KILX DRYING HANDBOOK. 13 
direct-acting thermostats by putting a single reverse- acting valve in 
the steam main and connecting it direct to the air supply. Both 
types are equipped with dials indicating the proper temperature ad- 
justment. Various combination instruments can be secured for dif- 
ferent services. One type consists of a combination recording ther- 
mometer and reverse-acting air-operated thermostat. Other types 
are discussed below. 
After the steam has passed through the various valves, it enter; 
the steam coils proper. If the coils are in one unit, steam enters 
all. However, they may be divided into several groups, each group 
controlled by a gate or globe valve. In the latter case, enough 
groups should be turned on to produce a temperature only slightly 
in excess of that desired. Care must be exercised to select the dif- 
ferent groups so that the kiln will be heated uniformly throughout. 
When steam is first turned on, the coils are full of air and will 
not heat properly until the air has been removed. The use of air 
valves to remove the air depends upon the method employed in 
removing the water of condensation from the coils. An automatic 
thermostatic air valve should be provided near each trap unless it 
is of the thermostatic type, for which no air valve is needed. 
Automatic air valves operate thermostatically. They remain open 
until a definite temperature is reached and then automatically close 
through the action of some element, such as a metal bar or a liquid- 
filled capsule, which expands with the heat. (See PI. VII.) This 
action permits the cold air to be blown out of the coils and prevents 
the passage of the hot steam. Other things being equal, air valves 
should be placed near the bottom of the coil, since the air is heavier 
than the steam and consequently settles to the bottom. They should 
be mounted on fittings projecting from the top of the drip pipe, 
so that they will not become water bound. 
THERMOSTATIC STEAM TRAPS. 
Steam imparts its heat mainly through condensation. This con- 
densed steam must be continuously removed from the heating coils, 
or they fill with water and become cold. Various devices are used 
to remove water from steam coils and several patented systems are 
in use. In most dry kilns steam traps which allow the escape of the 
water but trap the steam are used. They can be divided into two 
general classes, those depending upon temperature for their opera- 
tion, and those depending upon the weight of the accumulated water. 
The first class is known as thermostatic. (See PI. VII.) Most ther- 
mostatic traps have an operating bellows or diaphragm filled with 
some volatile liquid. One end of the bellows is attached to a valve 
stem and valve, and the motion of the bellows opens and closes the 
valve. The trap is connected to the lowest point in the heating 
system, so that the water will drain readily to it. 
The coils are cold and full of air, and the trap is cold and open 
when steam is first turned on. The steam displaces the air which 
is driven out through the open trap. A certain amount of steam 
is condensed, and this hot water flows to and through the trap. 
Warmed by the water, the trap partly closes, owing to pressure from 
heat expansion inside the bellows, but remains partly open until all 
the air and water have been forced out and steam starts blowing 
