There are several industries, such as paper making, textile, chemical, dyeing, sugar refining, and so on, in which there is a dual demand for power and steam for heating and process work. Producing steam separately for power and heating is wasteful. If the turbine is operated with normal exhaust pressure and the temperature of the exhaust steam is too low to be of any use for heating purposes, by suitable modification of the initial and exhaust pressures, it would be possible to generate the required power and still have available for process work a large quantity of heat in the exhaust steam.
Figure 7.19 Back pressure turbine plant
Figure 7.20 Thermodynamics of back pressure turbine
The back pressure turbine may be used in cases where the power, which may be generated by expanding steam from an economical initial pressure down to the heating pressure, is equal to, or greater than the power requirements. The layout of such a plant is shown in Fig. 7.19.
Steam is generated in the boiler at a suitable working pressure and admitted to the turbine. The exhaust steam from the turbine will normally be superheated and in most cases, is not suitable for process work. A de-superheater is used to make it suitable for process work by spraying a jet of water, thermostatically controlled, on the entering steam. The steam is cooled and the spray water evaporated. The new saturated steam enters the heaters and is entirely condensed. The condensed steam may or may not be returned to the boiler. The thermodynamics of back pressure turbine is shown in Fig. 7.20.
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