7.5 Draft Control Operation
The draft control trim functionality follows the same ideology to the E.G.A., where corrections are
made to air damper to compensate for the changes in exhaust gas values from their commissioned
values, caused by fluctuations in ambient conditions. For Autoflame draft control, corrections are made
to the stack damper to compensate for changes in wind and ambient conditions which cause the stack
pressure to differ from the commissioned stack pressure along the fuel curve.
Once the burner starts up and the main flame stabilised, there is a time delay where no draft control
operation occurs, set in expansion option 40.4. After this time delay elapses, as the air going into the
burner is increased from low fire to high fire, the boiler pressure may change and the draft servomotor
will move to follow these changes as commissioned. If the atmospheric or stack pressures have
changed from their commissioned values along the curve, the air damper will either move in the open,
or close direction to adjust the air pressure in the stack back to its set value.
The smallest angle that the draft damper can drive to while running is defined by expansion option
40.3; when the burner turns off, the draft damper will go to commissioned closed position to maintain
heat within the boiler.
If draft control is optioned off after being commissioned, then the draft servomotor will drive to the
open position and re-commissioning is not required. The draft servomotor must remain connected and is
still in operation so any channel errors will still shutdown the burner and generate an on screen
indication. A full re-commission with this turned off will be required to remove the servomotor from the
system. The air pressure sensor has no effect once draft control is optioned off and can be
disconnected without any lockouts occurring.
7.5.1 Deactivation Window
There is a deactivation window, outside of which no trim occurs, and the air damper remains at the
previously trimmed position until back within the window and trim is required. This deactivation window
is defined by expansion options 40.4 - delay before compensation and 40.5 - firing compensation
deactivation window. If the draft servomotor has moved more than the set angular degrees “x”, within
the time period set “y”, than no trim functionality will operate, if trim has been performed prior to this
deactivation then the trim is carried forward.
For example, when the burner is first switched on with a high demand, or if the burner comes out of
warming in lead-lag control, or is switched from low flame hold to auto mode. In these situations the
firing rate may ramp up quickly to meet load demand, at this point if the draft servomotor is required to
move greater than the set amount (40.5) quicker than the set time (40.3) then no trim functionality will
occur and any trim % will be carried forward.
See figure 7.5.1.i for a graphical representation of draft activation window.
To Next Page
Loading...
Need help?
General
