10
Eclipse ThermJet PCA, V2, Design Guide 206, 6/4/2010
Formula for calculating preheated air pressure drop
given a combustion air temperature:
h
2
= (Tabs
2
/ Tabs
1
) * h
1
h
2
= air pressure drop with preheated combustion air
h1 = air pressure drop with ambient combustion air
Tabs
2
= absolute temperature of preheated
combustion air, 460 + PCA°F (273 + PCA°C)
Tabs
1
= absolute temperature of ambient combustion
air, 460 + 60°F = 520 (273 + 15 = 288°C)
Example Static Air Pressure Required for
Preheated Combustion Air Calculation:
• ambient air temperature: 60°F
• preheated combustion air temperature: 700°F
• burner size: TJPCA0075
Tabs
1
= 60 + 460 = 520
Tabs
2
= 700 + 460 = 1160
h
1
= the ambient air pressure drop can be found
on the corresponding TJPCA Datasheet, 206-1 to
206-13. In this example the ambient air
requirement is 3.8"w.c.
h
2
= (1160/520) * 3.8 = 8.5" w.c.
The air pressure required to the inlet of the burner is
8.5" w.c.
When calculating the outlet pressure of the blower,
the total of these pressures must be calculated.
• The static air pressure required at the burner found
in the burner Datasheet, 206-1 to 206-13 (see
example above)
• The total pressure drop in the piping
• The total of the pressure drops across the valves
• The pressure in the chamber (suction or
pressurized)
Eclipse recommends a minimum safety margin of
10%
2.
Calculate the required flow.
The blower output is the air flow delivered under
standard atmospheric conditions. It must be enough
to feed all the burners in the system at high fire and
provide the eductor flow.
Combustion air blowers are normally rated in terms of
standard cubic feet per hour (scfh) of air.
An example calculation follows the information tables.
*Stoichiometric: No excess air. The precise amount of air
and gas are present for complete combustion.
Example Blower Calculation
A batch furnace requires a gross heat input of 2,900,000
BTU/hr (based on anticipated 60% efficiency with
preheated air). The designer decides to provide the
required heat input with four burners operating on natural
gas using 15% excess air.
Calculation Example:
a.
Decide which ThermJet PCA burner model is
appropriate:
• Select 4 model TJPCA0075 ThermJet burners
based on the required heat input of 725,000 BTU/hr
for each burner.
b.
Calculate required gas flow:
Table 3.1 Common Preheated Air Pressure Drop
Correction Factors
If Combustion Air
Temperature is:
Multiply 60°F
drop by:
400°F
1.65
600°F
2.04
800°F
2.42
1000°F
2.81
Table 3.2 Required Calculation Information
Description
Unit of
Measure
Formula
Symbol
Total system heat input
BTU/hr
Q
Number of burners
-
-
Type of fuel
-
-
Gross heating value of fuel
BTU/ft
3
(MJ/m
3
)
q
Desired excess air percentage
(Typical excess air percentage
@ high fire is 15%)
percent
%
Air/Gas ratio (Fuel specific,
see table below)
-
D
Air flow
scfh (Nm
3
/hr)
V
air
Gas flow
scfh (Nm
3
/hr)
V
gas
Table 3.3 Fuel Gas Heating Values
Fuel Gas
Stoichiometric*
Air/Gas Ratio
D
(ft
3
air
/ft
3
gas
)
Gross Heating
Value
q (BTU/ft
3
)
Natural Gas
(Birmingham, AL)
9.41
1,002 (40 MJ/m
3
)
Propane
23.82
2,564 (102,5 MJ/m
3
)
Butane
30.47
3,333(133,3 MJ/m
3
)
Q
(tot
a
l he
a
t input) o
f
2
,900,000 BTU/hr
4 burners
72
5,000
BTU/hr/burner
=
