Warm air at 35\'C is cooled sensibly to 20\'C at a constant pressure of 1 atm by
ID: 1862663 • Letter: W
Question
Warm air at 35'C is cooled sensibly to 20'C at a constant pressure of 1 atm by passing it over the evaporator coils of a refrigeration device, as shown in the figure. The mass flow rate of the air is 3 kg/s. The ambient air temperature is 35 C, while the evaporator temperature can be taken as constant and equal to -5'C. Calculate
(a) The heat transfer rate in kW from the air stream to the evaporator coils.
(b) The rate of increase in the entropy of the universe.
Figure:
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Warm air at 35'C is cooled sensibly to 20'C at a constant pressure of 1 atm by passing it over the evaporator coils of a refrigeration device, as shown in the figure. The mass flow rate of the air is 3 kg/s. The ambient air temperature is 35 C, while the evaporator temperature can be taken as constant and equal to -5'C. Calculate The heat transfer rate in kW from the air stream to the evaporator coils. The rate of increase in the entropy of the universe.Explanation / Answer
Since the Process is Constant Pressure
Heat Transfer rate(Q) = mass flow rate of air*Cp*dT
Q = m*Cp*dT
Q = 3*1.004*(35-20) KW
Q = 45.18 KW
Entropy of air(S1) = Q/T1 = 45.18/(273+20) = 0.1542 KW/K
Entropy of Refrigerent(S2) = Q/T2 = 45.18/(273-5) = 1.6835 KW/K
Entropy of universe(S) = S1 + S2 = 1.8377 KW/K