KILN DRYING HANDBOOK 9 
in or out. Since it is difficult to combine all these essentials in the 
highest degree in any one type of coil, different kinds have been found 
best adapted for various special conditions, in which a single require- 
ment is likely to predominate. 
Pipe coils for dry-kiln heating fall into two general classes, known 
as header and as return-bend coils. In header coils, a number of 
pipes lead from the same supply main, called a header, and return 
to a drip main, also a header, usually but not always located at the 
other end of the kiln. In the return-bend type, however, the lengths 
of pipe in each group are connected end to end by means of return 
bends; steam enters the top length, and condensate is removed from 
the bottom one. Figure 2 illustrates various types of pipe couls. 
Most kiln coils, regardless of detail characteristics, are located in 
the kiln proper, commonly between or under the rails. Many differ- 
ent methods of arranging, particularly of grouping, these heating 
coils have been designed to meet widely varying individual drying 
requirements. In most pipe-coil kilns, the pipes run lengthwise of 
the kiln, and are ordinarily grouped as plain header coils or as 
return-bend header coils. In several recent designs of cross-piled 
kilns, however, the pipes run crosswise, with a group under each 
truck. These groups are sometimes subdivided so that various num- 
bers of pipes in each one may be used as desired. 
PLAIN HEADER COIL 
' The plain header coil is one of the commonest forms of heating 
unit in present kiln practice. To secure satisfactory results, espe- 
cially in the quantity and the uniformity of the heat supply, coils 
of this type must be so designed and operated that there is steam in 
them all the time, and they must be arranged to drain freely. To 
meet the first requirement, the coils in each kiln must usually be 
divided into several groups, and just enough pipes in each group are 
used to give the desired amount of heat when the steam is on full 
all or most of the time. Trouble from uneven heating of header 
coils is confined largely to kilns having excessive heating surface, 
especially those with nonthrottling thermostatic control. 
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, however, 
and heating of the air will be practically uniform from end to end. 
The return-bend type also has disadvantages, among which are its 
first cost and the amount of head room that the vertical arrange- 
ment of the piping demands; the head room must be sufficient not 
only for the individual pipes and the return bends, but also for at 
least 0.1 inch of downward pitch per foot from the supply end to 
the discharge end of each group. ‘This pitch causes adjacent pipes 
to form a V with each other, and consequently the head room neces- 
sary increases rapidly with the length of the kiln. The return-bend 
coil is better adapted for short kilns requiring accurate temperature 
control and even heat distribution than for long kilns, 
