Description Of Progressive Two-Stage Operation With Diesel - baltur COMIST 122 DSPGM Manual De Instrucciones

D E S C R I P T I O N O F P R O G R E S S I V E
TWO-STAGE OPERATION WITH DIESEL
The term two-stage progressive operation indicates that transition
from the first to the second state (from minimum to maximum
operation) is progressive in terms of both amount of combustion
air let in and the amount of output fuel. The burner control and
command equipment (cyclic relay) is activated by means of the
switch on the panel (I).
The cyclic relay equipment carries out the ignition programme by
starting the fan and pump motor for the diesel pre-circulation and
pre-ventilation phases.
The fan must provide a sufficient air pressure to trigger the relevant
pressure switch, otherwise the equipment will shut down. The diesel
flows from the pump to the atomizer unit where it circulates without
flowing out since the passages to (delivery) and from (return) the
nozzle are closed. Closure is obtained using "closing pins" applied
at the ends of the rods. These "pins" are pressed against the seats
by strong springs placed on the opposite end of the rods.
The diesel circulates and flows out from the atomizer unit return
line, passes through the return pressure regulator, reaches the
pump return line, and is discharged in the unit return line. The oil
circulates at a slightly higher pressure (a few bars) than the minimum
pressure which the return pressure regulator is set to (10 ÷ 12 bar).
The diesel pre-circulation and pre-ventilation stage does not take as
long as set in the equipment since it is carried out with open air gate.
The pre-ventilation and post-circulation time is given by the total
time of the operations below:
- output servomotor opening stroke (fuel/air) +
- pre-ventilation time set in the equipment +
- output adjustment servomotor closure stroke (fuel/air) up to the
position of ignition air.
Then the equipment starts the ignition sequence by activating
the relevant transformer which, in turn, provides high voltage to
the electrodes. High voltage between the electrodes triggers the
electric discharge (spark) for the ignition of the fuel-air mixture. After
the ignition spark, the equipment provides voltage to the magnet
that uses levers to draw back the two rods that shut-off the diesel
flow (delivery and return) to the nozzle. When the rods move back
they close the atomizer unit internal passage (by-pass) bringing the
pump pressure to the nominal value of approx. 20 ÷ 22 bar. The
movement of the two rods from the closure seats makes it possible
for the fuel to enter the nozzle at the pump pressure at 20 ÷ 22 bar,
and exit from the nozzle in a suitable atomized state. The return
pressure, which determines the output in the furnace, is regulated
by the return pressure regulator. For the ignition flow rate (minimum
output) this value is approx. 10 ÷ 12 bar. The atomized diesel that
exits from the nozzle, is mixed with the air supplied by the fan, and
ignited by the electrode spark. The flame presence is detected by
the UV photocell. The programmer exceeds the shutdown point,
disables ignition, and the burner works at the minimum flow rate.
If the 2nd stage boiler thermostat (or pressure switch) allows it (set
to a temperature or pressure higher than that in the boiler), the
supply adjustment servomotor starts to turn, gradually increasing
the fuel and combustion air output up to the maximum level the
burner is set to. The increase in the fuel output is determined by the
variable profile disc that, when turning, increases the compression
of the return pressure regulator spring, with consequent increase
of both pressure and fuel output. The increase of diesel output
must correspond to a suitable increase of the combustion air. This
condition occurs during the first adjustment, by turning the screws
that vary the profile of the control disc that adjusts the combustion air.
The simultaneous fuel and combustion air output increases up to the
maximum value (diesel pressure at return pressure regulator equal to
approx. 18 ÷ 20 bar if the pump pressure is at 20 ÷ 22 bar). The fuel
and combustion air output remains at the maximum value until the
boiler temperature (or pressure in case of steam boiler) approaches
the value set in the thermostat (or pressure switch) of the 2nd stage.
The latter inverts the fuel/air output adjustment servomotor direction
to gradually reduce the diesel output and the relevant combustion
air to the minimum value. If also with minimum fuel and combustion
air output the maximum temperature (pressure in case of steam
boilers) is reached, the thermostat (pressure switch for steam
boilers), that completely stops the burner, triggers at the value it is
set to. As temperature (or pressure for steam boilers) drops below the
shutdown device's setpoint, the burner turns on as described above.
Upon standard operation the 2nd stage thermostat (or pressure
switch) detects boiler load variations and automatically commands
to the (fuel/air) adjustment servomotor to adapt the diesel and
relevant combustion air output. With this operation the fuel/air
output control system reaches a balanced condition with a fuel and
relevant combustion air output equal to the quantity of heat required
by the boiler.
The air pressure switch must be set upon burner
!
ignition, according to the pressure value necessary to
operate with ignition flame, otherwise the equipment
will shut down.
Keep in mind that the variation range of the flow rate that can be
achieved with a good combustion is approximately from 1 to 1/3
compared to the maximum rated flow rate.
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