Excess
air
Exit gas temperature Ambient or reference temperature Fuel type and analysis 
The
heat loss method disussed in ASME PTC 4.1 may be used for evaluating the
efficiency of steam generators. For quick estimates of oil and natural
gas fired steam generators,however,the following equations may be used.These
equations were arrived at by the author after performing several calculations.
Natural
gas
Efficiency,% HHV=89.4(0.001123+0.0195xEA)(TgTa) %LHV=99.0(0.001244+0.0216xEA)(TgTa) Oil fuels# 2
and #6
Tg,Ta=exit
gas and reference air temperatures,F
Efficiency on HHV and LHV basis can be related using the formula: HHVxEfficiency on HHV basis=LHVxEfficiency on LHV basis Example:A natural gas fired boiler with 15 % excess air has an exit gas temperature of 280 F,ambient=80 F. Determine efficiency on HHV and LHV basis. Efficiency%HHV=89.4(0.001123+0.0195x1.15)x(28080)=84.64
%

Excess air
from field data
If % oxygen on dry basis is known from field data,the excess air can be estimated as: EA=Kx21/(21%O2) K=0.98 for natural gas and 1 for oil Example:If % oxygen dry in an oil fired boiler is 3,the excess air EA=21/(2118)=1.17 or 17%.The same value of oxygen in a gas fired boiler would translate to about 0.98x1.17=1.15 or 15% excess air. Another formula that is used is: EA=100xO2/[.264N2(O2CO/2)] where O2,CO,N2 are from Orsat's analysis on dry basis. 
Casing Heat
Losses
This ranges from 0.3 to 1 % depending on the boiler size. The larger the boiler,lesser the loss. If say the hot face temperature is 366 F,wind velocity=100 fpm and ambient temperature=70 F: The casing temperature has to be arrived at based on a trial and error procedure;however here let us assume that it is 86 F if a 3 in mineral fiber insulation is used.Heat loss from casing : q=0.173x0.9x[5.46^{4}5.3^{4}]+0.296x(8670)^{1.25}x(169/69)^{0.5}=30.5 Btu/ft^{2}h (0.9=casing emissivitysee my books for details) Loss through insulation=0.316x(36686)/3=29.5 Btu/ft^{2}h,where 0.316 Btu in/ft^{2}hF is the average K value of the insulation If total surface area of the boiler is say 1000 ft^{2},then total loss=30,000 Btu/h.In a boiler generating 20,000 lb/h,the duty is about 20 MM Btu/h. Hence the loss=(0.03/20)x100=0.15 %.One may add a margin to this value. 
In
the above expressions it is assumed that there is no large amount of loss
due to CO formation or incomplete combustion and the total casing,unaccounted
loss=1 %. Loss
due to CO is:
L=10160xCxCO/(CO+CO2)/HHV,where
C=% carbon in fuel,CO,CO2,% volume of the gases. If CO=1000 ppm,CO2=12,C=87%,HHV=19000
Btu/lb, then L=(0.1/12.1)x10160x87/19000=0.38%.Typically CO is is the range
of 100200 ppm in well maintained boilers.Hence the CO loss is very low
and generally not computed and is included in the margin.
One
could also use the program discussed in Books,Software
on Boilers,HRSGS,Steam Plant Calculations to
perform these calculations more accurately.A thumb rule is that a 40 F
change in exit gas temperature is equivalent to about 1 % change in efficiency.
ECONOMIZER VS
AIR HEATER
Plant
engineers should always consider the use of economizers to improve boiler
efficiency. As an example,if the stack gas temperature is 550 F in a 100,000
lb/h boiler without an economizer and say 300 F with the economizer,the
approximate change in efficiency=(550300)/40=6.25 % or about 7.5 Mm Btu/h
energy savings or about $ 18.75/h based on 2.5 $/MM Btu fuel cost. Even
if the economizer costs $ 50,000,the payback is less than 2700 hours! Unfortunately
initial cost alone is often considered while making decisions on energy
equipment.
Note that air heaters have a few negative aspects
compared to economizers as heat recovery equipment: They increase the combustion
temperature,which increases NOx formation(not desirable) and the gas/air
side pressure drops are significantly higher compared to the economizer,resulting
in higher fan power consumption and larger fan size; airheaters could
be more expensive and also occupy more space.Hence the present trend is
to use economizers in packaged oil,gas fired steam generators. Air heaters
widely were used in designs developed about 2050 years ago.They
may be required in solid fuel fired boilers to improve the combustion process.
Books,Software
on Boilers,HRSGS,Steam Plant Calculations
email Ganapathy