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hw2223 - 6-41 E The heat absorption the heat rejection and...

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Unformatted text preview: 6-41 E The heat absorption, the heat rejection, and the The COP of the heat pump i power input of a commercial heat pump are given. - s to be determined. Assumptions The heat pump operates steadily. Analysis Applying the deﬁnition of th e heat pump coefﬁcient of performance to this heat pump gives Wnet,in 2 hp COPHP : .QH :15,090Btu/h lhp 2 2.97 2544.5 Btu/h 6-44 The cooling effect and the COP ofa r efrigerator are given. The power input to the refrigerator is to be determined. Assumptions The refrigerator operates steadily. Analysis Rearranging the deﬁnition of the refrigerator coefﬁcient of performance and applying the result to this refrigerator gives Wm: QL =5ﬂ=335kw ’ COPR 1.3 6-47 The COP and the power consump ' watermelons is to be determined " Assumptions 1 The refrigerator operates steadily. 2 The heat gain of the refrigerator through its walls, door, etc. is negligible. 3 The watermelons are the only items in the refrigerator to be cooled. Properties The speciﬁc heat of watermelons is given to be c = 4.2 kJ/kg.°C. Analysis The total amount of heat that needs to be removed from the watermelons is Q = (chT wa‘mbns = 5 x (10 ng4.2 kJ/kg - °c)(20 — 8)°C = 2520 k] The rate at which this refrigerator removes heat is QL = (c013R )(an): (2.5)(0.45 kw) = 1.125 kW That is, this refrigerator can remove 1.125 k] of heat per second. Thus the time required to remove 2520 kl of heat is At = = 2520 U = 2240 s = 37.3 min QL 1.125 kJ/s watermelons. 6-51 E The COP and the heating effect ofa heat pump are given. The power input to the heat pump is to be determined. Reservoir Assumptions The heat pump operates steadily. Analysis Applying the deﬁnition ofthe coefficient of performance, Q'H =1.4 W p w QH “100,000Btu/h lhp. )_ 28_1hp Wm“ copHp 1,4 , 2544.5Btu/h ._ . - QL 6—23 6-67C When an expansion process is non~quasiequilibrium, the molecules before the piston face cannot follow the piston fast enough, forming a'low pressure region behind the piston. The lower pressure that pushes the piston produces less work. 6—70C The four processes that make up the Carnot c ycle are isothermal expansion, reversible adiabatic expansion, isothermal compression, and reversible a diabatic compression. 6-71C They are (l) the thermal efﬁcienc reversible heat engine operating between reversible heat engines operating betwee y of an irreversible heat engine is lower than the efﬁciency ofa the same two reservoirs, and (2) the thermal efficiency of all the n the same two reservoirs are equal. 6—72C False. The second Carnot principle states that no heat engine cycle can have a higher thermal efficiency than the Carnot cycle 0 perating between the same temperature limits. 6-73C Yes. The second Carnot principle states that all re versible heat engine cycles operating between the same temperature limits have the same thermal efficiency. ...
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hw2223 - 6-41 E The heat absorption the heat rejection and...

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