KILN DRYING HANDBOOK 19 
heating coil being connected through a thermostatic trap to the pump 
suction main. Where the heating coils in a kiln are broken up into 
units, the best practice is to have a trap on each unit. Although the 
pump is very effective, the rapid relief obtained by it is not needed 
in most kilns. 
HUMIDITY IN THE KILN 
The earth’s atmosphere, our air, isa mixture of many invisible gases, 
principally oxygen, nitrogen, and water vapor. The amount of water 
vapor in the air, which is termed humidity, is usually expressed 
either in grains per cubic foot or as a percentage of saturation; the 
first method of expression is called absolute humidity, and the sec- 
ond is called relative humidity. Fortunately, the amount of water 
vapor that a given amount of air can hold at a given temperature is 
a fixed quantity; when this quantity is present the air is said to be 
saturated. 
The amount of water vapor at the saturation point of air increases 
rapidly with increase in temperature. At 60° I’., merely 5.8 grains of 
water vapor saturate the ordinary atmosphere, whereas at 212° F-., 
the boiling point of water under a pressure equal to that of a column 
of mercury 29.92 inches high, it will hold about 260 grains per cubic 
foot. 
RELATIVE HUMIDITY 
- In kiln drying it is more convenient to express the amount of 
water vapor in the air in terms of relative humidity than as absolute 
humidity, and when the term “ humidity ” is used in this pubhcation, 
relative humidity is meant. As already intimated, relative humidity 
is always expressed as a percentage of saturation. 
The lower relative humidities represent dry air and the higher 
ones moist air. Air at a temperature of 125° F., for instance, can 
hold a maximum of 40 grains of water vapor per cubic foot. If a 
certain atmosphere at that temperature had only 10 grains of water 
per cubic foot it would have only ten-fortieths of the maximum 
amount it could hold, which is a relative humidity of 25 per cent. 
Air with 25 per cent relative humidity is comparatively dry. At 
125° F., the relative humidity of air having 30 grains of water vapor 
would be thirty-fortieths, or 75 per cent; such air would be consid- 
ered moist. Air at 155° F. can hold 80 grains per cubic foot, twice 
as much water vapor as at 125° F. At 155° F. air containing 10 
grains of water per cubic foot would be very dry, having a relative 
humidity of only 12% per cent, and air containing 30 grains per 
cubic foot would still be moderately dry, having a relative humidity 
of 37% per cent. 
The preceding examples may be expressed by the following 
formula : 
amount of water vapor actually present in a 
siven space 
maximum amount of water vapor possible 
(the saturation value) in the same space 
under the same temperature 
Relative humidity per cent= x 100 
At any given temperature dry air is heavier than moist air, and 
hot air always is lighter than cold air at the same relative humidity 
and the same pressure. When water is evaporated from wood, the 
