The execution of performance tests is no easy task, it may require the installation of a vast amount of high precision instruments, the coordination and synergy of plant operators, site engineers and test coordinators. It requires careful planning, creation of detailed test procedures, and revision of hundreds of plant documents and station instruments datasheets.

The first question to answer is whether to run a direct measurement test or an indirect method. There is not single correct answer; the test objective will dictate the way forward. If one requires the most accurate efficiency test involving solid fuels, such as coal, the indirect method should be used.

If you are trying to isolate the most significant losses in your system, only the indirect method will allow you to do that. But, what is the indirect method? This article will discuss what is and how this method of testing is advantageous to measure efficiency of solid fuelled boilers/plants.

The direct method only requires the measurement of the fuel flow for the calculation of heat input to the steam generator and the heat output.

In simple terms, the efficiency of a steam generator can be evaluated with the following expression in a percentage basis:

(01)

The heat output from the steam generator can be calculated by the algebraic sum of all incoming and outgoing heat flows from the steam generator as described in ASME PTC 6. (American Society of Mechanical Engineers, 2004)

The indirect method for calculating the thermal efficiency of a steam generator unit consists of carrying out a heat and mass balance around the steam generator unit by quantifying all the incoming and outgoing energy streams, as well as a bookkeeping of the credits and losses of heat.

The energy input and energy output streams are the same as in the direct method, i.e. the energy input (QrF) to the steam generator is the heat input from the fuel and the energy output (QrO) is the algebraic sum of all incoming and outgoing heat flows from the steam generator. This method also requires the determination of all the heat losses (QpL) and heat credits (QpB) of the cycle. The energy output can be defined as:

(02)

or:

(03)

And:

With the above definitions in mind, the steam generator efficiency (fuel efficiency) can be expressed in a percentage basis in the following fashion:

(04)

Comparing uncertainties of both methods:

Table 1. Typical Code Test Uncertainties for Efficiency. (American Society of Mechanical Engineers (ASME), 2013)