HUMIDITY-REGULATED AND RECIRCULATING DRY KILN. 5 
of 26 inches or less is almost as rapid as under ordinary air pressure. 1 
The viscosity of the gas is a factor in the convection through small 
spaces, such as between the layers of lumber, and as this is almost 
as great at low pressures as at atmospheric pressure, it follows that 
the actual circulation would nevertheless be very much cut down. 
Thus, by drawing a vacuum the means of heating the wood is re- 
duced. Later on it will be shown, however, that drying at low 
pressure in absence of air should give the highest theoretical heat 
efficiency, 'but the volume of vapor required is excessive. 
RATE OF EVAPORATION CONTROLLED BY HUMIDITY. 
It is essential, therefore, to have an ample supply of heat through 
the convection currents of the air; but in the case of wood the rate 
of evaporation must be controlled, else checking will occur. This can 
be done by means of the relative humidity. It is clear now that 
when the air — or, more properly speaking, the space — is completely 
saturated no evaporation can take place at the given temperature. 
By reducing the humidity, evaporation takes place more and more 
rapidly. 
Another bad feature of an insufficient and nonuniform supply of 
heat is that each piece of wood will be heated to the evaporating 
point on the outer surface, the inside remaining cool until consider- 
able drying has taken place from the surface. Ordinarily in dry 
kilns high humidity and large circulation of air are antitheses to 
one another. To obtain the high humidity the circulation is either 
stopped altogether or greatly reduced, and to reduce the humidity a 
greater circulation is induced by opening the ventilators or otherwise 
increasing the draft. This is evidently not good practice, but as 
a rule is unavoidable in most kilns. The humidity should be raised 
to check evaporation without reducing the circulation. 
ELEMENTARY PRINCIPLES OF HYGROMETRY. 
RELATIVE HUMIDITY AND DEW POINT. 
It is necessary to know something of hygrometry in order to under- 
stand the drying operations. As stated before, at any given tempera- 
ture the same quantity of water vapor is required to saturate a given 
1 Bottomly gives for radiation of a bright platinum wire to a copper envelope, at differ- 
ent air pressures, the temperature of the inclosure being 16° C. and the difference in 
temperature 408° C. expressed in the heat lost in c. g. s. units per square centimeter of 
inclosure (Smithsonian Table 250) : 
At 740 mm. absolute pressure 0. 8137 
At 42 mm. absolute pressure . 7591 
At 0.44 mm. absolute pressure . 2683 
At 0.01 mm. absolute pressure .0539 
These figures evidently include radiation and convection. They show comparatively 
small change at pressures above 42 millimeters of mercury, which corresponds to a 
vacuum of about 28.4 inches. 
