7221 Steam Turbine Systems

Steam turbines are currently used as prime movers in topping, bottoming and combined cycles. There are many types of steam turbines to accommodate various heat/power ratios and loads. For limited expansion (pressure drop) and smaller loads (<4000 HP) lower cost single stage backpressure turbines are used. When several pressure levels are required (and usually for larger loads) multi-stage condensing and non-condens ing turbines with induction and/or extraction of steam at intermediate pressures are generally used. Fig. 7.5 shows a variety of condensing and non-condensing turbines.

Four factors must be examined to assure that the maximum amount of power from a CHP steam plant is economically generated based on the process heat required. These factors are: (1) prime-mover size, (2) initial steam conditions, (3) process pressure levels, and (4) feedwater heating cycle.

1. Prime-Mover Type and Size. Process heat and plant electric requirements define the type and size of the steam generator. The type of CHP system and its corresponding prime mover are selected by matching the CHP system heat output to the process heat load.

If process heat demands are such that the plant power requirements can be satisfied by cogenerated power, then the size of the prime mover is selected to meet or exceed the "peak" power demand. However, co-generation may supply only a portion of the total plant power needs. The balance has to be imported through a utility tie. In isolated plants, the balance is generated by

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