2 BULLETIN 509, U. S. DEPARTMENT OF AGRICTJLTTJEE. 



well as Tei\v small ones, irregularly arranged. Consequently, strains 

 are produced when the wood dries, which cause warping and check- 

 ing. AVhile air drying is undoubtedly the safest method, the process 

 is ordinarily so slow, requiring a year or longer according to species 

 and size, that forced " artificial " drying becomes a business neces- 

 sity. Moreover, air drying is by no means always to be preferred to 

 kiln drying from the standpoint of the quality of the product. 



A correct understanding of the principles of drying is rare, and 

 opinions in regard to the subject are very diverse. The same lack of 

 knowledge exists in regard to dry kilns. The physical properties 

 of the wood which complicate the drying operation and render it 

 distinct from that of merely evaporating free water from some sub- 

 stance like a piece of cloth must be studied experimentally. It can 

 not well be worked out theoretically. 



The thermal process of the drying operation, however, is capable 

 of exact theoretical analysis. It is the purpose of this article to 

 interpret the conditions which exist in the various stages of the dry- 

 ing operation with respect to the heat quantities and the changes 

 which occur in the drying medium, from a tl^eoretical standpoint. 

 The object of this analysis is to show the limiting conditions which 

 may be approached, but can not be exceeded. 



ELEMENTARY PRINCIPLES OF DRYING. 



Before taking up the theoretical discussion, a few remarks upon the 

 elementary principles of drying will be of assistance. 



EVAPORATION REQUIRES HEAT. 



In the first place, it should be borne in mind that it is the heat 

 which produces evaporation and not the air nor any mysterious 

 property assigned to a " vacuum." For every pound of water evapo- 

 rated at ordinary temperatures approximately 1,000 British thermal 

 units of heat are used up, or " become latent," as it is called. This 

 is true whether the evaporation takes place in a vacuum or under a 

 moderate air pressure. If this heat is not supplied from an outside 

 source it must be supplied by the water itself (or the body being 

 dried), the temperature of which will consequently fall until the sur- 

 rounding space becomes saturated with vapor at a pressure cor- 

 responding to the temperature which the water has reached ; evapora- 

 tion Avill then cease. The pressure of the vapor in a space saturate^d 

 with water vapor increases rapidly with increase of temperature. 

 At a so-called vacuum of 28 inches, which is about the limit in com- 

 mercial operations, and in reality signifies an actual pressure of 2 

 inches of mercury column, the space will be saturated with vapor 

 at about 101° F. Consequently, no evaporation will take place in 

 such a vacuum unless the water be warmer than 101° F., provided 



