Modern Engineering Thermodynamics

580 CHAPTER 14: Vapor and Gas Refrigeration Cycles
Warm environment
3
Expansion
valve
Q H
Q L
W in
Condenser
Compressor
2
T
3
2
W in
Q H
Q L
4
Evaporator
1
4h 1
Cold environment
s
FIGURE 14.36
A simple vapor-compression refrigeration cycle.
Because of the simple flow structure of most refrigeration systems, it is easy to use the indirect method to find
the irreversibility rate of various components. For a steady state (SS), steady flow (SF) system component with a
single inlet and a single outlet (SI, SO) having isothermal boundaries (IB), the irreversibility rate is
_I SS, SF
= T 0 ð _S P Þ SS, SF
SI, SO IB
SI, SO IB
Q
= T 0 _m ðs out − s in Þ − _
T b
For example, for the simple vapor-compression refrigeration system shown in Figure 14.36, the irreversibility
rate produced by the adiabatic compressor is
and, in the condenser, we have
_I adiabatic
= T 0 ð _S P Þ adiabatic
compressor
compressor
_I condenser = T 0 ð _S P Þ condenser
= T 0
= _m ref T 0 ðs 2 − s 1 Þ (14.30)
_m ref ðs 3 − s 2 Þ − _ Q condenser
T condenser
(14.31)
where _Q condenser = _Q H , and across the adiabatic expansion valve, we have
_I adiabatic
expansion valve
= T 0 ð _S P Þ adiabatic
expansion valve
= _m ref T 0 ðs 4h − s 3 Þ (14.32)
Finally, the irreversibility rate produced inside the evaporator is
" #
_Q evaporator
_I evaporator = T 0 ð _S P Þ evaporator = T 0 _m ref ðs 1 − s 4h Þ −
T evaporator
(14.33)
where _Q evaporator = _Q L is the refrigeration or cooling rate of the system. Note that, since a phase change is
usually a reversible process (see Chapter 7), the irreversibilities that occur in the condenser and evaporator come
from viscous pressure losses between the inlet and outlet and, if the refrigerant exits the evaporator or enters the
condenser in a superheated state, then irreversibilities exist for all heat transfer processes that occur outside the
vapor dome.
The use of this technique is illustrated in the following example.
EXAMPLE 14.14
The following preliminary design information is available for the vapor-compression refrigeration cycle shown in Figure 14.36
using R-134a:
Station 1 Station 2s Station 3 Station 4h
Compressor Inlet Compressor Outlet Condenser Outlet Expansion Valve Outlet
x 1 = 1:00 p 2s = 800: kPa x 3 = 0:00 h 4h = h 3
T 1 = −20:0°C s 2s = s 1 p 3 = 725: kPa p 4h = 160: kPa