This document summarizes key concepts about vapor compression refrigeration cycles. It discusses that a vapor compression refrigeration cycle used for cooling is called a refrigerator, while one used for heating is called a heat pump. The coefficient of performance is used to measure efficiency for both. Ideal refrigeration cycles are shown on pressure-enthalpy diagrams, including the four processes of compression, heat rejection, expansion, and heat absorption. Factors that reduce efficiency in practical cycles like pressure drops and subcooling/superheating are also covered.

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

2. VCR used for Cooling
C
COP
in
E
cycle
Q
W
   
energy sought
energy that costs
The concept of an efficiency being greater than 100%
makes people uneasy. Therefore, the conversion
efficiency for a refrigerator is called the Cooling
Coefficient of Performance (COPC). A refrigeration
system that is used for cooling is called a refrigerator.
Observation:  
may be >1 100%
E E
  
H
T
L
T
2

3. VCR used for Heating
H
COP
out
E
cycle
Q
W
   
energy sought
energy that costs
The concept of an efficiency being greater than 100%
makes people uneasy. Therefore, the conversion
efficiency for a refrigerator is called the Heating
Coefficient of Performance (COPH). A refrigeration
system that is used for cooling is called a heat pump.
Observation:  
may be >1 100%
E E
  
H
T
L
T
3

4. Theoretical Carnot Refrigerator
R
1
COP
/ 1
in in
th
cycle out in out in
Q Q
W Q Q Q Q
    
 
For the Refrigeration cycle …
 
H
H, H,
1
COP
1 /
1 1
COP COP
1 / 1 /
out out
th
cycle out in in out
H
Carnot Carnot
in out L H H L
rev
Q Q
W Q Q Q Q
T
Q Q T T T T
    
 
  
  
For the Heat Pump cycle …
H
T
L
T
 
R,
1 1
COP
/ 1 / 1
Carnot
out in H L
rev
Q Q T T
 
 
R,
COP L
Carnot
H L
T
T T


4

5. Terminology
• Refrigeration cycle
– The cycle is operating in a refrigeration cycle
• The goal is to keep the cold space cold
– Transfer heat from a low-temperature source
• Heat pump cycle
– The cycle is operating in a refrigeration cycle
• The goal is to keep the hot space hot
– Transfer heat to a high-temperature sink
The words refrigeration or heat pump define the goal of the cycle.
5

6. What’s Inside?
6
H
T
L
T

7. Refrigerant Properties
• Two phase changes
– Boiling (evaporator)
– Condensing (condenser)
• Low temperature boiling fluids
Working fluid = Refrigerant
7

8. Refrigerant Naming Convention
Most refrigerants are halogenated hydrocarbons. The naming
convention adopted by ASHRAE is,
R(a-1)(b+1)d = CaHbClcFd c = 2(a – 1) – b – d
 
 
1 0 1
1 2 1
2
2 1
2 1 1 1 2 1
a a
b b
d
c a b d
c
   
   

   
    
Example: R22 (R022)
C
H
F
Cl
F chlorodifluoromethane
8

9. The Pressure-Enthalpy P-h Diagram
9

10. Refrigeration Cycle Analysis
 
2 1
m h h
 
 
2 3
m h h
 
 
1 4
m h h
 
3 4
h h

Performance
C
COP in
c
Q
W

10

11. P
h
The Ideal VCR Cycle on the P-h Diagram
 
2 1
m h h
 
 
2 3
m h h
 
 
1 4
m h h
 
3 4
h h

1
2
3
4
1-2: Isentropic compression
2-3: Isobaric heat rejection
3-4: Isenthalpic expansion
4-1: Isobaric heat addition
/
in in
q Q m
 /
c c
w W m

/
out out
q Q m

11

12. Refrigeration Effect and Capacity
1 4
in
q h h
 
Refrigeration Effect:
Refrigeration Capacity:  
1 4
in
Q m h h
 
Refrigeration capacity is often expressed in tons of
refrigeration. Definition …
1 ton of refrigeration is the steady state heat transfer rate
required to melt 1 ton of ice at 32°F in 24 hours.
1 ton = 12,000 Btu/hr = 3.516 kW
12

13. VCR Cycle Irreversibilities
P
h
1
2
2s
3
4
Isentropic efficiency
of the compressor
Pressure drop through
the condenser
Pressure drop through
the evaporator
13

14. Practical VCR Cycle
P
h
1
2
2s
3
4
3
T SCT
1
T
SET
SCT = Saturated Condensing Temperature
DSC = Degrees of Subcooling = SCT – T3
SET = Saturated Evaporating Temperature
DSH = Degrees of Superheat = T1 – SET
Subcooling increases the
refrigeration capacity
Superheating provides a dry
vapor at the compressor inlet
14