Two moist air streams flow into a modified cooling tower to cool liquid water fr

Question

Two moist air streams flow into a modified cooling tower to cool liquid water from 120 F to 75 F. The first moist air stream has a relative humidity of 60 percent and enters the cooling tower at 80 F. The second moist air stream enters the cooling tower at 40 F with a relative humidity of 40 percent  and a dry air mass flow rate that is 1/4 that of the 80 F air stream. The air exiting the cooling tower exits with a relative humidity of 90 percent at 110 F. Assuming that no makeup water is supplied to the return water stream and that atmospheric pressure is 14.7 lb/in2, determine

(Using English units)

a. the amount of water evaporated from the incoming stream, in lb/mass dry air.

b. the mass flow rate of the hot water stream, in lb/mass dry air. (Hint: let the %u201Cmass of dry air%u201D for the units be the mass of air in the 80 F stream)

c. If all of the same conditions hold (including the mass evaporation rate that you found) from a-b, determine the rate of air flow for the 80 F stream (in lb/hr) if a 7 hp fan is added to homogenize the air mixture within the cooling tower.

Explanation / Answer

The water cycle is all about storing water and moving water on, in, and above the Earth. Although the atmosphere may not be a great storehouse of water, it is the superhighway used to move water around the globe. Evaporation and transpiration change liquid water into vapor, which ascends into the atmosphere due to rising air currents. Cooler temperatures aloft allow the vapor to condense into clouds and strong winds move the clouds around the world until the water falls as precipitation to replenish the earthbound parts of the water cycle. About 90 percent of water in the atmosphere is produced by evaporation from water bodies, while the other 10 percent comes from transpiration from plants.

There is always water in the atmosphere. Clouds are, of course, the most visible manifestation of atmospheric water, but even clear air contains water%u2014water in particles that are too small to be seen. One estimate of the volume of water in the atmosphere at any one time is about 3,100 cubic miles (mi3) or 12,900 cubic kilometers (km3). That may sound like a lot, but it is only about 0.001 percent of the total Earth's water volume of about 332,500,000 mi3 (1,385,000,000 km3), as shown in the table below. If all of the water in the atmosphere rained down at once, it would only cover the globe to a depth of 2.5 centimeters, about 1 inch.

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