11
2.4
Safety measures for preventing internal flue gas recirculation
The heat exchanger of the S-CB
+
400 & 550 boiler contains two burners. The air supply channels of both burners
are internally connected to each other, so the installer only needs to connect one air supply duct to the boiler. The
controller allows the situation in which only one burner is burning, while the other one is off. When in this situation
the flue gas canal resistance is (too) high, flue gases may go into the burner room of the burner that’s off, instead of
into the flue gas canal. These flue gases will flow into that burner room, pass its fan, and eventually enter the air
supply to the other fan and back to the burning burner, influencing combustion of that burner negatively.
The phenomenon is called recirculation.
Three
safety features have been installed in the boiler to prevent this recirculation from happening.
1.
Non-return valve.
Behind both fans a non-return valve is installed. Both burners communicate with each other
checking the non-return valve to be closed on the burner that is off so the other burner can burn. The non-return
valve is equipped with a proximity switch which is signalling on the burner controller whether the valve is closed
or opened. The communication between these burner controllers is checked every 20 seconds and when this fails
the other burner(s) will be switched off. In normal operation of the boiler the fan of the burner that is not burning
will be switched off because recirculation is prevented by the non-return valve and checked by the burner control-
ler.
2.
Fan counter pressure
. If the electronic safety features are functioning well, a non-return valve fitted behind the
fan will prevent recirculation (see 1). The position of this non-return valve is monitored continuously by the
electronics. If the non-return valve is not closed the electronics will activate the fan to run at a speed relative to
that of the fan of the other burner so enough pressure is created to prevent recirculation.
-
So, if the non-return valve is not functioning its function is taken over by the fan. In this case the unit will still be
able to burn for 100% of its total capacity when both burners are used.
-
When the fan is not functional (of the burner that is not burning), this fan is not able to run and build up any
pressure to prevent recirculation. The burner controller that controls that burner/fan then checks if the
non-return
valve
is closed. If closed the burner controller communicates to the other burner controller(s) that it is able to
burn. The unit has then only 50% of its total capacity because only one burner is available.
-
If both non-return valve and fan fail the complete unit will go into a lock out, if cascaded the other units will be
blocked because this unit is in a lock out.
3.
Flue gas monitoring
. The tendency to re-circulate also depends of the flue gas resistance. Within normal oper-
ational specifications the boiler will not re-circulate. If the flue gas resistance (by blocking the canal) rises too high,
a
pressure switch
will put the whole boiler into lockout.
2.5
Benefits of the S-CB
+
400 & 550 Boiler
The special features of this boiler range are:
a. The weight is low:
Maximum 450 kg (empty) and filled with water 493 kg.
b. Small dimensions: due to the width of 750 mm, the boiler can access premises using a standard door to
bring it to the place, where it will be installed. It is easy to put the boiler on its exact position, by using the two
wheels at the back.
c. The small diameter of the air supply and flue gas discharge tube.
d. The boiler consists of one heat exchanger with two burners that are controlled by the internal cascade man-
agement system.
e. Extremely low noise production.
f.
Very small water content.
g. Reliable through use of twin burner and smart electronics.
