Compressed air is used in a heat-pump system to heat water that is subsequently used to warm a house. The house demand is 95,000 Btu/hr. Air enters the exchanger at 200’F and leaves at 120’F; water enters and leaves the exchanger at 90’F and 125’F,respectively. Choose, from the following alternatives, the unit that is most compact.
(a) A counter flow surface with U = 30Btu/hr-ft=’F and a surface-to-volume ratio of 130ft21W.
(b) A cross-flow configuration with the water unmixed and air mixed, having U = 40 Btu/hrft2-‘F and a surface-to-volume ratio of 100 fe/ft3•
(c) A cross-flow unit with both fluids unmixed with U = 50 Btu/hr-ft2-‘F and a surface-to volume ratio of 90 ft2 /fe.