26 BULLETIN 894, U. S. DEPARTMENT OF AGRICULTURE. 
This allows a velocity of about 25 feet per second and is used for 
low-pressure or exhaust systems. 
Rule II* — Allow a constant of 0.375 square inch per 100 square feet heating surface 
in coils for exhaust or low-pressure steam. Allow a constant of 0.19 square inch per 
100 square feet heating surface in coils for high -pressure steam. 
CONDENSATION RETURN. 
The water of condensation from the heating coils contains a 
considerable amount of heat, and the question of saving this heat 
by returning the water to the boiler is one of economy and efficiency. 
This question can only be decided by the engineer or architect 
designing the installation, who will be familiar with local conditions, 
such as cost of water, distance between kilns and boiler, whether 
gravity or pump return to receiver, cost of installing return pumps, 
traps, receivers, etc. 
The return from each unit pair of coils should be drained separately 
through a swing check valve and a globe valve, after which the drains 
ma} r be connected and extended to a separate trap for each kiln. The 
ceiling coil must drain into a trap separate from that of the heating 
coils. The proper size of return pipes may be found in Table 9. 
TRAPS. 
There are a great many traps on the market which will give quite 
satisfactory results when properly installed, and the type selected 
must depend upon the conditions occurring in each individual case. 
For low-pressure systems, thermostatic, ball, float, bucket, and tilt 
traps of the nonreturn type are used; while for high pressures the 
tilt trap gives the most satisfaction, though bucket traps may be 
used. 
All traps give more or less trouble, on account of their becoming 
choked with scale, dirt, or other foreign matter, and require frequent 
inspection. In this respect the tilt trap is superior to the others; 
for it tilts at each discharge so that the operator can determine at a 
glance whether it is working properly. This type of trap is more 
easily cleaned than most other types. However, more height is 
required between the coils and waste for this trap than for other 
types, and it should be used only where at least 1| feet of height is 
available between condensation return and floor. 
If the condensation water is to be wasted, the traps for low- 
pressure systems should be connected to the sewer in the most con- 
venient manner, preferably through a small basin. If the condensa- 
tion is to be returned to the boiler, the traps are connected to a re- 
ceiving tank or pump. 
If a thermostat is used on the steam supply, the trap must have a 
check valve and discharge under a water seal; otherwise air will be 
4 From Van Nostrand's Science Series No. 68. 
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