Page 224 - Mechanical Engineers' Handbook (Volume 4)
P. 224
3 Furnace Types 213
Table 1 Symbols and Abbreviations
A area in ft 2
a absorptivity for radiation, as fraction of black body factor for receiver temperature:
combustion gases
a g
furnace walls
a w
load surface
a s
combined emissivity–absorptivity factor for source and receiver
a m
C specific heat in Btu/lb F or cal/g C
cfm cubic feet per minute
D diameter in ft or thermal diffusivity (k/dC)
d density in lb/ft 3
e emissivity for radiation as fraction of blackbody factor for source temperature, with
subscripts as for a above
F factor in equations as defined in text
fpm velocity in ft/min
2
G mass velocity in lb/ft hr
2
g acceleration by gravity (32.16 ft/sec )
2
H heat-transfer coefficient (Btu/hr ft F)
for radiation
H r
for convection
H c
r
H t for combined H H c
HHV higher heating value of fuel
h pressure head in units as defined
k thermal conductivity (Btu/hr ft F)
L length in ft, as in effective beam length for radiation, decimal rather than feet and inches
LHV lower heating value of fuel
ln logarithm to base e
MTD log mean temperature difference
N a constant as defined in text
psi pressure in lb/in 2
psig, pressure above atmospheric
psia, absolute pressure
Pr Prandtl number ( C/k)
Q heat flux in Btu/hr
R thermal resistance (r/k) or ratio of external to internal thermal resistance (k/rH)
Re Reynolds number (DG/ )
r radius or depth of heat penetration in ft
T temperature in F, except for radiation calculations where S ( F 460)/100
T , combustion gas temperature
g
T , furnace wall temperature
w
heated load surface
T , s
T , c core or unheated surface of load
t time in hr
viscosity in lb/hr ft
wc inches of water column as a measure of pressure
V volume in ft 3
v velocity in ft/sec
W weight in lb
X time factor for nonsteady heat transfer (tD/r )
2
x horizontal coordinate
y vertical coordinate
z coordinate perpendicular to plane xy