Power plant cycles have the steam from the high pressure turbine reheated before it goes to the medium pressure and then to the low pressure turbine. This increases the cycle efficiency of the plant. Designers sometimes use double-reheat turbines to increase cycle efficiency. As the steam from the high pressure turbine has done work, the pressure and temperature are low. Hence the steam around 40 bar is heated to even above the main steam temperature. The temperature at the outlet of reheater will have to be maintained within a narrow band. Designers adopt many methods to achieve this depending upon the type of furnace and burner.
In designs where the reheater surface is kept in a more convective region, it is possible to control the reheater temperature by simple mechanisms like increasing or reducing the flue gases flowing over the sections. As the convective heat transfer coefficient depend more on the mass flow, and in turn the velocity, this method has been adopted by designers successfully. This method is most often used in wall fired units. Here the second pass of the boiler is divided in to two parallel paths up to the economizer. The two parallel paths are so designed to have a normal one third and two third ratio of flow area. The two third ratios of area being adopted for low temperature superheater surface and the one third flow area for the reheater surface. Dampers at the bottom of this flue gas passage are used to control the flow. A set of dampers is kept in the bigger area so that closing of these dampers will divert flue gas to the smaller area side where the reheater surface is. This increases the pickup in the reheater steam and thus the outlet temperature of the reheater is increased. Reducing the flow by opening the damper in the other parallel path will reduce the flue gas flow through the reheater section and thus reduce the reheater steam outlet temperature. Even though the logic of this design is simple, the construction and maintenance aspect suffer considerably when coal and low grade fuels are used.
In models where the designer has opted to keep more of the radiant surface in the reheater heat transfer area, the method of shifting the flame is used. Such is the case for tangential fired boilers. Here the burners in the corners are tilted up or down in unison to increase the radiant heat going to the reheater surface. This also affects the superheater heat absorption. The burner tilting mechanism is so designed that all the burners in all corners tilt up or down based on the signal from the reheater outlet steam temperature. It has been the experience when low grade coal is being used for power generation keeping the burner tilt in one position for a long duration can lead to seizure of the tilting mechanism. Hence designers generally advise putting the tilt on manual and move from the maximum to minimum position once a day before bringing it back to the original required position and putting it back on auto.
The third alternative is to use injection of water to de-superheat the reheater steam. This method affects the cycle efficiency of power plant. Hence this method is generally used only as an emergency system to control the reheater steam temperature during upset operation. However it is one of the simple methods and results in a good economy over the life period mainly for sliding pressure operation designs.
There are other methods like Gas Recirculation, Excess Air Control, Selective Burner Operation, etc. In the case of the gas recirculation method, the flue gas is tapped from the second pass at a convenient location and re-circulated into the furnace. This changes the convective heat transfer coefficient in the boiler thus changing the absorption in the convective heat transfer surface. The amount re-circulated is sized in such a way the temperature control is achieved. Excess air control to vary steam temperature is currently not in practice as it brings down the boiler efficiency and performance. However in fractional loads this method can be useful. Selective burner operation is adopted by some designers and operating groups depending upon the furnace outlet temperature requirement. This further controls the steam temperature in reheaters.
In all these methods it has to be remembered that whatever is done for reheater temperature control will also affect the superheater temperature, the effect of which the designers take care while designing the total process.