Air heater

AIR PREHEATER

INTRODUCTION:

HEAT EXCHANGER

HEAT TRANSFER FROM FLUE GAS TO AIR

HEAT REJECTED TO ATMOSPHERE REDUCED

INCREASE BOILER EFFICIENCY BY STABILITY OF COMBUSTION WITH HELP OF HOT AIR.

HOT AIR USED FOR DRYING THE COAL AS WELL AS FOR TRANSPORTING.

FOR EVERY 20°C DROP IN FLUE GAS EXIT TEMPERATURE THE BOILER EFFICIENCY INCREASE BY ABOUT 1%.

10% IMPROVEMENT IN BOILER EFFICIENCY WHEN COMPARED TO AN IDENTICAL UNIT WITHOUT AN APH.

Why APH in Boiler System

• APH - Tail ender• APH & Economizer are heat recovery• surface• Designers always look at this in pair• Economizer - self limiting characteristics • Can steam if not properly sized • Minimum 25 – 30 deg C Eco out F W

& saturation temperature• APH can be sized for any requirement• APH has ability to absorb changes

TYPES OF AIR PREHEATERS:

AIR PREHEATERS CAN BE CLASSIFIED AS RECUPERATIVE AND REGENERATIVE TYPES BASED ON THEIR OPERATING PRINCIPLE.

IN RECUPERATIVE TYPE HEATING MEDIUM I.E. FLUE GAS 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.

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.

TYPES

RECUPERATIVE RE-GENERATIVE

STATIC CONSTRUCTION ROTARY BY CONSTRUCTION

TUBULAR TYPE

PLATE TYPE

SCAPH

TRI-SECTORBI-SECTORQuad-Sector

ROTHEMUHLE

MATRIX ELEMENT STATIONARYLJUNGSTROM TYPE

MATRIX ELEMENT ROTATING

HEAT PIPE or THERMOSYPHONE

TUBULAR AIR PREHEATER (RECUPERATIVE):

•LARGE NUMBER OF STEEL TUBES OF 40 TO 65 MM DIA.•EITHER WELDED OR EXPANDED INTO THE TUBE PLATES.•EITHER GAS OR AIR FLOW THROUGH THE TUBE.•GAS THROUGH THE TUBE NORMALLY REQUIRES HIGHER SIZE TUBE AND VERTICAL FLOW TO REDUCE FOULING.•SINGLE OR MORE PASSES ON THE GAS SIDE AND MULTIPASS CROSS FLOW ON THE AIR SIDE USUALLY FITS IN WITH THE OVERALL PLANT DESIGN.•THE PORTION OF AIRHEATER AT LOW TEMPERATURE ZONE IS DESIGNED NORMALLY WITH A SHORTER TUBE LENGTH SO AS TO FACILITATE MAINTENANCE OF SURFACES DUE TO CORROSION AND FOULING.

PLATE TYPE AIR PREHEATER (RECUPERATIVE):

•THESE COMPRISE OF PARALLEL PLATES.• WHICH PROVIDE ALTERNATE PASSAGE FOR GAS AND AIR.•THIS TYPE IS SIMPLE AND COMPACT COMPARED TO THAT OF TUBULAR TYPE.•THE NARROW PASSES BETWEEN PLATES MAKE THE CLEANING TEDIOUS BUT WITH SHOT CLEANING METHOD IT IS IMPROVED.•REPLACEMENT IS A MAJOR TASK.

LJUNGSTROM REGENERATIVE AIR - HEATER

•THE HEAT TRANSFER ELEMENTS ARE ROTATED AT A CONSTANT SPEED AND THEY PASS ALTERNATELY THROUGH GAS AND AIR PASSES.

•THE AXIS OF ROTATION MAY BE HORIZONTAL OR VERTICAL. •THE DRIVE IS NORMALLY ELECTRICAL OPERATED THROUGH REDUCTION GEAR WITH COMPRESSED AIR MOTOR AS STAND-BY.

