14 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE 
tions in the reduced pressure are less than those in the high-pressure 
main. 
Whenever a battery of kilns is run part time on exhaust steam and 
part time on live steam it is highly desirable, if not essential, to have 
a reducer between the boilers and the exhaust-steam main to the 
kilns, so that the live steam may be supplied to this main at about 
the exhaust pressure. If desired, the back-pressure valve on the ex- 
haust line and the reducing valve on the boiler line can be so adjusted. 
that the boiler line will automatically supply any deficit in the ex- 
haust steam. To accomplish this, the back-pressure valve should be 
set at a slightly higher pressure than the reducing valve; tandem re- 
ducing v alves are required with such an arrangement. Steam pres- 
sure gauges should invariably be provided on a live-and-exhaust- 
steam heating system so that the operator may always know just 
what pressures he has available. 
The intelligent manipulation of reducing valves assists materially 
in maintaining good temperature control. “The pressure to the kilns 
may be so adjusted that it is barely sufficient to keep the desired tem- 
perature with the steam-control valve wide open. Excessive tem- 
perature rises can thus be prevented, and the coils may in conse- 
quence be kept full of steam most of the time. Under hand control 
this arrangement is unusually sensitive, since a comparatively large 
change in the setting of the hand valve will then make only a small 
change in the amount of steam supplied. Hand control, of course, 
is sensitive to weather changes and the temperature of a hand-con- 
trolled kiln will fluctuate with the outside temperature unless con- 
tinual readjustment is made. 
The use of automatic control valves is recommended for practically 
all kinds of kiln drying, because with them a temperature more even 
than that possible with hand control may be maintained, injury from 
excessive temperatures may be avoided, and loss of time from un- 
necesarily low temperatures may be prevented. Automatic control 
effects material savings in steam and in time of attendance over hand 
control. 
Reducing valves should always be so installed that they can readily 
be removed for repairs. If the kiln is provided with automatic con- 
trol, the control valve will usually be placed next to the kiln. 
AUTOMATIC TEMPERATURE CONTROL 
Automatic control of temperature is secured by means of instru- 
menis known as thermostats, which regulate the amount of steam 
suppled to the kiln. Two classes of thermostats, self-contained and 
auxiliary-operated, are in common use in dry kilns. The self-con- 
tained ones combine in a single unit a motor valve and a liquid or 
vapor filled tube system comprising the bulb—which is placed in the 
kiln—the capillary connecting tube, and the motor head. The tem- 
perature variations in the kiln change the pressure inside the bulb, 
which in turn causes corresponding pressure changes in the motor 
head. This action results in a movement of the valve, the stem of 
which is connected to the motor head, which is a bellows-type dia- 
phragm. The valve itself is usually of the balanced type, to provide 
ease of movement. A constant counter pressure, tending to keep 
the valve open by opposing the varying pressure in ‘the motor head, is 
