Steam Temperature Control

Super heater temperature depends on boiler working pressure. The degree if super heat varies directly with respect to boiler working pressure. The various steam outlet temperature corresponding to a pressure is fixed up by cycle efficiency and turbine manufactures practice. Operating variables like flue gas temperature inlet. Load. Manufactures practice. Operating variables like flue gas temperature inlet, load, excess air, fuel creates fluctuation in steam temperature leaving the final super heater. If there is any fluctuation in steam temperature there is a change in volume of steam which will affect the turbine  performance, since turbines are designed for predetermined steam volume flow between stator and rotor and also the exhaust steam temperature quality will varies with steam fluctuation which will affect condenser performance. Hence it is mandatory to have a steam temperature control.

In practice two types of temperatures( de- super heater) is widely used in boiler industry , they are

• spray type
• surface type

In the above two types, spray type temperature gives a faster temperature control compared to surface type. However quality water spraying into the super heater should be taken care for boiler and turbine life. Attemperators temperature technique is nothing but a simple energy balance. It consists of spraying water in mist form in between stages, depends on the final steam temperature and pick up by individual stages. The quantity of spray inside de superheated accomplished by mean of nozzle arrangements.

Surface type attemperators are further divided into two types submerged type and shell and tube type. In submerged type the steam coils is submerged below the durm water level. Part of the steam flows through the submerged coil is regulated in such a way that the  outlet steam temperature is of desired level after mixing.

In shell and tube type attemperators super heater header itself modified into an exchanger. Steam from super heater coils meter this intermediate header and leaves to second stage. In the super heater header the cooling water coils enter in both the sides and leaves. The steam gets cooled when it contact with low temperature cooling coils. In this steam temperature is adjusted by regulating the flow of cooling water flow inside the coils. Higher the water flow team cooling will be more. The simplest type of desuperheater is an unlagged section of pipe, where heat can be radiated to the environment.  The water which absorbs heat usually mixed with feed water to avoid the heat loss.