Page 260 - Mechanical Engineers' Handbook (Volume 4)

8 Heat Transfer 249

it becomes 0.87 116,347/708 143. Values of R for use with Figs. 23 and 24, will vary
correspondingly (R k/4H).

8.14 Convection Heat Transfer
Heat transferred between a moving layer of gas and a solid surface is identified by ‘‘con-
vection.’’ Natural convection occurs when movement of the gas layer results from differen-
tials in gas density of the boundary layer resulting from temperature differences and will
vary with the position of the boundary surface: horizontal upward, horizontal downward, or
vertical. A commonly used formula is
H 0.27(T T ) 0.25
c
g
s
2
where H Btu/hr ft F
c
T T temperature difference between gas and surface, in F
g
s
Natural convection is a significant factor in estimating heat loss from the outer surface
of furnace walls or from uninsulated pipe surfaces.
‘‘Forced convection’’ is heat transfer between gas and a solid surface, with gas velocity
resulting from energy input from some external source, such as a recirculating fan.
Natural convection can be increased by ambient conditions such as building drafts and
gas density. Forced convection coefficients will depend on surface geometry, thermal prop-
erties of the gas, and Reynolds number for gas flow. For flow inside tubes, the following
formula is useful:
k
H 0.023 Re Pr Btu/hr ft F
0.8
0.4
2
c
D
where k thermal conductivity of gas
D inside diameter of tube in ft
Re Reynolds number
Pr Prandtl number
Forced convection coefficients are given in chart form in Fig. 28 for a Prandtl number
assumed at 0.70.
For forced convection over plane surfaces, it can be assumed that the preceding formula
will apply for a rectangular duct of infinitely large cross section, but only for a length
sufficient to establish uniform velocity over the cross section and a velocity high enough to
reach the Re value needed to promote turbulent flow.
In most industrial applications, the rate of heat transfer by forced convection as a func-
tion of power demand will be better for perpendicular jet impingement from spaced nozzles
than for parallel flow. For a range of dimensions common in furnace design, the heat-transfer
coefficient for jet impingement of air or flue gas is shown in Fig. 29, calculated for impinge-
ment from slots 0.375 in. wide spaced at 18–24 in. centers and with a gap of 8 in. from
nozzle to load.
Forced convection factors for gas flow through banks of circular tubes are shown in the
chart in Fig. 30 and for tubes spaced as follows:

A: staggered tubes with lateral spacing equal to diagonal spacing.
B: tubes in line, with equal spacing across and parallel to direction of flow.
C: tubes in line with lateral spacing less than half longitudinal spacing.
D: tubes in line with lateral spacing over twice longitudinal spacing.