Page 51 - Handbook of Energy Engineering Calculations

Related Calculations. In the above procedure it is assumed that the carbon is
burned in dry air. Also, the nitrogen coefficient of 3.78 used in the chemical
equation in step 1 is based on a theoretical composition of dry air as 79.1
percent nitrogen and 20.9 percent oxygen by volume, so that 79.1/20.9 =
3.78. For a more detailed description of this coefficient see Section 3 of this

handbook.

DETERMINATION OF THE SAVINGS PRODUCED BY

PREHEATING COMBUSTION AIR

2
2
A 20,000 ft (1858 m ) building has a calculated total seasonal heating load
of 2,534,440 MBH (thousand Btu) (2674 MJ). The stack temperature is
600°F (316°C) and the boiler efficiency is calculated to be 75 percent. Fuel
oil burned has a higher heating value of 140,000 Btu/gal (39,018 MJ/L). A
pre-heater can be purchased and installed to reduce the breeching discharge

combustion air temperature by 250°F (139°C) to 350°F (177°C) and provide
the burner with preheated air. How much fuel oil will be saved? What will be
the monetary saving if fuel oil is priced at $1.10 per gallon?

Calculation Procedure:

1. Compute the total combustion air required by this boiler
3
3
A general rule used by design engineers is that 1 ft (0.0283 m ) of
combustion air is required for each 100 Btu (105.5 J) released during
combustion. To compute the combustion air required, use the relation CA =

H/100 × Boiler efficiency, expressed as a decimal, where CA = annual
3
3
volume of combustion air, ft (m ); H = total seasonal heating load, Btu/yr
(kJ/yr). Substituting for this boiler, CA = (2,534,400)(1000)/100 × 0.75 =
3
3
33,792,533 ft /yr (956.329 m /yr).

2. Calculate the annual energy savings
3
The energy savings, ES = (stack temperature reduction, deg F)(ft air per
year)(0.018), where the constant 0.018 is the specific heat of air. Substituting,
ES = (250)(33,792,533)(0.018) = 152,066,399 Btu/yr (160,430 kJ/yr).
With a boiler efficiency of 75 percent, each gallon of oil releases 0.75 ×