﻿12 BULLETIN 1136, U. S. DEPABTMENT OF AGRICULTURE. 



of the hand valve makes only a small change in the amount of steam 

 supplied. If the kiln is provided with automatic control, the con- 

 trol valve will usually be located next to the kiln. The use of auto- 

 matic control valves is recommended for practically all kinds of 

 kiln drying, because a more even temperature may be maintained, 

 injury from excessive temperatures avoided, and loss of time from 

 unnecessarily low temperatures prevented. 



AUTOMATIC TEMPERATURE CONTROLS. 



There are two classes of automatic temperature control in common 

 use in dry kilns. These are known as self-contained and auxiliary 

 operated. The self-contained thermostats are operated by means of 

 the direct pressure of vapor or liquid upon the valve stem. The 

 action is very similar to that of the recording thermometers already 

 described. A large bulb in the kiln is connected by means of a 

 capillary tube to a diaphragm or capsule in the head of the valve 

 located in the steam line. The temperature variations in the kiln 

 change the pressure inside the bulb, which in turn causes correspond- 

 ing pressure changes in the capsule. This results in the opening and 

 closing of the valve, the stem of which bears upon the capsule. The 

 valve itself is usually of balanced type to provide ease of movement. 

 A counter force or pressure is provided by means of an adjustable 

 spring or sliding weights, and the instrument is set for the desired 

 temperature by changing the tension of the spring or the position of 

 the weights. (See PI. V.) The principal advantages of the self- 

 contained thermostat are that no auxiliary source of power is re- 

 quired for its operation and that the first cost is comparatively small. 

 This type is not so sensitive as the auxiliary operated type. The 

 manufacturers claim regulation within 2° of the temperature for 

 which the instrument is set, but in kiln operation the variation is 

 often much greater than that. The auxiliary operated instruments 

 are supposed to control with a variation of only 1° and in kiln 

 operation usually maintain this accuracy. 



The auxiliary operated instruments using air as the operating 

 medium are usually provided with a small bulb inserted in the kiln 

 and connected to a capsule in the instrument by means of a capillary 

 tube. (See PI. VI.) The movement of the capsule top in response 

 to temperature changes in the kiln is transmitted to a small valve 

 connected on one side to a supply of air compressed to 15 pounds 

 pressure and on the other side to a diaphragm-motor valve on the 

 steam main. Sometimes a bimetallic sj'stem is used in place of the 

 capsule to operate the air valve. This small air valve is so arranged, 

 in instruments using direct-acting diaphragm valves, that as the 

 temperature rises, air pressure is admitted to the head of the dia- 

 phragm-motor valve. This forces the diaphragm down, which closes 

 the valve and shuts the steam off from the kiln. As the temperature 

 falls, the air pressure is shut off. and a means of escape is provided 

 for the air in the valve head. The valve then opens through spring 

 action and admits steam to the kiln. Eeverse-acting diaphragm 

 valves are so constructed that the air pressure opens them and the 

 springs close them. The air valve must be modified accordingly. 



The advantage of the reverse-acting type is that a failure of the 

 air supply causes the valves to shut, which prevents a dangerous 

 rise in temperature. The same effect may be secured in a battery of 



