KILN DRYING HANDBOOK. 11 
ence the accuracy of the instrument, especially in the case of the 
mercury-filled and gas-filled types. In these the record is influenced 
by the bulb temperature, the tube temperature, and the case tempera- 
ture. Variations in any one of the three will change the reading of 
the instrument, except when compensation is made for variations in 
case temperature. The vapor-filled instrument is nearly free from 
this particular defect, since the bulb is partially filled with a volatile 
liquid, and 'the pressure of gas or vapor in the tube and the capsule 
is virtually the vapor pressure of the liquid in the bulb at the bulb 
temperature. If the bulb is large and filled with the proper amount 
of liquid, the thermometer is practically free from errors due to case 
and tube temperatures, and this type is recommended for dry-kiln 
work, 
Charts recording temperature for one- week periods are satisfactory 
for most purposes. It is desirable to use charts at least 10 inches in 
diameter. The divisions on the charts of most vapor-filled instru- 
ments are not uniform, because the vapor pressure does not vary in 
direct proportion with the temperature. The divisions spread as 
the temperature rises. This drawback has been overcome by intro- 
ducing a cam movement which compensates for the lack of uni- 
form^ and produces a uniform pen movement. 
The temperature in the kiln is controlled by the use of auxiliary 
apparatus, such as valves and thermostats. The pipe leading from 
the steam main to the kiln is almost always provided with a simple 
globe or gate valve, by which the steam supply to the kiln may be 
turned on or shut off. This valve can also be used for hand control 
of the temperature in case no other means is available. The pressure 
in the steam main is usually higher than necessary to furnish the 
desired temperature in the kiln, and it then becomes desirable to 
place a reducing valve (Pis. Ill and IY) between the steam main 
and the kiln. With this the pressure may be reduced to almost any 
desired point; the variations in this reduced pressure are less than 
those in the high-pressure main. If the pressure reduction is very 
great, from 100 pounds down to 1 or 2 pounds per square inch, it 
may be necessary to install two reducing valves in tandem, the first 
one reducing to perhaps 10 pounds and the second making the final 
reduction. In an installation of this kind a steam receiver or a couple 
of lengths of pipe should be placed between the two reducers to pro- 
vide a cushion, and thus prevent the first reducer from chattering. 
Reducing valves should always be so installed that they can be 
readily removed for repairs. Whenever a battery of kilns is run 
part time on exhaust steam and part time on live steam it is very 
desirable to have a reducer between the boilers and the exhaust-steam 
main to the kihis, so that the live steam may be supplied to this main 
at about the exhaust pressure. Steam-pressure gauges should inva- 
riably be provided so that the operator may always know just what 
pressure 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 
temperature with the steam-control valve wide open. Excessive 
temperature rises may thus be prevented and the coils kept full of 
steam most of the time. Under hand control this arrangement is 
unusually sensitive, since a comparatively large change in the setting 
