Modern Engineering Thermodynamics

14.5 Vapor-Compression Refrigeration Cycle 543
WHERE DID “MECHANICAL REFRIGERATION” COME FROM?
The first vapor-compression refrigeration system was patented by Jacob Perkins (1766–1849) in 1834 (Figure 14.9). Though
Perkins was an American, his refrigerator was made in England and was not an economic success. A similar machine was made
in the United States in 1856 by Alexander Catlin Twinning (1801–1884), again with little financial success. In each case, the evaporator
was immersed in a salt brine solution and the cold brine was used to make ice, but it attracted little attention for more
than 20 years, after which natural refrigeration had begun to cause changes in people’s dietary habits. In 1855, James Harrison
(1816–1893), a Scotsman who emigrated to Australia, produced a commercially successful refrigerator similar to Perkins’s for
the manufacture of ice. Since natural ice is difficult to find in Australia, Harrison’s artificial ice machine was an instant success.
Outdoor
heat exchanger
Supply duct
Heated air
Heat
Indoor
heat exchanger
Return duct
FIGURE 14.9
Jacob Perkins’s 1834 refrigeration apparatus.
Compressor
EXAMPLE 14.3
A refrigeration system for a supermarket is to be designed using R-22 to maintain frozen food at −15.0°C while operating in
an environment at 20.0°C. The refrigerant enters the condenser as a saturated vapor and exits as a saturated liquid. Determine
the COP for this refrigerator, using
a. A reversed Carnot cycle operating between these temperature limits.
b. An isentropic vapor-compression cycle with an isentropic expansion turbine installed between the high-pressure
condenser and the low-pressure evaporator.
c. An isentropic vapor-compression cycle with an aergonic, adiabatic, throttling expansion valve installed between the highpressure
condenser and the low-pressure evaporator.
Solution
First, draw a sketch of the system (Figure 14.10).
Q H
Condenser
3 2
4
W T
Evaporator
Q L
1
W C
T
3 2s
4s 1
s
FIGURE 14.10
Example 14.3, system sketch.
a. Here, T H = 20.0 + 273.15 = 293.15 K, and T L = −15.0 + 273.15 = 258.15 K. Then, Eq. (14.5) gives
T
COP Carnot
= L
=
T H − T L
refrigerator
258:15
293:15 − 258:15 = 7:38 (Continued )