Heat Loss Method

Heat loss (or indirect method as per BS) is

1. Practical because it calls for no flow measurements to calculate efficiency

2. More accurate because it involves measuring only the loss and not the efficiency

In measurement of any loss, the sensitivity of any inaccuracy is considerably reduced on the final result. For example, in a boiler of 80% efficiency, a 2% error of a parameter reflects as 1.6% in the final result from the input-output method whereas it reduces to only 0.4% from the heat loss method. This is because it is 2% of 80% in the former, whereas it is 2% of

20% in the latter. This method has been overwhelmingly adopted in the industry, particu­larly for acceptance testing.

ASME PTC 4.1 (1964 and reaffirmed in 1991) specifies an abbreviated test procedure more suitable for

1. Routine testing of all boilers

2. Acceptance testing of industrial boilers

Only the major loss is considered and only the chemical heat of the fuel is taken as the input. Humidity loss is small and is not considered in abbreviated testing.

For utility boilers a long test, considering all loss and heat credits, is recommended, but many times the abbreviated procedure is adopted for its practical approach and simplicity. Besides, the costs of boiler testing are lower.

The main measurements in the heat loss method for calculating efficiency are

• Temperatures (feed in, SH out, RH in, RH out, exit gas, and ambient air)

• Pressures (SH out, drum, SH in, and RH out feed)

• Flue gas analysis at exit preferably by Orsat analyzer

• Heating values of ash and fuel

Steam flows of SH and RH are also measured to establish the evaporation, but they do not figure in calculating the efficiency. They are measured to ensure that the flow condi­tions are met. The salient points with regard to the performance testing of boilers are as follows:

• For setting the boiler parameters correctly and tuning the performance and con­trols, the boiler should be handed over to the manufacturer a couple of weeks or so before the test, as required. Larger boilers require longer tuning time. For indus­trial boilers a week should be sufficient.

• More time is required to set up the test instrumentation, particularly the thermo­couples, gas sampling lines, and ash-collecting probes. In a large utility this can take typically 6 weeks. The time may be reduced if more gauge instrumentation is accepted instead of test instrumentation.

• The actual test should be short, >4 h as per code and <6 h by practice, as running the boiler with constant load is not easy in power and process plants. Also there should be no soot blowing and blowdown.

• Peak and minimum output demonstration tests are kept outside of this main test period.

• Part load efficiency test or testing on alternate fuels, if required, is scheduled for different days.

• The duration of the test, procedure, scope, fuels, tolerances, etc. should be agreed in advance.

• The testing of the boiler should be a steady state operation; the boiler should be running at the prescribed load for a few hours before and after the test.

• All the instruments are required to be of test quality and must be calibrated. It is normally necessary to perform trials for a couple of days to get systems, instru­ments, and manpower properly tuned.

• All test readings are taken usually at 15 min intervals except for fuel and ash sam­ples that are taken every hour.

The test codes explain the requirements in detail, and they must be referred to for more information on procedures, precautions, and calculations.

The enthusiasm to determine the exact efficiency should be tempered with the reality it is an expensive and difficult task to isolate a boiler for testing purposes for a week or two. Many times, therefore, the testing is not undertaken, particularly when the operating staff is convinced that the boiler meets needs.

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