Operation - Siemens LEC1/8853 Manual

Construction of the
Control Unit and the
Flame Safeguards

Operation

Prerequisites for the
burner start-up
Important
Program sequence
during burner start-up
2
The control unit LEC1... as well as the flame safeguards LAE10... and LFE10... are of
plug-in design and are suitable for the mounting in any position at the burner, on control
panels and in control cabinets. The spacious terminal bases and the housings are made
of impact proof and heat-resistant plastic. The switching mechanism of the unit driven by
a synchronous motor, its relays as well as all other switching, control and adjusting
elements are mounted on solid printed circuit boards.
The following description of the mode of operation refers to the start-up and supervision
of one single burner.
With multi-flame supervision all of the burners connected to the control unit are put into
operation and supervised simultaneously in the same manner. A defect leading to a lock-
out at one of the burners therefore results in the shutdown of all burners. The prerequisite
for the immediate restart of the non-defective burners is the bridging of the flame
safeguard of the defective burner by means of an operating switch. This switch must
simultaneously interrupt all control lines to the ignition transformer and the fuel valves. For
connection examples please refer to data sheet 7781.
The burner is started only if:
-
the switching mechanism of the control unit is in its start position
-
the control unit is not in lock-out position, e.g. with defective UV-tube
-
the contacts of all the control and safety devices in the control loop between
terminal 8 and 9 are closed
-
the air pressure monitor does not indicate any air pressure
Defects in the flame safeguard or in the control unit prevent the start or lead to a lock-out
during the start-up.
If the air damper is not controlled by the control unit, terminal 20, 21 and 22 must be
interconnected.
First the fan motor is switched on via terminal 3 and the air damper actuator is driven via
terminal 22. As soon as the air damper has reached its fully open position, the switching
mechanism of the unit starts its run - the prepurge time begins. The minimum air pressure
set at the air pressure monitor must be reached within 10 s (or 7 s in operation with post-
purge) and must be maintained until the controlled shutdown, otherwise lockout occurs. A
flame signal during the prepurge time also leads to a lockout. On completion of the set
prepurge time the air damper receives the control command to return to the minimum air
position. During the air damper's closing time the switching mechanism remains
stationary. As soon as the signal contact for the minimum butterfly position is operated by
the air damper actuator, the switching mechanism starts again and now controls the
program sequence which can no longer be influenced externally:
-
Pre-ignition (3 s)
-
Release of the 1st fuel valve via terminal 5 (the fuel valve of a pilot burner which must
be closed on completion of the 2nd safety time must, however, be connected to
terminal 10)
-
Programming of the set safety time. If no flame is established during this time, lockout
occurs (always with interlocking of the control unit)
-
Following an interval of 11 s after the release of the 1st valve, the 2nd fuel valve is
released
-
The pilot burner, if connected to terminal 10, is switched off
-
The load controller is switched on after a further interval of 12 s. Now the burner has
reached its operating position. From now on the load controller controls the burner
capacity by either increasing or decreasing the fuel throughput and the air volume
depending on the heat demand (fuel/air ratio control). This can be carried out in stages,
i.e. by means of thermostats or continuously (modulating) by using a continuous
controller.
The loss of flame during operation always leads to burner lockout.
CC1N7761E