User Manual

13
version has been ignored. This loss is subtracted from the efficiency.
CO Air frEE
Certain standards ( ANSI Z21.1) for Carbon Monoxide are stated in terms of air-free. Air-free refers to the
concentration of CO in combustion gases undiluted with flue, or other gases containing little CO. This value is com-
puted using an equation that takes into account the O2 concentration of the flue gas.
 • If5%O2ismeasured(O2m)inthefluethentheCOgasvaluewillberecalculatedasif0%weremeasured.
The equation for air-free is as follows:
- COaf = CO PPM x [(20.9) / (20.9 - O2m)]
 • Inourexampleifareadingof325PPMweremeasuredthentheair-freevaluewouldbecalculatedasfollows:
- COaf = 325 PPM x [(20.9) / (20.9 - 5)] COaf = 325 PPM x [(20.9) / (15.9)] COaf = 427
We may be given a limit on our gas range by the local authority, which stated that we must not emit more than
400-PPM Carbon Monoxide air-free. In the example we would be breaking the limit and corrective action should be
taken to reduce the level of CO. Air-free values prevent false readings being submitted, e.g. allowing more air into the
boiler will increase the oxygen level in the flue and dilute any toxic gas reading. Air-free referencing gives readings as if
they were undiluted.
COmbUstiOn EffiCiEnCy CALCULAtiOns
This identifies three sources of loss associated with fuel burning:
  •Lossesduetofluegasses:
- Dry Flue gas loss, Moisture and hydrogen,
- Sensible heat of water vapor, Unburned gas
  •Lossesduetorefuse:
- Combustible in ash, riddling and dust
  •Otherlosses:
- Radiation, convection, conduction other unmeasured losses
Net efficiency calculations assume that the energy contained in the water vapor (formed as a product of
combustion and from wet fuel) is recovered and the wet loss term is zero. Gross efficiency calculations assume that
the energy contained in the water vapor is not recovered. Since the fuel air mixture is never consistent there is the
possibility of unburned/partially unburned fuel passing through the flue. This is represented by the unburned carbon
loss. Losses due to combustible matter in ashes, riddling, dust and grit, radiation, convection and conduction are not
included.
Efficiency Calculation:
•KnownDataFuel:
- Qgr = Gross Calorific Value (kJ/kg)
- Qnet = Net Calorific Value (kJ/kg)
 -K1=ConstantbasedonGrossorNetCalorific••
•KnownDataValue:
 -K1g=(255x%Carboninfuel)/Qgr
 -K1n=(255x%Carboninfuel)/Qnet
 -K2=%maxtheoreticalCO2(drybasis)
 -K3=%WetLoss
 -H2=%Hydrogen
 -H2O=%Water
•MeasuredData:
-Tf = Flue Temperature
-Ti = Inlet Temperature
 -O2m=%Oxygeninfluegas
 -O2r=Oxygenreference%
•Calculateddata:
- Tnet = Net Temperature
 -%CO2contentinfluegas
 -%DryFlueGaslosses
 -%Wetlosses
 -%Unburnedcarbonloss
 -%Efficiency
•Tnet=Flue Temperature - Inlet Temperature (or ambi-
ent)
•Dryfluegasloss%
 =20.9xK1x(Tnet)/K2x(20.9-O2m)
•Wetloss% 
 =9xH2+H2O/Qgrx[2488+2.1Tf-4.2Ti]
•Simplified 
= [(9xH2+H2O)/Qgr]x2425x[1+0.001Tnet]
•Wetloss% 
 =K3(1+0.001xTnet)
•WhereK3 
 =[(9xH2+H2O)/Qgr]x2425
•NetEfficiency%
= 100 - dry flue gas losses
= 100-20.9xK1nx(Tnet)/K2x(20.9-O2m)
•GrossEfficiency%
= 100 - {dry flue gas losses + wet losses}
= 100–{[20.9xK1gx(Tnet)/K2x(20.9-O2m)]+
  [K3x(1+0.001xTnet)]}
•ExcessAir
= [20.9 / (20.9 - O2m) - 1] x 100
•CO2%
 =[(20.9-O2m)xK2/20.9]
•Unburned
 =K4xCO/(CO+CO2)
 Note:COscaledin%fuelLoss% 
•WhereK4
= 70 for coke
= 65 for anthracite
= 63 for Bituminous coal
= 62 for coal tar fuel
= 48 for liquid petroleum fuel
= 32 for natural gas
TheformulaforK4isbasedonthegrosscalorificvalue
Qgr. To obtain the loss based on net calorific value multi-
ply by Qgr/Qnet. Since this loss is usually small this con-