baltur BT 75 DSPN-D Mode D'emploi page 39

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, warming 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
preventilation 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 discharge (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 programmer 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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