FOREST SERVICE WATER SPRAY DRY KILN". 29 
The selection of the type of thermostat depends upon the tempera- 
ture range of the kilns. This in turn depends on the variety of stock 
being dried and the availability of suitable air pressure, etc. The 
direct-acting thermostats have a temperature range covering from 
60° to 80° F. This range is sufficient where one kind of material is 
being dried. It is not sufficient where the kiln is used for a low- 
temperature run, as oak, and then for a high-temperature run, as 
yellow pine. The air-controlled type is actuated by diaphragm motor 
valves, with air pressure, and has a temperature range covering from 
150° to 250° F., wide enough to include the drying temperatures 
suitable for any wood. An air pressure of 15 pounds gauge is required 
to operate the valves on the steam supply, but the size of the dia- 
phragm must be suited to the steam pressure, whether high or low. 
Where the air-operated type is installed, a reverse acting diaphragm 
valve, kept open by the air pressure, should be placed on the steam 
supply, so that it will close off the steam entirely in case of failure 
of air supply. The water-operated type is similar in action and tem- 
perature range to the air-controlled type but is liable to get out of 
order on account of clogging with dirt, sediment, or grit. The elec- 
trically-operated type at present available is hardly suitable for dry- 
kiln use, since it does not throttle the steam through a graduated 
valve but closes it off entirely or opens it up full, through a positive- 
seating valve. There is also considerable danger of failure in con- 
tact, which would mean ruin to the lumber. The possibilities of this 
type for dry-kiln use, however, have not been fully developed. 
The control bulb of the thermostat should be placed in the entering 
air flue and in such a position that its action will be positive and 
immediate. It is well to shield it against direct radiation and from 
,0 effects of cold walls or wet lumber. Where the temperature changes 
of the thermostat are controlled by a key at the bulb inside the kiln 
an extension handle should be installed at some convenient place 
outside the kiln to facilitate operation. 
STEAM CQNSUMPTION. 
HEATING COILS. 
An estimate of the approximate amount of steam required to heat 
a kiln to the temperatures given in Table 7 may be determined by 
the formula: 
T = b 
§ 
When R t =Total radiation surface in kiln in square feet. 
D =Difference in temperature between steam and air entering lumber pile, in 
degrees Fahrenheit, 
k = Constant from Table 7. 
T = Latent heat of steam, B. t. u. 
S = Pounds of steam condensed per hour. 
