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808
CHAPTER 31
1957 Gui&re
Table 9. Weights of Black and Galvanized Sheets
U. S.
Gaob
Black Sheets
Approximate Thickness, In.
oieei
Iron
Weight Per Square Foot
Ounces Pounds
Galvanized Sheets*
.. *
Approximate Thickness, In.
Steei
Iron
Weight Per1. Square Foot
Ounces Pounds
30
0.0123
0.0125
8
0.500 > 0.0163
0.0165
1C.5
0.656'
28
0.0153
0.0156
10
0.625
0.0193
0.0196
12.5
0.781
26
0.01S4
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
04306
0.0313
20
1.250
0.0346
0.0353
22.5
1.408
20
0:0368
0.0375
24
1.500
0.0408
0.0415
26.6
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 5.156r 5.781
* Galvanized aheeta are gaged before galvanizing and are therefore approximately 0.001 in. thicker.
Table 10. Weights and Thicknesses of 2S Aluminum (Density 0.098 lb/cu in.)
B. & S. Gaoe
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 11. Weights and Thicknesses of Standabd Copper Sheets* Rolled to Weight
Weight peb Square Foot
Thickness, Inches
Nearest Gaob No.
Ounces
Pounds .
Decimal Equivalent
Nearest Fraction
B. & S.
Stubs
U. S. Std.
10
0.625
0.0135
Hi
12
0.750
0.0162
Hi
14
0.875
0.0189
16
1.000
0.0216
Hz
18
1.125
0.0243
Hz
27 29 29 26 27 28 25 26 26 23 24 25 22 23 24
20
1.250
0.0270
Hz
24
1.600
0.0324
Hz
28
1.760
0.0378
Ht
32
2.000
0.0432
H.
36
2.260
0.0488
<
21 22 23 20 21 22 19 20 20 17 19 19 16 18 18
40
2.500
0.0540
Hi
44
2.750
0.0594
H
48
3.000
0.0848
Hi
66
3.500
.0.0756
Hi
64
4.000
0.0864
Hi
15 17 17 15 17 17 14 16 16 13 15 14 11 14 13
--
Variations from these weights must be expected in practice.
Air Duct Design
809
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,
but also on long duct runs within conditioned space. Proper insulation
will remedy this situation considerably, but sometimes a redistribution of
the supply air is necessary in order to compensate for the heat exchanges
that occur.
The heat loss from a given length of duct can be expressed by:
Q. = UPl [(^p) - <]
<14>
where
Q. = heat loss through duct walls, Btu per hour. P -- perimeter of duct, feet. 2 = length of duct, feet. ii = temperature of air entering duct, Fahrenheit. U = temperature of air leaving duct, Fahrenheit.
= temperature of air surrounding duct, Fahrenheit.
To obtain the temperature drop in warm air for a given distance of trans mission, or the temperature rise if the duct carries air cooler than the room through which it passes, the following formulas can be used:
= tr(y + 1) - 2U (y - l)
ti(y -- 1) +2It
(y + i)
(16)
where
y -- --------- - for rectangular ducts or, XJPl
y = 7--.2--DV--p for round ducts,
A = cross-sectional area of duct, square feet.
p = density of air, pounds per cubic foot.
y = mean velocity of fluid, feet per minute.
D = diameter of round duct, feet.
In using Equations 15 and 16, one of the duct air temperatures will be unknown and will be obtained by substitution of the other known or as sumed values.
. Heat loss coefficients for insulated ducts with various conductivities are given in Fig. 16. The conductivities of various materials, which are based on mean temperatures, about 70 F, will be found in Table 4 of Chapter 9. For cases where the mean temperature is other than that at which the test was conducted, a correction should be made. However, in most cases the effect of this factor will be small and may be neglected.
Example 7: Determine the entering air temperature and heat loss for a duct 24 X