Modern Engineering Thermodynamics

14.6 Refrigerants 547
T
3
2s
2
p
3
2s
2
4h
1
4h
1
s
FIGURE 14.13
Example 14.4.
Exercises
10. After several years of use, the isentropic efficiency of the compressor in Example 14.4 decreases from 75.0% to 55.0%
due to wear and a lack of maintenance. Determine the new COP for this system. Answer: COP = 3.47.
11. Determine the power required to drive the compressor _W c in Example 14.4 if the refrigeration system is to produce
_Q L = 20 tons of cooling. Recall that 1 ton of refrigeration is equal to 214. kJ/min. Answer: _W c = 19:8 hp.
12. Determine the mass flow rate of refrigerant required in Example 14.4 if this system produces _Q L = 20 tons of cooling.
Recall that 1 ton of refrigeration is equal to 214. kJ/min. Answer: _m = 23:9kg=min.
h
Even if the compressor had an isentropic efficiency of 100%, the COP in this example would be only 4.74/0.750 =
6.32, which is still slightly less than the 6.37 of part c in Example 14.3. Thus, adding superheat to the cycle
usually does not increase the COP because both j _W c j/ _m and _Q L / _m are increased. However, the required mass flow
rate _m is significantly reduced by the addition of superheat. Also, because condensers and evaporators are not 100%
effective as heat exchangers, the temperature difference between the working fluid in these devices and their local
environment is typically about 15.0°F.
14.6 REFRIGERANTS
Whereas the working fluid of the steam engine (water) was nearly ideal for vapor power cycles, it was totally
unsuitable for the refrigeration cycles of commercial interest. The major problem faced by the early developers
of refrigeration technology was not the design of the machinery per se but the search for a suitable nontoxic,
safe, inexpensive working fluid with satisfactory low-temperature thermodynamic characteristics.
Though water is the cheapest and safest refrigerant available, it is limited to high-temperature applications such
as steam-jet refrigeration. Since most refrigeration needs are at temperatures near the freezing point of water,
other refrigerants had to be found that boiled at lower temperatures.
Perkins used ethyl ether as his refrigerant. It was a good refrigerant, but it was also toxic and flammable. Also,
the entire ethyl ether refrigeration system operated below atmospheric pressure, making it difficult to prevent air
from leaking into the system. The danger and complexity of ethyl ether refrigerators caused other inventors to
search for alternative refrigeration technologies, which ultimately lead to the rapid development of gas expansion
refrigeration cycles between 1860 and 1890.
The French inventor Charles Tellier (1828–1913) introduced methyl ether (CH 3 Cl) as a replacement for ethyl
ether in 1863. Though methyl ether was also toxic and flammable, it had a higher vapor pressure, and that
allowed the entire refrigeration system to operate above atmospheric pressure, thus eliminating the problems
caused by air leaking into the system.
IS ETHER A REFRIGERANT OR AN ANESTHETIC?
The di in diethyl ether is often dropped, and it is called either ethyl ether or simply ether. This is the same ether that was first
successfully used as an anesthetic in 1846 by the Massachusetts dentist William T. G. Morton. Since the boiling point of
ether at atmospheric pressure is 35°C (95°F), slightly below the temperature of the human body, it was common practice
in the late 19th and early 20th centuries for physicians to use liquid ether as a local anesthetic by spraying it onto parts of
the body where it would then freeze the tissue as it boiled away and consequently numb the local sensations. This is the
source of the term freezing as a synonym for a local anesthetic (especially in dentistry) today.