Document NNZ3yLxaeVdmLjjVgvDNJkwD
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CHAPTER 31
1958 Guide
Table 10. Weights of Black and Galvanized Sheets
u. s.
Std.
Gage
Black Sheets
Approximate Thickness, In.
Weight Per Square Foot
Galvanized Sheets*
Approximate Thickness, In.
Weight Per Square Foot
Steel
Iron
Ounces Pounds
Steel
Iron
Ounces Pounds
30
0.0123
0.0125
8
0.500
0.0163
0.0165
10.5
0.656
28
0.0153
0.0156
10
0.625
0.0193
0.0196
12.5
0.781
26
0.0184
0.0188
12
0.750
0.0224
0.0228
14.5
0.906
24
0.0245
0.0250 16
1.000
0.0285
0.0290
18.5
1.156
22
0.0306
0.0313
20
1.250
0.0346
0.0353
22.5
1.406
20
0.0368
0.0375
24
1.500
0.0408
0.0415
26.5
' 1.656
18
0.0490
0.0500
32
16
0.0613
0.0625
40
14
0.0766
0.0781
50
12
0.1072
0.1094
70
11
0.1225
0.1250
80
10
0.1379
0.1406
90
2.000 2.500 3.125
4.375 5.000 5.625
0.0530 0.0653 0.0806 0.1112
0.1265 0.1419
0.0540 0.0665
0.0821 0.1134
0.1290 0.1446
34.5 42.5 52.5
72.5 82.5
92.5
2.156 2.656
3.281
4.531 5.156 5.781
* Galvanized sheets are gaged before galvanising and are therefore approximately 0.004 in. thicker.
Table 11. Weights and Thicknesses of 2S Aluminum (Density 0.098 lb/cu in.)
B. & S. Gage
Thickness, Inches
Decimal
Nearest Fraction
Weight peb Square Foot
Ounces
Pounds
28
0.012
1/64
2.7 0.169
26
0.016
1/64
3.6 0.226
24
0.020
1/64
4.5 0.282
22
0.025
1/32
5.4 0.353
20
0.032
1/32
7.2 0.452
18
0.040
3/64
9.0 0.563
16
0.051
3/64
11.5
0.720
14
0.064
1/16
14.4
0.903
Table 12. Weights and Thicknesses of Standard Coppeb Sheets* Rolled to Weight
Weight peb Square Foot
Thickness, Inches
Nearest Gage No.
Ounces
Pounds
Decimal Equivalent
Nearest' Fraction
B. A S.
10
0.625
0.0135
12
0.750
0.0162
14
0.875
0.0189
16
1.000
0.0216
18
1.125
0.0243
20
1.250
0.0270
24
1.500
0.0324
28
1.750
0.0378
32
2.000
0.0432
36
2.250
0.0488
40
2.500
0.0540
44
2.750
0.0594
48
3.000
0.0648
56
3.500
0.0756
64
4.000
0.0864
w.
\U
Hi
Hi
H*
Su H* H*
Me M. **4
w.
* Variations from these weights most be expected in practice.
27 26 25 23 22
21 20 19 17 16
15 15 14 13 11
Stubs
29 27 26 24 23
22 21 20 19 18
17 17 16 15 14
U. S. Std. 29 26 23
17 14
' `tJ
Air Duct Design
819
adhesive tapes are available for this purpose and for sealing joints in duct work. The fan discharge connections shown in Fig. 16 are marked good, fair, and poor in the order of the amount of turbulence produced. An inspection of the heater connections shown in Fig. 16 will readily show that uniform velocity through the heater cannot be expected in the diagram noted poor. When obstructions cannot be avoided, the duct area should never be decreased more than 10 percent, and then a streamlined collar should be used. Larger obstructions require an increase in the duct size in order to maintain as nearly uniform velocity as possible. Branch take offs should always be arranged to cut or slice into the air stream in order to reduce as far as possible the losses in velocity head.
Wherever ducts pass through fire walls or connect two fire areas of a building, automatic fire dampers should be provided. For design of such dampers and other fire-protective details, see Pamphlet No. 90 of the National Board of Fire Underwriters?*
The recommended gages for steel (or iron) and aluminum sheet-metal rectangular ducts are given in Table 9. Steel or iron sheets are specified according to the manufacturers or U. S. Standard Gage System. Alu minum sheets are specified according to the American or Brown & Sharpe Gage System. Weights of black and galvanized steel and iron sheets per square foot of surface for various gages are given iii Table 10. Similar data for 2S aluminum sheets will be found in Table 11. Weights of standard copper sheets are given in Table 12. In calculating the total weight of a given length of duct work from these tables, it is customary to add 20 percent for the weight of joints and bracings.
Aluminum sheets of the 2S and 3S type alloy and % hard temper are readily workable, and can be used for practically all duct work. The 2S type (commercially pure aluminum) is suitable for all, except very large ducts. For large ducts, where more strength is desired, the 3S alloy with % or % hard temper is frequently used. The higher tempers, particularly full hard, do not have the formability of the lower tempers. For very large ducts, where considerable strength is required, aluminum sheets should be 2 gages heavier than indicated in Table 9, and should be amply stiffened. Joints can be of any of the standard designs, and can be fabri cated in the same manner as iron. Repeated sharp bending and rebending should be avoided, as aluminum has a tendency to crack under such treat ment. Aluminum of 16 B. & S. gage or heavier can readily be welded by the metallic arc or acetylene process. Soldering is difficult and is not gen erally recommended. Riveting is done in the same manner as in iron or steel sheet. Self-tapping screws tend to loosen because of the softness of aluminum.
HEAT LOSSES FROM DUCTS
In designing duct systems, the heat gains or losses of the ducts can be quite considerable, not only if the duct passes through unconditioned space,
. t also on long duct runs within conditioned space. Proper insulation will remedy this situation considerably, but sometimes a redistribution of thatSUPPly 'S necessary 'n order to compensate for the heat exchanges
The heat loss from a given length of duct can be expressed by:
(14)