FOREST SERVICE WATER SPRAY DRY KILN. 19 
full length of the coil back and forth until condensed. Thus the 
temperatures are even from end to end. A return bend coil may be 
throttled without interfering with the end-to-end-heat distribution 
in the kiln. This is not the case with the header system. 
The return-bend type is somewhat more difficult to install than 
the header type and is more limited as to the position in which it 
may be placed to dram properly. These factors, however, are of no 
consequence hi comparison with the uniform temperatures obtained 
and the flexibility of control. 
The return-bend type of coils is recommended, and its use will be 
assumed hereafter in this bulletin. 
LOCATION OP COILS. 
The recirculating air leaving the baffle plates should be so controlled 
and directed as to travel directly toward the entering air flue. The 
heating coils should be in the path of this moving air and not close 
up under the floor under the lumber pile, where the circulation is 
usually more sluggish. 
Whether the center or side spray chambers are installed, the heat- 
ing surface is separated into two main sections, one on each side of 
the kiln, in order that it may lie in the path of the moving air. 
Where side-spray chambers are used (see figs. 2 and 5), the coils are 
disposed across the cross section of the kiln in the form of a V. 
When a center-spray chamber is used (see fig. 1), the form is inverted. 
MULTIPLE UNITS. 
For flexibility of temperature control the total heating surface 
should be divided into three units of varying sizes, regardless of the 
division into sections mentioned above. Each unit consists of two 
sets of coils, one on each side of the kiln, and is supplied through an 
independent valve. The units are proportioned so that the radia- 
tion in the upper set is about 25 per cent, the middle set about 35 
per cent, and the lower set about 40 per cent of the total. 
SUPFORTS. 
The coils should be supported by hook plates or other equally 
substantial supports placed so that the coils will drain in a continu- 
ous fall in the direction of the flow of steam. Any possibilit}^ of sag 
in the pipe should be avoided. The greater the slope the better, but 
it should not be less than one-tenth of an inch to each foot of length. 
ESTIMATING HEATING SURFACE REQUIRED. 
The amount of radiation required may be determined by using the 
following factors, or by means of constants and the formula given on 
page 21. The factors entering into the calculation of the heating 
surface required are maximum temperature, minimum dew-point 
