﻿14 BULLETIN 113<i, U. S. DEPARTMENT OF AGRI< [JLTXJRE. 



through. The higher temperature of the steam causes a further ex- 

 pansion of the bellows and a complete shutting off of the trap. A 

 screw adjustment is necessary so that the trap may be set for vari- 

 ous steam temperatures, since the temperature of saturated steam in- 

 creases with the pressure. After the trap has closed, condensed steam 

 accumulates back of it, until the trap cools enough to allow it to 

 open and blow out. It is desirable to locate the thermostatic trans 

 and air valves in the operating room where they will be under better 

 supervision and will be more sensitive, since they will cool more 

 quickly than if they were located in the hot kiln. 



Properly installed thermostatic traps are very useful in dry-kiln 

 -work, especially on coils built in groups. One installed on each 

 group prevents trouble which arises when all the groups are oper- 

 ated by one trap. They also operate on coils which are controlled 

 by auxiliary operated thermostats, almwing the coils to heat uni- 

 formly and thoroughly in a minimum time. 



Several types of traps used on dry kilns are operated by the w r eight 

 of the water of condensation. Among these are tilt, float, and 

 bucket traps. The water of condensation flows into a receptacle 

 within the trap and by its weight or buoyancy opens a valve that 

 allows the water to be blown out, after which the valve returns to 

 its closed position. Such traps do not, as a rule, provide for the 

 escape of air from the coils, and for this reason automatic air valves 

 or hand-operated pet cocks are fitted to them. 



VACUUM PUMPS. 



Kilns are sometimes equipped with a vacuum pump for the rapid 

 removal of air and water from the coils. One is sufficient for a 

 battery of kilns, each heating coil being connected through a thermo- 

 static trap to the pump suction main. Dependence must be placed 

 on the traps, since the pump will not work properly without them. 

 Although the pump is very effective in removing air and water, 

 especially on low-pressure systems, the rapid relief obtained by it is 

 not needed in most kilns. 



HUMIDITY IN THE KILN. 



ABSOLUTE HUMIDITY. 



Humidity or water vapor in the air is the most puzzling factor 

 with which the average kiln operator must contend. The amount 

 of water vapor in the atmosphere may be expressed in terms of the 

 weight of water vapor for every unit volume of atmosphere. The 

 unit of weight used is the grain and the unit of volume the cubic 

 foot. The absolute humidity is the number of grains water vapor 

 per cubic foot. This alone is no indication of the drying capacity 

 of the air, since its capacity to hold water varies greatly with the 

 temperature. 



RELATIVE HUMIDITY. 



Air containing the total number of grains of water vapor it can 

 hold at a given temperature is saturated. The ability of air to dry 

 any substance varies with (he amount of additional moisture it can 

 hold before becoming saturated. The amount of vapor in the air 

 expressed in percentage of the amount held at saturation is called 



