18 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE 
the air and the water have been forced out and steam starts blowing 
through. The temperature of the steam, higher than that of the 
water, causes the bellows to expand further, enough to close the 
trap completely. A screw adjustment to set the trap for various 
-steam temperatures is necessary, since the temperature of saturated 
steam increases with the pressure. After the trap has closed, con- 
densed steam accumulates back of it until the bellows cools and conse- 
quently contracts enough to allow the trap to open again, thus 
discharging the condensate. It is desirable to place the thermo- 
static traps and air valves in the operating room rather than in the 
hot kiln, since they will then be under closer supervision and, on 
account of cooling more quickly, will be more responsive. 
Thermostatic traps are used only on heating systems operating 
under compartively low steam pressures—say, below 20 pounds per 
square inch. When the system is broken up into a number of units, 
a thermostatic trap is sometimes fitted to each unit. ; 
Gravity traps are of various types, such as tilt, fioat, and bucket. 
Of these, the bucket is far and away the most popular type of dry- 
kiln trap; it 1s suitable for high and low pressure systems, can be 
made rugged, and is in general reliable and easy to repair. (PI. 8.) 
The special kind known as a return trap is perhaps most suitable for 
the occasional cases in which the condensate is to be returned to the 
boiler direct from the trap. Most return traps are the tilt type, 
that is, the whole body of the device tilts up and down as it empties 
and fills, and this tilting opens and closes the valves that control 
the flow of the water. 
ATR-RELIEF VALVES 
Under certain circumstances the presence of air in the steam coils 
may prevent the steam from filling them, a situation that results in 
uneven and insufficient heating. Coils in this condition are said to 
be airbound; the obvious remedy is to remove the air. Air binding is 
usually confined to low-pressure heating systems that are equipped 
with gravity-type steam traps. Vacuum systems, low-pressure sys- 
tems with thermostatic traps, and high-pressure systems should give 
no trouble from this cause. Adjusting the amount of active heating 
surface so that there is always some steam in the coils will avoid 
trouble from air binding. When thermostats of nonthrottling types 
are used, alr is apt to get into the pipes each time the thermostat 
shuts off the steam. The customary remedy for air binding is to 
equip the heating system with an automatic air-relief valve, which 
permits the escape of air but prevents the escape of steam. These 
valves are thermostatically operated; the expansion of a bar or of a 
liquid-filled capsule closes them when hot, and the contraction opens 
them when cold. The connection for an air valve should, in general, 
be taken off near the bottom of the system, preferably near the trap, 
and should run as high as the steam piping; the actual tap should be 
made on the top, not the bottom, of a pipe or fitting. 
VACUUM PUMPS 
Kilns operating on low-pressure steam are sometimes equipped 
with a vacuum pump for the rapid removal of air and condensate 
from the coils. One pump is sufficient for a battery of kilns, each 
