KILN DRYING HANDBOOK. y 
of headroom which the vertical arrangement of the groups of pipe 
demands. This headroom must be sufficient not only for the pipes 
and the return bends, but also for at least 0.1 inch of the downward 
pitch per foot from the supply to the discharge end of each group. 
This pitch causes adjacent pipes to form a V with each other, and 
the headroom required for the pipes increases rapidly with the length 
of the kiln. For short kilns requiring accurate temperature control 
and even heat distribution the return-bend coil is specially adapted. 
EETURN-BEND HEADER COIL. 
Various modifications of the two types have been introduced, re- 
taining the advantages of both and eliminating the disadvantages. 
Among these are the return-bend header coil, with horizontal headers 
and two or more layers of pipe connected with return bends ; and the 
vertical header coil, with both headers at one end of the kiln and 
return bends or double elbows with a short run of pipe at the other 
end. These compromise types have merit and will operate advan- 
tageously under conditions to which they are adapted. 
WALL COILS. 
Several types of kiln use pipe-coil radiators on the side walls. 
These radiators do not need to differ materially from those located 
under the lumber, and the great amount of headroom available makes 
it a simple matter to get rid of the water of condensation from almost 
any type of coil. It also permits the use of return-bend coils in long 
kilns without the sacrifice of the pitch required for proper drainage. 
Cast-iron radiators of various kinds have recently been introduced 
for use in dry kilns. They can be had in a wide range of sizes and 
shapes adapted to practically any space or heating requirement. This 
type of radiation is higher in first cost than some other types, but 
great durability is claimed for it on account of the resistance of cast 
iron to rust. 
Blower kilns of several types have the heating units located outside 
of the kiln as shown in Plate II, Figure 1. These units are usually 
of the standard types used in blower systems for heating buildings. 
Practically all of these consist of compactly arranged groups of pipes 
or pipe coils made up into cast headers, which form the base of the 
heater. Sometimes special forms of cast-iron radiators are used. It 
is good practice to equip the heater with valves, so that various por- 
tions of it may be used as desired. Such heaters give little trouble, 
since their design permits unusually easy removal of air and water 
and the short pipes are free from difficulties caused b} T uneven expan- 
sion and contraction. 
In addition to the heating equipment described, some kilns are 
equipped with ceiling coils. These usually consist of a few runs of 
pipe spaced a foot or more apart and hung a few inches below the 
ceiling. They are connected independently and are used most or all 
of the time. Their function is to replace the heat lost through the 
ceiling and so prevent the latter from acting as a condenser. During 
cold weather especially, and when high humidities are used, the 
ceiling is likely to accumulate a great deal of condensation, which 
drips down upon the lumber and prevents humidity control. 
