Page 66 - Boiler plant and distribution system optimization manual

Chapter 6

Efficiency Calculation Methods

A key part of a boiler efficiency improve- cedures. However, for practical boiler tests with
ment program is knowing the operating efficien- limited instrumentation, comparisons between
cy of the boiler and the corresponding increase in the two methods are generally poor. The poor re-
efficiency from as-found conditions to the final sults are primarily from the inaccuracies associat-
optimized condition. This may require several ef- ed with the measurement of the flow and energy
ficiency tests over an extended period. content of the input and output streams.
The following paragraphs discuss the vari- The efficiencies determined by these meth-
ous test methods and computational procedures ods are “gross” efficiencies as opposed to “net”
available for measuring efficiency. values which would include as additional heat
input the energy required to operate all the boiler
auxiliary equipment (i.e. combustion air fans, fuel
ASME COMPUTATIONAL PROCEDURES pumps, fuel heaters, stoker drives, etc.)
These “gross” efficiencies can be considered
The basis for testing boilers is the American essentially as the effectiveness of the boiler in ex-
Society of Mechanical Engineers Power Test Code tracting the available heat energy of the fuel. It
4.1. is important to take complete data when using
Figure 6.1 shows the ASME Test Form for Ab- this test form to fully document the test results no
breviated Efficiency Test or so called “ASME Short matter what procedure is used.
form” which is used for both the Input-Output
and heat loss methods. Figure 6.2 is the calculation
sheet for the abbreviated ASME efficiency test.
Both Heat Loss and Input-Output boiler ef- THE INPUT-OUTPUT METHOD
ficiency calculations are included in the ASME
Output
short Form. This power test code has become
Efficiency (percent) = ———— x 100
the standard test procedure in many countries. It Input
neglects minor efficiency losses and heat credits,
considering only the Higher Heating Value of the This method measures the heat absorbed by
input fuel. the water and steam and compares it to the total
energy input of the higher heating value (HHV)
of the fuel.
COMPARISON OF THE INPUT- This method requires the accurate measure-
OUTPUT AND HEAT LOSS METHODS ment of fuel input. Also, accurate data must be
available on steam pressure, temperature and
Both methods are mathematically equivalent flow, feed water temperature, stack temperature,
and would give identical results if the required and air temperature to complete energy balance
heat balance (or heat loss) factors were consid- calculations.
ered in the corresponding boiler measurements Figure 6.3 illustrates the envelope of equip-
could be performed without error. ment included in the designation of “Steam Gen-
When very accurate instrumentation and erating Unit.”
testing techniques are used, there is reasonably Figure 6.4 shows the relationship between
good agreement between the two calculation pro- input, output, credits and losses.

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