1. Chapter 14A: VC AND AC REFRIGERATION CYCLES AND SYSTEMS
Agami Reddy (July 2016)
Standard vapor compression (VC) refrigeration cycle
Use of refrigerant property tables and p-h diagrams
Analysis of different processes
Actual VC refrigeration cycle
Chiller systems and effect of HX
Chiller maps and manufacturer tables
Absorption systems description
Components
Types of systems
Thermodynamic analysis
HCB 3- Chap 14A: Refrigeration Cycles and Systems

2. HCB 3- Chap 14A: Refrigeration Cycles and Systems
Introduction
Basic types of refrigeration systems:
- vapor compression refrigeration and heat pumps
- gas refrigeration
- absorption
- adsorption
- thermoelectric
Applications:
- building air conditioning
- automotive
- industrial
- food processing, food distribution…..
HCB 3- Chap 14A: Refrigeration Cycles and Systems

3. HCB 3- Chap 14A: Refrigeration Cycles and Systems
Recall: Carnot Refrigeration Cycle
Here work is put in
(in the form of electricity) so as to
achieve heat extraction or cooling
HCB 3- Chap 14A: Refrigeration Cycles and Systems

4. Practical Modifications to Carnot Cycle
Figure 14.2 Schematic diagram of the standard VC refrigeration cycle
arrangement of mechanical components,
plot on T-s coordinates showing wet compression,
plot on p-h coordinates showing wet compression.
HCB 3- Chap 14A: Refrigeration Cycles and Systems

5. HCB 3- Chap 14A: Refrigeration Cycles and Systems
Quantities of Interest
Table 14.1
HCB 3- Chap 14A: Refrigeration Cycles and Systems

6. HCB 3- Chap 14A: Refrigeration Cycles and Systems

7. HCB 3- Chap 14A: Refrigeration Cycles and Systems

8. HCB 3- Chap 14A: Refrigeration Cycles and Systems

9. HCB 3- Chap 14A: Refrigeration Cycles and Systems
Expander Turbine
Throttling process of Rankine cycle produces no useful work
If throttling is replaced by energy recovery, external work input requirement is less and COP will increase
HCB 3- Chap 14A: Refrigeration Cycles and Systems

10. Modified VC Cycle: Subcooling
Practical advantages: -Ensures that compressor is dry
(but may result in greater power reqd.)
-Results in smoother flow of refrigerant
at expansion valve (bubbles are reduced
Figure 14.3 Modified VC cycle with a heat exchanger meant to superheat vapor leaving the compressor and subcool condenser liquid (a) System diagram, (b) P-h diagram
HCB 3- Chap 14A: Refrigeration Cycles and Systems

11. Isentropic efficiency of compressor
Second difference:
Isentropic efficiency of compressor
Figure 14.4 p-h diagram showing the various state points for the three cycles: the standard VC cycle ( ), the modified VC cycle (1’-2’-3’-4’), and the effect of isentropic compression (1’-2”-3’-4’)
HCB 3- Chap 14A: Refrigeration Cycles and Systems

12. Third difference between actual refrigeration cycle
and standard VC cycle:
Depends on specific system piping and layout
Fig Pressure losses due to friction
Compressor discharge valves, Compressor suction valves
Discharge lines, Liquid lines, Suction Lines
Condenser, Evaporator
HCB 3- Chap 14A: Refrigeration Cycles and Systems

13. HCB 3- Chap 14A: Refrigeration Cycles and Systems
Modified VC cycle
2 2’ 2”
HCB 3- Chap 14A: Refrigeration Cycles and Systems

14. HCB 3- Chap 14A: Refrigeration Cycles and Systems

15. HCB 3- Chap 14A: Refrigeration Cycles and Systems

16. HCB 3- Chap 14A: Refrigeration Cycles and Systems
Effect of Change in Evaporator Temperature
Fig
As the evaporator temperature increases with fixed condenser temp. :
Compression work 1-2 is markedly less than that of 1a-2a,
So refrigerant flow rate will increase
Cooling effect is reduced a little due to the shape of the saturation dome (from 4-1 to 4a-1a),
However, the effect of the increased refrigerant flow rate is to increase the cooling capacity.
HCB 3- Chap 14A: Refrigeration Cycles and Systems

