FTFS Chap08 P122 - Chapter 8 Power and Refrigeration Cycles...

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Unformatted text preview: Chapter 8 Power and Refrigeration Cycles The Reversed Carnot Cycle 8-122C Because the compression process involves the compression of a liquid-vapor mixture which requires a compressor that will handle two phases, and the expansion process involves the expansion of high-moisture content refrigerant. 8-123 A steady-flow Carnot refrigeration cycle with refrigerant-134a as the working fluid is considered. The coefficient of performance, the amount of heat absorbed from the refrigerated space, and the net work input are to be determined. Assumptions 1 Steady operating conditions exist. 2 Kinetic and potential energy changes are negligible. Analysis ( a ) Noting that T H = 30 C = 303 K and T L = T sat @ 120 kPa = -22.36 C = 250.6 K, the COP of this Carnot refrigerator is determined from ( 29 ( 29 4.78 =- =- = 1 K 6 . 250 / K 303 1 1 / 1 COP C R, L H T T ( b ) From the refrigerant tables (Table A-11), h h h h g f 3 30 4 30 263 50 91 49 = = = = @ @ . . C C kJ / kg kJ / kg Thus, and ( 29 kJ/kg 142.3 = = = = =- =- = kJ/kg 172.01 K 303 K 250.6 kJ/kg 01 . 172 49 . 91 50 . 263 4 3 H H L L L H L H H q T T q T T q q h h q ( c ) The net work input is determined from /kg kJ 71 . 29 =- =- = 3 . 142 01 . 172 net L H q q w 8-96 s T Q H Q L 120 kPa 1 2 3 4 30 C Chapter 8 Power and Refrigeration Cycles 8-124E A steady-flow Carnot refrigeration cycle with refrigerant-134a as the working fluid is considered. The coefficient of performance, the quality at the beginning of the heat-absorption process, and the net work input are to be determined. Assumptions 1 Steady operating conditions exist. 2 Kinetic and potential energy changes are negligible. Analysis ( a ) Noting that T H = T sat @ 90 psia = 72.83 F = 532.8 R and T L = T sat @ 30 psia = 15.38 F = 475.4 R. ( 29 ( 29 8.28 =- =- = 1 R 475.4 / R 532.8 1 1 1 COP C R, L H T / T ( b ) Process 4-1 is isentropic, and thus ( 29 ( 29 ( 29 0.237 =-- = - = =- + = + = = 0364 . 2209 . 0364 . 0801 . R Btu/lbm 0.0801 0729 . 2172 . 05 . 0729 . psia 30 @ 1 1 psia 90 @ 4 4 1 fg f fg f s s s x s x s s s ( c ) Remembering that on a T-s diagram the area enclosed represents the net work, and s 3 = s g @ 90 psia = 0.2172 Btu/lbmR, ( 29 ( 29 ( 29 [ ]( 29 Btu/lbm 7.88 R Btu/lbm 0801 . 2172 . R 15.38 72.83 4 3 in net, = --- =-- = s s T T w L H 8-97 s T Q H Q L 1 2 3 4 Chapter 8 Power and Refrigeration Cycles Ideal and Actual Vapor-Compression Cycles 8-125C Yes; the throttling process is an internally irreversible process. 8-126C To make the ideal vapor-compression refrigeration cycle more closely approximate the actual cycle. 8-127C No. Assuming the water is maintained at 10 C in the evaporator, the evaporator pressure will be the saturation pressure corresponding to this pressure, which is 1.2 kPa. It is not practical to design refrigeration or air-conditioning devices that involve such extremely low pressures....
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FTFS Chap08 P122 - Chapter 8 Power and Refrigeration Cycles...

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