UNITED STATES DEPARTMENT OF AGRICULTURE 
BULLETIN No. 509 { 
tit- \ 
Contribution from 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 Haery D. Tiemann, In Charge, Section of Timber Physics, Forest Products 
Laboratory. 
Introduction 
Elementary principles of drying 
Elementary principles of hygrometry. 
Types of kilns 
Drying by superheated steam 
CONTENTS. 
Importance of proper piling of lumber. 
Theory and description of the Forest Sendee 
Page. 
kiln. 
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, which 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 having 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 
