Department of Mechanical Engineering ME 322 Mechanical Engineering Thermodynamics. Lecture 29. The Vapor Compression Refrigeration (VCR) Cycle

Transcription

1 Department of Mecanical Engineering ME 3 Mecanical Engineering Termodynamics Lecture 9 Te Vapor Compression Refrigeration (VCR) Cycle

2 Refrigerator used for Cooling T H energy sougt Q in E energy tat costs W cycle COP C Observation: may be > 00% E E T L Te concept of an efficiency being greater tan 00% makes people uneasy. Terefore, te conversion efficiency for a refrigerator is called te Cooling Coefficient of Performance (COP C ). A refrigeration system tat is used for cooling is called a refrigerator.

3 Refrigerator used for Heating T H energy sougt Q out E energy tat costs W cycle COP H Observation: may be > 00% E E T L Te concept of an efficiency being greater tan 00% makes people uneasy. Terefore, te conversion efficiency for a refrigerator is called te Heating Coefficient of Performance (COP H ). A refrigeration system tat is used for cooling is called a eat pump. 3

4 Analysis of te Carnot Refrigerator For te Refrigeration cycle T H COP R, Carnot Qin Qin t COPR W Q Q Q / Q cycle out in out in Q / Q T / T out in rev H L COP R, Carnot T H TL T L For te Heat Pump cycle T L COP Qout Qout t COPH W Q Q Q / Q cycle out in in out COP H H, Carnot H, Carnot Qin / Qout TL / T rev H TH TL T 4

5 Terminology Refrigeration cycle Te cycle is operating in a refrigeration cycle Te goal is to keep te cold space cold Transfer eat from a low-temperature source to te cycle Heat pump cycle Te cycle is operating in a refrigeration cycle Te goal is to keep te ot space ot Transfer eat to a ig-temperature sink from te cycle Te words refrigeration or eat pump define te goal of te cycle. 5

6 Te Vapor Compression Refrigeration Cycle T H T L 6

7 Te Vapor Compression Refrigeration Cycle Working fluid = Refrigerant Two pase canges Boiling (evaporator) Condensing (condenser) Low temperature boiling fluids 7

8 Refrigerants Most refrigerants are alogenated ydrocarbons. Te naming convention adopted by ASHRAE is, R(a-)(b+)d = C a H b Cl c F d c = (a ) b d Example: R (R0) a 0 a H b b d c a b d c F C Cl F clorodifluorometane 8

9 9 Te Pressure-Entalpy Diagram

10 Vapor Compression Refrigeration Analysis m 3 Performance COPC Q W in c 3 4 m m 4 0

11 Te Ideal VCR Cycle on te P- Diagram P -: Isentropic compression -3: Isobaric eat rejection 3-4: Isentalpic expansion 4-: Isobaric eat addition 3 q Q / m out out m m q Q / m w W / m in in c c m 4

12 Refrigeration Effect and Capacity Refrigeration Effect: q in 4 Refrigeration Capacity: Qin m4 Refrigeration capacity is often expressed in tons of refrigeration. Definition ton of refrigeration is te steady state eat transfer rate required to melt ton of ice at 3 F in 4 ours. ton =,000 Btu/r = 3.56 kw

13 VCR Cycle Irreversibilities P Pressure drop troug te condenser Pressure drop troug te evaporator 3 4 s Isentropic efficiency of te compressor 3

14 Practical VCR Cycle P T3 SCT SCT = Saturated Condensing Temperature DSC = Degrees of Subcooling = SCT T 3 s Subcooling increases te refrigeration capacity 3 4 Supereating provides a dry vapor at te compressor inlet SET T SET = Saturated Evaporating Temperature DSH = Degrees of Supereat = T SET 4

15 Example Given: A vapor compression refrigeration cycle is operating wit a saturated evaporating temperature of -0 F and a saturated condensing temperature of 80 F. Te refrigerant, R, leaves te condenser as a saturated liquid and enters te compressor wit 5 degrees of supereat. Te pressure drops troug te evaporator and condenser can be considered negligible. Te compressor as an isentropic efficiency of 85%. Te cycle as a capacity of 5 tons. Find: (a) te mass flow rate of te R (lbm/r) (b) te power requirement of te compressor (p) (c) te coefficient of performance of te cycle 5

16 Example P T3 80F 3 SCT 80F s T SET 5F 4 SET 0F T4 0F 5 tons T SET 5F 6

17 T3 80F Example T4 0F T SET 5F P 5 tons 3 SCT 80F s 4 SET 0F T SET 5F 7

18 T3 80F Example Strategy: Build te property table ten do te termodynamics! T4 0F P 5 tons T SET 5F Te ig and low pressures in te cycle can be found since te saturation temperatures are given. 3 SCT 80F s 4 SET 0F T SET 5F Te pressures at all four states are known! 8

19 T3 80F Example T4 0F T SET 5F P 5 tons 3 SCT 80F s 4 SET 0F T SET 5F 9

20 T3 80F Example Te property table is complete! T4 0F T SET 5F P 5 tons 3 SCT 80F s 4 SET 0F T SET 5F 0

21 T3 80F Example EES Solution (Key Variables) T4 0F T SET 5F P 5 tons Comparison (same units) 3 SCT 80F s,000 Btu Btu Qin 5 tons 80,000 ton-r r 4 SET 0F Wc 545 Btu Btu 4.45 p 6,5 p-r r T SET 5F