Air Preheaters in Steam Boilers: Function, Types and Working Principles

In this article, I will share the function, types and working principles of different types of air preheaters in steam boilers.


The main function of air preheaters in steam boilers is to enhance the stability of the boiler, absorption of maximum waste heat from the flue gases and also allow proper combustion of poor-quality fuels.


All these functions of air preheaters aim in saving energy and increasing the efficiency of the steam boilers.


When using air that is heated the combustion process is going to be more efficient than when one is using usual atmospheric air with the usual temperatures.


And this is where the air preheaters come. They are very significant in steam boilers and we need to talk about them now.


Types of air preheater used in steam boilers.

1. Tubular/Recuperative air preheater.

The name “tubular” suggests that the tubular air preheater consists of tubes that are within the outlet ducting of a boiler.


These tubes are opened at both ends and held by baffles to keep them in place within the outlet ducting of the boiler.


A tubular air preheater has two inlets; one consists of a point that allows the flue gas to enter the outlet ducting of the boiler.


We also have the second inlet opening that allows atmospheric air to enter the duct.


The atmospheric air enters the bundle of tubes that are within the ducting of the boiler.


It also has two outlet openings, the first one allows hot air out of the tubular preheater while the second one allows the flue gasses to go to the chimney.


Working principle of tubular air preheaters.

When the flue gasses enter the outlet ducting of the boiler, they pass around the bundle of tubes that are within the air preheater thus transferring heat.


Since these tubes are opened at both ends, this allows the tube to allow cold air into the tubes and the hot air is displaced from the tubes.


This ensures that the heat from the flue gasses is used to the maximum and continuous heating of the air goes on.


The flue gasses temperature reduces as it moves along the tubes within the tubular preheater.


The flue gasses are then directed to the chimney and the hot air that is used to heat the air entering the furnace.


2. Regenerative air preheater.

A regenerative air preheater is the most efficient and compact air preheater that is used widely.


The efficiency is brought about by the design enhancements it has offered other air preheaters.


You can term regenerative air preheater as a heat exchanger because it absorbs heat from the flue gasses and transfers the hot air where it’s needed for use.


Regenerative air preheater has a rotating steel heat transfer element that transfers the heat from the flue gasses.


You also have to remember that the regenerative air preheater has a single flow path that allows cold and hot fluids to pass through alternatively.


Its compact design allows it to occupy less space in a plant layout.


Working principle of the regenerative air preheater.

When the flue gasses enter the regenerative air preheater, the heater inside the air preheater absorbs the waste heat from the flue gasses and transfers it to the rotating steel heat transfer element.


The rotating steel heat transfer elements transfer the heat to the boiler to enhance the combustion process and increase the overall efficiency of the boiler.


And flue gasses are directed to the stack.


Final thoughts

Don’t confuse yourself between these two air preheaters I have shared in this article.


You have to know that a recuperative air preheater is also called a tubular air preheater.


Tubular air preheaters are not that effective compared to regenerative air preheaters.


The main difference between a tubular air preheater and a regenerative air preheater is that in a tubular the cold air and the hot air pass through two different paths (flue gasses out of the tubes, while the cold air in the tube) while in the regenerative air preheater, the flue gasses and cold air follow the same path.


Since regenerative air preheater is more effective than recuperative, it’s costlier.


If you have any questions about these two types of air preheaters you should leave a comment.


This is just a summary of the air preheaters, you can still learn more about the limitations of these air preheaters and their design by doing further research.


Thanks for reading, see you in my next article.


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