3.1.4   The reheat cycle

 

The desire for further increases in cycle conditions and consequent increases in cycle efficiency, led to the addition of steam reheat during turbine expansion. The reheat cycle is shown schematically in Fig 1.26 and on the T-S diagram in Fig 1.27.

reheat cycle plant

reheat cycle T-S diagram

The reheat cycle incorporates an improvement in thermal efficiency over the superheat cycle.  The thermal efficiency from the T-S diagram is r?(Reheat) = (Area ABCDEF)/(Area GABCDEFH).

The reheat cycle benefits from reduced wetness in the turbine exhaust, but presents an increased capital outlay in terms of reheater pipework to, from and within the boiler. The turbine is usually split into HP and LP cylinders to avoid the high thermal gradients which would be introduced between stages of reheat on a single-cylinder machine.

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