Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

11–5 ■ SELECTING THE RIGHT REFRIGERANT

When designing a refrigeration system, there are several refrigerants from

which to choose, such as chlorofluorocarbons (CFCs), ammonia, hydrocarbons

(propane, ethane, ethylene, etc.), carbon dioxide, air (in the air-conditioning of

aircraft), and even water (in applications above the freezing point). The right

616 | Thermodynamics

Analysis The T-sdiagram of the refrigeration cycle is shown in Fig. 11–8.

We note that the refrigerant leaves the condenser as a compressed liquid

and enters the compressor as superheated vapor. The enthalpies of the

refrigerant at various states are determined from the refrigerant tables to be

h 1 246.36 kJ/kg

h 2 286.69 kJ/kg

h 3 hf @ 26°C87.83 kJ/kg

h 4 h 3 (throttling) ⎯→ h 4 87.83 kJ/kg

(a) The rate of heat removal from the refrigerated space and the power input

to the compressor are determined from their definitions:

and

(b) The isentropic efficiency of the compressor is determined from

where the enthalpy at state 2s(P 2 s0.8 MPa and s 2 ss 1 0.9724

kJ/kg · K) is 284.21 kJ/kg. Thus,

(c) The coefficient of performance of the refrigerator is

Discussion This problem is identical to the one worked out in Example

11–1, except that the refrigerant is slightly superheated at the compressor

inlet and subcooled at the condenser exit. Also, the compressor is not isen-

tropic. As a result, the heat removal rate from the refrigerated space

increases (by 10.4 percent), but the power input to the compressor increases

even more (by 11.6 percent). Consequently, the COP of the refrigerator

decreases from 3.97 to 3.93.

COPR

Q

#

L

W

#

in



7.93 kW

2.02 kW

3.93

hC

284.21246.36

286.69246.36

0.939 or 93.9%

hC

h 2 sh 1

h 2 h 1

W

#

inm

# 1 h

2 h 12 ^1 0.05 kg>s^231 286.69246.36^2 kJ>kg^4 2.02 kW

Q

#

Lm

# 1 h

1 h 42 ^1 0.05 kg>s^231 246.3687.83^2 kJ>kg^4 7.93 kW

P 3 

T 3 

0.72 MPa

26°C f

P 2 

T 2 

0.8 MPa

50°C f

P 1 

T 1 

0.14 MPa

10°C f

T

s

3

0.72 MPa

26 °C

4

QH

Win

QL

0.15 MPa

2

0.8 MPa

2 s 50 °C

1

0.14 MPa

–10°C

FIGURE 11–8

T-sdiagram for Example 11–2.