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PFP7/REV C/01-08
8.2
Combustion and ignition system description
Figure 8.2.1
: Combustion and ignition system
The PFP7 can be used with propane, butane, natural gas or L.P.G. at pressures as defined in
Section 2. Immediately behind the rear panel fuel bulkhead is the
fuel
solenoid valve. This
solenoid valve is normally closed when there is no power to the instrument or where there is
power to the instrument but the flame is not alight. The solenoid will be latched in the open
position when the flame is alight and has been detected by the flame thermocouple. The fuel
solenoid can be held open manually during the flame ignition sequence by holding down the
front panel
ignition
switch; fuel then flows through small bore copper tubing to the fuel valve on
the front panel. This is a very fine needle valve and to avoid damage, this control should never
be screwed hard fully clockwise and should not be used as an on/off valve. From the fuel valve
the gas then passes to the mixing chamber and then on to the burner. A restrictor screw is fitted
to the mixing chamber fuel inlet for use with propane, butane or L.P.G. but must be removed if
natural gas is used.
Air is fed through the rear panel to the air regulator. The air regulator is factory set to 12psi.
Adjustment should not be necessary but can be achieved by lifting the yellow locking ring and
rotating the knob on the side of the regulator until the correct pressure is shown on the pressure
gauge. From the air regulator the air passes to the nebuliser where it acts as an atomising
agent.
Ignition is achieved by passing a high voltage spark between the ignition electrode and the
centre of the burner. The spark is generated by the self-contained ignition unit mounted inside
the instrument. This is activated by depressing the front panel
ignition
switch, which
simultaneously opens the fuel solenoid, allowing gas to flow through the stream of sparks and
hence ignite. The ignition switch must be manually held down until the thermocouple, mounted
at the top of the back of the chimney, detects the temperature rise due to the flame, latches the
solenoid in the open state and energises the FLM indicator in the display window. If the fuel flow
ceases and the flame is extinguished, the thermocouple will detect the fall in temperature and
the solenoid will automatically be closed, stopping any further gas escaping to atmosphere.
Air at 14 to 30 psi,
6 litres/min
Ignition switch
& flame
detect circuits
Needle valve
(fuel)
To ‘Fuel in’ on
mixing chamber
To nebuliser
Solenoid
Rear panel
Regulator & gauge
assembly
Fuel gas regulated
to less than 0.5 psi
Air at 14 to 30 psi,
6 litres/min
Ignition switch
& flame
detect circuits
Needle valve
(fuel)
To ‘Fuel in’ on
mixing chamber
To nebuliser
Solenoid
Rear panel
Regulator & gauge
assembly
Fuel gas regulated
to less than 0.5 psi