What is Reheater on Boiler? – Reheater is a part of boiler which to reheat steam output from the first level of steam turbine. Reheated steam will again absorb the heat energy from boiler to be used in the next level steam turbine. Reheater is one way to improve the thermal efficiency of the Rankine Cycle. Visit the following article to find out how to improve the thermal efficiency of the Rankine Cycle.
Let us consider the following Rankine T-s diagram.
The T-s diagram above is the ideal, standardized Rankine Cycle diagram, without the use of the reheater concept. Superheated steam produced by the boiler only once flows the turbine blades, and ends with a condensation process in the condenser. Next let’s compare it with the Rankine Cycle with the reheater system.
The Rankine Cycle equipped with a reheater, will have at least two levels of steam turbine: high pressure turbine and low pressure turbine. High pressure steam turbine output steam, commonly known as cold reheat steam, has lower pressure and temperature of main steam, before entering the turbine. Although the cold reheat pressure is drop significantly, it still has not reached its saturation point. So, if this steam reheated, it will return to superheated steam. Therefore the cold reheat steam that exits the high pressure turbine, will be reheated back into the boiler.
Modern boilers equipped with reheater pipes, specially designed so that the reheated steam absorb heat just like the main steam. The reheated steam, also called hot heat reheat steam, will reach the same temperature as the main steam vapor. To achieve this, the reheater pipes will be placed not far from the final pipes of the superheater. More specifically, let us refer to the following supercritical boiler diagram.
Seen in above boiler diagram, reheater pipes are divided into two heating stages. First the cold reheat steam passes through the Low Temperature Reheater pipes, which shown by the name “LTRH” at the diagram. Furthermore, LTRH output steam flows to the pipes called Final Reheater. The steam produced by the Final Reheater is then called Hot Reheat steam. Hot Reheat steam then goes to the low pressure steam turbine, so that the heat energy contained in it is converted into mechanical energy of the turbine shaft rotation.
Theoretically, the addition of one stage reheater utilization will increase the thermal efficiency of the rankine cycle by 3-4%, the addition of two stages of the reheater increases the efficiency by 1.5-2%, the addition of the three-stage reheater increases efficiency by 0.75-1%, and so on. Commonly, modern boiler only use one or two stage reheater.