﻿KILN DRYING HANDBOOK. 19 



trollers by providing the bulb or sensitive element with a suitable 

 wick and water supply. The Forest Products Laboratory has 

 used with success air-operated wet-bulb controllers of both the 

 extension bulb (vapor filled) and the bimetallic or differential 

 expansion types. Self-contained thermostats can also be used, 

 but their sensitiveness is not so great as that of the air-operated 

 instruments. Wet-bulb controllers can keep the wet-bulb tem- 

 perature constant. If the dry-bulb temperature is also kept con- 

 stant, the humidity will remain constant. If it does not, how- 

 ever, the humidity will vary, even if the wet-bulb temperature 

 is accurately controlled. To overcome this difficulty a differential 

 type of self-contained humidity control has been developed. In 

 this instrument there is a dry bulb as well as a wet bulb ; the two 

 bulbs are connected to their respective motor diaphragms on the 

 body of a balanced steam valve so that an increase in the wet- 

 bulb temperature will close the valve and an increase in the dry- 

 bulb temperature will open it. Balance between the two is se- 

 cured by a lever and sliding weights. This system provides for 

 a constant difference between the vapor pressures in the wet and 

 dry bulb motor diaphragms, no matter what the dry -bulb tem- 

 perature may be. This results in an approximately constant differ- 

 ence between the wet and dry bulb temperatures. 



A glance at the humidity table shows that, with a constant 

 difference between wet and dry bulb temperatures, even quite a 

 considerable variation in the dry-bulb temperature has but little 

 effect upon the relative humidity. 



To secure satisfactory service from wet-bulb thermostats, care 

 and attention should be given especially to the wicks, which should 

 be changed as often as they become hard and dirty. 



Humidity controllers almost without exception operate valves 

 controlling steam jets, just as temperature controllers operate 

 valves upon the heating system. The same kind of valves are 

 ordinarily used, each valve being adapted to the needs of the partic- 

 ular service it is to render. As the use of humidity controllers 

 on steam- jet lines presupposes that the humidity will always need 

 to be increased, means must be provided to insure this need. Ordi- 

 narily in ventilated kilns the fresh-air inlets and the moist-air vents 

 are open sufficiently to require continuous humidification. If 

 necessary in special cases, the controllers can be made to operate 

 dampers of various sorts, and also to control the flow of water in 

 condenser pipes. Control in the various kiln types will be con- 

 sidered more in detail later. 



Several special types of temperature and humidity-control instru- 

 ments have been designed or adapted for dry-kiln use. Among these 

 are double-duty air-operated instruments which have two sensitive 

 bulbs and extension tubes with but a single case, in which are housed 

 the capsules and air valves. These instruments c&n be used for 

 temperature and humidity control, or for temperature control and 

 the removal of condensation from the heating coils. This latter 

 use is not common in dry kilns. 



The recorder regulator' has already been mentioned under temper- 

 ature control. This air-operated instrument provides for the con- 

 trol of either wet or dry bulb temperature and for a graphic record 

 of the controlled temperature. 



