Types of Air-preheater and their Advantages and Disadvantages
Types of Air-preheater and their Advantages and Disadvantages
today we discussed about a very important topic related to boiler combustion process helping which is air preheaters in this article we discussed about Types of Air-preheater and their Advantages and Disadvantages .Air preheater is a device which is used for the preheating the combustion air supplied to boiler for combustion by F.D ( forced draft ) fan and also sustain the temperature of burning fuel it is essential to supply combination air preheated to higher temperature. Perfect and efficient combustion of fuel can be achieved in a boiler, if the atmospheric air can be preheated by the low temperature flue gas, which is otherwise going to be wasted through the chimney. Air preheater is situated at the exit end of the flue gas path, after economizer.
Air preheating is helpful in following ways:-
-Greater load flexibility
-Stabilizing fuel ignition
-Increase thermal efficiency
-Decreases fuel combustion
-Increase steam generation capacity
-Recovery of waste heat from the boiler protects the environment from the pollution.
Types of air heaters: based on the operating principal, the air preheater is classified into two different types
1. Recuperative air preheater
2. Regenerative air preheater
Recuperative type air preheater:-
Tubular air heaters are known as Recuperative type of air preheater.
Recuperative air heater is generally called static air heater. These type air preheater are of totally enclosed rectangular box type construction, consisting of several straight tubes connecting two thick end plates at the top and bottom. The flue gas which is the heating medium passes through inside of the tubes whereas the air entering the air preheater passes through outside of the tubes by conduction. Due to very low heat transfer co-efficient between flue gas and air, the heat transfer surface of the air heater is large. These are of statics construction and hence problem of leakage through expansion joint access doors and casing is very minimum. Thin wall of tubes are however susceptible for getting eroded and punctured due to abrasive nature of Indian coal ash in flue gas . Normally steel tubes of size 40 to 60 mm in diameter are used and direction of flows is vertically down the tubes.
2. Regenerative type of air preheater:-
Regenerative preheater is used in large capacity boilers due to its compactness. Tri sector regenerative preheater is commonly used in power plant. this design permits a single heat exchanger to heat both primary air which dries and transports coal from coal mills to the furnace and also secondary air which is used at furnace for combustion .Regenerative type is dynamic and continuously keeps on rotating about vertical or horizontal axis. It consists of a circular diaphragm shaped construction, which is divided into several sectors. Each sector is stacked with several blocks of baskets, arranged in three tires. The baskets are filled with several corrugated sheets of metallic plates kept close to each other and tightly packed.
The sheets are so assembled, that each plant is butting against the other and at the same time there are gaps through corrugated shape, for passage of air or gas medium. One half of the air preheater will be exposed to hot flue gas path and at the same time the other half of the air preheater is exposed to incoming air. The ducting is so arranged that the air and flue gas path is always kept separated and each half of the air preheater elements is alternatively exposed to flue gas and air. When the air preheater elements is alternatively exposed to flue gas and air. When the air preheater keeps on rotating ,one half of the baskets of the corrugated sheets , which are exposed to flue gas path the hot sheeting materials give away the heat to the air passing through. Thus alternatively the sheet elements absorbed the heat from flue gas and give away the heat to air when the rotation takes place.
Flue gas temperature towards the outlet of air heater is less , so there is chance of corrosion there is a chance of corrosion there. The dew point of sulfuric acid is around 140 degree Celsius. The exact dew point is calculated as per “ verhoof equeation ” as per this equation, the dew point of sulfuric acid depends upon the partial pressure of SO3 gas and H2O water vapor present in the flue gas . so3 gas which is formed due to combustion of sulfur present in the fuel reacts with moisture in the flue gas and forms sulfuric acid . So mostly the gas outlet end of the air heater is corroded. During low load this temperature is very less, so chance of acid formation is more in this condition. To avoid this situation some time steam coil preheater is used.
Advantage and disadvantage of air preheating system
Advantage of air preheater:
In addition to increase in boiler efficiency the other advantages that may result are listed below:
1. Stability of combination is improved by use of hot air.
2. Intensified and improved by use of hot air.
3. Permitted to burn poor quality coal.
4. High heat transfer rate in the furnace and hence lesser heat transfer area requirement.
5. Less unburned fuel particle in flue gas thus combustion and boiler efficiency is improved.
6. Intensified combustion permits faster load vibration and fluctuation.
7. In the case of pulverized coal combustion, hot air can be used for drying the coal as well as for transporting the pulverized coal to burners.
Disadvantage of air heaters: –
Air heaters are non pressure part which will not allow shutdown of units due to corrosion of heat transfer surface which is inherent with lowering of flue gas temperature.
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