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For power plant boiler, it is a crutial way to increase thermal economy of thermal power station by improving parameter of superheated steam. The increase of superheated steam parameters are limited by metal materials. The design of superheaters must make sure that the temperature of outer wall of heating surface pipes is lower than allowable temperature of oxidation resistance of steel, and meanwhile make sure its mechanical strength. with the development of metal material used in boilers, pwoer station boilers in our country already generally apply high pressure, high temperature(9.8MPa, 540 degrees) and superhigh pressure parameters(13.7 MPa, 540 and 555 degrees), and have developed subcritical pressure parameters(16.7MPa, 540 and 555 degrees). Now lots of boilers apply supercritical pressure parameters(24.5MPa, 540-570 degrees), and even very few units apply higher pressure and temperture parameters.

With the increase of steam pressure, in order to reduce the steam humidity of steam turbine tail as well as further improve the thermal economy of power station, middle reheating system is widely used in high parameters power station, e.g. sending the exhaust of the high pressure cylinder in steam turbine again back to be re-heated in the boiler to high temperature, and then re-sending them back to the medium-pressure and low-pressure cylinder of the steam turbine, expanding and working. The element for reheating is called reheater.

In modern boilers, the heat absorption consumption of the superheaters and reheaters is over half of total heat absorption for the working fluid, thus the heating surfaces of superheaters and reheaters account for a large proportion in total heating surfaces of boilers, and they must be placed in the areas where smoke temperature is higher, the working conditions are the worst among all the heating surfaces.Tube wall temperature of the heating surfaces is close to the max. allowable temperature of steel, so the reasonable arrangement a dnd design of superheaters and reheaters matter so much for economy and reliability to the while boiler. When it comes to their design, on the basis of assurance of safety and reliability of superheater and reheater, it is very necessary to save metal consumption at the best, especially to save usage of alloy steel.

Functions and characteristics of superheaters and reheaters

functions of superheaters and reheaters:

Superheaters and reheaters are important parts for the heating surfaces, used to increase steam temperature, aiming to improve steam enthalpy and further to increase the thermal circulation efficiency for power station.

Superheaters work to heat surnatured steam to superheated steam with a certain tempetature. When boiler loads or other working conditions change, it is necessary to make sure superheated steam temperature to be normal, and within allowable range of temperature variation.

From the thermal circulation in power station, the original parameters of pressure and temperature is higher, the thermal efficiency of circulation is better, or the steam humidity of the steam turbine rear is too high, influencing its safety. While the superheaer steam temperature is limited by metal materials, at present, due to limitation of metal materials, superheated temperature for most power station boilers are still within the range of 540-555 dgrees, and to avoid the steam humidity of blade of steam turbine rear too big, thus middle reheating system is applied.

Functions of reheaters are to heat exhaust of steam turbine high pressure cylinder to the reheated temperature which is close or equal to superheated temperature, and then send them to middle pressure cylinder and low pressure to expand and work to increase the dryness of blace of steam turbine rear. Usually, the pressure of reheated steam is one fifth of that of superheated steam. Application of reheating system can increase the thermal efficiency for power stations by 4-5%.