixture-5 have lower liquid density than R134a. For the alternative mixtures the density is 45% to 56% lower than R134a for the considered operating range. Due to this it can be inferred that less mass of alternative refrigerant mixtures is needed when compared with R134a in an existing system [13, 11].

23 Pressure in bar R134a Mix-1 Mix-2 Mix-3 Mix-4 Mix Temperature in 0 C Figure 3.2 Variation of vapour pressure with saturation temperature 1600 R134a Mix-1 Mix-2 Mix-3 Mix-4 Mix-5 Liquid density in kg/m Temperature in 0 C Figure 3.3 Variation of liquid density with saturation temperature

24 In the present study the system considered is working with vapour compression refrigeration system principle, it is necessary to study the vapour density of the selected refrigerants. The vapour density of considered alternative refrigerants and R134a is plotted in Figure 3.4. It shows that vapour density of the mixture % to 61.3% and mixture % to 36.68% is lower than that of the R134a and the range of other mixtures falls in between mixture-1 and mixture-5. Hence the considered mixtures charge quantity by mass as compared to R134a will be lesser, when R134a compressors are used. 80 R134a Mix-1 Mix-2 Mix-3 Mix-4 Mix Vapour Density in kg/m Temperature in 0 C Figure 3.4 Variation of vapour density with saturation temperature The important thermodynamic property that plays a dominating role while deciding the refrigeration effect is the latent heat. For a given compressor it can handle a particular volume flow rate, from the above

25 discussion it has been found that the mass flow rate of alternative mixtures is less than that of R134a due to its lower density. The latent heat of vaporization of the mixtures doesn t match with that of R134a, results in decreasing cooling capacity. The latent heat of considered alternative refrigerants and R134a is plotted in Figure 3.5. It shows that the alternative mixtures mixture-1, mixture-2, mixture-3, mixture- 4 and mixture-5 have 34% to 76% higher latent heat than R134a.From the graph it was observed that latent heat of vaporisation of the alternative mixtures decreases from mixture-1 to mixture-5, which is due to increase in the mass quantity of R134a.Thus there is a scope for the lower mass of alternative mixtures to have the same or better cooling effect compared with R134a. Viscosity is one of the important thermo physical properties of the refrigerant which influences the flow-ability of refrigerant through the system. It influences the flow characteristics of the refrigerants inside the capillary tube. Pressure loss increases with the increase of viscosity. The viscosity of the considered alternative refrigerants and R134a in liquid and vapour form is plotted separately in Figure 3.6 and Figure 3.7 respectively. As the viscosity values of the alternative refrigerant mixtures have more or less same values, mixture-3 was chosen in comparison with R134a.

26 Liquid viscosity Latent heat in kj/kg 425 R134a Mix-1 Mix-2 Mix-3 Mix-4 Mix-5 HC Temperature in 0 C Figure 3.5 Variation of latent heat with saturation temperature 450 R134a Mix Temperature in 0 C Figure 3.6 Variation of liquid viscosity with saturation temperature

27 Vapour viscosity 14 R134a Mix Temperature in 0 C Figure 3.7 Variation of vapour viscosity with saturation temperature Figures 3.6 and 3.7 show that viscosities of the selected mixtures are 40% to 47% and 23 to 26% which is lower than R134a in liquid and vapour phase respectively. Hence for alternative refrigerants, due to their lesser viscosity higher capillary lengths are required for the same pressure drop as compared with R134a[53, 24].

28 Thermal conductivity in W/m/K Thermal conductivity in W/m/K R134a Mix Temperature in 0 C Figure 3.8 Variation of liquid thermal conductivity with saturation temperature R134a Mix Temperature in 0 C Figure 3.9 Variation of vapour thermal conductivity with saturation temperature

29 The thermal conductivity of the selected alternative refrigerants and R134a in liquid and vapour form is plotted in Figure 3.8 and Figure 3.9 respectively. From the Figures 3.18 and 3.19, it is inferred that thermal conductivity of the selected mixtures is higher than R134a.For example Micture-3 is showing 6.8 to 7.7% and 23.2 to 24.5% higher than R134a in liquid and vapour phases respectively. Hence higher heat transfer coefficients can be expected for the selected refrigerants in the evaporator and condenser. This results in better heat transfer rates. Specific volume of the refrigerant plays an important role in influencing the work of compression. The specific volume of the considered alternative refrigerants, R134a and HC mixture is plotted in Figure It shows that specific volume of the alternative mixtures increases from mixture-5 to mixture-1 and it is highest for HC mixture, lowest for R134a. Even though the specific volume of the selected refrigerants is higher than that of R134a, since the mass flow rates of selected refrigerants is lesser it would not result in higher compressor displacement rates. Hence for the alternative mixtures there is no need to change the compressor, the same compressor used for R134a can be used.

30 Specific volume in m 3 /kg 0.35 R134a Mix-1 Mix-2 mix-3 Mix-4 Mix-5 HC Temperature in 0 C Figure 3.10 Variation of specific Volume with saturation temperature Vapour specific heat is one of the influencing parameter that decides the super heat of the refrigerant at inlet to the compressor. For the same heat transfer the refrigerant which is having high specific heat leads to decrease in degree of super heat which decreases the work of compression. The specific heat of the considered alternative refrigerant, R134a is plotted in Figure It shows that specific heat of the alternative mixture is 60 to 66% which is more than that of the R134a.Hence the degree of super heating would be lesser for selected refrigerants as compared to R134a, thereby better performance can be expected from the alternative mixtures.

