CONSTRUCTION FEATURES OF

AIR PRE-HEATER (APH)

By: Dhruv GargEngineer/BoilerPS-TS, BHEL

Purpose of APH

• Preheats combustion air by transferring heat from

outgoing flue gas of boiler.

• Using hot air for combustion, aids rapid & efficient

combustion in furnace.

• Increases the boiler efficiency.

Location of APH

• Located in 2nd pass of boiler after economizer in the flue

gas path.

• Last heat recovery equipment in the boiler.

TYPES OF APH

Classification According to Operating Principle

• Recuperative Heaters

1. Tubular Air Pre-Heater

2. Plate Air Pre-Heater (Not used by BHEL)

Regenerative Heaters

1. Ljungstrom (Howden) Air Pre-Heater

(Rotating Heating Elements of Bisector and Trisector Type arrangement )

2. Davidson (Rothmuhle) Air Pre-Heater

(Rotating Air Hood) (Not used by BHEL)

• Transfer of heat in cross flow direction through the separating partition of either plates or tubes with the cold side continuously recuperating (absorbing) the heat conducted from the hot side.

• Separating surfaces may be composed of steel or cast iron tubes or plates.

• Mixing of air and flue gas is not possible during normal operation except in case of leakage of tubes.

RECUPERATIVE AIR PRE-HEATER OPERATING PRINCIPLE

Tubular Air Pre-Heater

• Consist of a series of long straight tubes of 40 to 65 mm diameter either welded or expanded into the tube plates (perforated) at the both ends and an enclosing casing provided with inlet & outlet openings for the cold and hot air.

• Cold air is flowing inside the tubes and flue gas is flowing over the tubes and passing heat to the cold air.

• In between the inter joining ducts , baffle plates are provided to direct the air over the entire surface of the tubes in cross-flow direction.

GIRAL CFBC BOILER TUBULAR AIR HEATER

Type of Tubular APH

No. of Passes

Heating Surface Area (m2)

Total Heating Surface Area (m2)

Primary APHSection

2 882027165

Secondary APHSection

3 18345

Nomenclature of Tubular APH in Contract Data Sheet of

GIRAL CFBC Boiler

Advantages of Tubular APH

• Maintenance involved is less and no use of auxiliary power because of absence of moving parts.

• No possibility of fly ash carry over by the heated primary air and thus reduces chances of mill erosion.

Disadvantages of Tubular APH

• Occupies more surface area.

• Any puncture in the tube results into leakage of air into the flue gas stream thereby increasing the I.D fan load.

• Pressure drop of flue gas across the AH is high.

• Deposits on the tube surface reduces the heat transfer.

• Cross flow heat transfer is less efficient.

• Severely affected by cold end corrosion.

• Replacement of punctured tubes is a major task requiring more downtime of the boiler.

• Indirect heating type APH.

• Rotating heavy structure serves as an intermediate heat storage surface.

• Cold surface converted into heated surface when exposed in the passage of hot flue gas. This heated surface is then brought in contact with cold air to give the heat.

• Heat energy is again regenerated in the structure.

REGENERATIVE APH OPERATING PRINCIPLE

• Extensive use in all modern utility power plants.

• Placed vertically in the 2nd pass in the flue gas path .

• Consist of a casing divided into 2 flow paths (Bi-sector Type) or 3 flow paths (Tri-sector Type), with the hot flue gases confined to one path and 1 or 2 streams of air to be heated on the other paths.

• Flow paths are composed of heating elements rotated at 1 to 3 rpm, passes through the gas stream absorbing heat, then through a seal sector to the air stream where heat is given up & again returns through another seal sector to the gas stream for absorbing the heat.

Ljungstrom Air Pre-Heater

Nomenclature of Ljungstrom RAPH

27.5 VIM 2150

Stands for

27.5 - Radius of Rotor in Decimeter (Heater Size)

VIM - Vertically Inserted Module

2150 - ???

• 500 MW & above ratings boiler has usually Bisector type air pre-heaters used to independently heat the primary & secondary Air.

• Two APH used each for Primary and Secondary Air (Total 4 APH).

• Usually both Secondary APH (SAPH) are larger in size and is located in between the two Primary APH (PAPH).

• Provides greater flexibility of controlling temperature of primary and secondary air.

Ljungstrom Air Pre-Heater(Bi-sector Type)

LJUNGSTROM (HOWDEN) REGENERATIVE APH

• 250 MW & lower ratings boiler has usually Tri-Sector Type APH.

• Two APH for both Primary & Secondary Air.

• Provides a means for heating primary air at high pressure and secondary air at low pressure within the same air pre-heater without mixing.

Ljungstrom Air Pre-Heater(Tri-sector Type)

DAVIDSON (ROTHMUHLE) AIR PRE-HEATER OPERATING

PRINCIPLE

• Heating & Cooling the Stationary Mass by shifting Flue Gas and Air Stream arrangement.

