105.What is Re-heater? How to control re-heat outlet steam temperature?

In large boilers, to increase the efficiency, the steam from the HP cylinderr outlet  will be taken to the boiler again and passed through the set of heating coils and used at the MP cylinder. This heating coil is known as re-heater.

In a wall fired boiler the flue gas from Eco-inlet will be pumped through a fan to the furnace. This is known as gas re-circulation. This way the flue gas mass will be more than the steam flow. This will help to increase the temperature pickup in the re-heater. In this case the re-heater pickup may be mainly by convection.

Some times flue gas will be bypassed over the re-heater.

In a corner fired boiler, burners will be tilted up or down to control re-heat temperature. In this case the Effective Projected Radiant Surface (EPRS) of the re-heater is changed. In this type of arrangement, re-heater coils are in such a way that the effective heat pickup is by radiation.
Two methods are in practice: Reheat Spray control, Burner tilt/Bypass damper/Gas re-circulation control systems.