31 Specific heat in kj/kg/k R134a Mix Temperature in 0 C Figure 3.11 Variation of Vapour specific heat with saturation temperature From the above discussion it can observed that with the increasing R134a quantity in the ternary mixture (R134/R290/R600a) from mixture-1 to mixture-5 both the specific volume and latent heat decrease in comparison with HC mixture which is presently used as alternative refrigerant in the place of R134a. This is because pure R134a has low specific volume and low latent heat values and HC mixture has high specific volumes and high latent heat values. When R134a and HC mixture are mixed together the final mixture results in lower specific volumes than HC mixture, better latent heat of vaporisation values than R134a, by taking this advantage the ternary mixture is expected to perform well when compared with the existing refrigerants R134a and HC mixture. From Table 3.2 it can be observed that mixture-1 to mixture-3 compared to decrease of refrigeration effect, decrease of work of compression is more, which is due to more

32 decrease of specific volume. Hence it leads to better COP values than R134a and HC mixture at Mixture-3. From mixture-3 to mixture-5 decrease of refrigeration effect will be more than the decrease of work of compression which leads to lower COP values. Hence for the selected mixtures mixture-3 will result in maximum COP values HANDLING HC CYLINDERS Cylinders containing HC refrigerants should be clearly labelled to show the type of refrigerant and that it is flammable. The guidelines given below are recommended as good practices when handling HC cylinders which are very similar to the guidelines for any refrigerant cylinder [15] The valve cap should be fitted when the cylinder is not being used; The cylinder should not be heated. Refrigerant cylinders can usually withstand temperatures up to 45 to 50 0 C. If a cylinder needs to be heated (e.g. to remove refrigerant more easily), it should be placed in a container of water not hotter than 45 to 50 0 C. The cylinder and its valve should not be modified. The cylinder should not be refilled unless it is designed for recovered refrigerant. It should be noted that the weight of the same volume of HC refrigerant is only 40% to 44% of the weight of R12/R134a refrigerant. A cylinder, which can safely contain 10kg of R12/R134a, will only be able to contain 4 to 4.4 kg of HC. The volume of the liquid refrigerant in

33 the cylinder should never exceed 80% of the total cylinder volume or the weight of refrigerant filled should be 80% or less of the maximum permitted fill weight 3.12 PREPARATION OF REFRIGERANT MIXTURE The proposed ternary mixture of HFC(R134a)/HC (R600a/R290) in the present study are zeotrope in nature. Hence mixing of the refrigerants, handling and charging should be done carefully. Many guidelines have been reported in the literature regarding procedure and characteristic of the zeotrope mixtures. The five mixtures mixture-1, mixture-2, mixture-3, mixture-4 and mixture-5 were prepared in separate cylinders before they were charged into the system. To control the concentration shifts, the minimum liquid level of the charge quantity in the refrigerant mixture cylinder should not be less than 10% volume while charging the system. Hence the mixture quantity has been prepared sufficiently to maintain the 10% level. To have an accurate quantity the weight of the mixtures were prepared in small cylinders of 1kg capacity. The following are the steps that have been followed by the researcher for preparing ternary mixture Initially cylinders were cleaned and flushed with R134a twice. Evacuate the cylinder by vacuum pump up to 0.1mbar. Cylinders were kept at a low temperature bath while filling to avoid cross contamination and quick transfer of refrigerant.

34 Initially cylinders were filled with required quantity of HC, as HC has a lower vapour pressure than R134a [54]. Later the required quantity of R134a is filled in to the cylinder. Each cylinder was properly labeled to indicate the name and quantity of filled refrigerant mixture While charging the refrigerant into the system it was ensure that only liquid has to enter into the system which is done by placing the cylinder in upright down position. The photographic view of the charging procedure is as shown in Figure Charging The charging of refrigeration systems with hydrocarbon refrigerants is similar to those using halocarbon refrigerants. As with all blend refrigerants, hydrocarbon refrigerant blends should also be charged in the liquid phase in order to maintain the correct composition of the blend [15]. The following additional requirements should be adhered to:- Ensure that contamination of different refrigerants does not occur when using charging equipment. Hoses or lines are to be as short as possible to minimize the amount of refrigerant contained in them.

35 a) vacuum process b) charging of the refrigerant c) weighing scale d) charging kit and low temperature bath Figure 3.12 Photographic views of the preparation of the ternary mixture

36 It is recommended that cylinders be kept upright and refrigerant is charged in the liquid phase. Ensure that the refrigeration system is earthed prior to charging the system with refrigerant. Label the system when charging is complete. The label should state that hydrocarbon refrigerants have been charged into the system and that it is flammable. Position the label in a prominent position on the equipment. Extreme care should be taken as to not to overfill the refrigeration system. (Note that hydrocarbon charge sizes are typically 40% to 50% of CFC, HCFC and HFC charge sizes) EQUIVALENT CHARGE QUANTITY OF THE MIXTURES The density difference is important when charging the systems. When charging hydrocarbons by weight, only 43% of the R134a charge is used. When charging hydrocarbons by volume, the same volume as for the halocarbon is used. The system should always be charged with liquid refrigerant in case of blends. It is essential that the system should be filled with an exact charge for better performance. HFC/HC refrigerant is zeotropic blends therefore, while charging with mixture, make sure that the refrigerant drawn from the cylinder is in the liquid form. It is recommended that charging should be done by weight using an electronic weighing scale along with charging equipment.

37 For the given volume of the visi cooler considering the instrumentation of the system the manufacturer specified quantity of R134a is 240 grams. The Table 3.3 shows the equivalent quantity of considered HFC/HC mixtures in comparison with R134a. Table 3.3 Equivalent mass of selected alternative refrigerants and HC blend Refrigerant Equivalent Charge to 240 grams of R134a in grams Mixture Mixture Mixture Mixture Mixture HC mixture 104