Thermodynamics II course taught at air university Islamabad by Sijal Ahmed Vapor compression cycle : Heat pump and refrigeration.

1. THERMODYNAMICS II
LECTURE 10-11
VAPOR COMPRESSION CYCLE (VCR)
1

2. Contents
• Introduction
• Heat pump and refrigeration cycle
• Vapor compression cycle
• Solved example and home work
• Refrigerants
2

3. Introduction
• Refrigeration : Transfer of heat from lower temperature to higher
temperature
• Heat pump : Transfer of heat from lower temperature to higher
temperature!
• Well known cycle is vapour compression cycle, in which the refrigerant
(working fluid) is vaporized and condensed alternatively, and is
compressed in vapor phase
• Performance is measured using coefficient of performance (COP)
3

4. Introduction
• Because, we are just transferring the energy from one place to
another, so we need less energy to accomplish the task of refrigeration
or heat pump.
• Ideal vapor compression cycle dates back to 1834 by Jacob Perkins
4

5. Refrigerators and Heat Pumps
• Heat flows from high temperature to lower
temperature, reverse is possible with the help
of external device
• Working fluid is refrigerant
• Heat pump and refrigerators are same devices,
only difference is objective
• at worst heat pumps = resistance heaters
5

6. Refrigerators and Heat Pumps
6
R = Refrigeration
HP = Heat Pump

7. Refrigerators and Heat Pumps
• Cooling capacity is measured in tons of refrigeration
• 1 ton = can freeze 1 ton of liquid water at 0 oC into ice at 0 oC in 24 h
• 1 ton = 211 KJ/min (3.5 kW) or 200 Btu/min (12000 Btu/h)
• Cooling load of typical 200-m2 is 3 ton (~ 10 KW)
7

8. Reversed Carnot Cycle
8

9. Ideal Vapor Compression Cycle
9

10. Ideal Vapor Compression Cycle
10
Non Ideal process,
enthalpy is constant

11. Ideal Vapor Compression Cycle
• Steady flow energy equation (SFEE)
11

12. Solved Example
12

13. Solved Example
13

14. Solved Example
14

15. Solved Example (Heat Pump)
15
Example: A vapor-compression heat pump cycle with R-134a as
the working fluid maintains a building at 20oC when the outside
temperature is 5oC. The refrigerant mass flow rate is 0.086 kg/s.
Additional steady state operating data are provided in the table.
Determine the
(a) compressor power, in kW,
(b) heat transfer rate provided to the
building, in kW,
(c) coefficient of performance.
TC = 278 K (5oC)TH = 293 K (20oC) TC = 278 K (5oC)TH = 293 K (20oC)
State
h (kJ/kg)
1
244.1
2
272.0
3
93.4

16. Solved Example (Heat Pump)
16
(a) The compressor power is
)( 12c hhmW  







kJ/s1
kW1
kg
kJ
)1.2440.272(
s
kg
086.0cW 2.4 kW
(b) The heat transfer rate provided to the building is
)( 32out hhmQ  







kJ/s1
kW1
kg
kJ
)4.930.272(
s
kg
086.0outQ 15.4 kW
State
h (kJ/kg)
1
244.1
2
272.0
3
93.4 TC = 278 K (5oC)TH = 293 K (20oC) TC = 278 K (5oC)TH = 293 K (20oC)

17. Solved Example (Heat Pump)
17
State
h (kJ/kg)
1
244.1
2
272.0
3
93.4 TC = 278 K (5oC)TH = 293 K (20oC) TC = 278 K (5oC)TH = 293 K (20oC)
(c) The coefficient of
performance is
c
out
W
Q
COP


 
kW.42
kW5.41
COP 6.4
Comment: Applying Eq. 10.9, the maximum theoretical
coefficient of performance of any heat pump cycle
operating between cold and hot regions at TC and TH,
respectively is



K872K932
K932
maxCOP 19.5

18. Cascade Refrigeration System
18

19. Home Work
19

20. Home Work
20

21. Refrigerants
21
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

22. Refrigerants
• Refrigerant selection is based on several factors:
►Performance: provides adequate cooling capacity cost-effectively.
►Safety: avoids hazards (i.e., toxicity).
►Environmental impact: minimizes harm to stratospheric ozone layer and
reduces negative impact to global climate change.
• Global warning potential is based on Co2 (GWP = 1)
22

23. Refrigerant Types and Characteristics
23
Global Warming Potential (GWP) is a simplified index that estimates the potential
future influence on global warming associated with different gases when released
to the atmosphere.