AIR PREHEATER

• An APH is a  heat trap device.
• Air heater is a heat exchanger in which air temperature is raised by transferred heat from flue gas.
• Air heater transfer heat from flue gas to air.
• Air heater is the last equipment in the flue gas path where remaining heat of flue gas is utilized.
• Hot air leaving from the air pre heater is further supplied to boiler furnace involving in combustion process.
• A lower stack outlet temperature are obtained as per local pollution regulation.
• APH out let air temperature is higher than the inlet air temperature which increase the efficiency at all loads.

• The inlet gas temperature to the air prerheater is 315⁰C to 450⁰C.
• The exit gas temperature from air preheater vary from 135⁰C to 180⁰C & dissipate the cold end corrosion in the last stage.
• After installing air preheater boiler efficiency increased.
• In a modern boilers air preheater heated both primary air & secondary air.

ADVANTAGE OF AIR PREHEATERS

• Increase the boiler efficiency & saving the fuel.
• Hot air improve the stability of combustion.
• Possible to burn poor quality of fuels.
• High heat transfer in furnace.
• Hot air is used for drying the coal & transporting of pulverized coal.
• Decrease the temperature of flue has leaving the boiler.
• Lessening the energy losses due to incomplete combustion & unburnt carbon.
• hot air increase the theoretical flame temperature.

SOOT BLOWING OF AIR PREHEATER

For APH soot blowing following system is used.

• Steam operated soot blower.
• Air soot blower.
• Sonic sound operated soot blower.

• From soot blower unwanted deposits are removed.
• Fine grains of ash deposits on the cold end of the APH as flue gas temperature goes down.
• Air heater soot blower installed in the cold end where deposition exists.
• For steam operated soot blower a sawing arm is provided in which steam is opposite directed of flue gas flow.
• A motor & gear system drive the soot blower.
• This soot blower consist of single nozzle or multiple nozzle.
• The swing arm travel in arc follow path & nozzle face of heat transfer surface in upper side.
• As kinetic energy of steam coming out from the nozzle is greater than 100% of steam.
• For APH soot blowing high pressure dry steam is used up to 18 kg/cm².

TYPES OF AIR PREHEATERS

Air preheaters can be classified as based on their operating principle.

• Recuperative types
• Regenerative types

RECUPERATIVE TYPES

• Generally tubular type or plate types.
• In Recuperative type the heating medium is on one side and air is on the other side of tube or plate and the heat transfer is by conduction through the material which separates the media.
• One side of surface is hot flue gas which transfer their heat to air & become cold & other side of air to be heated.
• In tubular type APH vertical tube bundles are used in which flue gas flowing & outside the tube cold air is flowing horizontally.
• These are of static construction and hence there is only nominal leakage through expansion joints, access doors, casings etc.

REGENERATIVE AIR PREHEATER

• Heat exchange takes place indirect from the hot flue gas to the air with the help of heating elements.
• These closely packed matrix elements are acted as heat storage element.
• In it hot flue gas enter from the top of the APH & heat is absorbed by APH elements.
• After this these elements travels and enter in to the cold air zone & cools down.
• Cold air is enter from the bottom of APH air section & absorb the heat from the heating element & directs to upward.
• The heating medium flows through a closely packed matrix to raise its temperature and then air is passed through the matrix to pick-up the heat.
• Either the matrix or the hoods are rotated to achieve this or hence there is slight leakage through sealing arrangements at the moving surfaces.
• The heating element heated & cools down from both sides.
• The efficiency of this type of APH is very high.
• It has disadvantages that it has leakage up to 10%.

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