Cold air at 40 OF, 760 mm Hg pressure, and 80 % relative humidity is conditioned by being passed through a bank of heating 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 80 OF. 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 a 70 OF and 35 % relative humidity. What is the mass of water evaporated (lbm) per cubic foot of air fed to the conditioning unit?