Combustion air blowers are normally rated in terms of
standard cubic feet per hour (scfh) of air.
An example calculation follows the information tables
below:
Required Calculation Information
Table 3.3 Fuel Type
Description
Total system heat input
Number of burners
Type of fuel
Gross heating value of fuel
Desired excess air
percentage
(Typical excess air
percentage @ high fire is
15%)
Air/Gas ratio
(Fuel specific, see table
below)
Air flow
Gas flow
Fuel Gas Heating Values
Table 3.4 Fuel Type
Stoichiometric*
Fuel Gas
Air/Gas Ratio
α (ft
Natural Gas
(Birmingham, AL)
Propane
Butane
*Stoichiometric: No excess air: The precise amount of air
and gas are present for complete combustion.
Application example:
A designer of a spray washer has determined the heat
input for the water tank requires 857,500 Btu/h. Based on
the size of his tank, he has selected a tube efficiency of
70% which results in a gross burner input of 1,225,000
Btu/h.
Calculation example to determine the air flow
requirement:
a. Decide which ImmersoJet model is appropriate:
• From the capacity table, either the 4" with a
remote blower (1,800,000 Btu/h), or the 6" with
the low pressure packaged blower (2,000,000
12
Unit of
Formula
Measure
Symbol
Btu/h


3
Btu/ft
percent

scfh
scfh
V
Gross Heating
Value
3
3
q (Btu/ft
/ft
)
air
gas
9.41/1
1002
23.82/1
2572
30.47/1
3225
Btu/h)
example, the designer selects the 4" tube
because his tank size limits the amount of the
larger 6" tube that will fit.
• Select an IJ004, 4" diameter tube ImmersoJet
burner with a remote blower for a maximum
firing rate of 1,225,000 Btu/h.
b. Calculate the required gas flow:
V
gas
Q

• Gas flow of 1,223 ft

c. Calculate the required stoichiometric air flow:
q
V
airstoichiometric
%
• Stoichiometric air flow of 11,508 scfh required
d. Calculate the final blower air flow requirement
based on 15% excess air at high fire:
α
V
air
V
air
•
gas
requirement is 13,234 scfh at 15% excess air.
NOTE: It is common practice to add an additional 10% to
the final blower air flow requirement as a safety margin.
3. Find the blower model number and motor
horsepower (hp).
3
)
With the output pressure and the specific flow,
you can find the blower catalog number and the
motor hp in Bulletin 610.
4. Select the other parameters:
• inlet filter or inlet grille
• inlet size (frame size)
• voltage, number of phases, frequency
• blower outlet location, and rotation direction
Clockwise (CW) or Counter Clockwise (CCW
NOTE: The use of an inlet air filter is strongly
recommended. The system will perform longer and the
settings will be more stable.
have
sufficient
= Q/q = 1,225,000 Btu/h/1,002 Btu/ft
3
=1,223 ft
/h
3
/h is required.
= a (air/gas ratio) x V
3
= 9.41 x 1,223 ft
/h = 11,508 ft
= (1 + excess air %) x V
= ( 1 + 0.15) x 11,508 ft
For this example, final blower air flow
capacity.
For
this
3
gas
3
/h
airstoichiometric
3
3
/h = 13,234 ft
/h
3200069—01