17 Fully Metered Combustion Control
Mk8 MM Manual Page | 300
17.3 Fully Metered Combustion Control Operation
17.3.1 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.
17.3.2 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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