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Modern Engineering Thermodynamics

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560 CHAPTER 14: Vapor and Gas Refrigeration Cycles<br />

EXAMPLE 14.7 (Continued )<br />

c. The total compressor power is obtained from Eq. (14.15) as<br />

∑ _W compressors = _m A ½ðh 2sA − h 1A Þ/ðη s Þ c−A + ð1 − x flash Þðh 2B − h 1B Þ/ðη s Þ c−B Š<br />

= ð0:330 kg/sÞ½ð292:33 − 256:60 kJ/kgÞ/0:800 + ð1 − 0:339Þð264:25 − 231:35 kJ/kgÞ/0:800Š<br />

= 23:7 kJ/s = 23:7kW<br />

Exercises<br />

19. If the refrigerating capacity of the two-stage system described in Example 14.7 is tripled, determine the required<br />

refrigerant mass flow rate. Assume all the other variables remain unchanged. Answer: _m ref = 0:654 kg=s.<br />

20. We just found another manufacturer that can provide the compressors for the unit described in Example 14.7 with an<br />

isentropic efficiency of 90.0% instead of 80.0%. Determine the new system coefficient of performance, assuming all the<br />

other variables remain unchanged. Answer: COP = 1.88.<br />

21. Using a spreadsheet or equation solver (like EES), develop a plot of system COP vs. flash chamber pressure for the unit<br />

discussed in Example 14.7. Note that the maximum COP occurs at a flash chamber pressure of about 500 kPa.<br />

14.10 ABSORPTION REFRIGERATION<br />

Ammonia was discovered in 1774 by the British chemist Joseph Priestley (1733–1804), who noted that his new<br />

gas dissolved easily in water (one volume of water dissolves over 1000 volumes of ammonia at STP). The<br />

French engineer Ferdinand Carré utilized this property of ammonia’s affinity for water to create the first absorption<br />

refrigeration system in 1859.<br />

Carré’s absorption refrigeration technique is an important vapor refrigeration technology, because it does not<br />

require a vapor compressor. It is based on dissolving the refrigerant vapor (ammonia) in a carrier liquid<br />

(water) and pumping this liquid to a high pressure. A liquid can be pumped more efficiently than a vapor<br />

can be compressed, so this technique has a decided advantage over vapor-compression technology. The pressurized<br />

liquid is then fed into a generator, where the refrigerant vapor is boiled off, now at a much higher<br />

pressure, and the carrier liquid returned to the absorber to continue the process. The high-pressure refrigerant<br />

vapor then continues through the refrigeration cycle in the normal reversed Rankine manner. Standard and<br />

absorption refrigeration systems are shown schematically in Figure 14.19. These are both vapor-compression<br />

Q H<br />

Condenser<br />

Expansion<br />

valve<br />

W compressor<br />

Evaporator<br />

Q L<br />

Compressor<br />

(a) Standard vapor-compression refrigeration.<br />

Q H<br />

Q G<br />

Condenser<br />

Generator<br />

Expansion<br />

valve<br />

W pump

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