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4.9.1
Flue gas recirculation line sizing
The Tab. G can be helpful to correctly size the FGR pipes taking
flue gases from boiler stack base up to the burner intake port.
Diameter of FGR pipe
3"
4"
6"
8"
To account for the flow resistance at 90° elbows of the flue gas cir-
cuit, the following equivalent straight pipe length values can be
considered, and shall be deducted from the maximum pipe length
values indicated in Tab. G to obtain the maximum length of FGR
pipe development in the considered instance:
Size of pipe
3"
4"
6"
8"
Exemple for RS 500/EV FGR:
➤ foreseen overall pipe development: 80'
➤ foreseen number of bends: 3
Start considering a 6" pipe.
In Tab. G you see that maximum allowed length is 100', then we
must take off 3 x 15' = 45' to account for bends. We then obtain a
maximum pipe value of 55'.
You understand that pipe diameter is too small so we must choose
a 8" diameter.
Maximum allowed pipe length
FGR = 250'- 3 x 15' = 205' that is < 80' so it is fine.
4.9.2
Calculating the percentage of recirculated flue
gas
As a general rule, recirculated flue gas quantity must be adjusted
so as to recirculate the smallest quantity necessary to obtain the re-
quired Nox rate.
Adjustment is carried out through the throttle valve located on FGR
pipe. It is necessary to consider that too high a quantity of recircu-
lated flue gas could lead to flame instability and excessively high
CO rate. To calculate the % of recirculated flue gas, use the formu-
la below: % IFGR= (CO
)/(CO
R
2
Where:
➤ (CO
) is the percentage of CO
R
2
pling
➤ (CO
) is the percentage of CO
f
2
20037075
RS 300/EV FGR
RS 400/EV FGR
10'
30'
200'
-
Pipe fittings
90° elbow
45° elbow
7,5'
4'
10'
5,5'
15'
8'
20'
11'
) x 100.
f
2
measured at the burner cou-
2
measured at the stack
2
Installation
Maximum allowed pipe length (considering smooth steel pipes) is
specified for every burner and pipe diameter.
To prepare the Tab. G a flue gas maximum temperature of 500°F
was considered.
RS 500/EV FGR
-
-
20'
16'
150'
100'
-
250'
Tab. H
28
GB
RS 650/EV FGR
RS 800/EV FGR
-
10'
80'
200'
150'
-
-
60'
Tab. G
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