17. HCB 3- Chap 14A: Refrigeration Cycles and Systems
Effect of Change in Condenser Temperature
Fig
As the condensing temperature increases:
- Refrigerant flow decreases because the compressor has to pump the refrigerant through a higher pressure ratio.
Cooling effect also reduces (from 4-1 to 4a-1)
Cooling capacity decreases
Even with reduced refrigerant flow rate, the power input increases due to the higher pressure ratio.
COP decreases as the pressure ratio decreases.
HCB 3- Chap 14A: Refrigeration Cycles and Systems

18. HCB 3- Chap 14A: Refrigeration Cycles and Systems
Chiller Systems
Actual heat exchangers need a certain temperature difference in order for them to operate.
Refrigerant temperature in the condenser > Tsink (=Tl) while the boiling refrigerant temperature in the evaporator < Tsource (=Th).
The wider refrigerant temperature levels will reduce the cycle COP
Fig Schematic diagram showing essential components of basic liquid chiller and relation to vapor compression cycle.
HCB 3- Chap 14A: Refrigeration Cycles and Systems

19. HCB 3- Chap 14A: Refrigeration Cycles and Systems
Figure 14.9 The standard VC refrigeration cycle drawn on a p-h diagram along with the sink and source temperatures
HCB 3- Chap 14A: Refrigeration Cycles and Systems

20. HCB 3- Chap 14A: Refrigeration Cycles and Systems
Figure Vapor compression cycle equipment with typical R-22 operating temperatures and pressures.
A direct expansion (DX) evaporator is used.
The state points are based on a compressor efficiency of 85 percent and an evaporator outlet superheat of 9° F (5° C).
HCB 3- Chap 14A: Refrigeration Cycles and Systems

21. HCB 3- Chap 14A: Refrigeration Cycles and Systems
Larger chillers have orifice plates or guide vane refrigerant control and flooded type of condensers and evaporators and water loops
Figure Sketch of a water cooled refrigeration system with flooded evaporator and condenser showing the two coolant water loops
(14.24)
(14.25)
HCB 3- Chap 14A: Refrigeration Cycles and Systems

22. HCB 3- Chap 14A: Refrigeration Cycles and Systems
Example 14.4
HCB 3- Chap 14A: Refrigeration Cycles and Systems

23. HCB 3- Chap 14A: Refrigeration Cycles and Systems

24. HCB 3- Chap 14A: Refrigeration Cycles and Systems

25. HCB 3- Chap 14A: Refrigeration Cycles and Systems
Chiller Performance Maps
Figure Example of “performance map” for reciprocating chiller; based on R-22, 10 F subcooling, 20o F superheat, and 1725 r/min compressor speed.
For an evaporating temperature of 45 oF (7.2 oC) and a condensing temperature of 118 oF (47.8 oC):
Power= 13.5 kW and
Capacity = 160,000 Btu/hr (46.9 kW)
So COP = about 3.5 or 1.0 kW/ton (typical value for a small unit).
HCB 3- Chap 14A: Refrigeration Cycles and Systems

26. HCB 3- Chap 14A: Refrigeration Cycles and Systems

27. HCB 3- Chap 14A: Refrigeration Cycles and Systems
Caution: In many cases, the data is not experimental data but generated
by a regression model fit to experimental data
HCB 3- Chap 14A: Refrigeration Cycles and Systems

28. HCB 3- Chap 14A: Refrigeration Cycles and Systems
Example 14.11: Regression Model to Table 14.5 Data
HCB 3- Chap 14A: Refrigeration Cycles and Systems

29. HCB 3- Chap 14A: Refrigeration Cycles and Systems
Absorption Cooling
Invented by Ferdinand Carre who took out a U.S. patent in 1860
Used for refrigeration by the South during the Civil War
Aqua-Ammonia systems used extensively for early refrigeration systems
Lithium-Bromide/Water systems were predominantly used for large chillers during 40’s and 50’s in the U.S.
Absorption still accounts for 75% of tonnage in Japanese Industrial / Commercial Applications
Renewed interest as a result of increase in combined heat and power systems
HCB 3- Chap 14A: Refrigeration Cycles and Systems

30. Main Types of Absorption Systems
Aqua-ammonia ( ammonia is the refrigerant)
- caustic, toxic
- explosive with Cu and its alloys
- needs rectification
2) Lithium Bromide/Water (water is the refrigerant)
- water freezes at 32o F (limits cooling to 3-4o C)
- must operate below atmospheric atmosphere
- LiBr is a solid at room temperature and pressure
HCB 3- Chap 14A: Refrigeration Cycles and Systems

