Eclipse ThermJet TJ0015 Design Manual page 20

Step 5:
Combustion Air
System: Blower and
air pressure switch
(continued)
0
2. Calculate the required flow
The blower output is the air flow delivered under standard at-
mospheric conditions. It must be enough to feed all the burners
in the system at high fire.
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:
Table 3.1: Required calculation information
D
esCriptioN
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
Table 3.2: Fuel gas heating values
f g
uel as
Natural gas
(Birmingham, AL)
Propane
Butane
Eclipse ThermJet Design Guide No. 205, 3/5/07
u
Nit of
m
easure
-
-
Btu/ft
3
percent
-
scfh
scfh
s * g
toiChiometriC ross
a /g r
ir as atio iNg
a (ft
/ft ) q(b
3 3
air gas
9.41
23.82
30.47
f
ormula
s
ymbol
-
-
q
%
a
V
air
V
gas
h -
eat
v
alue
/ 3)
tu ft
1,002
2,572
3,225