What is Air Preheater?
A very basic word to word meaning is a device used to heat the air before further use is called as Air Preheater. They are also recognized as air heaters or air-heating pipe. It is designed to exchange heat energy with desuperheaters. Desuperheater is a Device which is been used to reduce the temperature of the steam in a high heat generation plants where large amount of heat energy or steam is released in the atmosphere.
Working of Air Preheater
Mode of operation of the air preheater is in counter flow, co-current flow, co-current flow-countercurrent or cross-countercurrent or cross-co-current flow. The design of the device is depend on the media and the temperature conditions of the application. For small construction, finned tubes are suited to arrange in a small space much heating surface. Is in the exhaust gas or in the air containing dust or other particles, so that deposits are expected, preferably smooth tubes are used, which are less prone to deposits and will be clean easier.
In most of the applications, an Air preheater is performed in several stages that mean, it consists of several individual heat exchangers connected in series on the air side.
They are heating the air passing through it by means of wasted heat energy hence air preheateras are used in various areas such as boilers, industrial furnace units and in hot-air ventilation systems. Design of an Air preheater is depend on the application.
Types of Air Preheater
There are two basic types of Air preheater
- Recuperative or tubular type
- Regenerative or feed water type
Let’s discuss one by one type in brief
Recuperative or Tubular Type
Recuperative type of air preheaters is preferably used in small and medium size boilers. In this type heat energy exchange is carried out through the walls of heating surface that separate the heat energy carrier and air to be heated.
Ambient air is forced by a fan through ducting at one end of the preheater tube and at other end the heated air from inside the tubes emerges into another set of ducting Tubular preheaters consist of straight tube bundles which pass through the outlet ducting of the boiler and open at each end outside of the ducting. The hot furnace gases pass around the preheater tubes inside the ducting which transfers that carries it to the furnace of a boiler for combustion.
Regenerative or Feed Water Type
In feed water air preheaters exchange of heat energy takes place through alternate heating and cooling of metallic or ceramic nozzles of the fixed or rotating heating surfaces. Regenerative air preheaters are compact and can have stationary rotating hood.
Combine Use of Recuperative and Regenerative Type
The applications of very high capacity boilers uses combination of tubular and regenerative type of air preheater. For such applications primary air heating is performed by tubular type and secondary air heating is carried out using regenerative type. If in case the designers of boiler don’t want to use the combination of recuperative and regenerative type of Air preheater, then they can go for tri-sector regenerative air heater.
Advantages of Air Preheater
For heating the air it uses heat energy of flue gases hence there are many more advantages of Air preheaters over the normal preheaters such as
- Increases thermal efficiency of the system
- Payback period is very short and increase combustion rate
- Improve and strengthen the combustion
- Reduces the loss of furnace and the exhaust gas temperature, improve the boiler thermal efficiency.
- Air preheater is suitable for all standard fuels
- They are easy for retrofit into existing systems
- These air preheaters are suitable for all waste heat boiler / flue gas boiler / boiler types.
- They strengthen the heat transfer
Applications of Air Preheater
There are various kind of applications from household devices to industrial purpose where we find air preheaters
- In refrigeration economizer system
- Ovens used in home appliances and industrial uses
- The dryer applications are heavy energy consumers in the food and non-food industry. Air preheater transfers hot exhaust stream to the cold air inlet stream.
- In Indirect fired air heater applications.