44 
BULLETIN 1373, U. S. DEPARTMENT OF AGRICULTURE 
elbows from a common supply branch line (fig. 33). In an installa- 
tion of this nature, very little overhead space is required to maintain 
a constant air velocity over the entire area of the hood opening. 
For a hood opening 42 inches long a single hood would be 42^ inches 
high to an 8-inch pipe connection; 
twin hoods are only 1S% inches to 
a 6-inch pipe connection, a reduc- 
tion in height of 23% inches. 
INSPECTION TRAPS 
Properly designed traps in the 
branch lines serving dust-collecting 
hoods place the application of suc- 
tion under the supervision and con- 
trol of an inspector, thus removing 
the suspicions usually encountered 
in applying suction. 
The design of an inspection trap 
is similar to that of the large grain 
trap used in the main trunk lines of 
floor-sweep systems. The cross- 
sectional area of the expansion 
chamber should be at least 12 times 
as great as the area of the branch 
pipe in order to insure a velocity* 
sufficiently low to permit the heavier 
material to fall to the bottom of the trap. The inspection trap, how- 
ever, does not have a continuous discharge as in the case of the floor- 
sweep grain trap. The material separated remains on the bottom of 
the trap to indicate to the inspector the nature of the products 
Fig. 33. 
-Hood suggested for use over 
belt 
42-inch 
Side — Docr £nd 
Fig. 34.— Inspection trap designed by the Underwriters Laboratories 
