3.2.2 Heat rate
The principles of stage and cylinder efficiency having been introduced, consideration is now given to the definition of turbine heat rate. The heat rate is determined by measurement of various plant operating parameters. These include:
- Flow rate, using a differential pressure device.
- Enthalpy, based on calibrated pressure and temperature measurement.
- Electric power, based on calibrated voltage and current measurements.
Consider a steam cycle with single reheat and regenerative feedheating, shown in Fig 1.33.
The heat rate is defined by:
A means of measuring cycle heat rate having been defined, consideration can now be given to variations between the design heat rates quoted by the turbine manufacturers and the heat rates achieved in operation.
First, consider the variation of the hourly heat consumed by the turbine-generator with the load produced. This characteristic is known as Willans line (Fig 1.34), and is based on turbine test runs at 100%, 80%, 60% and 40% load. It is British practice to specify performance and test at these four loads. A linear relationship exists and extrapolation to the no-load output condition reveals a no-load heat consumption of about 3% of the full-load value. The variation in heat rate with load (Fig 1.35) shows the high thermal cost of operating plant on part-load. The optimum heat rate (i.e., the minimum condition) should correspond to the design output (100% load). The major contributory loss comes from the throttling loss across the turbine governor valves. The effects of throttling are discussed later in this chapter.
Operating the unit at part-load is one of the 'external' factors preventing the achievement of the design heat rate. The other major external factor, which increases average heat rate, is the need to start the unit as required by the operating regime. During start-up, the unit is unloaded and additional works power is needed for start-up systems.
Plant operating losses cause the other major increase in design heat rate. In practice, turbine heat rates increase due to:
- Deterioration of cylinder clearances.
- Deterioration in feedheating efficiency.
- Poor control of superheat and reheat temperature, (particularly in older designs).
These factors become more prevalent as the plant ages.