6-125 The claim of a manufacturer of ice cream freezers regarding the COP of these freezers is to be evaluated.
Assumptions The refrigerator operates steadily.
Analysis The maximum refrigerator coefficient of performance would occur if the refrigerator were completely reversible,
Since the claimed COP is less than this maximum, this refrigerator is possible.
6-77 Two pairs of thermal energy reservoirs are to be compared from a work-production perspective.
Assumptions The heat engine operates steadily.
Analysis For the maximum production of work, a heat engine operating between the energy reservoirs would have to be completely reversible. Then, for the first pair of reservoirs
For the second pair of reservoirs,
The second pair is then capable of producing more work for each unit of heat extracted from the hot reservoir.
6-78 The source and sink temperatures of a power plant are given. The maximum efficiency of the plant is to be determined.
Assumptions The plant operates steadily.
Analysis The maximum efficiency this plant can have is the Carnot efficiency, which is determined from
6-79 The source and sink temperatures of a power plant are given. The maximum efficiency of the plant is to be determined.
Assumptions The plant operates steadily.
Analysis The maximum efficiency this plant can have is the Carnot efficiency, which is determined from
7-197 Air flows in an adiabatic nozzle. The isentropic efficiency, the exit velocity, and the entropy generation are to be determined.
Properties The gas constant of air is R = 0.287 kJ/kg.K (Table A-1).
Assumptions 1 Steady operating conditions exist. 2 Potential energy changes are negligible.
Analysis (a) (b) Using variable specific heats, the properties can be determined from air table as follows
alternatively, to get h2s, get T2s using eq 7-34 then get h2s from T2s
Energy balances on the control volume for the actual and isentropic processes give
The isentropic efficiency is determined from its definition,
(c) Since the nozzle is adiabatic, the entropy generation is equal to the entropy increase of the air as it flows in the nozzle