Tabla de contenido
Publicidad
DESCRIPTION OF TWO STAGE PROGRESSIVE
E
OPERATION (See BT 8721/3)
N
This is referred to as a 2-stage progressive operation because the
passage from the 1st flame to the 2
G
to the maximum pre-established rate) takes place gradually both as
delivery of combustion air and as delivery of fuel. During the fuel oil
L
pre-heating phase, voltage passes through the regulation thermostat
I
of the pre-heater and reaches the relay coil of the resistances. The
relay closes and takes current to the resistances which, in turn, heat
S
the fuel contained in the pre-heater. By operating Panel Switch ( I
), the resistances which heat the pump and the atomizer group are
H
also inserted (only for version ...D).
The control box will be connected only when the temperature in the
pre-heater reaches the value at which it has been set and causes
the resistances The minimum thermostat of the pre-heater closes
when the temperature reaches the value at which it has been set.
to be disinserted (an operating 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 disinserting the relative relay.
The cyclic relay control box carries out the ignition pro-
gramme 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 rea-
ches 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 passage 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 (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 special 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.
flame (from the minimum rate
nd
8 / 28
0006081143_201107
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, mo-
ves 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 pro-
gramme proceeds and, after 5 seconds, surpasses the "shut down"
position, disconnects the ignition and by now, the burner is operating
at minimum output.
If the boiler thermostat (or pressure switch) of the 2nd stage allows it
(regulated at a temperature or pressure value superior to that existing
in the boiler), the servomotor which regulates the fuel delivery starts
turning and determines a gradual increase in the fuel delivery and
in the relative combustion air until it reaches the maximum delivery
value at which the burner has been regulated. The increase in fuel
delivery is determined by a disk with a varied profile which, by ro-
tating, can determine a greater compression of the return pressure
regulator spring and thus an increase in the pressure itself. When
the return pressure increases, there is also a corresponding incre-
ase in fuel delivery. There should also be an adequate increase in
combustion air to meet the increase in fuel delivery.
Adjustment can be carried out at first regulation by operating the
screws which vary the profile of the command disk of the combustion
air regulator. Fuel and combustion air delivery both increase at the
same time until they reach maximum value (fuel pressure at the
return pressure regulator is equal to about
18 ÷ 20 bar if the pressure at the pump is at the value of 20 ÷ 22 bar). The
burner remains in the maximum delivery position until the temperature or
pressure reaches the limit set for the intervention of the boiler thermostat
(or pressure switch) of t 2
stage and makes the servomotor
nd
regulating delivery rotate in the opposite sense of direction. The
return movement of the modulation motor causes a reduction in
fuel delivery and a relative reduction in combustion air. Should the
maximum temperature (pressure, if steam boiler) be reached even
with fuel and combustion air delivery at a minimum, the thermostat
(pressure switch, if steam boiler) will intervene when the value at
which it has been set is reached, and bring the burner to a standstill.
When the temperature (pressure, if steam boiler) drops below the
intervention limit of the "shut down" device, the burner will start up
again as previously described. During normal operations, the boiler
thermostat/pressure switch of the 2
the variations requested and automatically proceeds with adapting
stage fitted to the boiler detects
nd
Publicidad
Tabla de contenido
