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Combustion Data and Basic Calculations
For more detail and expanded information on multiple fuels, see North American Combustion Handbook, Vol. 1, B&W Steam book, and Perry’s Chemical Engineers’ Handbook.

FINDING EXCESS AIR BASED ON STACK OXYGEN

Shortcut to find/check approximate level of excess air %:

Excess air % = 90 (O2%/(21-O2%)) where O2% is dry basis

Example:  Excess air at 11.1% O2 = 90(11.1/(21-11.1)) = 100%

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

 COMPONENT Mol Weight HHV Btu per lb fuel lb air reg'd per lb fuel Methane 16 23,896 17.3 Ethane 30 22,282 16.2 Propane 44 21,523 15.7 Hydrogen 2 60,991 34.3 Carbon Monoxide 28 4,323 2.42 Carbon Dioxide 44 -- -- Oxygen 32 -- -- Nitrogen 28 -- --

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

HEAT EXCHANGER EFFECTIVENESS

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 