8 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE 
products of combustion pass directly upward through the lumber, 
which is open piled on a platform above the fire. Kilns of this type 
are known as smoke kilns. A number of years ago they were very 
popular through the southern pine region, but their use is now limited 
almost entirely to small portable or semiportable mills in that 
region. ; | 
Indirect furnace heat—As in an ordinary hot-air furnace, the in- 
cirect-furnace-heat system leads the incoming air around the fire pot 
and the radiators on its way to the kiln, and the products of com- 
bustion pass directly up the chimney instead of going 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. 
Llectricity —Electric heat offers many advantages in the way of 
cleanliness, ease of control, and efficient installation, but the cost of 
operation is in most cases prohibitive. The successful operation of a 
smal] electric-kiln installation is reported in the trade press. 
fot water—The hot-water system can readily be adapted to the 
heating of kilns that do not demand too high a temperature. A suit- 
able hot-water supply rarely is available, however, in the absence of 
steam. : | 
Steamv.—aAt present steam is almost universally employed for heat- 
ing dry kilns of all types, and consequently a knowledge ‘Sf its proper 
use 1s 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, that which has passed through engine, pump, or 
turbine on its way from the boilers to the kilns. As a rule high- 
pressure steam is far drier than low-pressure, 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. 
The steam cooling in the kiln radiators gives up its heat to the kiln 
air and the charge of lumber is dried accordingly. 
PIPE COILS AND OTHER RADIATORS 
The form, construction, and arrangement of the kiln radiators 
is of importance. Those built of pipe coils are most common; the 
coils are made of ordinary merchant pipe or of wrought-iron pipe, 
the rust-resisting qualities of the latter making it particularly suit- 
able for severe corrosion conditions. Among the advantages of 
such radiators are low first cost, ease of manufacture and of installa- 
tion, ready adaptability to a great range of shapes and of sizes, and 
ease of repair by any shop mechanic. 
A good pipe coil must possess several essentials: (1) the size, the 
shape, and the location to heat properly and in some cases to recircu- 
late the air in the kiln; (2) mechanical strength and durability, with 
provision for the expansion and contraction of the individual pipes 
in the coil; (3) provision for the ready escape of air and of water of 
condensation from the entire system; and (4) provision for adjust- 
ment in the amount of active heating surface, by cutting some pipes 
