Super heater is useful for increasing the cycle efficiency of the power plant. super heater converts the saturated steam that is generated from drum into super heated steam and is sent to turbine.
Super heater increases the steam temperature by keeping the pressure constant. super heated steam are usually in the range of 540 deg Celsius.
Types of super heater:
Latent super heater:
This super heater is placed at the back pass of the boiler. It gains heat through convection from flue gas. It is the primary super heater that heats the steam coming from the steam drum. It helps to remove the water carry over, hence the name latent.
It is place in low temperature zone of furnace so it is also called as low temperature super heater. The steam from latent super heater passes to the platen super heater for further heating.
Platen Super heater:
platen is a vertical single loop placed in the furnace with many parallel tubes carrying steam. platen has set of closly packed tubes appearing like plate. they are place in the upper part of the furnance. they pick up heat enterily by radiation as it is placed in the furnace top.
The platen coils are place 600mm- 1000mm apart to avoid formation of ash build up.
with increase in steam flow the steam temperature from the platen super heater decreases. and with decrease in steam flow the steam temperature increases This is because at lower steam flow steam absorbs greater heat and becoming hotter at low load.
Pendant Super heater:
Pendant super heater is a vertical or horizontal multi loop super heater placed in the convection section of boiler with one or many parallel tubes carrying steam. the pendant tubes are place with space.
It is normally vertically hanged from top, but also can be found as horizontal. The heat transfer is by a combination of radiation and convection but predominantly by the latter. With increase in steam load the temperature of the steam increases.
both pendant and platen super heater are required in a boiler to get to attain high steam temperature needed.Their characters can complement each other to get a fairly constant temperature at all loads thus minimum attemperation is required.