Page 69 - Boiler plant and distribution system optimization manual

54 Boiler Plant and Distribution System Optimization Manual

Because of the many physical measurements 4. Heat loss due to combustibles in refuse. (for
required at the boiler and the potential for sig- coal)
nificant measurement errors, the Input/Output 5. Heat loss due to radiation.
method is not practical for field measurements at 6. Unmeasured losses.
the majority of industrial and commercial boiler
installations where precision instrumentation is This procedure neglects minor efficiency
not available. losses and heat credits and only considers the
Large errors are possible because this meth- chemical heat (Higher Heating Value) of the fuel
od relies on the difference in large numbers. If the as energy input.
steam flow is off by 2-3% and other instrumenta- In addition to being more accurate for field
tion have a similar level of error, then the cumu- testing, the heat loss method identifies exactly
lative error can become unacceptable, producing where the heat losses are thus aiding energy sav-
false information. ings efforts.
The Input-Output test method is also very This method might be termed the flue gas
labor intensive. Precision instrumentation must analysis approach since the major heat losses con-
be specified and installed. Test runs are usually sidered by this method are based on measured
more than four hours and must be rejected for any flue gas conditions at the boiler exit together with
inconsistent data. Trial runs are often required to an analysis of the fuel composition.
check out instrumentation and identify problems This method requires the determination of
with the boiler as well as to train test personnel the exit flue gas excess O (or CO ), CO, combus-
2
2
and observers. tibles, temperature and the combustion air tem-
Often plants cannot support testing for long perature.
periods because of load considerations. For ex- The heat loss method is a much more accu-
ample, a plant may not be able to provide either rate and more accepted method of determining
full load or partial load conditions for extended boiler efficiencies in the field provided that the
periods for various reasons which can cause pre- measurements of the flue gas conditions are accu-
mature curtailment of tests. rate and not subject to air dilution or flue gas flow
Problems like inconsistent water level con- stratification or pocketing.
trol or variations in outlet steam pressure can pre-
vent the stable thermal balance required for accu-
COMBUSTION HEAT LOSS TABLES
rate test information.
Tables for stack gas heat losses for different
THE HEAT LOSS METHOD types of fuels have been prepared separately and
are available for determining flue gas losses due
Efficiency(%) = 100 % – Heat Loss %
to dry gas and moisture losses.
The Heat Loss method subtracts individual
energy losses from 100% to obtain percent effi-
ciency. It is recognized as the standard approach HEAT LOSS DUE TO RADIATION
for routine efficiency testing, especially at indus-
trial boiler sites where instrumentation quite of- Radiation loss, not associated with the flue
ten is minimal. gas conditions can be estimated from the stan-
dard curve given in the ASME Power Test Code.
The losses measured are: Radiation Joss can also be measured using
1. Heat loss due to dry gas. a simple direct method, using an infrared instru-
2. Heat loss due to moisture in fuel. ment which has specially designed to detect ra-
3. Heat loss due to Hp from the combustion of diation losses from the boiler surface and gives a
hydrogen. read out in Btu/sq-ft/hr.