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
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