•THE PLATES FORMING THE ELEMENTS (MATRIX) MAY BE VARIED IN SPACING AND THICKNESS.

•COLD ENDS ARE MADE OF SPECIAL CORROSION RESISTANCE ALLOY SUCH AS CORTEN OR ENAMELED TO ACHIEVE CORROSION RESISTANCE.

•THIS TYPE IS VERY COMPACT AND LENDS EASILY FOR DUCTING ARRANGEMENT EFFECTIVE CLEANING OF HEAT-TRANSFER SURFACE BY SOOT BLOWING IS POSSIBLE.

• THE BASIC COMPONENT OF THE CONTINUOUSLY ROTATING CYLINDER, CALLED THE ROTOR, THAT IS PACKED WITH THOUSANDS OF SQUARE FEET OF SPECIALLY FORMED SHEETS OF HEAT TRANSFER SURFACES.

• AS THE ROTOR REVOLVES, WASTE HEAT IS ABSORBED FROM THE HOT EXHAUST GAS PASSING THROUGH ONE HALF OF THE SURFACE.

• THIS ACCUMULATED HEAT IS RELEASED TO THE INCOMING AIR AS THE SAME SURFACES PASS THROUGH THE OTHER HALF OF THE STRUCTURE. THE HEAT TRANSFER CYCLE IS CONTINUOUS AS THE SURFACES ARE ALTERNATELY EXPOS ED TO THE OUTGOING GAS AND INCOMING AIR STREAMS.

• FUEL SAVINGS WITH THE LJUNGSTRÖM AIR PREHEATER ARE ABOUT 1-1½% FOR EVERY 4.4°C TO 10°C INCREASE IN COMBUSTION AIR TEMPERATURE, DEPENDING ON THE APPLICATION.

•THEIR SIMPLIFIED DESIGN AND OPERATING INTEGRITY ASSURE CONTINUOUS RELIABLE SERVICE THROUGHOUT THE LIFE OF THE PLANT.

Size Type Rotor Dia (Meters) Heat Duty(M.K.Cals/Hr)

7 - 16.5 K 1.2 - 3.0 2.5 - 60

17 - 18.5 S 3.2 - 3.8 50 - 70

19 - 24 R 4.2 - 6.6 70 - 200

24.5 - 36 LARGE 6.9 - 20.0 > 200

Range of RAPH

Designation of RAPH

27 VI M T 2000

Size number

Vertical Shaft; Inverted gas Flow

Modular Rotor

Trisector

Element depth in mm

BI-SECTOR TRI-SECTOR

BI-SECTOR

THE MAJORITY OF LJUNGSTRÖM AIR PREHEATERS SUPPLIED ARE IN THE BI- SECTOR DESIGN. THESE HEATERS HAVE TWO BASIC STREAMS, ONE OF GAS AND ONE OF AIR.

TRI-SECTOR

TRI-SECTOR AIR PREHEATER PERMITS A SINGLE HEAT EXCHANGER TO PERFORM TWO FUNCTIONS: COAL DRYING AND COMBUSTION AIR HEATING.

THE DESIGN HAS THREE SECTORS - ONEFOR THE FLUE GAS, ONE FOR THE PRIMARY AIR THAT DRIES THE COAL IN THE PULVERIZER, AND ONE FOR SECONDARY AIR THAT GOES TO THE BOILER FOR COMBUSTION

• QUADSECTOR

•THE QUAD-SECTOR IS WITH FOUR FLOW STREAMS THROUGH THE ROTOR.

•THE FOUR SECTORS COMPROMISE ONE GAS AND ONE PRIMARY AIR AS IN THE TRI- SECTOR, BUT THERE ARE TWO SEPARATE SECONDARY AIR SECTORS.

•THE DESIGN HAS THE PRIMARY AIR SECTOR’ FLANKED' ON EITHER SIDE BY SECONDARY AIR, AND THIS HAS A BENEFIT ON THE TOTAL AIR-TO-GAS LEAKAGE OF THE UNIT.

