Page 52 - Handbook of Energy Engineering Calculations

140,000 Btu/gal = 105,000 Btu (110.8 jk). Hence, the fuel saved, FS =
ES/usable heat in fuel, Btu/gal. Or, FS = 152,066,399/105,000 = 1448.3
3
gal/yr (5.48 m /yr).
With fuel oil at $1.10 per gallon, the monetary savings will be $1.10

(1448.3) = $1593.13. If the preheater cost $6000, the simple payoff time
would be $6000/1593.13 = 3.77 years.

Related Calculations. Use this procedure to determine the potential savings
for burning any type of fuel—coal, oil, natural gas, landfill gas, catalytic

cracker offgas, hydrogen purge gas, bagesse, sugar cane, etc. Other rules of
thumb used by designers to estimate the amount of combustion air required
3
3
3
3
for various fuels are: 10 ft of air (0.283 m ) per 1 ft (0.0283 m ) of natural
3
3
3
gas; 1300 ft of air (36.8 m ) per gal (0.003785 m ) of No. 2 fuel oil; 1450 ft 3
3
3
3
of air (41 m ) per gal of No. 5 fuel oil; 1500 ft of air (42.5 m ) per gal of No.
6 fuel oil. These values agree with that used in the above computation—i.e.
3
3
100 ft per 100 Btu of 140,000 Btu per gal oil = 140,000/100 = 1400 ft per
3
3
gal (39.6 m /0.003785 m ).
This procedure is the work of Jerome F. Mueller, P.E. of Mueller
Engineering Corp.
COMBUSTION CALCULATIONS USING THE MILLION BTU
(1.055 MJ) METHOD

The energy absorbed by a steam boiler fired by natural gas is 100-million
Btu/h (29.3 MW). Boiler efficiency on a higher heating value (HHV) basis is

83 percent. If 15 percent excess air is used, determine the total air and flue-
gas quantities produced. The approximate HHV of the natural gas is 23,000
Btu/lb (53,590 kJ/kg). Ambient air temperature is 80°F (26.7°C) and relative
humidity is 65 percent. How can quick estimates be made of air and flue-gas

quantities in boiler operations when the fuel analysis is not known?

Calculation Procedure:

1. Determine the energy input to the boiler
The million Btu (1.055 MJ) method of combustion calculations is a quick