Page 130 - Boiler_Operators_Handbook,_Second_Edition

What the Wise Operator Knows 115

an assumed fuel. They’re sufficiently accurate to deter- bustion efficiency of 78% at a 40% load. In this case
mine relative efficiency over the load range and to com- the operating boiler efficiency is 73% (0.82 +0.015 ÷
pare the boiler performance to another boiler burning 0.4 × 0.75)
the same fuel but if you use those results to challenge
the boiler manufacturer’s higher prediction you’ll lose Why bother with the radiation loss? To ignore it is to
the argument. Calculations in Appendix L permit deter- invite some crucial errors in operating decisions. Radia-
mination of boiler efficiency using the heat loss method tion losses are, for all practical purposes, constant regard-
and a fuel analysis for those purposes. less of firing rate so their proportional effect varies with
The most common value used today is what we load. My favorite example is a plant with an old HRT
call “combustion efficiency.” When the technician visits boiler and a newer cast iron boiler. Since the HRT furnace
your plant to do your annual combustion optimization was substantially hotter it was easier to get low excess air
(typically required by EPA [Environmental Protection with a newly installed burner than possible with the cast
Agency] or its equivalent in your State) or you draw iron boiler at the same loads. The predicted full load ra-
stack samples that allow a calculation of boiler efficiency diation loss for the HRT boiler was slightly more than 8%
that’s combustion efficiency. It’s basically a heat loss while the cast iron boiler had a predicted radiation loss of
efficiency that assumes a fuel analysis and determines 4%. At the normal load of 50% the combustion efficiency
the energy lost up the boiler stack. It’s the one that is of the HRT has to be 8% higher than the cast iron boiler to
printed on that little strip of paper by the analyzer. As- overcome the higher (16% versus 8% of actual input) ra-
suming the analyzer was properly calibrated the value diation losses. The operators were firing the older boiler
is a reasonable indication of your boiler efficiency when because combustion analysis indicated it was 5% more
it is adjusted for radiation loss. efficient. Evaporation rate data later proved they couldn’t
That’s because the stack loss is the largest single rely on their combustion efficiency.
loss associated with boiler efficiency and the analyzer For years we have settled on the concept of boiler
does a pretty good job of determining it. efficiency being relative to the higher heating value
It isn’t much but radiation loss has to be con- (HHV) of the fuel fired. The advent of combined cycle
sidered in addition to that combustion efficiency. The and cogeneration plants has resulted in the return of
manufacturer will provide you with a value of radiation lower heating value (LHV) to our definitions. There is
loss, equal to a percent of input at a prescribed boiler a significant difference in the values expressed by these
load. All you have to do is determine its impact at the two references, with an efficiency at the LHV always
actual load. Divide the manufacturer’s predicted loss by being significantly higher than an efficiency at the HHV.
the percent of boiler load and, if the predicted loss is at a In those rare applications where a CHX is applicable,
load other than 100%, multiply the result by the percent an LHV efficiency could be greater than 100% because
load for the prediction. In most cases the manufacturer’s the system recovers heat the heating value doesn’t ac-
prediction is at 100% load so you only have to divide knowledge as existing. LHV doesn’t include the heat
the predicted loss by the percent load. A few examples that could be extracted if the water in the flue gas was
should suffice: condensed. When I talk efficiency I’m talking HHV,
you’ll have to be aware that someone can use the LHV.
• A boiler with a predicted radiation loss of 3% at Can a boiler efficiency be greater than 100%? Logic
full load is tested and found to have a combustion says the answer is no but by the definition of some ef-
efficiency of 79% at a 50% load. The radiation loss ficiency labels some of them can. My favorite example is
at that load is 6% (0.03 ÷ 0.5) so the operating boiler the Nevamar project we did back in 1974. That system
efficiency is 73% (0.79 less 0.06) used heated air off a process as combustion air. It con-
tains a small amount of hydrocarbons with negligible
• A boiler with a predicted radiation loss of 2% at heating value but can, when one particular process is
80% firing rate is tested and found to have a com- operating, produce 360°F combustion air. When sup-
bustion efficiency of 80% at full load. In this case plied to the one boiler with an economizer and a stack
the operating boiler efficiency is 81.6% (0.8 +0.02 ÷ temperature of 303° it can produce results in the ac-
1 × 0.8) cepted definitions that exceed 100%. That, by the way, is
efficiency at the HHV.
• A boiler with a predicted radiation loss of 1.5% at If we considered the true and full definition of
75% firing rate is tested and found to have a com- boiler efficiency we would have to include the heat in