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4.10.1 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
RLS 300/EV FGR
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 RLS 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 60', 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.
4.10.2 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
20037168
RLS 400/EV FGR
10'
-
30'
20'
200'
150'
-
-
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.
RLS 500/EV FGR
-
16'
100'
250'
Tab. H
30
GB
RLS 650/EV FGR
RLS 800/EV FGR
-
10'
80'
200'
-
-
60'
150'
Tab. G
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