Chapter 10: Refrigeration Cycles

Transcription

1 Capter 10: efrigeration Cycles Te vapor compression refrigeration cycle is a common metod for transferring eat from a low temperature to a ig temperature. Te above figure sows te objectives of refrigerators and eat pumps. Te purpose of a refrigerator is te removal of eat, called te cooling load, from a low-temperature medium. Te purpose of a eat pump is te transfer of eat to a ig-temperature medium, called te eating load. Wen we are interested in te eat energy removed from a low-temperature space, te device is called a refrigerator. Wen we are interested in te eat energy supplied to te ig-temperature space, te device is called a eat pump. In general, te term eat pump is used to describe te cycle as eat energy is removed from te low-temperature space and rejected to te igtemperature space. Te performance of refrigerators and eat pumps is expressed in terms of coefficient of performance (), defined as Capter 10-1

2 HP Desired output Cooling effect Q equired input Work input W Desired output Heating effect Q equired input Work input W net, in H net, in Bot and HP can be larger tan 1. Under te same operating conditions, te s are related by HP +1 Can you sow tis to be true? efrigerators, air conditioners, and eat pumps are rated wit a SEE number or seasonal adjusted energy efficiency ratio. Te SEE is defined as te Btu/r of eat transferred per watt of work energy input. Te Btu is te Britis termal unit and is equivalent to 778 ft-lbf of work (1 W Btu/r). An EE of 10 yields a of 2.9. efrigeration systems are also rated in terms of tons of refrigeration. One ton of refrigeration is equivalent to 12,000 Btu/r or 211 /min. How did te term ton of cooling originate? eversed Carnot efrigerator and Heat Pump Sown below are te cyclic refrigeration device operating between two constant temperature reservoirs and te T-s diagram for te working fluid wen te reversed Carnot cycle is used. ecall tat in te Carnot cycle eat transfers take place at constant temperature. If our interest is te cooling load, te cycle is called te Carnot refrigerator. If our interest is te eat load, te cycle is called te Carnot eat pump. Capter 10-2

3 Te standard of comparison for refrigeration cycles is te reversed Carnot cycle. A refrigerator or eat pump tat operates on te reversed Carnot cycle is called a Carnot refrigerator or a Carnot eat pump, and teir s are, Carnot HP, Carnot T H 1 / T 1 1 T / T T 1 T T H T H H TH T Notice tat a turbine is used for te expansion process between te ig and low-temperatures. Wile te work interactions for te cycle are not indicated on te figure, te work produced by te turbine elps supply some of te work required by te compressor from external sources. Wy not use te reversed Carnot refrigeration cycle? Easier to compress vapor only and not liquid-vapor mixture. Ceaper to ave irreversible expansion troug an expansion valve. Wat problems result from using te turbine? Capter 10-3

4 Te Vapor-Compression efrigeration Cycle Te vapor-compression refrigeration cycle as four components: evaporator, compressor, condenser, and expansion (or trottle) valve. Te most widely used refrigeration cycle is te vapor-compression refrigeration cycle. In an ideal vapor-compression refrigeration cycle, te refrigerant enters te compressor as a saturated vapor and is cooled to te saturated liquid state in te condenser. It is ten trottled to te evaporator pressure and vaporizes as it absorbs eat from te refrigerated space. Te ideal vapor-compression cycle consists of four processes. Ideal Vapor-Compression efrigeration Cycle Process Description 1-2 Isentropic compression 2-3 Constant pressure eat rejection in te condenser 3-4 Trottling in an expansion valve 4-1 Constant pressure eat addition in te evaporator Capter 10-4

5 Te P- diagram is anoter convenient diagram often used to illustrate te refrigeration cycle. Te ordinary ouseold refrigerator is a good example of te application of tis cycle. esults of First and Second aw Analysis for Steady-Flow Component Process First aw esult Compressor s const. W m ( ) in 2 1 Condenser P const. Q H m ( 2 3) Trottle Valve s > W net 0 Q net 0 Evaporator P const. Q m ( ) 1 4 Capter 10-5

6 HP Q W net, in Q H W net, in Example 10-1 efrigerant-134a is te working fluid in an ideal compression refrigeration cycle. Te refrigerant leaves te evaporator at -20 o C and as a condenser pressure of 0.9 MPa. Te mass flow rate is 3 kg/min. Find and, Carnot for te same T max and T min, and te tons of refrigeration. Using te efrigerant-134a Tables, we ave State1 Compressor inlet o T1 20 C x State3 Condenser exit P3 900kPa x U V S W T U V S W T s 3 3 s kg kg K kg kg K State2 Compressor exit P2s P2 900kPa s2s s kg K State4 Trottle exit o T4 T1 20 C 4 3 U V S W T x s 4 4 U V S T W T 2s 2s kg o C kg K Capter 10-6

7 Q m ( 1 4) W m ( ) net, in ( ) ( ) kg kg Te tons of refrigeration, often called te cooling load or refrigeration effect, are Q m ( 1 4) kg min (.. ) 1Ton kg 211 min 193. Ton, Carnot T H T T ( ) K ( ( 20)) K 391. Anoter measure of te effectiveness of te refrigeration cycle is ow muc input power to te compressor, in orsepower, is required for eac ton of cooling. Te unit conversion is p per ton of cooling. Capter 10-7

8 W Q net, in p 341. Ton p 11. Ton Actual Vapor-Compression efrigeration Cycle Capter 10-8

9 Heat Pump Systems Oter efrigeration Cycles Cascade refrigeration systems Very low temperatures can be acieved by operating two or more vaporcompression systems in series, called cascading. Te of a refrigeration system also increases as a result of cascading. Capter 10-9

10 Multistage compression refrigeration systems Capter 10-10

11 Multipurpose refrigeration systems A refrigerator wit a single compressor can provide refrigeration at several temperatures by trottling te refrigerant in stages. iquefaction of gases Anoter way of improving te performance of a vapor-compression refrigeration system is by using multistage compression wit regenerative cooling. Te vapor-compression refrigeration cycle can also be used to liquefy gases after some modifications. Capter 10-11

12 Gas efrigeration Systems Te power cycles can be used as refrigeration cycles by simply reversing tem. Of tese, te reversed Brayton cycle, wic is also known as te gas refrigeration cycle, is used to cool aircraft and to obtain very low (cryogenic) temperatures after it is modified wit regeneration. Te work output of te turbine can be used to reduce te work input requirements to te compressor. Tus, te of a gas refrigeration cycle is q w w q w net, in comp, in turb, out Capter 10-12

13 Capter 10-13

14 Absorption efrigeration Systems Anoter form of refrigeration tat becomes economically attractive wen tere is a source of inexpensive eat energy at a temperature of 100 to 200 o C is absorption refrigeration, were te refrigerant is absorbed by a transport medium and compressed in liquid form. Te most widely used absorption refrigeration system is te ammonia-water system, were ammonia serves as te refrigerant and water as te transport medium. Te work input to te pump is usually very small, and te of absorption refrigeration systems is defined as Desired output Cooling effect Q equired input Work input Q + W, gen pump in Q Q gen Capter 10-14

15 Termoelectric efrigeration Systems A refrigeration effect can also be acieved witout using any moving parts by simply passing a small current troug a closed circuit made up of two dissimilar materials. Tis effect is called te Peltier effect, and a refrigerator tat works on tis principle is called a termoelectric refrigerator. Capter 10-15