Carbon Steel Low Temperature Boiler Superheater in ASME/GB Standard

Carbon Steel Low Temperature Superheater and Reheater Tube Coils for Heat Exchanger and Vessels of Boiler System

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.

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.

The reheated steam pressure is low, and its temperature high and specific volumn big. Its mass flow rate is smaller compared with superheated steam, while its volume flow rate is far bigger than superheated steam(about 4-5 times). Therefore, not to make sure steam flow rate too fast and flow resistance too big, it is necessary to increase the cross section area of flow passage for the steam. Therefore, in the middle reheating boiler, the flow passage area of reheaters is always bigger than that of superheaters. For convective reheaters, usually their structures apply tubes with bigger diameters and surpentine pipes bundle with more tubes in parallel.

For reheated steam pressure is low, cofficient heat small, it is very sensitive to thermal deviation. In another word, at the same thermal deviation conditions, outlet temperature difference of reheated steam is bigger than that of superheated steam.

Outlet steam temperature of reheated steam is influenced by inlet steam temperature. To unit generator, under the condition of constant pressure operation, the exhaust temperature of steam turbine high pressure cylinder is decreased with the decrease of load. At this time due to low inlet steam temperature of reheaters, thus it makes outlet steam temperature is decreased correspondingly. For convective reheaters, their characteristics of convective steam temperature is more significiant, and their tributes of steam temperature adjustment is bigger than that of superheaters.

When the boiler is started, or the furnace is stopped, or safety door is on, there is no steam flow or small flow in reheaters, which possibly burn out reheaters. Thus, it is needed to install a high and low pressure bypass system between reheaters and superheaters, then when boilers get on or off, or the steam turbine sheds its load, reheaters can get enough cooling steam from the bypass device.