6 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE. 
HEAT IN THE KILN. 
Heat is used in a kiln to produce rapid evaporation and to hasten 
the transfusion of moisture from the interior to the surface of the 
wood. The correct temperature to use is determined by the char- 
acter of the wood and varies widely with different kinds of stock. 
Commercial kiln temperatures range from 100° to 250° F. 
The use of temperatures above that of the surrounding atmosphere 
introduces a problem in the heating of buildings, and imposes an 
added burden upon the heating system, namely, to keep the kiln 
building hot and to replace the heat lost through the walls of the 
kiln. The higher the kiln temperature, the greater will be these 
heat losses. The amount of heat actually used in the evaporation 
of the moisture is only a small part of the total heat supplied; it 
is seldom over 40 per cent and frequently as low as 5 per cent, 
depending upon the kind of drying being done. 
SOURCES OF HEAT. 
Many methods have been used to heat kilns, and although most 
of them are obsolete or impractical, brief mention will be made of 
the principal ones. 
Direct furnace heat, — Smoke and other products of combustion 
are led direct from an ordinary furnace into the kiln, from which 
they are exhausted by chimney or other suitable means. Kilns of 
this type are known as " smoke kilns." At one time it was thought 
that lumber dried in them was superior to steam-dried stock, but 
their use has been largely abandoned. 
Indirect furnace heat. — As in an ordinary hot-air furnace, the 
air passes around the fire pot and radiators on its way to the kiln, 
and the products of combustion pass directly up the chimney instead 
of through the kiln. 
Gas. — Occasionally natural or artificial gas is used to heat small 
dry kilns, the burners being arranged much as in an ordinary house- 
hold gas oven. 
Electricity. — Electric heat can be used in small kilns, although 
the cost of current is prohibitive, except possibly for experimental 
units. 
Hot water. — Hot-water heat can readily be adapted to the heating 
of kilns which do not demand too high a temperature. A suitable 
hot-water supply would rarely be available, however, in the absence 
of steam. 
Steam.— At present steam is almost universally used for heating 
dry kilns of all types, and a knowledge of its use is essential to 
intelligent kiln operation. It may be either high pressure, above 10 
pounds per square inch, or low pressure, below 10 pounds. High- 
pressure steam is almost invariably live steam — that is, steam direct 
from the boilers; low-pressure steam is frequently exhaust steam, 
or that which has passed through engine, pump, or turbine on its 
way from the boilers to the kilns. High-pressure steam is much 
drier, as a rule, than low-pressure steam, principally because exhaust 
steam generally carries with it much water condensed in its passage 
through the engine or other unit in which it has done work. As the 
steam circulates through the kiln radiators the kiln air is heated 
and the contained lumber is dried accordingly. 
