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the fuel and combustion air delivery by inserting the servomotor
which regulates delivery (fuel/air). This will rotate in such a way as
to obtain an increase or a decrease. With this manoeuvre, the fuel/
air delivery regulating system try to equilibrate the quantity of heat
supplied to the boiler with that which the boiler gives to be utilized. It
should be kept in mind that the field of variation in output obtainable
with a good combustion is, as an indication, from 1 to 1/3 of the
maximum output given on the rating plate.
Control box and re-
Safety Time
lative Programmer
in seconds
LAL 1.25
5
Cyclic relay
DESCRIPTION OF MODULATING OPERATIONS
(See BT 8712/3)
During the fuel oil pre-heating phase, voltage passes through the
regulation thermostat of the pre-heater and reaches the relay coil
of the resistances. The relay closes and takes current to the resi-
stances which, in turn, heat the fuel contained in the pre-heater. By
operating Panel Switch ( I ), the resistances which heat the pump
and the atomizer group are also inserted (only for version ...D). The
minimum thermostat of the pre-heater closes when the temperatu-
re reaches the value at which it has been set.
The control box will be connected only when the temperature in
the pre-heater reaches the value at which it has been set and cau-
ses the resistances to be disinserted (an opening of the contact of
the regulation thermostat); therefore, the fuel oil in the pre-heater
should be at maximum temperature. The control box (cyclic relay)
of the burner is therefore connected by the regulation thermostat
of the pre-heater when it excludes the resistances by disinser-
ting the relative relay. The cyclic relay control box carries out the
ignition programme by starting up the fan motor in order to effect
the pre-ventilation phase. If the air pressure supplied by the fan is
sufficient to cause the intervention of the relative pressure switch,
the pump motor will be turned on immediately and will carry out a
pre-circulation of hot oil in the burner pipes.
Oil from the pump reaches the pre-heater, is heated to the correct
temperature and when it comes out it passes through a filter before
reaching the atomizer unit. The hot oil circulates in the atomizer unit
without coming out of the nozzle because the passages leading to
the outward and return nozzles are closed. This closure is carried
out by "closing pins" applied to the ends of the rods.
These "pins" are pressed against their seats by strong springs which
are situated at the opposite ends of the rods.
The oil circulates, comes out of the atomizer group return, goes
through the sump where the TRU thermostat is inserted and
arrives at the return pressure regulator; it passes through this
and reaches the pump return and from there is discharged into
the return. Hot oil circulation, as described above, is carried out
at a pressure value slightly higher (some bar) than the minimum
pressure at which the return pressure regulator has been set
Note:
Control box specifications
Pre-Ventilation & Oil
Pre-ignition Time
Pre-circulation Time
in seconds
in seconds
22,5
(10 ÷ 12 bar). The duration time of the pre-ventilation and oil pre-
circulation phase is 22,5 seconds.
This time can be extended (theoretically indefinitely) because a spe-
cial device in the electric circuit does not allow the ignition programme
to be carried out until the temperature of the fuel in the nozzle return
pipeline has reached the temperature at which the TRU thermostat
(thermostat on the nozzle return) has been set.
This special device does not allow the fuel to pass through the
nozzle until it has reached at least the temperature at which the
TRU thermostat has been set. Normally, intervention of the TRU
thermostat takes place within the pre-ventilation time (22,5 seconds);
if it doesn't, pre-ventilation and fuel oil pre-circulation are extended
until the intervention of the TRU thermostat. The intervention of the
TRU thermostat (oil in circulation sufficiently hot) allows the control
box to proceed with carrying out the ignition programme by inserting
the ignition transformer which in turn feeds the electrodes with high
voltage. High voltage between the electrodes primes the electric
spark for ignition of the fuel/air mixture. 2,5 seconds after the ignition
spark appears, the control box carries voltage to the magnet which,
by means of appropriate levers, moves backwards the two rods
which intercept the flow (outward and return) of fuel to the nozzle.
This moving backwards of the rods also determines a closing of the
passage (by-pass) inside the atomizer unit. Consequently, the pump
pressure is taken to the normal value of about 20 ÷ 22 bar. Deviation
of the two rods from the closing seat, now permits the fuel to enter
the nozzle at the pressure at which the pump has been regulated
at (20 ÷ 22 bar), and comes out of the nozzle adequately atomized.
The return pressure, which determines delivery to the furnace, is
regulated by the return pressure regulator.
The value of the ignition flow rate (minimum delivery) should be about
10 ÷ 12 bar. The atomized fuel which comes out of the nozzle is
mixed with air supplied by the fan and is then ignited by the spark
of the electrodes. Flame presence is detected by the photoresistant
cell. The programme proceeds and, after 5 seconds, surpasses the
"shut down" position, disconnects the ignition transformer, and then
connects the modulation circuit. The modulation motor commends
an increase in the delivery of fuel and combustion air at the same
time. The increase in fuel delivery is determined by a disk with a
varied profile which, by rotating, can determine a greater compres-
sion of the return pressure regulator spring and thus an increase in
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The air pressure switch must be set when the burner is
started up and must be in function with the pressure value
verified during operation with the ignition flame.
Post-ignitio Time
in seconds
2,5
5
E
N
G
L
I
S
Time between 1st flame &
H
Start of Modulation
in seconds
20
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