31. HCB 3- Chap 14A: Refrigeration Cycles and Systems
Components
HCB 3- Chap 14A: Refrigeration Cycles and Systems

32. HCB 3- Chap 14A: Refrigeration Cycles and Systems
Example 14.3 :Lithium bromide cooling cycle
Condensing temperature 104°F
Evaporator temperature 50°F.
Heat is added to the generator at 212°F
and removed from the absorber at 86°F .
Pump flow rate is 4800 lbm/h,
Determine: COP and
the heat rates at generator, absorber, condenser, and evaporator?
Assume: State points 4 & 5 are saturated
HCB 3- Chap 14A: Refrigeration Cycles and Systems

33. Specific volume, ft3/lbm Internal energy, Btu/lbm
Saturated Steam Tables
Pressure, psia
Saturation temp.,  °F
Specific volume, ft3/lbm
Internal energy,  Btu/lbm
Enthalpy,  Btu/lbm
Sat. liquid
Sat. vapor
Evap. p
Tsat vf vg uf
ufg ug hf
hfg hg
32.018
3302
0.00
1021.2
0.01
1075.4 35
2948
2.99
1019.2
1022.2
3.00
1073.7
1076.7 40
2445
8.02
1015.8
1023.9
1070.9
1078.9 45
2037
13.04
1012.5
1025.5
1068.1
1081.1 50
1704.2
18.06
1009.1
1027.2
1065.2
1083.3
0.2563 60
1206.9
28.08
1002.4
1030.4
1059.6
1087.7
0.3632 70
867.7
38.09
995.6
1033.7
1054.0
1092.0
0.5073 80
632.8
48.08
988.9
1037.0
48.09
1048.3
1096.4
0.6988 90
467.7
58.07
982.2
1040.2
1042.7
1100.7
0.9503
100
350.0
68.04
975.4
1043.5
68.05
1105.0 1
101.70
333.6
69.74
974.3
1044.0
1036.0
1105.8 2
126.04
173.75
94.02
957.8
1051.8
1022.1
1116.1
HCB 3- Chap 14A: Refrigeration Cycles and Systems

34. HCB 3- Chap 14A: Refrigeration Cycles and Systems
Superheated Steam
Pressure
Temperature v u h s
psia °F
ft3/lbm
Btu/lbm
Btu/lbm·°R
1.0
(101.70°F)
Sat.
333.6
1044.0
1105.8
1.9779
200
392.5
1077.5
1150.1
2.0508
240
416.4
1091.2
1168.3
2.0775
280
440.3
1105.0
1186.5
2.1028
320
464.2
1118.9
1204.8
2.1269
360
488.1
1132.9
1223.2
2.1500
400
511.9
1147.0
1241.8
2.1720
440
535.8
1161.2
1260.4
2.1932
500
571.5
1182.8
1288.5
2.2235
600
631.1
1219.3
1336.1
2.2706
700
690.7
1256.7
1384.5
2.3142
800
750.3
1294.9
1433.7
2.3550
1000
869.5
1373.9
1534.8
2.4294
1200
988.6
1456.7
1639.6
2.4967
1400
1107.7
1543.1
1748.1
2.5584
HCB 3- Chap 14A: Refrigeration Cycles and Systems

35. HCB 3- Chap 14A: Refrigeration Cycles and Systems

36. HCB 3- Chap 14A: Refrigeration Cycles and Systems
Outcomes
Understanding the practical limitations of the Carnot Refrigeration Cycle
Knowledge of the Carnot cycle modifications considered in standard VC refrigeration cycles
Knowledge of the important quantities of interest while analyzing standard VC cycles and be able to solve problems
Knowledge of how actual VC cycles differ from standard VC cycles
Be able to analyze practical VC cycles
Knowledge of how changes in evaporator and condenser refrigerant temperatures affect practical VC cycle performance
Understanding actual chiller systems and how they differ from practical VC cycles
Knowledge of chiller performance maps and performance tables
Understanding of the operation and various components of an absorption chiller
Be able to solve simple problems involving absorption cycles
HCB 3- Chap 14A: Refrigeration Cycles and Systems