• Not used in Modern Power Plants

CONSTRUCTION OF APH(Ljungstrom Type)

Consist of following major components:-

1. Guide & Support bearing assembly

2. Upper & Lower connecting plates

3. Sector plates

4. Rotor housing (Octagonal shaped cross-section casing)

5. Rotor

6. Rotor Drive Unit (Electrical & Air Motor) with rack & pinion arrangement

7. Sealing system

8. Other systems

Ljungstrom Air Pre-Heater(Constructional Details)

Rotor Housing

Carbon Steel Fins Profile – Double Undulation with Notch

Corten Steel Fins Profile –Notch Flat (NF)

Top View of APH Elements

Rotor Bearings Lub Oil Skids

• Rotor Guide Bearing and Support Bearing of each APH has independent Lubrication Skids (consist of 2 oil pumps(one main & one standby), oil tank, 2 filters, 2 oil coolers etc).

• Support Bearing (spherical roller thrust bearing) housing is located at the bottom trunnion of the APH rotor & fill up with lub oil and takes care of entire load of the APH. Oil is circulated through lubrication skid removing heat out of the bearings.

• Guide Bearing (Spherical roller type) is located at the top trunnion of the APH rotor & takes care of axial expansion of the rotor. Oil is circulated from bearings through lubrication skid removing heat out of the bearings.

Bearing Lubrication Oil System

Support Bearing for APH

Typical

Cleaning Device

Rotor Drive Unit (Rack & Pinion Arrangement)

Flue Gas Outlet (Cold End) Soot Blowing with Steam

Now a days, Flue Gas Inlet (Hot End) soot blowing is also provided in utility boilers, due to increased content of ash in coal.

APH SEALING SYSTEM

Types of APH Sealing system

• Sealing system is provided to avoid the leakage of air into the flue gas across the rotating APH due to the differential pressure of air & flue gas.

Different Types of Sealing System are:

1. Radial Seals

2. Axial Seals

3. Bypass Seals

4. Rotor Post Seals

RADIAL SEALS

• Minimizes the leakage of air into gas side through the clearance between the rotor & sector plates in the radial direction at both hot and cold end.

• Sector plates are attached in the centre of the upper & lower connecting plates.

• Seals bolted along the top & bottom edges of the radial plates & bear against the machined surface of the sector plates during rotation of rotor.

RADIAL SEALS

AXIAL SEALS

• Minimizes the leakage of air around the rotor shell into the gas side of the APH.

• Seals attached to the outside of the rotor shell in the radial plates, parallel to the rotor axis.

• Seals bear against the axial seal plates which are mounted in the rotor housing through main pedestral & supporting feet.

AXIAL SEALS

AXIAL SEALING SYSTEM

AXIAL SEALING SYSTEM

BYPASS SEALS

• Minimizes the air & flue gas from bypassing the heat transfer elements through the gap between the rotor & the housing of the APH.

• The angles are bolted to the top & bottom periphery of the rotor shell and T- Bars are bolted to these angles.

• Bypass seals are bolted to the angles provided in the connecting plates.

• T-Bars bear against the bypass seals during the rotation of rotor to provide sealing.

BYPASS SEALING SYSTEM

BYPASS SEALING SYSTEM

BYPASS SEALING SYSTEM

ROTOR-POST SEALS

• Minimizes the leakage of air around the rotor post into the gas side of the APH.

• Seals is welded to the top & bottom ends of the rotor post.

• During rotation of rotor, seals bear against the static spool called as “Seal Ring” bolted to the stationary sector plates of the upper & lower connecting plates.

ROTOR POST SEALING SYSTEM

Other Systems of APH

• Oil carry over mechanical detection probe.

• Fire sensing device.

• Rotor stoppage alarm

• Steam Coiled Air Pre-heater (SCAPH)

• APH Bypass Secondary Air Duct.

• Water Wash/Deluge system for cleaning & fire fighting purpose.

• Steam connection for soot blowing.

• Service air line connection for soot blowing and air motor operation.

• Instrument air connection for solenoid valve operation.

Steam Coiled Air Pre-Heater (SCAPH)

• During cold boiler light up, the temp. of flue gas at the inlet of APH is in the range of 150-200 0C which can get reduce further to sulphur dew point temperature (around 80-90 0C) when comes in contact with the cold secondary air.

• SCAPH is used to heat the secondary air before APH during cold boiler light up to prevent the cold end corrosion in the flue gas path of APH.

• SCPAH used steam as a heating medium from the Auxiliary PRDS for heating the secondary air.

Steam Coiled Air Pre-Heater (SCAPH)Contd..

• In the Tri-sector APH, SCAPH is used to heat only the secondary air and not the primary air.

• In some units of 250/210 MW (New Parli-6, 250 MW), APH bypass secondary air duct is used for the cold Boiler light up secondary air requirement purpose to prevent the drop of flue gas temp. in APH.

APH Operating Parameters (210 MW)

S.N

Description Medium Operating parameters

Pressure (ksc)

Temp. (0C)

1 Flue Gas Inlet Gas - 350 - 400

2 Flue Gas Outlet Gas - 130 - 150

3 Primary/Sec. Air Inlet Air - 40 - 45

4 Primary/Sec. Air Outlet Air - 300 - 330

5 Support Bearing Lub Oil 1 >40

6 Guide Bearing Lub Oil 1 >40

7 Support Bearing LOP O/L Lub Oil 1-5 >40

8 Guide Bearing LOP O/L Lub Oil 1-5 >40

9 Soot Blowing Line Steam 14 315

10 Service Air line Air 5 - 7 40

11 Water Wash Line Water 5 40