HUMIDITY-EEGULATED AND RECIRCULATING DRY KILN. 15 



wood, the rate depends largely upon the relative humidity.^ There 

 is a balance between what might be termed the retentive or attractive 

 property of the wood, " hygroscopicity," and the tendency of the 

 moisture to vaporize. It is the difference between the tension of the 

 vapor at the higher temperature of the wood and the tension actually 

 existing in the space surrounding the wood. This retentive prop- 

 erty increases as the wood becomes drier and decreases as it ap- 

 proaches the wet condition. Experiments indicate that generally 

 it is nearly inversely proportional to the amount of moisture remain- 

 ing in the wood. 



EVAPORATION WHEN AIR IS PRESENT. 



When air is present with the superheated steam or water vapor 

 the conditions are quite different. Vaporization of a particle from 

 the surface of the free water is retarded by the air pressure, so that 

 the temperature of the water may be raised above the dew point.^ 



The air now, as well as the vapor, conducts heat to the water, so that 

 the rate of evaporation at given pressures depends not alone on the 

 quantity of heat supplied (by circulation and degree of superheat- 

 ing) but upon the relative amounts of vapor and air present. That 

 is to say, the lower the relative humidity the greater is the rate of 

 evaporation at a given temperature and pressure. The temperature 

 of the water will correspond to that of the wet bulb, and not to that 

 of the dew point. When the wood becomes partially dried its tem- 

 perature will rise, as in the case of superheated steam, and it may 

 be heated even above the boiling point at the given pressure without 

 giving up all of its moisture, provided there is some vapor in the air. 



CONCLUSIONS AS TO DRYING IN VAPOR ALONE AND IN AIR AND VAPOR. 



Thus it is seen that the rate of drying may be controlled by the 

 relative humidit}'', provided there be sufficient circulation to supply 

 the heat required. In the case of steam alone, the rate of drying, as 

 just shown, depends upon the quantity of circulation as well as the 

 degree of superheating. Hence the conclusion follows that moist 

 air, with ample circulation, should give more uniform drying 

 throughout than superheated steam, which varies with the rate of 

 circulation in each portion. 



1 In using the term relative humidity as applied to superheated steam it is understood 

 to mean the ratio of the actual vapor pressure to that of the pressure of saturated vapor 

 at the given temperature, as explained before. 



2 In reality what probably happens is that the layer of air in immediate contact with 

 the water becomes saturated and has a higher vapor pressure corresponding to the tem- 

 perature of the surface of the water, and the air retards the diffusion of this vapor. The 

 temperature of the water, however, can not exceed the boiling point for the given pres- 

 sure, at which point the conditions must become the same as those for superheated steam 

 alone, since then the air will become entirely displaced by the water vapor. 



