Alloy Steel Superheater And Reheater For Pulverized Coal Boilers With Natural Circulation

For boilers in large power stations, superheaters and reheaters are necessary parts, to a large extend, they have influced 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.

Details

• Superheating raises overall efficiency as well as avoids too much condensation in last stages of turbine which avoids blade erosion.

• The heat of combustion gases from furnace is utilised for removal of moisture from steam and to superheat the steam.

• Super-heaters ususlly have several tube circuits in parallel with one or more return bends,connected between headers.

• Heat from the hot gases to the vapour in the surperheater is transferred at high temperatures.

• Therefore promary section of superheater is arranged in counterflow and secondary section in parallel flow to reduce the temp.stressing of the tube wall.

Working characteristics

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 tempeature 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 parrallel tubes.

The inlet steam of reheaters is the outlet steam of steam turbine high pressure cylinder, its pressure is about 20% of that of main steam, whose temperatre is slightly higher than surnatured temperature, and thus increases both steam and heat consumption. For example, the resistance of reheating system is increased by 1kg/cm2, the steam consumption of steam turbine is increased by 0.3%. To reduce the flow resistance of steam in reheaters and improve the thermal system efficiency, reheaters usually apply small mass flow rate, but meanwhile it makes worse for cooling conditions of tube wall of reheaters. Therefore, the design of reheating system requires simple to decrease flow resistance.

Application

use for heating saturated steam, steam temperature could be up to 900 degree C.

Superheater Capacity Upgrades to Increase Power Generation

Send us required technical data, such as steam capacity, presure, required temperature etc. We will design accordingly.

Differences