baltur BT 75 DSPN Manual De Instrucciones página 15

a fuel delivery and a relative combustion air delivery equal to the
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quantity of heat required by the boiler. It should be kept in mind that
the field of variation in output obtainable with a good combustion
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is, as an indication, from 1 to 1/3 of the maximum output given on
the rating plate.
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Note: The air pressure switch must be set when the burner is
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started up and must be in function with the pressure
value verified during operation with the ignition flame.
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DESCRIPTION OF MODULATING OPERATION
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(S BT 8712/3)
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H
While fuel oil is being pre-heated, voltage passes through the pre-
heater regulation thermostat and reaches the coil of the elements
remote-switch.
This remote-switch is cut-in and takes current to the pre-heater
elements which heat the fuel inside the pre-heater.
The elements that heat the pump and atomizer unit (only
for the ... D ver sion) are also c ut- in through panel
switch ( I ). The minimum level thermostat for the pre-heater is
cut-in when temperature reaches the level to which it is set. The
equipment is cut-in only when the temperature at which elements
are cut-off is reached in the pre-heater (the thermostat regulation
contact opens). This means fuel oil in the pre-heater must be at
maximum temperature. The burner command/control equipment
(cyclic relay) is therefore cut-in by the pre-heater regulation ther-
mostat when the latter cuts-off the elements by switching off the
relative remote-switch.
The cyclic relay equipment carries out the ignition programme by
operating the fan motor for the pre-ventilation stage.
If air supplied by the fan is sufficient to operate the relative pressure
switch, the pump motor which carries out hot oil pre-circulation in
the burner pipes is also cut-in immediately.
Oil flows from the pump to the pre-heater, passes through it, war-
ming to the pre-set temperature and flows out through the filter,
finally reaching the atomizer unit. Hot oil circulates in the atomizer
unit without flowing out of the nozzle, because the pipes in the
direction of the nozzle (delivery) and away from it (return) are closed.
Closing is carried out by "closing cones" fitted on the out-side of the
rods. These cones are pushed against the seats by strong springs
situated at the opposite ends of the rods.
The oil circulates and flows out of the atomizer unit return point,
passes through the trap where the TRU thermostat is fitted and
reaches the return pressure regulator. It passes through the latter,
reaches the pump return point and from this, discharges into the
main return pipe. Hot oil circulation as described above is carried
out at a slightly higher pressure value (a few bar) compared to the
minimum pressure set on the return pressure regulator (10 ÷ 12
bar). This oil preventilation and precirculation stage lasts for 22,5
seconds.
This time may be prolonged (indefinitely, in theory), because the
electrical circuit is built in such a ways that the ignition programme
cannot continue until fuel temperature in the nozzle return pipes
has reached the level set on the TRU thermostat. This special
construction does not allow fuel to pass through the nozzle until the
fuel reaches at least the temperature to which the TRU thermostat
is set. The TRU thermostat usually operates within normal preventi-
lation time (22,5 seconds). Otherwise, the fuel oil preventilation and
precirculation stages are prolonged until the TRU operates. TRU
operation (circulating oil is sufficiently hot) enables the equipment
to continue the ignition programme, by switching on the ignition
transformer that supplies high voltage to the electrodes.
High voltage between the electrodes triggers of the electric di-
scharge (spark) to ignite the air-fuel mixture.
Two seconds after sparking, the equipment takes voltage to the
magnet which, trough a series of levers, draws back the two fuel-
flow (forwards and return) stop-rods to the nozzle.
Rod withdrawal also closes the by-pass inside the atomizer unit.
Consequently, pump pressure reaches the nominal level of about
20 ÷ 22 bar.
As the rods move away from their closing seats, fuel can then
enter the nozzle at a pressure of 20 ÷ 22 bar (set on the pump)
and flow, adequately atomized, out of the nozzle. Return pressure,
which determines supply to the furnace, is regulated by the return
pressure regulator. For ignition flow (minimum supply level), this
value is about 10 ÷ 12 bar.
As atomized fuel comes out of the nozzle, it is mixed with air from
the fan and is ignited by the electrode spark.
Flame presence is detected by the photoresistance. The program-
mer continues operating, and after 5 seconds, it overcomes the
locked position, triggers ignition and then, cuts-in the modulation
circuit.
The modulation motor controls the increased supply of fuel and
combustion air simultaneously.
Fuel supply increase is determined by the variable profile disk.
This disk turns, and by doing so increases the pressure on the
spring of the return pressure regulator.
Therefore, increase of return pressure corresponds to increase
of fuel supply.
Fuel supply increase calls for an adequate increase of combustion
air. This condition is put into effect during the first regulation. This
is done by turning the screws that vary the profile of the disk (which
controls combustion air regulation). Simultaneous supply of fuel
and combustion air increases until maximum value is reached (fuel
pressure at the return pressure regulator equal to about 18 ÷ 20
bar, if pump pressure is 20 ÷ 22 bar).
Fuel and combustion air supply stays at maximum level until boiler
temperature (pressure if steam boiler used) is near to the set value.
This determines a reverse movement of the modulation control
motor, which in turn reduces fuel and combustion air supply. The
modulation system reaches perfect balance when the fuel and
combustion air supply is equal to the amount of heat called for
by the boiler. When the burner is in operation, the boiler probe
detects the boiler flow variations and automatically signals fuel
and combustion air supply modifications to the modulation motor.
If, even at minimum fuel and combustion air supply level, maximum
temperature (pressure for steam boilers) is reached, the thermostat
(pressure gauge for steam boilers) operates (at its pre-set value)
and brings burner operation to a complete stop. When temperature
(pressure for steam boilers) drops back below the stop device
intervention value, the burner ignites again as previously described.
Bear in mind that the effective capacity variation range, under good
combustion, is approximately from 1 to 1/3 compared to maximum
specified capacity. Note: The air pressure gauge must be adjusted
when the burner ignites, depending on the pressure value for
ignition flame operation.
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98321_2010/03