Stainless Steel Superheater And Reheater Tube Coils as Boiler Parts for Coal-fired Boilers

Stainless Steel Superheater and Reheater Tube Coils for Heat Exchanger and Vessels of Boiler System

Summary

For power plant boiler, it is a crucial 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, power 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 temperature 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.

Working characteristics of superheaters and reheaters:

Because what flow through tubes of superheaters and reheaters are high temperature steam, the convective heat release transfer coefficient of tube wall to steam is quite small, and heat transfer performance is poor, the cooling capacity of steam to tube wall is low. Meanwhile, to make sure reasonable temperature difference of heat transfer, usually superheaters and reheaters are placed at the areas where smoke temperatures are high, and their working temperature of tube wall is rather high, thus the working environment of superhearters and reheaters are quite bad. Especially reheaters, for reheated steam pressure is low, and their capacity to cool tubes are worse, thus that how to make sure tube metal can be safe working on the long term is an important issue when comes to design and operation of superheaters and reheaters. Diagram 4-1 lists the upper limit of allowable temperatures for common metal used in boilers. To avoid use of higher level of alloy steel, when it comes to design of superheaters and reheaters, used tube metal are almost close to the temperature limits when working. This time, if there is rise of temperature of 10-20 degrees, which also reduces the allowable stress a lot. Therefore, Attention should be paid for the design and running of superheaters and reheaters in aspects below:

1.to make sure the steam temperature stable when running. The variation of steam temperature is supposed to be not over+5- -10 degrees;

2.There needs reliable method to adjust temperatures for superheaters and reheaters when operation conditions changes to some extend to maintain rated steam temperatures.

3. to minimize the thermal deviation between parallel tubes.