Air Pre Heater

Air Pre HeaterĀ  is any device designed to heat air before another process 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

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 conveyed to the flue gas stack (or chimney) at a lower temperature, allowing simplified design of the conveyance system and the flue gas stack. It also allows control over the temperature of gases leaving the stack (to meet emissions regulations, for example).It is installed between the economizer and chimney.

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BRAZED PLATE TYPE HEAT EXCHANGERS FEATURES

TYPES OF AIR PRE HEATER
  • Shell and tube type single pass air pre-heaters
  • Shell and tube type dual-pass air pre-heaters
  • Finned tube air pre-heaters

Air Preheater Pros:

  • 2 pass design to extract maximum waste heat from flue gases leaving the boiler.
  • It is used to extract waste heat from the flue gases leaving the boiler exhaust thereby increasing the efficiency of a boiler by 2-3 %.
  • It is used to preheat the fuel which further increases the boiler efficiency.
  • Payback time is less.

Air Preheater Cons

Adding and equipment or accessory in the path of flue gases increases flue gas pressure drop down the line, so the size of Induced Draft Fan increases.

  • Air Pre Heater
  • Air Heater
REQUIREMENT:

Air heater is an important Boiler auxiliary which primarily preheats the combustion air for rapid and efficient combustion in the furnace. The air heater recovers the waste heat from the outgoing flue gas of a Boiler and transfers the same to the combustion air. In a utility Boiler the flue gas leaves the economizer at a temperature of around 3800C. As every 550C drop in flue gas temperature improves the Boiler efficiency by about 2.5%, having an air heater in the downstream of economizer the Boiler efficiency is considerably improved. Further the air heater may also be iised for heating the air to dry the coal in the pulverizing plant.

 

Advantages of Using Air Heater:

The recovery of waste heat from the flue gas and heating the air required for Boiler In an air heater gives the following advantages.

    1. a) The Boiler efficiency is increased.
    2. b) More stable combustion in furnace.
    3. c) The combustion is intensified with the use of hot air.
    4. d) Lower grades of coals can be burnt efficiently with hot-air.
    5. e) The use of hot air, improves the heat transfer rate and so less heat transfer area will be required.
    6. f) The combustion is made complete with negligible amount of carbon in ash.
    7. g) Faster load variations are possible.
    8. h) The coal can be dried effectively for easy pulverization and combustion.