Fgr Duct System - Riello DB 2 LSE FGR Installation, Use And Maintenance Instructions

4.12

FGR duct system

– Normally the duct would connect to the stack as shown in
Fig. 23, with a 45° cut facing the flue gas flow and with the
center of the cut centered in the stack.
The duct could be made to the flue gas duct of the boiler, but
must still be located with the same 45° cut facing the flue gas
flow stream and with the center of the cut in the centre of the
stream.
– The duct should be routed in a manner that has the minimum
number of elbows and provides for the normal expansion and
contraction of the piping.
Long duct runs can change length by several centimeters
and can put an extreme load on the connecting points that
could cause component failures.
The design must include offsets that will allow for the required
movement of the piping without undue force on the burner or
stack.
– Duct expansion and contraction can be managed by using
two relatively long duct runs that are 90° apposed to each
other.
A small movement in the angle between these two legs will
provide the space needed to absorb the expansion and con-
traction. The ends of the FGR duct must be securely attached
to allow this to work properly, and prevent high loads from be-
ing applied to the burner or stack.
– A condensation drip leg must be provided upstream of the
FGR control valve and the FGR shutoff valve (if used). There
must be sufficient condensate drip legs and catch space (vol-
ume of drip legs) to prevent the condensation from flowing
through the control valves and into the fan.
In cases of heavy condensation, a condensate drip leg may
3
6
5
2
10
8
11
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Key (Fig. 23)
1 Primary gas supply inlet
2 Inducted FGR modulating damper
3 Flue gas recirculation pipe
4 Boiler stack
5 Air/flue gas mixing chamber
6 Burner
7 Boiler
8 Drain valve (manual ball valve, stainless steel)
9 Drain line
10 Condensate trap
11 FGR fan
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1
10
8
Installation
be required on the bottom of the duct 8)(Fig. 23), to remove
condensate.
Uncontrolled condensation can cause premature
failure of the control valves, fan and motor.
Adequate means must be provided to remove
condensation from the system.
CAUTION
Cold start-up will generate significant amounts of
condensation.
– Determine if pipe reducers are needed for the connection to
the FGR control valve and the FGR shutoff valve.
– The duct must be properly supported, handling both the
weight of the duct and to control the thermal expansion and
contractions. The supports may need to be anchored to pro-
vide this stability in the FGR duct.
– The FGR duct 3)(Fig. 23) is normally made from DN200 pipe
because it is easily obtainable and inexpensive.
DN150 pipe can also be used for this application.
– The duct components must be seal welded, flanged or
screwed together to provide flue gas tight duct.
Flue gas leakage into the duct will prevent the system from
working properly. It is sufficient to only inspect the welds for
a proper seal or gaskets, they do not need to be leak tested.
The flue gas recirculation ducts must be made of
stainless steel to prevent corrosion due to con-
densation.
WARNING
4
8
9
20 GB
Fig. 23