Methods of Determining Boiler Efficiency

The thermal efficiency of steam generators can be usually determined using the following two methods.That is:

Input-output method

Here, the efficiency is determined as the ratio of heat absorbed by the working fluids (water and steam) to the heat input (chemical heat plus heat credits added to the steam generator).

Heat loss method

The heat loss method requires the determination of all accountable heat losses, heat credits and the heat in the fuel. The efficiency is then equal to 100 minus the ratio of the sum of all heat losses to the sum of heat in the fuel plus heat credits.

It is recommended that the major heat losses also be determined when using this method.

The two methods are equally acceptable. Which method is to be given preference depends on the technical resources. Where solid fuels are used, for example, it is not possible or extremely difficult to accurately measure large mass flows. Here, the only choice is the heat loss method, which should also be adopted when the fuel properties are subject to large fluctuations. However, it is normally possible to take accurate measurements of fuel flow in gas or fuel oil firing installations. In this case, the input-output method may be the better choice, especially fo small steam generators, owing to the uncertainty involved in the measurement of radiation and convection losses.

The two methods have different levels uncertainty. The method with the higest accuracy should always be employed.

From: Acceptance Testing of Steam Generators DIN 1942