1 Introduction
Page 2 Mk8 EGA Set-Up and Trim Guide 12.04.2016
1.2 3-Parameter Trim
The 3-parameter trim function can be enabled when the Mk8 EGA is used in conjunction with an MM
module to manage the combustion. When the EGA detects any differences in the online exhaust gas
readings to the original commissioned values, the trim function will make small corrections to the air
damper (and channel 5 if optioned for trim), to bring those online values back to the commissioned
readings. The trim function controls the combustion of the burner by adding air (air rich) or taking
away air (fuel rich) from the commissioned air positions to keep the volume of O
2, CO2 and CO close to
their commissioned values without compromising safety. The air rich and fuel rich limits are set by
adding trim data when commissioning the burner or through single point change for ‘quick
commission.’ This trim data is translated into a combustion map, which shows how the burner reacts
when air is added or removed from the combustion process during commissioning. The Autoflame
system continually monitors 3 parameters O
2, CO2 and CO to create the safest and most efficient way
of trimming the combustion process.
Ingress of tramp air through an ill-fitting boiler or flue section will distort the O
2 reading and show an
increase in this value. This results in the EGA reading the tramp air influence in the sample rather than
just the actual combustion gases.
Single parameter O
2 trim systems would see both of these conditions as rich (excess air) combustion
and start to trim back on the air by closing the air damper. In reality this trim process is not trimming the
combustion gases at this point, but is in fact trimming the exhaust gases with the excess O
2. This can
potentially lead to the formation of excessive amounts of CO but more importantly this can lead to
incomplete combustion with dangerous consequences.
Similarly, single parameter CO
2 trim systems would interpret of air as lower CO2 levels in the flue,
inflicting similar dangerous conditions in the boiler.
Another benefit of the 3 parameter trim is that the EGA is continually measuring the formation of CO
compared to its commissioned value. A higher CO reading can be contributed to both lean and rich
combustion. A lack of air will produce incomplete combustion and the formation of CO. Also, excess
air around the flame envelope can chill the flame edge causing incomplete combustion and higher CO
levels.
By referencing all 3 parameters against mapped combustion performance the burner can be trimmed
back to the original commissioned values whilst maintaining the highest degree of safety.
When the trim function adds air to bring back the combustion to the commissioned values, as the
burner modulates to a new position, the deviation in air damper movement is added to each air
position. In this way, optimum combustion is maintained during modulation, through carry forward trim.
As a safety feature, as the air is being taken away, the fuel to air ratio will return back to the
commissioned positions when the burner modulates, for every 10 degrees of fuel valve movement.
Once this new position is held the system will determine whether the air damper should be closing. This
ensures safe combustion at all times without any compromise.
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