Steam Temperature Adjustment and Control.

Improvement in the heat rate of modern boiler units and turbines results mostly from the high cycle efficiency possible with high steam temperatures.

Important reasons for accurate steam temperature regulation are to prevent failures due to excessive metal temperatures in the superheater, reheater,or turbine; to prevent thermal expansion from dangerously reducing turbine clearances; and to avoid erosion from excessive moisture in the last stages of the turbine.

The control of temperature fluctuations from variables of operation, such as slag or ash accumulation, is also important.

With drum - type boilers, steam output is maintained by firing rate, while the resulting superheat and reheat steam temperatures depend on basic design factors, such as total surface quantity and the ratio of convection to radiant heat absorbing surface. Steam temperatures are also affected by other important operating variables such as excess air, feedwater temperature, changes in fuel that affect burning characteristics, ash deposits on the heating surfaces, and the specific burner combination in service. In the once- through boiler, which has a variable steam- water transition zone, coordination of the firing rate and the boiler feedwater flow rate control steam output pressure and temperature, leaving reheat steam temperature as a dependent variable. Standard performance practice for steam generating equipment usually permits a tolerance of ±10F (6C) in a specific steam outlet temperature.