Classification Of Super Heaters

A super heater is a device used to convert saturated steam or wet steam into dry steam. Super heaters are normally classified as radiant and convection super heaters. Radiant super heaters are located in the radiant zone receiving energy directly from the flame in the furnace. Convective super heaters do not receive furnace radiation. A few super heaters receive energy partly from the flame are called semi radiant. One of the most important accessories of a boiler is a super heater. We have fabricated a number of super heater coils, most of them very complicated in shape, gauge and size.

The other classification of super heaters depends on location arrangements and flow pattern.

• radiant super heater located at radiant zone of boiler
• Convection super heater located at the convection zone of boiler
•  In modern fluidized bed boilers to achieve higher steam temperature a portion of super heater located in the fluidized bed called bed super heaters.

Depends on arrangement

• Horizontal
• Vertical
• Inline
• Staggered

Depends on the flow pattern

• cross flow super heater
• parallel flow super heater
• counter flow super heater

Horizontal arrangement of super heaters have a advantage of easy drainage, which is quite important in boilers where shut down will be for longer periods. Drain ability helps in non accumulation of salts in water or steam inside the tubes.

In vertical arrangement the problem of expansion can be tackled easily than horizontal arrangement.

Staggered bundles are difficult to clean but they offer a marginal improvement in heat transfer coefficient. Inline arrangement provides lower gas side pressure drop.

Counter flow arrangement offers a slight improvement in log mean temperature difference and there by decreasing surface area compared to parallel flow. The parallel flow arrangement leads to cooler tubes. Metal temperature can be higher with counter flow arrangement since both gas and steam temperature are higher at steam exit portion.

Designing A Super Heater:

Designing a super heater or any other heat transfer involves three steps one is the thermal design, second is the mechanical design and other is the performance calculation.

In a thermal design we have to perform the heat duty , heat transfer and surface area required for the known thermal input and output parameters.

In a mechanical design thickness and material with standing capacity are to be checked.

Performance calculation involves confirmation of the designed equipment for various loads.