AIR HEATER –(SCHEMATIC)

RADIAL SEAL

AXIAL SEAL

BYPASS SEAL

COLD END

HOT END

HOT INTERMEDIATE

GUIDE BEARING

SUPPORT BEARING

ROTOR:The rotor is the heart of the equipment radially divided open ended cylinder which contains the heating surface elements. The center shaft of the rotor is called the post. Diaphragm plates extend out ward from the post dividing the rotor into 12 or 24 sectors which are further divided to form compartments into which the element baskets are packed-A pin rack is located around the outside of the rotor to allow it to be rotated by the drive mechanism.

HEATING ELEMENTS:

They are packed in a reversible containers called baskets, are placed in rotor in three tiers: - Hot, Intermediate and Cold.

The notches are used for maintaining the spaces between the elements and minimizing the pressure drop across the air preheater.

• HOT END BASKETS & HOT INTERMEDIATE : -

Hot End is the first layer & Hot Intermediate is second layer of heating element packing from hot end side. The elements are usually made from 24 gauge / 22 gauge(0.5 – 0.8MM) open hearth steel (IS 513 Gr. DD). They are having a profile called “Double Undulation. The notches run parallel with the rotor axis and provide the correct spacing of sheets and the undulations run at 60° to the notches to impart turbulence. Open-channel element, where the notches, which provide the required plate spacing, rest on a series of point contacts on the adjacent sheet. Flow can move across the element pair because there are openings between the two sheets along the flow length between point contacts.

COLD END BASKETS

The elements are made of 18 gauge / 22 gauge carton steel. Enameled elements are also used in severe corrosive conditions like for more percentage of sulphur in fuel or for low gas duct temperature. All the heating surface elements are packed into reversible containers called baskets to facilitate easy removal and handling. Cold end baskets are arranged for removal through the basket removal door in the housing. A closed element profile is the notched flat 6-mm element (NF6), the element pair is formed by a series of notches that rest on an adjacent flat sheet with contact along the total flow length. They provide the necessary spacing and form discrete individual flow channels of fixed cross-sectional area along the flow length or element depth. There is no flow communication from one channel to the adjacent one.

AIR LEAKAGE

• Leakage of the higher pressure air to the lower pressure flue gas through the clearances between the rotor seals and the sector plates.

• Air to gas leakage can be increase with time, to more than twice the immediate post overhaul level.

• Can be increase with increase in differential pressure of two fluids

• Leakage paths for a tri-sector APH are more complex, compared to a bi-sector .• In a tri-sector , primary air leaks into the flue gas and secondary air streams, while SA leaks into the flue gas stream. • Leakage occurs both on the cold end (CE) and hot end (HE). • Due to large difference in pressure between the PA and SA streams, as well as the PA and flue gas streams, leakage in a tri-sector APH is higher than in a bi-sector APH.

• Air leakage has the largest single effect on APH performance.

• Two penalties to boiler performance occur with excessive radial seal leakage. 1.Thermal losses associated with the leakage air cooling the APH. 2. Additional auxiliary horsepower consumed by the fans for pushing more flow.

TYPE OF AIR LEAKAGEA. ENTRAINED LEAKAGE :

B. DIRECT LEAKAGE :

• THIS IS MAINLY ENTRAPED AIR IN BETWEEN THE HEATING ELEMENTS

We = . v . rpm. 60 = Sp. Wt of air in Kg/m3v = Volume of Rotor air space

• THIS IS MAINLY DUE TO DIFFERENTIAL PRESURE OF TWO FLUID • INCREASE OF SEAL CLEARANCE AT HOT CONDITION• EROSION OF SEALS• IMPROPER SEAL SETTING

Wd = A *√(2g * * ∆ P) = Coeff. Of flow rate (0.6 – 0.7)

g= Acceleration due to gravityA= Gap area at hot condition (m2)∆P= Diff. pr. Of two fluids = Sp. Wt of air in Kg/m3

Variable factors are∆P

* Hence amount of entrained Leakage is independent of operating and maintenance condition

Circumferential bypass seal leakage into the warm airflow

Peripheral bypass seal leakage into the gas path

Circumferential bypass seal leakage into the cold gas flow.

