10 Fully Metered Combustion Control
Page 144 Mk8 MM Expansion Features Installation and Set-Up Guide 04.09.2016
10.2 Fully Metered Combustion Control Operation
10.2.1 Philosophy
The fully metered system will add a layer on top of the standard commission map, with the aim of
maintaining the fuel-air ratio for each firing rate. The system can either directly measure mass flow or
use corrected volume flows to maintain this ratio.
The Mk8 MM continuously measures the fuel and air flows to compensate for any variations from
stored values, in an effort to maintain the commissioned burner efficiency. To compensate for changes
the MM will trim the air damper position to try to maintain the commissioned excess air. In addition the
MM will move the fuel valve, to try to achieve the firing rate required to maintain the commissioned
heat input.
The fully metered combustion control works with the commissioned fuel valve and air damper positions,
storing the mass or volume flow of the fuel and air at each point. The flow data is recorded using two
4-20mA inputs, which can be the data from a mass flow meter or calculated from volume flow meter.
When using a volume flow meter the fuel density is used to calculate and display a mass flow using
either default values or temperature and Autoflame pressure sensors.
If variations occur from the commissioned fuel or air flow, the MM will trim servomotors up to an option
limited percentage of their commissioned positions at that time. Unlike other systems, the Autoflame
fully metered operation is based on the commissioned fuel-air curve, so combustion deviations are
compensated for faster than those systems without a base firing curve. Should any faults occur with the
meters, the control can be optioned to revert to the default fuel-air curve to allow the burner to
continue to run.
As the fuel valve moves to reach the commissioning firing rate, based on the measured mass flow rate,
the air damper will also adjust to achieve the commissioned excess air, due to proportional change
required in air flow.
10.2.2 Firing Rate
When measuring the mass flow, the control process will aim to maintain the same fuel-air ratio as the
commissioned fuel flow and air flow ratio. The burner’s firing rate can be controlled by the Autoflame
internal PID, external modulation, hand mode, DTI firing rate or Modbus firing rate. Without fully
metered combustion control, the MM would map the fuel valve angle through the fuel flow curve,
whereas with this control, the fully metered system firing rate is proportional to the mass flow. The firing
rate is then determined by:
100%
The maximum mass fuel flow rate is the fuel flow rate recorded at the high fire position during
commissioning. From this, once the burner commission is complete, the fuel flow curve is mapped out
automatically based on the formula above. This means that if the flow meters fail, the MM can revert
back to the default behaviour and use the pre-stored fuel flow curve. Also, if after commission the high
fire position was moved in single point change, the MM will update the fuel flow curve automatically.
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