The function of a superheater in a thermal power plant is to superheat the wet or saturated steam up to the desired temperature by transfer of heat energy.
Generally, the superheaters are placed in the passage of hot flue gases or directly over the furnace depending upon its design and applications.
The superheating and reheating of steam are done in a range of 440˚C to 650˚C in modern power plants.
A superheater utilizes 40% of the total heat generated by a modern boiler.
- The advantages of superheater and reheating of steam are;
- It improves the overall efficiency of the plant.
It reduces the moisture content in the last stages of the turbine. It helps in preventing the erosion of blades and corrosion of various components of a steam turbine.
The superheaters are made of special high-strength steel (in case of superheating is done above 450˚C) and less costly carbon steel (in case of superheating is done below 450˚C).
Classification of Superheater
The superheaters are classified according to the method of heat transfer and their location.
- Convective Superheater (CSH)
- Radiant Superheater (RSH)
- Pendent Superheater (PSH)
Convective Superheater (CSH)
This type of superheater is often called a primary superheater. In convective superheaters, the steam from the boiler drum flows into superheaters, and heat is transferred from flue gases.
By energy balance,
In most cases, the superheaters are placed ahead of the economizer in a convective zone of the boiler furnace.
The superheated steam from the convective superheater is transferred to the radiant superheater (RSH). The RSH is placed in the radiant zone of the boiler furnace. Therefore, the heat is transferred in steam by radiation.
For temperature control, the steam from the radiant superheater is supplied to the desuperheater. And them it supplied to the combined convective and radiant superheater. These are usually pendent-type superheaters and the temperature of the steam is controlled up to the desired value.
Desuperheater is located between the primary and secondary superheater to reduce the quality of steam by spraying highly pure water either directly into the steam or outside of the chamber containing steam.
Radiant Superheater (RSH)
The heat transfer in radiant superheater is proportional to . So, these superheaters are able to absorb a high amount of radiant heat.
In a convective superheater, the temperature of steam at exit increases with an increase in load due to increased mass flow rate of fuel and temperature of flue gases.
The combined effect of convective and radiant superheater in series is to give an almost constant temperature of superheated steam.
In a radiant superheater, the steam velocity is kept in a range of 7-8 m/s.
Pendent Superheater (PSH)
In a pendent superheater (PSH), the heat is received by both convection and radiation. And it is used for applications needing high temperatures of superheated steam.
It has firm structural support. But the disadvantage is that the flow of steam is blocked due to condensation of steam after a cold shut down.
Therefore, the pendant superheater needs a slow restart to purge water that gets accumulated at the bottom of the superheater coil from the previous run.
The velocity of steam is kept in the range of 15-20 m/s.
Types of Coils or Platens for Radiant and Pendent Superheater
Radiant and Pendent superheaters are used in form of coils or platens. These coils may be single, double with one tube, or double coil with a double tube.
This heat exchanger provides a very large surface area per unit volume.
The platen-type superheater may be triangular or rectangular. These are compact heat exchangers that can be hung from the top. And it also provides a large surface area per unit volume.
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