An air heater is installed into the convective gas flue of the boiler and the coal-grinding devices with the purpose of heating the air – by using the heat of the flue gases – directed to the furnace to increase the efficiency of fuel combustion.
The optimum value for air preheating in an air heater depends on the type of the fuel burnt, its moisture content and the type of the combustion device, and ranges from 200 °C for the coal burned on the chain grates to 250 °C for the peat burned on the same grates and up to 350-450 °С when burning liquid and pulverized fuel in chamber furnaces.
In order to obtain a high temperature of air preheating, two-stage heating is used. For that purpose, the air heater is divided into two parts, between which (“in series”) a part of the water economizer is installed.
The types of air heaters:
– two-way ones for air and gas intended for boilers with a steam capacity of 2.5 and 4 t/h;
– single-way ones for gas and two-way ones for air intended for boilers with a steam capacity of 6.5 and 10 t/h;
– single-way ones for gas and air.
Tubular air heaters are the most widespread. By design, they constitute a bundle of parallel tubes that are attached to the tube sheets. Altogether, the structure forms a section or a cube.
Also, according to their types, air heaters are divided into vertical and horizontal ones. And according to the degree of air heating, they are divided into low-temperature (150-200 °С), medium temperature (200-350 °С), high temperature (350-450 °С) and radiation (450-700 °С) ones.