Document 4Jemg0y7Nr8dBjzp42K0pNEQQ
American Society, of Heating and Ventilating Engineers Guide, 1929
sum of the connections, and still lower power consumption can be ob
tained by using larger branches and mains of equal area. While the rule
of thumb method of determining size of mains works very well in many
cases, yet it is always desirable to figure the mains and branches of the
proper size to give the velocity which has been found best suited to the
work to be done.
-
In certain special cases where explosive or poisonous dusts such as
aluminum buffings, grain dust, powdered sugar, or lead dust are handled,
increasing the size of the mains unduly would introduce a serious hazard.
Table 4. Cubic Feet of Air Handled Per Minute Through Average Collecting Hoods
Based on Coefficient of Orifice of 0.71 with 10 Per Cent Added for Leakage
Diameter of
Connection
Pipe In.
1
Maintained Suction--In. Water Gage
iM 2 2M 3
4
5
2 2H 3
3Yt
4
4H
5
6 7
8 9 10
38 68 107 153 209 273 345 427 614 ,, 835 1092 1381 1705
47 84 131 188 256 334 423 523 751 1023 1337 1694 2090
54 97 161 217 296 386 488 605 867 1181 1546 1953 2409
61 108 168 243 330 431 546 676 970 1322 1727 2184 2695
67 118 185 266 362 473 598 741 1062 1448 1892 2387 2959
76 136 214 306 418 546 690 854 1228 1670 2184. 2762 3410
86 153 238 343 466 609 775 955 1373 1870 2440 3091 . .'3806
The maintained resistance of the exhaust system is comppsed of, three
factors: (1) loss through the hoods; (2) collector drop; and (3). friction
drop in the pipes.
'
Collector drop in inches of water is given roughly by `the following
formula; but where possible the resistance of the .particular collector
to be used should be ascertained from the manufacturer as these resis
tances differ quite widely] r; v
;
where
Dropp
=
cV(
--V 40p0 )
-
C = a constant which depends upon the type of collector and is found to range
' :.
from 0.25 to 0.75.
.
'
V = velocity in feet per minute of air entering the collector.
Friction drop in the pipes must be computed for each section where there is a change in area or in velocity. Find the velocities in each section of pipe starting with the branch furtherest from the fan. The friction drop for these sections can be determined by reference to Table 5. Total friction loss in the piping system is the friction drop in furthest branch, plus the drop in. the various sections of the main, plus the drop
in the discharge pipe.
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Chapter XXXII--Dust, Exhaust and Collecting Systems
Table S. Frictional Resistance of Straight Conveyor Pipe To Flow of Air Per 100 Feet of Pipe
Vel. op Air
per Min.
2000 2200 2400 2600 2800 3000 3200 3400 3600 3800 4000 4200 4400 4800 5200 5600 6000
Loss of Pressure in Inches for Given Diameter Pipe
4' '
1.92 2.32 2.77 3.26 3.76 4.33 4.93 S.56 6.23 6.95 7.69 8.48 9.26 11.05 13.00 15.25 17.30
5'
1.53 1.85 2.22 2.60 3.01 3.46 3.94 4.45 4.98 5.55 6.15 6.78 7.41 8.85 10.50 12.05 13.85
6'
1.28 1.55 1.84 2.17 2.52 2.88 3.28 3.71 4.15 4.62 5.13 5.65 6.18 7.38 8.66 10.05 11.52
7'
1.09 1.32 1.58 1.86 2.15 2.47 2.82 3.18 3.56 3.97 4.40 4.85 5.30 6.32 7.44 8.61 9.89
8'
0.962 ' 1.16
1.39 1.63 1.89 2.08 2.47 2.78 3.12 3.48 3.85 4.25 4.63 5.55 6.50 7.55 8.66
10'
0.770 0.932 1.01 1.30 1.51 1.73 1.97 2.22 2.49 2.78 3.08 3.49 3.71 4.43 5.21 6.03 6.92
12'
0.640 0.778 0.924 1.08 1.26 1.44 1.64 1.85 2.08 2.32 2.57 . 2.83 3.09 3.69 4.34 5.05 5.76 .
14' 16' 18' 20' 22' 24' 30'
2000 2200 . 2400 2600 2800 3000 . 3200 3400 3600 3800 4000 4200 4400 4800 5200 5600 6000
-0.550 0.655 0.790 0.930 1.07 1.24 1.41 1.59 1.78 1.99 2.20 2.43 2.66 3.17 3.72 4.32 4.95
0.482 0.582 0.693 0.810 0.932 1.08 1.23 1.43 1.56 1.74 1.92 2.12 2.33 2.77 3.25 3.78 4.33
0.428 0.578 0.617 0.722 0.838 0.961 1.09 1.24 1.38 1.54 1.71 1.88 2.06 2.46 2.89 3.35 3.85
0.385 0.465 0.553 0.650 0.754 0.865 0.985 1.11 1.25 1.39 1.54 1.70 1.85 2.22 2.61 3.02 3.46
0.350 0.423 0.504 0.590 0.685 0.788 0.895 1.01 1.13 1.26 1.40 1.54 1.68 2.02 2.36 2.74 3.14
0.320 0.388 0.462 0.542 0.628 0.722 0.820 0.925 1.04 1.16 1.28 1.42 1.54 . 1.85 . 2.16 2.52 2.89
0.257 0.310 0.369 0.434 0.503 0.577 0.657 0.742 0.832 0.926. 1.03 1.13 1.24 1.48 1.75 2.01 2.31 '
FRICTIONAL RESISTANCE OF ELBOWS Elbows having a throat radius equal to the pipe diameter, set up a resistance equivalent to a section of straight pipe approximately 10 diameters long. With a throat radius of 1M times the diameter the resistance is about the same as seven diameters of straight pipe.
SELECTING THE FAN
Having determined the volume of air' and static head required, the size of exhaust fan, speed and horsepower can be'found by reference to the manufacturers' performance tables or charts covering the type of exhaust fan selected.
The usual types of ventilating fans are unsuitable for exhaust systems which are required to handle materials such as shavings, sawdust, emery dust, etc.- Higher pressures are required than in ventilating work and in addition housings and blast wheel must be so constructed that the materials handled do not deposit in same. While the fans used in different
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