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

11–9 ■ ABSORPTION REFRIGERATION SYSTEMS

Another form of refrigeration that becomes economically attractive when
there is a source of inexpensive thermal energy at a temperature of 100 to
200°C is absorption refrigeration.Some examples of inexpensive thermal
energy sources include geothermal energy, solar energy, and waste heat
from cogeneration or process steam plants, and even natural gas when it is
available at a relatively low price.
As the name implies, absorption refrigeration systems involve the absorp-
tion of a refrigerantby a transport medium. The most widely used absorp-
tion refrigeration system is the ammonia–water system, where ammonia
(NH 3 ) serves as the refrigerant and water (H 2 O) as the transport medium.
Other absorption refrigeration systems include water–lithium bromide
and water–lithium chloride systems, where water serves as the refrigerant.
The latter two systems are limited to applications such as air-conditioning
where the minimum temperature is above the freezing point of water.

Chapter 11 | 631

(a) The maximum and minimum temperatures in the cycle are determined

from the isentropic relations of ideal gases for the compression and expansion

processes. From Table A–17E,

T 1 
460 R ⎯→ h 1 
109.90 Btu/lbm and Pr 1 
0.7913

Pr 2 
 Pr 1 
(4)(0.7913) 
3.165 ⎯→

T 3 
540 R ⎯→ h 3 
129.06 Btu/lbm and Pr 3 
1.3860

Pr 4 
 Pr 3 
(0.25)(1.386) 
0.3465 ⎯→

Therefore, the highest and the lowest temperatures in the cycle are 223 and

97°F, respectively.

(b) The COP of this ideal gas refrigeration cycle is

where

Thus,

(c) The rate of refrigeration is

Discussion It is worth noting that an ideal vapor-compression cycle working

under similar conditions would have a COP greater than 3.

Q

#

refrig
m

# 1 q

L^2 
^1 0.1 lbm>s^21 23.2 Btu>lbm^2 
2.32 Btu/s

COPR

23.2

53.642.36

2.06

Wcomp,in
h 2 h 1 
163.5109.9
53.6 Btu>lbm

Wturb,out
h 3 h 4 
129.0686.7
42.36 Btu>lbm

qL
h 1 h 4 
109.986.7
23.2 Btu>lbm

COPR

qL

wnet,in

qL

wcomp,inWturb,out

e

h 4 


T 4 

86.7 Btu/lbm

363 R (or 
 97 °F)

P 4

P 3

ehT^2 


2 

163.5 Btu/lbm

683 R (or 223°F)

P 2

P 1