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Boiler Reheater Brief Introduction

1. In a normal three-pressure reheat steam cycle the intermediate-pressure output of the heat recovery boiler joins the returning cold reheat line from the steam turbine and the combined flow goes through the reheater.

2. At low loads, below 20%, there is no steam flow through the transition pieces and all of the intermediate-pressure output goes to the cold reheater inlet as in a conventional three-pressure heat recovery boiler.

3. As the load builds up, the intermediate-pressure flow to the cold reheater is gradually reduced as steam is diverted to the transition pieces. By the time that the gas turbinereaches full load, most of the intermediate-pressure steam is going through the transition pieces, where it enters at 275 °C and leaves at 485 °C.

Processing

1. The superheated steam then flows through the main steam piping to the high-pressure turbine. Exhaust steam from the high-pressure turbine is guided to the boiler for reheating and from there to the intermediate and low-pressure turbines.

2. High reheating temperatures improve the output and efficiency of a power plant.

Reheater Function

1. Therefore the number of reheat stages can not be many as the pressure drop will reduce efficiency of thermal cycle.The steam pipes and reheater coils are to be so designed to limit the drop to 2–3%.

2. The control valves in reheat steam path is not desirable.Systems using HP LP bypass require these valves to control steam flow during bypass operation.

3. The efficiency of the Reheat cycle can be improved by increasing the pressure and temperature of steam entering into the turbine. As the initial pressure increases the expansion ratio of the turbine also increases and the steam becomes quite wet at the end of expansion.

Main Advantage

The main advantage of reheat cycle is to reduce the specific consumption and consequently reduce the size of the boiler and other auxilaries for the same power.The reheat cycle is preffered for the high capacity above 100 MW plant.

Differences

Accessory(SS Shield)