﻿KILN DRYING HANDBOOK. 11 



nee the accuracy of the instrument, especially in the case of the 

 lercury-filled and gas-filled types. In these the record is influenced 

 y the bulb temperature, the tube temperature, and the case tempera- 

 iire. Variations in any one of the three will change the reading of 

 be instrument, except when compensation is made for variations in 

 ase temperature. The vapor-filled instrument is nearly free from 

 bis particular defect, since the bulb is partially filled with a volatile 

 iquid, and the pressure of gas or vapor in the tube and the capsule 

 3 virtually the vapor pressure of the liquid in the bulb at the bulb 

 Bmperature. If the bulb is large and filled with the proper amount 

 f liquid, the thermometer is practically free from errors due to case 

 nd tube temperatures, and this type is recommended for dry-kiln 

 rprk. 



Charts recording temperature for one- week periods are satisfactory 

 or most purposes. It is desirable to use charts at least 10 inches in 

 iameter. The divisions on the charts of most vapor-filled instru- 

 lents are not uniform, because the vapor pressure does not vary in 

 irect proportion with the temperature. The divisions spread as 

 he temperature rises. This drawback has been overcome by intro- 

 ucing a cam movement which compensates for the lack of uni- 

 ormity and produces a uniform pen movement. 



The temperature in the kiln is controlled by the use of auxiliary 

 pparatus, such as valves and thermostats. The pipe leading from 

 tie steam main to the kiln is almost always provided with a simple 

 lobe or gate valve, by which the steam supply to the kiln may be 

 urned on or shut off. This valve can also be used for hand control 

 f the temperature in case no other means is available. The pressure 

 a the steam main is usually higher than necessary to furnish the 

 .esired temperature in the kiln, and it then becomes desirable to 

 'lace a reducing valve (Pis. Ill and IV) between the steam main 

 nd the kiln. With this the pressure may be reduced to ahnost any 

 esired point; the variations in this reduced pressure are less than 

 hose in the high-pressure main. If the pressure reduction is very 

 ;reat, from 100 pounds down to 1 or 2 pounds per square inch, it 

 lay be necessary to install two reducing valves in tandem, the first 

 ne reducing to perhaps 10 pounds and the second making the final 

 eduction. In an installation of this kind a steam receiver or a couple 

 f lengths of pipe should be placed between the two reducers to pro- 

 ide a cushion, and thus prevent the first reducer from chattering, 

 deducing valves should always be so installed that they can be 

 eadily removed for repairs. Whenever a battery of kilns is run 

 ►art time on exhaust steam and part time on live steam it is very 

 Lesirable to have a reducer between the boilers and the exhaust-steam 

 nam to the kihis, so that the live steam may be supplied to this main 

 t about the exhaust pressure. Steam-pressure gauges should inva- 

 iably be provided so that the operator may always know just what 

 •ressure he has available. 



The intelligent manipulation of reducing valves assists materially 

 n maintaining good temperature control. The pressure to the kilns 

 nay be so adjusted that it is barely sufficient to keep the desired 

 emperature with the steam-control valve wide open. Excessive 

 emperature rises may thus be prevented and the coils kept full of 

 team most of the time. Under hand control this arrangement is 

 musually sensitive, since a comparatively large change in the setting 



