Modern Engineering Thermodynamics

14.11 Commercial and Household Refrigerators 567
c. Now we can find the enthalpy at the exit of the refrigeration evaporator from the energy balance on the refrigeration
compartment as
h 5 = h 4h +
_ Q R
_m ref
= 91:49 kJ/kg +
ð422 kJ/hÞð1 h/60minÞ
0:0970 kg/min
= 164:0 kJ/kg
Then, we can find the quality at the exit of the refrigeration evaporator as
x 5 = h 5 − h f ð4:00°CÞ
h fg ð4:00°CÞ
=
164:0 − 55:35
194:19
= 0:559 = 55:9%
Exercises
25. A high-efficiency compressor has just been developed. It has an isentropic efficiency of 95%. Determine the new COP
for the refrigerator design presented in Example 14.9. Answer: COP = 4.00.
26. Explain why the COP of the dual-evaporator refrigerator in Example 14.9 does not depend on the quality of the
refrigerator evaporator’s exit. What characteristic of the system depends on this variable? Answer: The intermediate state
h 5 to h 6h cancels out in the COP evaluation. The amounts of cooling in the refrigeration and freezing sections depend on
this variable.
27. If the refrigerator in Example 14.9 runs only 50% of the time, determine its annual operating cost if the price of
electricity is $0.15 per kWh. Answer: Annual operating cost = $45.05.
HOW LONG HAVE WE HAD AIR CONDITIONING?
Before 1930, air conditioning for human comfort was available only for large installations, such as movie theaters, department
stores, and offices. In 1919, the first air-conditioned movie theater opened in Chicago, Illinois, and the first air-conditioned
department store (Abraham and Straus) opened in Brooklyn, New York. The first completely air-conditioned office building
was built in San Antonio, Texas, in 1927. By the late 1930s, smaller units became available for restaurants, shops, and hotels.
Air conditioning using cold or chilled water circulating throughout the air handling system of a building was introduced in
1929. The water was chilled by the evaporator in a large central refrigeration unit (the evaporator was usually called the chiller).
Small home window air conditioners were not introduced until after 1946, and mass-produced automobiles were not air
conditioned until after 1948.
WHERE DID HEAT PUMPS COME FROM?
Lord Kelvin seemed cognizant of the possibility that a “reversed heat engine” could be used for heating and cooling. The
first use of the term heat pump appears in the British literature in about 1895. The first application of heat pump technology
was made by T.G.N. Haldane to heat his London office in 1927. Modern heat pumps (Figure 14.25) use refrigerants
R-113 or R-114 to reach hot-side temperatures as high as 150°C.
Cool water
Warm water
Refrigerant
FIGURE 14.25
A heat pump.