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0006080103_201309
DESCRIPTION OF PROGRESSIVE 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’s control box (cyclic
relay) is connected by operating panel switch ( 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. Oil from
the pump reaches the atomizer unit and is obliged to circulate within
it 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 the seats by
strong springs placed on the opposite end of the rods.
The oil circulates, comes out of the atomizer unit return and arrives
at the return pressure regulator. It passes through this and reaches
the pump return and from there it is discharged back into the return.
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 pre-circulation time is given by the total time
of the operations below:
- output servomotor opening stroke (fuel/air) +
- pre-ventilation time set in the equ
- 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 primes the electric
spark for 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 atomization
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
delivery to 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 light oil which comes out of the nozzle is
mixed with air supplied by the fan and is then ignited by the spark
of the electrodes. The presence of the flame 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 shut-
down device’s set point, 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. Thus the delivery regulation system
(fuel/air) reaches a balance position corresponding to a fuel delivery
and to its combustion air delivery equal to the heat amount 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.
ENGLISH
Summary of Contents for GI MIST 350 DSPGM
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