FOREST SERVICE WATER SPRAY DRY KILN. 27 
sucked up into the trap when the thermostat is closed, and the system 
will become air bound. For high-pressure steam, connect waste to 
the vent pipe, one end of which is open to the outside atmosphere, 
the other end in a water seal. This permits any steam passing 
tiirough the trap, or vapor from the hot water, to escape outside the 
operating room. 
The proper operation of traps is of utmost importance; for if they 
fail to function properly, uniform heating is impossible. 
AIR RELIEF VALVES. 
To get the highest efficiency from the heating coils some means of 
automatic removal of the air must be provided. Pet cocks and 
valves are sometimes used but are apt to be neglected by the operator. 
There are a number of automatic valves on the market which give 
perfect satisfaction. 
In determining the type of air valve to be selected the steam 
pressure used must be considered, and where a thermostat is used on 
the steam supply, the air valve must seal against a vacuum, and not 
allow any air to enter the coil. 
The air relief valve should be located near the condensation trap, 
since air is heavier than steam at the same temperature, and will 
therefore gather in the lowest part of the coils. If possible, the 
relief valve should be accessibly placed outside the kiln. 
TEMPERATURE REGULATION. 
Close and accurate control of humidity can be maintained only 
when the kiln temperatures are under absolute control of the oper- 
ator. Temperatures may be controlled by "hand," by pressure- 
regulating valves, or automatically by means of thermostatic valves. 
For hand control globe or gate valves are operated to allow more 
or less steam into the system. With this method of control there is 
considerable variation in the temperatures obtained. Such control 
means constant attention of the operator and otherwise is very 
imperfect. The only feature recommending it is the saving in first 
cost. Figure 9 illustrates a kiln with hand-controlled heating coils. 
Pressure regulating valves are slightly better than hand control, 
as they balance the pressure in the system, thus regulating the amount 
of heat given off. With such means of control it is very difficult, 
however, to maintain low temperatures accurately. The control of 
humidity is jeopardized at the most critical period of drying and the 
temperature rises or falls gradually at all times, seriously disturbing 
the control of drying. 
The most practical form of temperature regulation is by means of 
thermostats, which automatically operate special valves to admit or 
shut off steam as the kiln temperatures fall or rise. A thermostat 
