75.What is the need for an Airheater? How many types are there? What is cold end corrosion? How to protect from cold end corrosion?

If the heat input from fuel is extracted as much as possible, the efficiency of the boiler will increase. The flue gas temperature at the outlet of economizer is around 300 Deg.C. The process media, which is less than 300 Deg.C., is air used for combustion. The heat ex changer used to extract the heat from flue gas after economizer to air is called Air heater.

Two types are in the market. One is Tubular and the other one is Rotary air heater. Tubular air heater is used in small boilers. Rotary air heaters are used in large size boilers above 110 MW.

The average temperature between the exist flue gas and entering air is the temperature at the exit of air heater. This end is called cold end.

If the cold end temperature goes below sulphur dew point, suphuric acid will form on the metal. This will cause corrosion of the metal. The metal shall be protected from this possible corrosion.

One method is using non metallic material at the cold end. Rotten iron and Ceramic are used in rotary air heaters. Another method is to keep the cold end temperature higher than sulphur dew point. For this purpose, the air will be pre-heated by steam before admitting to rotary air heater. Both of them are practiced in majority cases.