18 BULLETIN 894, U. S. DEPAETMENT OF AGRICULTUKE. 
HEATING COILS. 
The necessity of proper planning and installation of the heating 
equipment can not be too strongly emphasized. Whether or not a 
kiln can dry refractory woods successfully is absolutely dependent 
upon the heating installation. Even temperatures must be main- 
tained, otherwise it is impossible to maintain an even control of the 
humidity. The heat from' the coils must be evenly distributed 
throughout the kiln from end to end if any degree of uniformit}^ in 
the drying is to be obtained. The critical period in the seasoning 
of green stock is during the early stage of drying, when low tempera- 
ture and high humidities are necessary. A few degrees difference in 
temperature in different parts of the kiln at this time means a very 
marked difference in the humidity. This may result in saturating 
and moulding the lumber in one end of the kiln and checking and 
honeycombing it at the other. Under such conditions it is impos- 
sible to dry the lumber without great loss. All refinements that are 
introduced in a dry kiln are for the purpose of bringing about even 
distribution of heat and humidity. The coils must be so designed 
and installed that the temperature of the air after it has passed over 
them will be the same throughout the length of the kiln. 
There are two general types of coils in common use, the header 
and the return bend. 
HEADER TYPE. 
The usual manner of installing header coils is to start the pipes 
from a supply header at one end of the kiln and bend them down hi to 
a drainage header at the opposite end, from which a drain leads to 
a steam trap or pump. The steam enters the supply header at one 
end of the coil and starts through all of the pipes more or less simul- 
taneously. Naturally, the steam loses heat very rapidly on account 
of the amount of radiation surface exposed, and before it reaches , 
the return header it has become cooled far below the temperature at 
the supply header. Oftentimes the steam is condensed before it has 
gone one-third the length of the coil, with the result that one end of 
the kiln may be 15 or 20 degrees hotter than the other end. This 
means a very low humidity at the hot end and a high humidity at 
the cooler end, a condition that would be disastrous in a kiln loaded 
with heavy green oak or other refractory wood. 
Header coils may be used for medium and high temperature runs 
where kilns are not over 25 feet long. They are not practical for 
high humidities in low-temperature runs. 
RETURN BEND TYPE. 
In return bend coils the pipes extend continuously from end to 
end of the coil and are connected like the folds of an accordion or 
bellows, with return bends. Obviously the steam has to travel the 
