ASME Standard Carbon Steel Boiler Superheater Tube Coil for Boilers

Carbon Steel Superheater Tube Coil for Heat Exchanger of Boiler System in Power Station

Details

For boilers in large power stations, superheaters and reheaters are necessary parts, to a large extend, they have influnced the safety and economy of boilers. Superheaters are the heating surfaces which heat the main steam from the surnatured temperature to the rated superheated temperature, and the reheaters are the heating surfaces which heat the exaust from the steam turbine high pressure cylinder to a certain temperature.When running conditions like boiler loads, coal types change, it is to adjust them to make sure the outlet temperature at the rated level of -10-+5 degrees.

For large boilers in power stations, superheaters and reheaters account for a big proportion in the total heating surfaces, they are to be arranged in the areas where smoke is with higher temperature. To improve the efficiency of thermal circulation in power stations, it is needed to improve the original parameters f steam gradually, the increase of steam pressure requires the corresponding increase of temperature for superheated steam, or the humidity of large stage exhaust of steam turbine will be too high, which will influence its safety. Thus, the steam flowing through superheaters and reheaters are high temperature steam, their heat transfer performance is quite bad, which determines that tube wall temperatures of superheaters and reheaters are high.

Working characteristics of superheaters and reheaters:

To improve the thermal efficiency, the pressure of superheated steam have been increased from superhigh pressure to subcritical and supercritical pressure. But limited by the metal performance, the steam temperature of superheaters and reheates are kept within 540-555 degrees.

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.