RULES OF THUMB FOR COMBUSTION AIR ESTIMATION

For air volume:
Btu/hr input/6,000 = combustion air flow, scfm

For air mass:
765 lb/air required to burn 1 MM Btu of fuel

Example:
A 10 MM Btu/hr burner running at 0% excess air requires:
10,000,000/6,000 = 1,670 scfm air or 10 x 765 = 7,650 lb/hr air

CALCULATE ADIABATIC FLAME TEMPERATURE (AFT)

Q = M x Cp x temperature difference

Temperature difference  = Q/(M x Cp)

Example:  1 mole CH4, 16 lb X LHV 21,500 Btu/lb = 344,000 Btu

At 100% excess air,  M = 568 lb for air plus fuel

Temp diff = 344,000/(568 lb x 0.282) = 2,150 F

With 60 F base temperature at standard conditions:

Adiabatic Flame Temp = 60 + temp diff = 2,210 F

Caution:  Cp varies with gas temp and gas; Use integrated heat capacities of gases in place of specific heat via equations or tables

 

SELECTED COMBUSTION CONSTANTS

E = Ch * (Th in - Th out)/Cmin * (Th in - Tc in)

Where Cmin is the smaller of mhcph or mccpc, the hourly heat capacity

The rate of heat exchange is

Q = E Cmin * (Th in - Tc in)

Reference: F. Kreith, 3rd ed. Principles of Heat Transfer, Intext Education Publisher, NY, 1973