Compressed air is used in a heat-pump system to heat water, which is subsequently used to warm a house. The house demand is 95,000 Btu/h. Air enters the exchanger at 200 F and leaves at 120 F, and...

Compressed air is used in a heat-pump system to heat water, which is subsequently used to warm a house. The house demand is 95,000 Btu/h. Air enters the exchanger at 200F and leaves at 120F, and water enters and leaves the exchanger at 90 and 125F, respectively. Choose from the following alternative units the one that is most compact.

a. A counterflow surface with
 and a surface-to-volume ratio of
 .

b. A crossflow configuration with the water unmixed and air mixed having
 and a surface-to-volume ratio of 100 ft²/ft³.

c. A crossflow unit with both fluids unmixed with
 and surface-to-volume ratio of 90 ft²/ft³.