Air Heater

Air heater is a heat recovery unit, which is employed t recover heat from lower temperature levels usually 240ºC and below. Air heater is used in most of the industrial and utility boilers. The construction of this equipment being a simple and also a non pressure part. Gas temperature drop 240ºC to 170 ºC can be effectively achieved by means of air heater there by 45 boiler efficiency can be improved. In cases of low capacity, low pressure boilers manufactures prefers to have an air heater instead of economizer. This is to avoid problems raised due to economizer like oxygen pitting,, corrosion higher feed water temperature, necessity of having generator more over economizer is a pressure part. These are avoided by means of air pre heater.

Types Of Air Heater

The basic classification of air heater is based on their operating principle

• recuperative
• regenerative

Recuperative heat exchanger:

In recuperative heat exchanger heating and cooling medium are separated by partition and the heat is transferred from one fluid to other by means of conduction and convection and there is no moving part employed in recuperative heat exchangers. According to the construction it is again sub divided into tubular or plate type air heater.

Tubular air heater

This usually consists of large number of steel tubes of either welded or expanded into tube plate at the ends. Either gas or air may be designed to flow through the tube. Gas through the tube normally requires higher size tube and vertical flow to reduce fouling. Single or more passes on the gas side and multi pass to and fro on the air side usually fits in with shorter tube length so as to facilitate maintenance on surface due to corrosion and fouling . in some cases instead of using of boiler flue gas separated external firing is used particularly during staring.

Plate air heater

This comprise of parallel plates which provide alternate passage for gas and air. This type is simple and compact to that of tubular type. The narrow passes between plates make cleaning tedious but with shot cleaning method it is improved. But replacement is a major task and nowadays it is used in industry or utility boilers.

Regenerative heat exchangers:

In regenerative heat exchangers the heating fluid heat is closely packed matrix to raise its temperature which is again placed in cooling medium to transfer the heat to cooling fluid. In regenerative exchangers either matrix or the hoods are rotated to achieve this. This is sub divided by the names or  inventors into

• Ljungstrom type
• Rothemuble type