KILN DRYING HANDBOOK 11 
COMPROMISE TYPES 
Various modifications of the two primary types, which in differ- 
ent degrees retain some advantages of both and eliminate some disad- 
vantages, have been introduced. Among these are the return-bend 
header coil, with horizontal headers and two or more layers of pipe 
connected by means of return bends; and the vertical header coil, 
with both headers at one end of the kiln and either return bends or 
elbows and nipples at the other end. Such compromise types have 
merit and will operate advantageously under conditions to which 
they are adapted. 
WALL COILS 
Although pipe-coil radiators on the side wall can not be recom- 
mended for general application, they properly form a part of the 
design of certain types of kilns. These radiators need not differ ma- 
terially from those located under the lumber. In fact, the great 
amount of head room available for them facilitates getting rid of the 
condensate from almost any type of coil; it even permits the use of 
return-bend coils in long kilns without the sacrifice of the pitch re- 
quired for proper drainage. 
CAST-IRON RADIATORS 
Cast-iron radiators have been used in a few kilns, but their high 
first cost has no doubt been an important factor in preventing their 
more general adoption. Although they can be obtained in a variety 
of shapes and sizes, in general they are best adapted to conditions that 
require concentrated radiation. Care must be exercised, especially 
with low-pressure steam, to secure proper venting of the air from 
them, and it is desirable to have some means of determining from the 
outside of the kiln whether they are working. 
EXTERNAL HEATING UNITS 
External fan kilns of several types have the heating units located 
outside of the kiln, as shown in Plate 2, B. These units are usually 
of the standard types common in blower systems for heating build- 
ings. Substantially all of them consist of compactly arranged groups 
of pipe coils made up with cast headers, each of which forms the 
base of a unit, although sometimes special forms of cast-iron radia- 
tors are used. Good practice equips each unit with valves, so that 
various portions of it may be cut out as desired. Such heaters give 
little trouble, since their design permits unusually easy removal of 
air and of water and the short pipes are free from difficulties caused 
by uneven expansion and contraction. 
CEILING COILS 
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 an inch or two below the 
ceiling. Since their function is to replace the heat continuously lost 
through the ceiling, thus preventing the ceiling from acting as a 
condenser, they are made entirely independent of the main heating 
units, so that they may be in service most or all of the time. During 