Hot radial seal leakage

Cold radial seal leakage

VARIOUS LEAKAGE PATHS THROUGH THE APH

It is noted that nearly 80-85% leakage is from Radial Seal

10-15% through By-Pass Seal

5-10% through Axial

LEAKAGE PATH

THERMAL TURNDOWN

When the APH rotor is heated from a cold condition (blue), thermal expansion (yellow) can cause the rotor to droop or “turn down” up to 3 inches on the periphery. Knowing the amount of turndown is important when presetting the seal position before operation, because seal positions will change as the rotor warms to its operating temperature.

• SEALING SYSTEM COMPRISES THE FOLLOWING:

– RADIAL SECTOR & AXIAL SEALING PLATE

– RADIAL, CIRCUMFRENCIAL & AXIAL SEALS

– ROTOR POST SEALS

– INBOARD & OUTBOARD STATIC SEALS

SEALING SYSTEM

SEALING SYSTEM: -Usually air leaks into the gas stream due to static pressure differential. This leakage air decrease the air leaving temperature. Various arrangements to reduce the leakage are as follows.

The sealing arrangement consists of Radial, Axial, Bypass, Axial Seal Plate to Sector Plate, Static, Rotor Post Seals and Mechanical Sealing Plates designed to minimize leakage between the gas and air streams of the pre-heater.

THE RADIAL SEALS are located along the edges of the diaphragm plates and bear against the sector plates, housed under centre sections.

THE AXIAL SEALS are located axially in line with the outer edge of diaphragm plates and bear against the axial seal plates, mounted in the housing pedestals.

THE BYPASS SEALS are located on the housing around the periphery of the rotor and bear against the T-bar attached to the periphery of the rotor.

THE AXIAL PLATE TO SECTOR PLATE SEALS are attached to the axial seal plats and bear against the sector plates.

THE STATIC SEALS are fixed under the centre sections and housing pedestals and bear against sector plates and axial seal plates respectively.

THE ROTOR POST SEALS are attached to the ends of the rotor posts and bears against the sector plates.

SEALING SYSTEM: -

– SINGLE LEAF TYPE• Only one sealing strip passing under sector plate & axial seal plate

any one time.• Adjusted with no gap due to sharp edge & bend.• 6% reduction in radial seal leakage than channel type.

RADIAL SEALS

AXIAL & BY-PASS SEALS

Gaps are observed around the Baskets and with Diaphragm/Stay plates. This will by-pass the flue gas: thereby loosing the efficiency of the boiler. This is revealed by the high flue gas outlet temperature.

The newly developed erosion resistant ‘Basket Bypass Seal’, which will permanently close the gaps till the next replacement of the baskets. This will result in lower flue gas outlet temperature, which will improve the efficiency of the Boiler.

BASKET BYPASS SEAL

CIRCUMFERENTIAL SEALS DEFLECTORS

Ideas for deflector Plates and Clay Cloth seals which cater for thermal load dependant rotor position fluctuations have been devised previously.

EROSION

Erosion caused by fly ash has resulted in the rapid loss of a heat exchange element as well as damage to perimeter seals, radial seals, and rotor diaphragms. Two other factors with regard to erosion are actually more important than ash content: abrasiveness and ash velocity.

The abrasiveness of fly ash increases as the amount of silica and alumina increases.

Ash velocity is as much as three times more important than ash content or abrasiveness when it comes to determining the rate of erosion. One way to defeat high ash velocity is to increase the fineness of the coal particles leaving the pulverizers and balancing the coal and air flows to each of the burners.

CORROSION, PLUGGING AND FOULING

A lower flow rate of combustion air results in a lower furnace excess air (excess O2) level, increased CO emissions, increased levels of unburned fuel in fly ash, higher furnace exit gas temperature, increased slagging and fouling rates, and higher net unit heat rate.

