Humidity and Dehumidification

Dehumidifier capacity is often rated in pints per 24 hours (p/d, pints/day), for example W.W. Grainger offers small dehumidifiers rated 15 pints per 24 hours, up to 50 pints per 24 hours, and one model even 105 pints per 24 hours. (Reference 1998-99 catalog 389 pages 3446-3447, models 3ZG61, 3ZM70, 3H292, 3H335, 3H350, 4E404, 4E403, 4E402, 4E971, 3H356, 5E821, 5E822.)

The capacity is dependent upon condition of the air being dehumidified, the higher the ambient humidity and temperature, the more water can be condensed out of the air. Grainger data shows the same unit (5E821) rated 6 p/d at 40'F, 20 p/d at 60'F, 45 p/d at 80'F. Dehumidifiers are standard rated at 60%RH at 80'F, per ANSI standard B149.1.

To dehumidify effectively, (1) don't let the room get colder than the dehumidifier can handle, heat the room if necessary, and (2) don't let new moist air get in the room and/or (3) properly size the equipment for the continuous inflow of new moist air. Heating a room also reduces the relative humidity and reduces the possibility of condensation, although heat does not actually reduce water vapor content of the air. Also don't excessively overdry a room, causing a static electricity spark problem, in some cases a dangerous or destructive condition.

Measurement of Humidity.
(1) Relative humidity (RH) is % of the fraction - actual moisture vapor in the air divided by the maximum amount of moisture vapor air can hold at this temperature, beyond this maximum amount (100%RH) the moisture cannot be held as vapor, instead it forms small droplets (fog) or bigger droplets (rain).

(2) Pounds of water per cubic foot of air, or pints of water per cubic foot of air are very useful measurements to the engineer. These measurements are commonly converted from another unit - grains, where one grain = 1/7000 pound = 0.0001428 pound. Also, 7000 grains = 1 pound of water, 7304 grains = 1 pint of water. (Other conversions - 1 pint = 16 fluid ounces, 8 pints = 1 gallon).

How much moisture is in air? Rain and hail conditions contain the most water and are not analyzed here. Man-made Fog from a spray may contain rain, and is not analyzed here. Natural fog and clouds contain the same density of total water as the naturally occurring moist air that condensed and became the fog or clouds.

1.2 pints of water vapor in a 10'x10'x10' room (0.0012 pint per cubic foot) is approximately the maximum amount of water that air can be expected to contain in the U.S. (126 grains per pound dry air at 14.55 cu ft per pound dry air, Corpus Christi TX in August 102'F dry bulb, 81'F wet bulb) For more humid locations, look to Southeast Asia.

0.64 pints of water vapor in a 10'x10'x10' room (0.00064 pint per cubic foot) is a maximum for Sacramento California 104'F dry bulb, 72'F wet bulb (a summer extreme), (68 grains per pound dry air at 14.44 cu ft per pound dry air).

0.56 pints of water micro-droplets in a 10'x10'x10' room (0.00056 pint per cubic foot) is a maximum for San Francisco fog (58 grains per pound dry air at 14.02 cu ft per pound dry air, a maximum summer humid condition condensed to a cold fog.)

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