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KILN DRYING HANDBOOK 17 
Some types of air-operated thermostats are equipped with gradu- 
ated dials, indicating the instrument adjustment, and with two air- 
pressure gauges, one indicating the pressure in the air-supply line 
and the other the air pressure in the motor head, which shows when 
the valve is open or closed. This information is useful to the operator 
when he is making new temperature adjustments. 
Various combination instruments can be secured for different 
services; one type consists of a recording thermometer and an air- 
operated thermostat. Additional information concerning thermo- 
stats will be found under “ Humidity in the kiln.” 
DISTRIBUTION OF ACTIVE HEATING SURFACE 
After the steam has passed through the various valves in the sup- 
ply piping, it enters the steam coils proper. If the coils are in one 
large unit, steam enters all of the pipes at the supply header at one 
time. This is a disadvantage when the kiln is operating under the 
low temperatures customary at the beginning of a run because the 
small amount of steam then required condenses on the large pipe 
surface it thus encounters before it penetrates any great distance 
into the coil; the result is uneven heating. The coils may, however, 
be divided into several smaller units, each of which can be con- 
trolled by a gate or a globe valve. Im the latter case, enough units 
should be turned on to produce, if unregulated, a kiln temperature 
only slightly in excess of that desired. Care must be exercised to 
select the active units so that the kiln will be heated uniformly 
throughout. 
STEAM TRAPS 
Steam imparts its heat mainly through condensation. The water 
resulting must be continuously removed from the heating coils; 
otherwise they fill with it and become cold. Various devices are 
employed to remove water from steam coils, and several patented 
systems are in use. For most dry kilns, steam traps that allow the 
water to escape but retain the steam are selected. They can be di- 
vided into two general classes, those depending upon temperature 
for their operation, and those depending upon the weight of the 
accumulated water. The first class is known as thermostatic and 
_ the second as gravity. Most thermostatic traps contain an operating 
bellows, commonly called a diaphragm, which is filled with a liquid, 
generally volatile, that expands suitably under heat. One end of the 
- bellows is attached to the stem of a valve, a part of the trap, in such 
a way that expansion of the bellows closes the valve, and conversely. 
The trap is connected to the lowest point in the heating system, so 
that the condensate will readily drain into it. 
The heating coils are cold and full of air, and the trap with its 
contracted bellows (diaphragm motor) is cold and open when steam 
is first turned on. The entering steam gradually displaces the air, 
which is driven out through the open trap. A certain amount of 
steam is condensed, and the resulting hot water, which is far heavier 
than the obstructing air, flows to the trap and then through it. 
Warmed by the water, the trap partly closes, because of the expan- 
sion of the liquid in the bellows, but remains slightly open until all 
32006°—29——_2 
