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KILN DRYING HANDBOOK 25 
CONDENSERS 
Water vapor may be removed from the kiln atmosphere by con- 
densation, since it condenses as it passes over a substance colder than 
the dew point. Pipes with cold water flowing through them are 
commonly used for condensing. When cold water is not available, 
a refrigeration plant may be installed, with brine circulating through 
the condenser pipes. 
WATER SPRAYS 
Water sprays may be used either to raise the humidity within the 
kiln or to lower it; the result depends upon relative temperatures 
and upon the method of application. The spraying of water di- 
rectly into the kiln atmosphere, in such manner that it can be evap- 
orated for absorption by this atmosphere, will raise the kiln humid- 
ity. To lower the humidity with water sprays, it is necessary (1) 
to pass part or all of the air in the kiln through water sprays cold 
enough to cool the air below its original dew point, thus actually 
condensing moisture out of it, (2) to separate the spray water and 
condensed moisture from the air, and (8) to reheat the air to the 
kiln temperature and return it to the kiln. 
COMMON PRACTICE 
In the chemical laboratory air is dried by passing it through chem- 
- jcals that have affinity for moisture, principally calcium chloride and 
sulphuric acid. ‘The process has not been developed for commercial 
wood drying. In actual practice, most dry kilns depend upon leak- 
age and ventilation to lower the humidity and upon steam jets to 
raise it as required. 
WET-BULB CONTROLLERS 
The control of humidity is more difficult than temperature con- 
trol, and greater attention must be given to the humidity-regulating 
apparatus in order to secure satisfactory results. One principal 
reason is that a small difference in the wet-bulb temperature pro- 
duces a comparatively large difference in humidity, so that securing 
good control requires an accurate instrument. 
The humidity controllers of greatest importance in kiln drying 
are those that depend upon a wet-bulb of one type or another. Tem- 
perature controllers of various types can be made into wet-bulb 
humidity controllers by providing the bulb, the temperature-sen- 
sitive element, with a suitable wick and a water supply. Wet-bulb 
control on dry kilns is generally carried out by means of air-operated 
instruments. Self-contained thermostats can also be used for this 
purpose, particularly under conditions that do not require the 
greatest accuracy and quickness of response to changes in temper- 
ature. Wet-bulb controllers, however, can keep only the wet-bulb 
temperature constant. If the dry-bulb temperature is also kept con- 
stant, the humidity will remain constant. If it is not, the humidity 
will vary, even if the wet-bulb temperature is accurately controlled. 
It is obvious, therefore, that the wet-bulb controller alone can not 
keep the humidity constant, and that some form of temperature 
(dry-bulb) control must be used in conjunction with it. It also is 
