Advantages Of Air Heater

In addition to increase in boiler efficiency the other advantages that may result are

• Stability of combustion is improved by use of hot air.
• Intensified and improved combustion which results in faster load variation and fluctuation.
• Permitting to burn poor quality coal.
• High heat input to the furnace and hence high heat flux and high heat transfer rate.
• Less unburnt fuel particle in flue gas, thus combustion and boiler efficiency further improved.
• In case of FBC and pulverized coal combustion hot air can be used for drying the coal as well as for transporting the coal into furnace
• This being a non pressure part shut downing of unit is not required due to corrosion of heat transfer surface or failure.
• High temperature of inlet cause reduction in carbon mono oxide.

Heat Transfer In Air Heater

The operation region involves low temperatures, where heat transfer takes place predominantly through convection and little conduction. Since the thickness of tube is very less conduction heat transfer can be neglected.

Convection heat transfer takes place in two modes

• gas to metal heat transfer.
• metal to air heat transfer

as the air heater is located in cold end of heat transfer side, non-luminous radiation will be very negligible and hence the same can be eliminated.(above 400ºC non luminous radiation heat transfer will be negligible).

Designing air heater

• Gas or air velocity inside tube has to be limited to maximum of 15 to 18 meters/sec and out side tube has to be limited to 6 to 7 meters/sec beyond which hysterics will occur.
• The air or gas passes over the tubes, where the lesser number of tubes deep arranged this is to avoid higher pressure drop.
• The unit height of tube should not exceed 3.5 meters to avoid vibration problem and preferable height be 2 to 3 m between tube sheets.
• In FBC and convectional boilers it is desirable to heat the air not more than 180ºc, in order to have a better cooling for grate or distributed plate.
• Normally a tube diameter 50.8 mm or 63.5 mm will be used.

Air Preheater

An air preheater (APH) is a general term to describe any device designed to heat air before another process with the primary objective of increasing the thermal efficiency of the process. The purpose of the air preheated is to recover the heat from the boiler flue gas which increases the thermal efficiency of the boiler by reducing the useful heat lost in the flue gas.