Cold air at 20°F, 760 mm Hg pressure and 70% relative humidity is conditioned by being passed through a bank of heatin2 coils, then through a water spray and finally through a second set of heating coils. In passing through the first coil bank, the air is heated to 75°F. The temperature of the water supplied to the spray chamber is adjusted to the wet-bulb temperature of the air admitted to the chamber, so that the humidifying unit may be assumed to operate adiabatically. It is required that the air emerging from the conditioning unit be at 70°F and 35% relative humidity. Use Figure 8.4-2 to solve the following problems.

(a) Calculate the temperature of the water supplied to the spray chamber and the relative humidity and dry-bulb temperature of the air leaving the spray chamber.

(b) Calculate the mass of water evaporated (lbm) per cubic foot of air fed to the conditioning unit.

(c) Calculate the required heat transfer rates (Btu/ft3 entering air) in each of the heating coil banks.

(d) Sketch a psychometric chart and show the path followed by the air in each of the three steps of this process.