14 BULLETIN" 1136, U. S. DEPARTMENT OF AGRICULTURE. 
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 traps 
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, allowing the coils to heat uni- 
formly and thoroughly in a minimum time. 
Several types of traps used on dry kilns are operated by the weight 
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 the 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 
