Question

Calculate the amount of work input a refrigerator needs to make ice cubes out of a tray of 0.5 lbm liquid water at 50 F. Assume the refrigerator has β = 3.5 and a motor-compressor of 750 W. How much time does it take if this is the only cooling load? 

Answer 1

C.V. Water in tray. We neglect tray mass.

Energy Eq.: m(u₂ − u₁) = ₁Q₂ − ₁W₂ 

Process : P = constant = P_o

Comment: We neglected a baseload of the refrigerator so not all the 750 W are available to make ice, also our coefficient of performance is very optimistic and finally the heat transfer is a transient process. All this means that it will take much more time to make ice-cubes.