Module 4. Refrigeration and Heat Pump Systems The Vapor ...

Assumptions: SSSF, Negligible ∆KE and ∆PE, Single-Inlet, Single-Outlet conditions
K. Nasr,c:\thermo \module4.doc
Process Device 1st Law 2nd Law
.
.
1→2: Isentropic Compression Compressor
WComp . = m(h1 − h
2)
s 2 = s 1
.
.
2 →3: p = C. Heat Rejection Condenser
QCond.
= m(h3 − h
2)
s 2 > s 3
3 →4: Adiabatic Throttling Throttling Device h 3 = h 4 s 4 > s 3
(O-Tube, TXV)
.
.
4 →1: p = C. Heat Absorption Evaporator
Q = m(h − h )
s 1 > s 4
COP = Q Evap . / W Comp .
Evap.
1 4
¯
Actual Cycle Operation: Deviates from ideal cycle operation by having:
1. Pressure drops due to friction in connecting pipes
2. Heat transfer exists across connecting pipes
3. Pressure drops occur through the condenser and evaporator tubes
4. Heat transfer occurs from the compressor
5. Frictional effects and flow separation occur on the compressor blades
6. Refrigerant vapor entering the compressor may be slightly superheated
7. Refrigerant temperature exiting the condenser may be slightly below saturation.
A desirable effect out of the above list is item 7. Having a subcooled liquid exiting the
condenser results in having a larger refrigeration effect.
Problem:
A vapor-compression refrigeration system for a household refrigerator has a refrigerating capacity of
1000 Btu/h. The refrigerant, R134a, enters the evaporator at –10 °F and exits at 0 °F. The isentropic
compressor efficiency is 80%. The refrigerant condenses at 95 °F and exits the condenser subcooled
at 90 °F. There are no significant pressure drops in the flows through the evaporator and condenser.
We want to determine the evaporator and condenser pressures, the mass flow rate of the refrigerant,
the compressor power input, and the coefficient of performance.
Solution:
• First, the refrigerating capacity is the rate of heat transfer into the refrigerant flowing in the
evaporator. It is the cooling effect, that is the rate of heat transfer extracted from air on the outside
of the evaporator. The refrigerating capacity is normally given in tons of refrigeration for a system
where one ton of refrigeration is equivalent to 12000 Btu/hr.
• Second, since the inlet to the evaporator is always a mixture of liquid and vapor, the pressure
therefore is the saturation pressure at the given temperature. It is also assumed that frictional effects,
between the refrigerant and the tubes of the evaporator, are neglected so that the pressure at the
inlet equals the pressure at the exit of the evaporator.
Copyright © 2004, K. Nasr. All Rights Reserved Page 3