Page 480 - Boiler_Operators_Handbook,_Second_Edition

Appendices 465

calculation sheet) by the quantity of air (f). Wet flue gas combustion air temperature or simply use 1040 which
(h) is the sum of the fuel, air, and moisture in the air, (e) is usually close enough. Divide that result by 1,000 to
+ (f) + (g). The water in the flue gas that is produced get percent.
by burning the fuel (i) is determined by multiplying the
fuel (e) by the calculated ratio [3] on the combustion You should not plan on having a carbon monoxide loss
calculation sheet. when predicting boiler efficiency and normally carbon
monoxide (CO) is so small that its loss is insignificant
The total moisture in the flue gas (k) is the sum of the but occasionally a problem occurs where there is sig-
moisture from the air (g) and the moisture from combus- nificant loss so you want to determine it. Multiply the
tion (i). Subtract that from the wet flue gas (h) to get the dry flue gas (l) by the CO measurement in ppm and
quantity of dry flue gas (l). divide by 230,200 to get the percent loss from incomplete
combustion.
Now you’re ready to determine the boiler efficiency by
the heat loss method. The loss in sensible heat, where Radiation losses (p) are difficult to determine and fre-
you’re just heating up the fuel, air, and water then quently much of the heat lost to radiation is recovered
throwing it away is called sensible heat loss and is cal- in the combustion air making a true analysis even more
culated by multiplying the wet flue gas quantity (h) by difficult. You have the option of ignoring the radiation
the difference between the flue gas and combustion air loss (that’s what all those modern analyzers do) or us-
temperatures (subtract (c) from (d)) and multiplying the ing the boiler manufacturer’s predicted radiation loss.
result by the specific heat of the flue gas. The specific If using the manufacturer’s number it’s important to
heat varies according to the ratio of carbon and hydro- consider that value is at full boiler load. When calculat-
gen and you can get a more precise value from Figure ing efficiency at partial loads you should use a radiation
13
2-3 in PTC-4.1 but a value of 0.25 for gas or 0.245 for loss equal to the manufacturer’s prediction divided by
coal or oil is close enough for most calculations. The the percent load on the boiler when the data was taken
result of that calculation is a loss in Btu for the fuel and multiply by 100 to get percent loss.
burned so divide the result by 1,000 to get the loss in
percent. (That’s the same as dividing by a therm then Add up all the losses to get the total losses (q) subtract
multiplying by 100 to get percent) that result from 100 to get the boiler efficiency in per-
cent. This is a more precise determination than those
The loss due to the moisture content of the flue gas is made with charts and electronic analyzers because
determined by multiplying the moisture in and from it considers the moisture in the fuel and air plus the
combustion of the fuel (i) by the difference between moisture from combustion of hydrogen in the fuel for
the enthalpy of the steam and the enthalpy of liquid the fuel you burn. A few calculations with different fuel
water at room temperature. You can look up the value analysis will show that the moisture loss is a significant
for steam at stack temperature and at one pound ab- consideration.
solute pressure and subtract the value for water at the