FOREST SERVICE WATER SPRAY DRY KILN. 9 
evaporated moisture. The circulation must be positive and ample 
throughout the pile and reasonably uniform in order to produce 
equal distribution of heat and humidity. A sluggish circulation 
means slow drying; stagnant air means no drying; and unequal 
circulation means unequal drying. This is especially significant in 
the case of green lumber. 
PRINCIPLES OF OPERATION. 
In a water-spray kiln the air entering the lumber pile is controlled 
at the maximum temperature and minimum humidity desired. As 
the air comes in contact with the moist lumber, it absorbs moisture, 
which causes a drop in temperature and an increase in humidity. 
Thus the temperature of the air is somewhat lower and the humidity 
higher when it leaves the pile than when it enters. 
To remove the moisture that has been taken away from the lumber, 
the air is cooled by passing through the water sprays. The air 
leaving the spray chambers is saturated at the dew point 2 of the 
air entering the pile. During the passage of this saturated air over 
the heating coils the temperature is raised and the relative humidity 
thereby is lowered. From the heating coils the air passes to the 
lumber again at the original temperature and humidity. The cycle 
is thus repeated. 
The temperature of the air as it leaves the spray chamber through 
the baffles is controlled by the temperature of the water admitted 
through the sprays. The temperature of the spray water itself is 
controlled by a thermostatic water-mixing valve, which automati- 
cally mixes warm and cold water to maintain the desired constant 
temperature condition; or in certain cases the automatic mixer is 
made unnecessary because the water may be held at the desired tem- 
perature in the well by admitting cold water with the spent water 
returning from the sprays. 
The heating coils are usually controlled by a thermostatic valve 
placed on the steam supply. The controlling bulb of the thermo- 
stat is placed on the entering air side of the lumber pile in which the 
temperature is to be controlled. The coils are divided into a number 
of units which are properly valved to be operated separately or 
together, according to the kind of material to be dried and the tem- 
perature required. 
2 The term relative humidity refers to the ratio of moisture present in the air to the amount of moisture 
the air can hold, and is expressed in per cent. The amount of moisture that air can hold varies with the 
temperature of the air. At any given temperature the greatest amount of water vapor that air can hold 
is fixed, but any less amount than this maximum may occur. Increasing the temperature of air increases 
its capacity to carry moisture and thus with the same amount of vapor present reduces the humidity. 
Lowering the temperature decreases its moisture-carrying capacity and increases the humidity. If air 
which is not saturated is cooled at constant atmospheric pressure, a definite temperature is eventually 
reached at which the air becomes saturated and any further cooling will cause condensation of some of the 
vapor into water. This temperature is called the "dewpoint." 
187385°— 20 2 