Cold End Heating elements were seriously affected by corrosion mainly in oil fired boilers. Corrosion causes due to formation of sulphuric acid.

To avoid corrosion: a. Complete combustion of fuel oil to be ensured. b. Soot blowing at regular intervals to be done. c. Maintaining minimum cold end average temperature. d. Heating elements to be dried properly after water washing. e. Oil quality to be ensured.

FOULING, PLUGGING AND CORROSION:

? Deposits are initiated by condensation of acid or moisture from flue gas on metal surface operating at temperature below dew point i.e. mainly at the cold end, where, as a result, most fouling and corrosion occur. ? Degree of fouling depends on heating element metal surface. ? As coal contains less sulphur, corrosion is not normally as much a problem as fouling and hence lower exit gas temperature to a level of 120 C is permissible. ? But in the case of oil firing, the corrosion and plugging due to corrosive products of combustion are very common.? The gas outlet temperature and/or air inlet temperature has to be raised to restrict the corrosion to the permissible level. ? Operating the oil fired boiler at very low excess air reduces the acid formation and hence corrosion.? During starting and at low loads the flue gas exit temperature falls to a low value that will lead to corrosion.

One or some of the following method is used to combat the problem:

1. Use of low sulphur oil during the above condition.

2. Air inlet temperature is increased mostly by steam air heating to maintain the recommended cold end average temperature for the installation.

2. Corrosion resistant alloys like corten steel can be used for cold end.

3. Easily and economically replaceable cold end portion of airheater without much outage period.

4. Effective on-load blowing of airheaters with superheated steam as moisture in steam accelerates fouling and corrosion.

APH THERMAL EFFICENCY

• Gas side efficiency ‘η’gas.

• Air side efficiency ‘η’air.

• X-Ratio.

IF: -

Tgi = APH Gas inlet temperature.

Tgo = APH Gas outlet temperature.

Tai = APH air inlet temperature.

Tao = APH air outlet temperature

Tgo (nl) = APH Gas outlet temperature at no seal leakage.

APH GAS SIDE EFFICIENCY

Flue gas inlet temp. – Flue gas outlet temp. at no seal leakage =

Flue gas inlet temp. – Air inlet temp. Tgi – Tgo (nl)

= Tgi – Tai

TgO (nl) = Corrected gas temp. for no seal leakage Al X Cpa (Tgo – Tai)

= + Tgo 100 X Cpg

AL = % Seal leakage on wt.

O2 out – O2 in

= X 100 X (0.9 for coal)

21 – O2 o

Cpa = Mean specific Heat between Tgi and TgoCpg = Mean specific Heat between Tgo and TgonlCpa / Cpg = 0.95 for Coal

Tao - Taiηai = X 100%

Tgi - Tai Tgi – Tgo (nl) ηgas

X-Ratio = = Tgi – Tai ηair

20°C rise in flue gas exist temperature there is decrease in boiler η 1% i.e. = loss of 26 Kcal approx.

DRIVE ARRANGEMENT : -The drive mechanism consists of:• Two electric motor connected to a gear reduction unit through

overrunning clutch and fluid coupling driving a pinion gear.• The pinion gear meshes with a pin rack on the rotor which allows the

rotor to rotate at a low speed.• Provision of Air Motor is also given for any failure of electric drive units. Rotor Drive Assembly (Down Shaft Design) 1. High Speed

Coupling 2. Drive Motor (Main)

3. Pin Rack 4. Rotor Housing. 5. Support Bracket 6. Pinion 7. Pinion Cover 8. Speed Reducer 9. Air Motor

(Auxiliary)

07. Motor – TEFC : 11 KW, 3 Phase,

50 Hz, 415 V, 1450 RPM, 05-06 Speed Reducer : Type – I Japan, Rissowai09. Coupling : 11.50 FCU

(Pembril)08. Air Motor = Chiago Pneumatic

Air Motor RSM 40010. Coupling = Bibby 124 A

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