Document 8R5Gkqd82n8M5zywBeE664jbo
726 CHAPTER 32
For uninsulated non-metallic ducts, 1
1954 Guide (21)
where U = overall coefficient of heat transfer, Btu per (hour) (square foot) (Fahren
heit degree). /i = surface conductance (inside) Btu per (hour) (square foot) (Fahrenheit
degree). /o = surface conductance (outside) Btu per (hour) (square foot) (Fahrenheit
degree).
x = thickness, inches. k -- unit conductivity of material, Btu per (hour) (square foot) (Fahrenheit
degree per inch thickness).
Table 11. Weights and Thicknesses of Standard Copper Sheets'1 Rolled to Weight
Weight per Square Foot
Thickness. Inches
Nearest Gags No.
Ounces 10
20 36
64
Pounds
0.625 0.750 0.875 1.000 1.125 1JS50 1.500 1.750 2.000 2.250 2.500 2.750 3.000 3.500 4.000
Decimal Equivalent
0.0135 : 0.0162
0.0189 0.0216 0.0243
0.0270 0.0324
0.0378 0.0432
0.0486 0.0540 0.0594 0.0648 0.0756 0.0864
.
Nearest Fraction
% Ut
'/
16 V6 V6
% V6 j5 A%
B.&S.
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.Srp.
29 28 26 25 24 23 22 20 19 18 17 17 16 14 13
Variations from these weights most be Expected in practice.
X' Where x is small and k is large, however, the factor -r is of little impor tance and may be neglected. Film conductance / i for air flowing in ducts apparently depends only on the velocity of the air and the diameter of the duct. A fairly reliable
,inside coefficient can be calculated from Schultz's modified equation: _ (22) J' ~ d0!6
where v = velocity of air in duct, feet per second. D = inside diameter of duct, feet.
Film conductance /<, depends on a number of variables including tem perature, diameter, and emissivity of the outer surface, and can be calcu lated from data in Chapter 5. From this explanation, it is seen that it is unwise to recommend a given value of U for all uninsulated metal ducts-
Air Duct Design
The heat loss from a given length of duct can be expressed by:
Q- " ^[(4^) ~ U]
where
Q* = heat loss through duct walls, Btu per hour. P = perimeter of duct, feet. I -- length of duct, feet. I, = temperature of air entering duct, Fahrenheit degrees. It = temperature of air leaving duct,. Fahrenheit degrees. It -- temperature of air surrounding duct, Fahrenheit degrees.
The heat given up by the air in the duct is:
727 (23)
where
Qw = 0.24w(f, - h) = 14.4iF^>,(l, - It)
(24)
to = weight of air through duct, pounds per hour. A -- cross-sectional area of duct, square feet. Vm = mean velocity-of fluid, feet per minute. Pv = density of air at specified temperature at which velocity Fm, is measured
pounds per cubic foot.
Equating (23) and (24):
ti "h t% -- 2g
U
28.84 UPl
Let y = ~
fr rectangular ducts, and
and solve for ti and k:
for round ducts
t\ ~ h(y + 1) -- 2ti
(y - i)
(25)
it = ti(y - 1) +2t, (y + l)
(26)
For low velocities and long ducts of small cross-section, a somewhat
more accurate formula may be used as follows:
where
, h ~~ U +. fs
(27)
UPL 14.4APvV,,' Naperian base of logarithms = 2.718.
In using Equations 25, 26, and 27, one of the duct air temperatures will n unknown and will be obtained by substitution of the other known or assumed values.
Heat loss coefficients for insulated ducts with various conductivities J='ven in Fig. 16. The conductivities of various materials, which are inTtab0lea 2meoaf nCtheamppteerra9t.ureFso, rracnagseinsgwfhroemre athbeoumte7a0ntote9m0pFe,rawtuilrlebies footuhnedr