KILN DRYING HANDBOOK. 7 
PIPE COILS AND RADIATORS. 
The form, construction, and arrangement of the kiln radiators is 
of importance. Those constructed of pipe coils are in most common 
use. Pipe coils are made of ordinary merchant pipe, extra heavy 
pipe of various kinds, and wrought-iron pipe, the last being par- 
ticularly suitable for severe drying schedules. Among the advan- 
tages of * pipe-coil radiators are low first cost, ease of manufacture 
and installation, ready adaptability to a great range of shapes and 
sizes, and ease of repair by the shop mechanic or millwright. There 
are several essentials which a good pipe coil must possess : First, it 
must be of such size. and shape and so located that it can properly 
heat the air in the kiln; second, it must be mechanically strong and 
durable and provided with means for permitting the expansion and 
contraction of the individual pipes in the coil; third, it must pro- 
vide for the ready escape of air and water of condensation from the 
entire system; fourth, it must provide for adjustment in the amount 
of heating- surface to be used by cutting certain pipes in or out. 
As it is difficult to combine all these essentials in the highest degree 
in any one type of coil, different ones have been found best adapted 
for various special conditions. 
A large portion of all pipe coils used for dry-kiln heating are 
located in the kiln proper, between or under the rails. These fall 
into two general classes, known as header and return-bend coils. In 
the former, a number of pipes spring from the same supply pipe or 
header and return to a similar drip pipe or header, usually but not 
always, located at the other end of the kiln. In the return-bend 
type, however, the pipes of each group are connected end to end by 
means of return bends or double-elbow fittings; steam enters at the 
front of the first or top pipe, and condensation is removed from 
the end of the last or bottom pipe. Figure 2 illustrates various types 
of header and return-bend coils. 
PLAIN HEADER COIL. 
The action of the two types of coils is quite different, especially when 
operated with a thermostat. When steam is turned on in a plain header 
coil with a header at each end of the kiln that end of the kiln nearest the 
supply header will heat up first ; the other end will not heat until the 
front end has become hot and ail the air has been exhausted from the 
coil. This uneven heating takes place each time the thermostat 
opens. If the heating surface is unduly large, as it may be when low 
temperatures are used, the thermostat will operate often, and there 
will be a marked difference in temperature between the two ends of 
the kiln. Another characteristic of the header system is that the 
large heating surface of the headers themselves causes an uneven 
distribution of heat by causing a "hot spot" at each header. 
RETURN-BEND COIL. 
In the return-bend type the top pipes in each group become hot 
first, since the steam must pass through them before reaching the 
lower ones. Each pipe runs the full length of the kiln, and heating- 
will be practically uniform from end to end. The return-bend type 
also has disadvantages, among which are the first cost and the amount 
