ASME Standard Tubular Heat Exchanger for Boiler Air Preheater

Air preheater is a general term and represents any heater designed to heat air. It may be for residential use namely for room heating, house heating, etc. by means of gas, electricity or oil etc. In the present context reference is to the airpreheaters used in steam generators (boilers) for generation of electricity in Power Generation Utilities.

The purpose of the air preheater is to recover the heat from the flue gas(exhaust gases) from the boiler to improve boiler efficiency. Incidentally the gases are sent to the chimney or stack at lowered temperature so that the design of the ducting and the stack becomes simplified. Also it controls the temperature of gases coming out of the stack.

Different types

For the use in Steam Generators (boilers)in Thermal power stations of Utility power stations, there are two types of design of the Airpreheaters. One is a tubular type built into the boiler outlet Flue_gas ducting and another one is of the regenerative type known as Ljungstrom (named after the name of inventor) Regenerative Air Pre Heater(RAPH),a rotating type. This may be horizontal (now very common in power generation utilities) or vertical.

Construction features

1. The vertical rotor of the same is supported on big thrust bearings at the lower side and has oil bath lubrication cooled by water circulating in the coils inside the oil bath. This arrangement is done for cooling of the shaft as this end of the vertical rotor is on the hot end of the ducting as well. The top end of the rotor has simple roller bearing to hold the rotating shaft in vertical position only.

2. The rotor is built up on the vertical shaft with radial supports and cages for holding the baskets in position. Radial and circumrential renewable seal plates are also provided to avoid leakages of gases or air between the sectors or between the duct and the casing, while in rotation.