Flue Gas Air Pre Heater
An air preheater (APH) is a general term to describe any device designed to heat air before another process (for example, combustion in a boiler) with the primary objective of increasing the thermal efficiency of the process. They may be used alone or to replace a recuperative heat system or to replace a steam coil.
In particular, this article describes the combustion air preheaters used in large boilers found in thermal power stations producing electric power from e.g. fossil fuels, biomasses or waste.
The purpose of the air preheater is to recover the heat from the boiler flue gas which increases the thermal efficiency of the boiler by reducing the useful heat lost in the flue gas. As a consequence, the flue gases are also sent to the flue gas stack (or chimney) at a lower temperature, allowing simplified design of the ducting and the flue gas stack. It also allows control over the temperature of gases leaving the stack.
There are two types of air preheaters for use in steam generators in thermal power stations: One is a tubular type built into the boiler flue gas ducting, and the other is a regenerative air preheater.These may be arranged so the gas flows horizontally or vertically across the axis of rotation.
Another type of air preheater is the regenerator used in iron or glass manufacture.
The total air heated in the RAPH provides: heating air to remove the moisture from the pulverised coal dust, carrier air for transporting the pulverised coal to the boiler burners and the primary air for combustion.
It rotates quite slowly around 3-5 RPM to allow optimum heat transfer first from the hot exhaust gases to the element, then as it rotates, from the element to the cooler air in the other sectors.
Air pre-heater transfer heat from flue gas to air by means of a rotary matrix in which heat is absorbed by the heating elements passing through the hot gas stream and transferred to the combustion air stream.