Modern Engineering Thermodynamics

542 CHAPTER 14: Vapor and Gas Refrigeration Cycles
EXAMPLE 14.2
The earth’s polar ice caps contain about 2.50 × 10 16 m 3 of ice. Determine the tons of refrigeration produced if all this ice
were to melt at 0.00°C in a 24.0 h period. The density of ice at 0.00°C is 917 kg/m 3 .
Solution
The mass of ice present in the polar ice caps is (2.50 × 10 16 m 3 )(917 kg/m 3 )(2.2046 lbm/kg) = 5.05 × 10 19 lbm = 2.53 ×
10 16 tons. Since a ton of refrigeration is equal to the amount of heat absorbed by melting 1 ton of ice at 0.00°C atatmospheric
pressure in one 24.0 h day, melting the earth’s polaricecapsat0.00°C in a 24.0 h period would produce 2.53 ×
10 16 tons of refrigeration.
Exercises
4. Suppose the polar ice caps in Example 14.2 melt over a period of 14.0 years. Then, how many tons of refrigeration
would be produced? Answer: 6.85 × 10 12 tons.
5. How many Btu per hour would be produced by the melting of the polar ice caps in Example 14.2?
Answer: 3.04 × 10 20 Btu/h.
6. How long would it take to melt the polar ice caps in Example 14.2 if the Earth receives an extra 10 15 kJ per year from
the sun? Answer: 7.56 million years.
14.5 VAPOR-COMPRESSION REFRIGERATION CYCLE
Like the steam engine, refrigeration technology had a significant impact on society and the way we live. First
of all, it changed the way we process food; it created large new agricultural markets and provided a healthier
diet for many people. Later, it was applied to making our living environment more comfortable and productive.
Initially, it was a spinoff technology from steam engine and gas power cycle prime movers that were
simply made to operate thermodynamically backward. Then, it became a powerful force in shaping our
culture.
The first vapor-compression refrigeration system using a closed cycle process was patented in 1834 by the
American Jacob Perkins (1766–1849). He chose ethyl ether (or, more accurately, diethyl ether, C 2 H 5 OC 2 H 5 )
as the refrigerant, because at low pressures, its temperature was low enough to freeze water on the outside of
the evaporator. The ether vapor was compressed in a piston-cylinder apparatus and condensed into a liquid at
a higher saturation pressure and temperature. Finally, the liquid ether was throttled through a valve back into
the low-pressure evaporator. This system is illustrated in Figure 14.8. Since this process occurs beneath the
vapor dome of the working fluid (ether), it is clearly a reversed Rankine cycle device.
All vapor-compression cycle refrigeration systems operate essentially on a reversed Rankine cycle, as shown in
Figure 14.8b. In these systems, the boiler is normally called the evaporator and the prime mover is replaced by a
compressor. Also, it would seem reasonable to replace the boiler feed pump of the forward-running Rankine cycle
with some form of prime mover in the reversed Rankine or vapor-compression cycle, whose work output could
be used to offset the work input to the compressor. Unfortunately, this is not economically feasible in most
small- to medium-scale refrigeration systems, as the following example illustrates.
Q H
Q L
W C T
s
Condenser
3
Throttling
2
(expansion)
4
valve
Compressor
Boiler 1
(evaporator)
3 2
4h 1 1s
(a) Equipment schematic
(b) The thermodynamic cycle
FIGURE 14.8
Jacob Perkins’s closed-loop vapor-compression refrigeration cycle.