UNITED STATES DEPARTMENT OF AGRICULTURE 



BULLETIN No. 5 



Contribution front the Forest Service 

 HENRY S. GRAVES, Forester 



Washington, D. C. 



PROFESSIONAL PAPER 



March 17, 1917 



THE THEORY OF DRYING AND ITS APPLICATION 

 TO THE NEW HUMIDITY-REGULATED AND RE- 

 CIRCULATING DRY KILN. 



By Harby D. Tiemann, In Charge, Section of Timber Physics, Forest Products 



Laboratory. 



CONTENTS. 



Page. 

 Introduction 1 



Elementary principles of drying 2 



Elementary prraciples of hygrometry 5 



Types of kilns 7 



Drying by superheated steam 8 



Importance of proper piling of lumber 9 



Theory and description of the Forest Service 



kihi 10 



Theoretical discussion of evaporation 13 



Theoretical analysis of heat quantities 18 



INTRODUCTION. 



The problem of satisfactorily drying lumber without checking, 

 honeycombing, or warping is one of very wide interest. Although 

 an old problem, it has not yet reached an entirely satisfactory solu- 

 tion, especially with hardwood lumber. Even air drying, Avhich is 

 the slowest and what might be called the most conservative method 

 of removing the moisture, is far from satisfactory for some species of 

 wood. The drying of softwoods, or wood from coniferous trees, 

 on the other hand, may be considered as havihg reached a fairly sat- 

 isfactory solution. With few exceptions, the softwoods present no 

 special difficulty to the lumber drier. The great trouble with the 

 hardwoods lies in their relatively excessive and very unequal shrink- 

 age. This is due largely to the structure of the wood. In soft- 

 woods the vertical elements are all of the same kind, regularly ar- 

 ranged and of approximately the same width (tangentially) . The 

 medullary rays also are very fine and regular. In hardwoods, on the 

 other hand, the elements are very complex, varying in diameter in 

 some species in the same section 20 to 30 times, and are often very 

 crooked. Many woods, such as the oak, have large medullary rays, as 



70253°— Bull, 509—17 1 



