R. KARTHIKEYAN

Objectives:

Primarily preheats the combustion air for rapid and efficient combustion in the furnace.

Flue gas temp at APH I/L-380c & O/L -130 to 140c (0.5% sulphur).

Every 20c temp drop in flue gas improves boiler efficiency by 1%.

Primary air is heated and used for drying the coal .

STATIC RECUPERATIVE APH: - In the recuperative type the flue gas in on one side of the surface and the air is on the other side.

- The heat from the flue gas is transferred to the air through the heat transfer surface normally in forms of tubes/ plates.

There are two types : Plate type APH.

b) Tubular APH.

These comprise of parallel plates which provide alternate passage for gas and air.

The narrow passes between plates- cleaning is difficult and the replacement of plates is a major task. As such these type of air heater is not much using in bigger boilers.

Consists large number of steel tubes of 40 to 65mm dia either welded or expanded into the tube plates both top and bottom and also placed in an shell. Baffles are provided to guide air flow across the tubes entire surface. Generally flue gas flows through inside the tubes and air flows over the tube surface picking the heat simultaneously.2 to 3 stages are provided. Last stage APH is made shorter with anti corrosion materials (corten) to facilitates easy maintenance and replacement due to fouling & corrosion.

TUBULAR AIR HEATER:

Not having any moving parts. Maintenance is less and no power consumption.

No possibility of fly ash carry over by the heated air.

ADVANTAGES OF RECUPERATIVE APH:

DISADVANTAGES OF RECUPERATIVE APH: Large heat transfer area is required . More material cost .

Deposit on the tube surfaces reduce the heat transfer. Pressure drop of flue gas across AH is high. severely affected due to cold end corrosion.

Replacement of punctured tubes is a major task and requires units shutdown.

ROTARY REGENERATIVE APH:

In the regenerative type, the flue gas flows through a closely packed matrix (heat transfer elements) and gives up heat to the air heater elements to raise the temperature of the matrix.

When the air heater elements passes through air, the heat is transferred to air and raise the temperature of air.

Either the matrix or the hoods may be rotated to achieve this heat transfer as a continuous process

There are two types :a) Rotating Element ( LJUNGSTROM AH)

b) Rotating Hood ( DAVIDSON AH)

ROTARY REGENERATIVE APH:

Rotor assyRotor housing assyHeating elements Sealing system (Radial, Axial & Bypass )Guide bearing assySupport bearing assyLub oil circulation systemMain Drive assy and air line componentsCleaning device assyFire sensing device assy

LJUNGSTROM TYPE APH: Matrix of heating elements is rotating at 2 to 3 rpm and alternatively passes through FG and air passage. Heating elements pick up heating when they passes through FG and release the heat to air when they passes through air passage.

APH can be BI-Sector or Tri-Sector type.

Tri-sector APH facilitates for heating SA & PA and cold PA system can be used.

Airpreheater Leakage:In Air heater leakage is inherent to varying degrees. The driving force that causes leakage is the difference in static pr. levels between the air & gas streams. The quantity of leakage is dependent on pressure differential, seal clearance and the length of the seals separating the two sides.Thermal gradients that are inherent in any heat exchanger Structural deformation takes place resulting in clearances or gaps between seal and sealing surfaces.

*The Paragon DuraMax radial seals incorporate unique, self-adjusting bellows that ensure positive contact with the sealing surfaces.The Paragon DuraFlex high-performance circumferential and bypass seals use a patented revolutionary interlocking design which reduces the air gaps.

Air preheater maintenance:

Air preheater leakage is determined by % AL = O2 % Gas out - O2 % Gas in x 100 (21- O2 % Gas out) AL = Air heater leakage (%)

O2 % Gas in = percent O2 in gas entering air heater

O2 % Gas out = percent O2 in gas leaving air heater

Advantages of APH:

Increase boiler efficiency.

More stable combustion.

Intensified combustion by using hot air.

Lower grade coals can be burnt efficiently.

Heat transfer rate is improved due to hot air.

Faster load variation is possible.