HUMIDITY-REGULATED AND RECIRCULATING DRY KILN. 5 



of 26 inches or less is almost as rapid as under ordinary air pressure.^ 

 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 . 0589 



These figures evidently include radiation and convection. They show comparatively 

 email change at pressures above 42 millimeters of mercury, which corresponds to a 

 vacuum of about 28.4 inches. 



