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Cross compound steam turbine arrangement

Compounding is the splitting up, into two or more stages, of the steam pressure or velocity change through a turbine. Pressure compounding of an impulse turbine is the use of a number of stages of nozzle and blade to reduce progressively the steam pressure. This results in lower or more acceptable steam flow speeds and a better turbine efficiency.



Velocity compounding of an impulse turbine is the use of a single nozzle with an arrangement of several moving blades on a single disc. Between the moving blades are fitted guide blades which are connected to the turbine casing. This arrangement produces a short lightweight turbine with a poorer efficiency which would be acceptable in, for example, an astern turbine.

The two arrangements may be combined to give what is called 'pressure-velocity compounding'. The reaction turbine as a result of its blade arrangement changes the steam velocity in both fixed and moving blades with consequent gradual steam pressure reduction. Its basic arrangement therefore provides compounding.

Cross compound steam turbine arrangement

Fig: Cross compound steam turbine arrangement

The term 'cross-compound' is used to describe a steam turbine unit made up of a high pressure and a low pressure turbine . This is the usual main propulsion turbine arrangement. The alternative is a single cylinder unit which would be usual for turbo-generator sets, although some have been fitted for main propulsion service.


Reheat

Reheating is a means of improving the thermal efficiency of the complete turbine plant. Steam, after expansion in the high-pressure turbine, is returned to the boiler to be reheated to the original superheat temperature. It is then returned to the turbine and further expanded through any remaining stages of the high-pressure turbine and then the low-pressure turbine.


Temperature measurement instruments - use of Thermometers, Thermocouple, Radiation pyrometer & Thermistor

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