Whenever electric power cannot be purchased from outside, or it is too expensive to bring to the plant, or the external grid is unreliable, or, alternatively, there is a significant amount of excess steam available from the process Plant, steam may be used as a driving force to generate electricity: this can be achieved by means of electrical generators driven by condensing or back-pressure turbines.

In the first case, the exhaust steam is condensed to water in a surface condenser: the volume of the steam is dramatically reduced in the condenser, thus reducing the pressure to vacuum conditions. This process maximizes the pressure drop across the turbine, which enables the maximum amount of

energy to be extracted from the steam. The condensate is then pumped back into the boiler as feed-water to be reused. Clearly, condenser systems need a constant ample supply of cooling water.

Back-Pressure Turbines do not use condensers: they discharge exhaust steam at positive pressure for process or heating services. This arrangement is commonly used where large amounts of heat are required, such as in refineries, gas plants, pulp and paper plants, desalination plants etc.. Exhaust steam may also be used to power mechanical drives for pumps, fans and materials handling. The boiler and turbine must of course be oversized to take into account all possible scenarios with possible peak demands.