Air Pre-heating

An Over view of

Air Pre Heaters, Soot Blowers andElectro Static Precipitators

Training & Development

Dr KC Yadav, Director,Jindal Institute of Power Technology

Training & Development

Air Pre Heaters

Dr. K.C. Yadav, Head,Training & Development

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Purpose

It recovers the heat from the boiler flue gas and increase the temperature of combustion air (PA & SA), which in turn increases the thermal efficiency of the boiler.

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Air Heater Air heater is a heat exchanger in which air temperature is raised by transferring heat from other fluids such as flue gas.

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Advantages of Air Heaters• Stability of combustion is improved by use of hot air.• Intensified and improved combustion.• Burning poor quality fuel efficiently.• Less unburnt fuel particle in flue gas thus complete

combustion is achieved.• Intensified combustion permits faster load variation.• In the case of pulverized coal combustion, hot air can be

used for drying the coal as well as for transporting the pulverized coal to burners.

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Types of Air heaters

Air heaters can be classified as:A. RecuperativeB. Regenerative

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RECUPERATIVE

In Recuperative type-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. These are of static construction and hence there is only nominal leakage through expansion joints, access doors, casings etc

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REGENERATIVE

In Regenerative type 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 and hence there is slight leakage through sealing arrangements at the moving surfaces

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Recuperative

1. Tubular Air heater This usually consists of large number of

steel tubes of 40 to 65 mm dia either welded or expanded into the tube plates at the end. Either gas or air may be allowed to flow through the tube.

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Tubular Type APH

Tubular pre heaters consist of straight tube bundles which pass through the outlet ducting of the boiler and open at each end outside of the ducting. Inside the ducting, the hot furnace gases pass around the pre heater tubes, transferring heat from the exhaust gas to the air inside the pre heater. Ambient air is forced by a fan through ducting at one end of the pre heater tubes and at other end the heated air from inside of the tubes emerges into another set of ducting, which carries it to the boiler furnace for combustion.

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Problems in Tubular Type APH

The tubular preheater ductings for cold and hot air require more space and structural supports than a rotating preheater design. Further, due to dust-laden abrasive flue gases, the tubes outside the ducting wear out faster on the side facing the gas current. Many advances have been made to eliminate this problem such as the use of ceramic and hardened steel.

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Recuperative

2. Plate Type Air heater These comprise of parallel plates which

provide alternate passage for gas and air3. Steam Air Preheater

This does not utilize the heat from boiler flue gas and hence does not improve boiler efficiency.

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Regenerative1. Ljungstrom Type

The heat transfer elements are rotated at a constant speed and they pass alternately through gas and air passes.

2. Davidson/Rothemuhle Type This type is the same as the Ljungstrom type-

except that the matrix element is stationary and the air gas hoods rotate.

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Stationary-Plate Regenerative Air Pre Heater

• The heating plate elements in this type of regenerative air preheater are also installed in a casing, but the heating plate elements are stationary rather than rotating. Instead the air ducts in the preheater are rotated so as to alternatively expose sections of the heating plate elements to the upflowing cool air.

• There are rotating inlet air ducts at the bottom of the stationary plates similar to the rotating outlet air ducts at the top of the stationary plates.

• Stationary-plate regenerative air preheaters are also known as Rothemuhle preheaters.

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Ljungström Air Pre Heaters

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1) Combined Cleaning Device for Blowing with Steam or Compressed Air and Washing with Water2) Rotor Drives3) Axial Seal Adjustable during Boiler Operation4) Radial Seal Adjustable during Boiler Operation5) Guide Bearing6) Rotor7) Heating Elements in Baskets8) Hot Layer (Material: Steel)9) Cold Layer (Material: Steel, Corten or Enameled Steel)10) Intermediate Layer (Material: Steel)11) Pin Rack

The Ljungström Regenerative Air Preheater Principle

The Ljungström® air preheater is a regenerative heat exchanger through which flue gas and air pass in countercurrent flow.

The heat transfer surface comprises thin, profiled heatingelements which are contained in a rotor and arranged parallel to the direction of flow. The rotor rotates at a constant speed. During each revolution the heating elements absorb heat while rotating through the flue gas side and impart this stored heat to the air when rotating through the air side.

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APH Driving Mechanism

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APH Driving Mechanism

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APH Rotating Mechanism

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APH Heating Elements

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Dew point corrosion

Dew point corrosion occurs for a variety of reasons. The type of fuel used, its sulfur content and moisture content are contributing factors. However, by far the most significant cause of dew point corrosion is the metal temperature of the tubes. If the metal temperature within the tubes drops below the acid saturation temperature, usually at between 190°F (88°C)and 230°F (110°C), but sometimes at temperatures as high as 260°F (127°C), then the risk of dew point corrosion damage becomes considerable.

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Regenerative Vs. Recuperative

Regenerative air heaters have the following advantages over recuperative ones (tubular):

1. Compact and hence save space and structure cost.2. This is the type that can be economically used for

high capacity boiler.3. From the above two reasons, it is clear that initial

cost as well as the operating cost can be kept minimum with regenerative air- heaters for higher size boilers.

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Regenerative Vs. Recuperative4. Less weight of metal permit economic usage of

alloy steel like corten steel.5. Holes in the elements due to corrosion etc. will not

materially affect the performance of the heater.6. Deposit on the heat transfer surface does not

reduce the heat transfer.7. Hot primary air for coal drying in mill is possible

with single air heater of tri-sector design. This also helps to make use of a cold primary air fan.

Soot Blowers

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Long Retractable Soot Blowers are available in various basic models designed for travel up to 12.2 m. to suit the cleaning requirements of different furnace sizes, heat recovery coil arrangements etc. These blowers are available with single motor drive, dual motor drive and non-rotating lance. They can be operated electrically or pneumatically.

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• The LRSB has one electric motor to move the lance in and out, a second to rotate it - always in the same direction. With no step-stop action at the end of travel, repeated cycles give an infinite number of double-helix patterns. This minimizes danger of tube cutting or erosion and ensures uniform cleaning of all surfaces. Low rotating speed increases range and penetration of cleaning, decreases wear, eliminates whip and permits proper cleaning with faster traversing speeds. Rotating and traversing speeds may be adjusted independently by speed-change gears or sprockets.

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• Lance and feed pipe have no inaccessible parts to clog, corrode or retard blowing-medium travel. Both are flanged for easy maintenance. Feed pipe is ground and polished stainless steel to prevent binding and wear from carriage movement. Single exposed stuffing box is easily repacked. Self-cleaning, low friction BHEL chain drive needs no lubrication, and is easy to inspect and maintain. No auxiliary carriage is needed or used. Method of support - never more than two points - is tailored to the needs of each installation. Both electric motors are stationary, away from hot furnace gases. Custom built for BHEL they are totally enclosed, non-ventilated, 3-phase, heavy-duty hoist type with special boiler room insulation and protection against internal corrosion.

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APH Soot Blower

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APH Soot Blower

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Electro Static Precipitator

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ESP

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ESP

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ESP Roof

ESP Roof

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ESP Roof

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ESP Bottom

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ESP Raping Mechanism

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ESP Raping Mechanism

Thank you