Nu-Way MDFL 3300, Installation And Maintenance Manual

Page: 14 / 49

SKP75 Air/Gas Ratio Controller
The air/gas ratio controller, shown below, varies the gas pressure in response to changes in
combustion air pressure to ensure that the air/gas ratio remains constant over the operating range of
the burner.
A separate gas pressure governor is not necessary. Two impulse pipes (both factory supplied) are
connected to the air/gas ratio controller.
The first is connected to the burner hinged extensions and supplies air pressure to the ratio controller.
Note that in installations with negative air pressure in the combustion chamber, this pipe must always
be under positive pressure. A second pipe connected to the gas line downstream of the valve set
provides gas pressure to the ratio controller.
SKP75 combustion chamber impulse connection
The impulse connection to the combustion chamber is not required in the majority of applications and
is therefore not supplied as part of the burner package.
This is because the resistance of the combustion chamber/flue assembly is assumed to remain
constant and that the pressure within this chamber will change in proportion to the burner gas and
combustion air pressure (as the burner output changes).
If however the pressure in the combustion chamber does not change in proportion to the burner gas
or air pressure, i.e. the plant is fitted with a flue gas fan, continuously operating flue gas damper, or
the combustion chamber pressure changes from negative to positive whilst moving from Low to High
flame, then a compensating circuit is required.
This means that the pressure in the combustion chamber must be connected to the SKP75 so that
the controller can automatically offset the pressure changes.
This compensating circuit should also be used if pressure shocks and vibrations, which adversely
affect the burner start-up, develop in the combustion chamber during the start-up phase.
Naturally, it must always be taken into consideration that the burner output decreases as the pressure
in the combustion chamber increases, and vice versa.
Installation of combustion chamber impulse pipe
A minimum inside pipe diameter of 8mm is recommended. The impulse pipe should be as short as
possible to allow the controller to respond quickly to sudden burner output changes. It must be
installed such that the gases will cool down in the area of the impulse pipe and condensing gases will
not enter the controller but run back into the combustion chamber. If necessary, a water trap must be
provided.
MDFL 3300 & MDFL 4000 Issue 2 08/08 Page 13
GAS CONTROL VALVE (SKP 75)
Item Function
1
Adjustment and indication of the
gas/air ratio (high fire settings)
2
Adjustment and indication of the
parallel displacement of the
characteristic (low fire setting)
3 Vented to atmosphere
4 Connection to the fuel gas line
5
Connection to the combustion air
supply at the burner head
6 Indication of the valve stroke