Document 6bznnZy9KZw4J6jpm3EY136Xm
698 CHAPTER 31
... For uninsulated non-metallic-ducts, l
U = lxl 7i + k+ To
1952`Guide (21)
where
T/ = overall coefficient of heat transfer, Btu per (hour) (square foot) (Fahren-
heitjiegree)..
'
fi 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* Rolled to.Weight
Weight per Square Foot
Thickness3, Inches
Nearest Gage Nol
; Ounces'
10 12 ' 14 16 18 20 24 28 32 36 40 44
AS 56 64
Pounds
0.625 0.750 0.875 1.000 1.125 1.250 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
14 Ut *6
34 " x933**4zt
*
&%
B.&&
27 26 25 23 22 21 20
1197
16 15 15 U 13 11
Stabs
29 27 26 24 23 22 21 20 19 18 . 17 17 : 16; 15 14
U.S.SO.
29 28 26 25 24 23 22 20 19 18 1 17 17 16 14 13
Variation* from these weights most be Expected in practice.-.
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:
0.32 /i - Do.,o
(22)
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: .
699
Q. = UPl
where '
Qw = 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. tt = temperature of air leaving duct, Fahrenheit, degrees. .; U = temperature of air surrounding duct, Fahrenheit' degrees.
The heat given up by the air in the duct is: .
where
Q. = 0!2W(f, - f,) =
-A)
to = weight of air through duet,' 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 pounds per cubic foot.
Equating (23) and (24):
tl -t t. -- 2- 28.8A Vnip
h + tt
UPl
(23)
(24) is measured
Let y =
for rectangular ducts, and'--for round ducts
and solve for t\ and fe:
,
tl = ttly + l) 21,
(y - l)
(25)
fc =
(y + 1)
(26)
For low velocities and long ducts of small, cross-section, a somewhat
more accurate formula may be used as follows:
-
tl = + it
(27)
where
UPL * 14.4Ap,,Vm e -- Naperian base of logarithms = 2.718.
In using Equations 25, 26, and 27, one of the duct air temperatures will be unknown and will be obtained by substitution of the , other known or assumed 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, ranging from about 70 to 90 F, will be found in Table 2 of Chapter 9. For cases where the mean temperature is other