CONSTRUCTION FEATURES OF AIR PRE-HEATER (APH) By: Dhruv Garg Engineer/Boiler PS-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) RECUPERATIVE AIR PRE-HEATER OPERATING PRINCIPLE • 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. . • 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 .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. 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 . of Passes Heating Surface Area (m2) Total Heating Surface Area (m2) Primary APH Section Secondary APH Section 2 3 8820 18345 27165 .Nomenclature of Tubular APH in Contract Data Sheet of GIRAL CFBC Boiler Type of Tubular APH No. 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. . • Pressure drop of flue gas across the AH is high. • Any puncture in the tube results into leakage of air into the flue gas stream thereby increasing the I. • Replacement of punctured tubes is a major task requiring more downtime of the boiler.Disadvantages of Tubular APH • Occupies more surface area.D fan load. • Deposits on the tube surface reduces the heat transfer. • Severely affected by cold end corrosion. . • Cross flow heat transfer is less efficient. • 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.REGENERATIVE APH OPERATING PRINCIPLE • Indirect heating type APH. • Heat energy is again regenerated in the structure. This heated surface is then brought in contact with cold air to give the heat. . • 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. • Placed vertically in the 2nd pass in the flue gas path . 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. • Flow paths are composed of heating elements rotated at 1 to 3 rpm. passes through the gas stream absorbing heat.Ljungstrom Air Pre-Heater • Extensive use in all modern utility power plants. . 5 VIM 2150 Stands for 27.??? .5 VIM 2150 .Radius of Rotor in Decimeter (Heater Size) .Vertically Inserted Module .Nomenclature of Ljungstrom RAPH 27. Ljungstrom Air Pre-Heater (Bi-sector Type) • 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). • Provides greater flexibility of controlling temperature of primary and secondary air. . • Usually both Secondary APH (SAPH) are larger in size and is located in between the two Primary APH (PAPH). LJUNGSTROM (HOWDEN) REGENERATIVE APH . . . . • Provides a means for heating primary air at high pressure and secondary air at low pressure within the same air pre-heater without mixing. • Two APH for both Primary & Secondary Air.Ljungstrom Air Pre-Heater (Tri-sector Type) • 250 MW & lower ratings boiler has usually TriSector Type APH. . • Not used in Modern Power Plants .DAVIDSON (ROTHMUHLE) AIR PRE-HEATER OPERATING PRINCIPLE • Heating & Cooling the Stationary Mass by shifting Flue Gas and Air Stream arrangement. CONSTRUCTION OF APH (Ljungstrom Type) . Ljungstrom Air Pre-Heater (Constructional Details) Consist of following major components:1. . 4. 8. 6. Guide & Support bearing assembly Upper & Lower connecting plates Sector plates Rotor housing (Octagonal shaped cross-section casing) Rotor Rotor Drive Unit (Electrical & Air Motor) with rack & pinion arrangement Sealing system Other systems 5. 7. 2. 3. . . . Rotor Housing . . Carbon Steel Fins Profile – Double Undulation with Notch . . Corten Steel Fins Profile –Notch Flat (NF) . . . Top View of APH Elements . . oil tank. 2 filters. • Guide Bearing (Spherical roller type) is located at the top trunnion of the APH rotor & takes care of axial expansion of the rotor. • 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. 2 oil coolers etc). Oil is circulated from bearings through lubrication skid removing heat out of the bearings.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). . Bearing Lubrication Oil System Support Bearing for APH Typical Cleaning Device . Rotor Drive Unit (Rack & Pinion Arrangement) . . . due to increased content of ash in coal.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. APH SEALING SYSTEM . Radial Seals Axial Seals Bypass Seals Rotor Post Seals . 4.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. 2. Different Types of Sealing System are: 1. 3. • Seals bolted along the top & bottom edges of the radial plates & bear against the machined surface of the sector plates during rotation of rotor. . • Sector plates are attached in the centre of the upper & lower connecting plates.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. . RADIAL SEALS . • Seals attached to the outside of the rotor shell in the radial plates. . parallel to the rotor axis.AXIAL SEALS • Minimizes the leakage of air around the rotor shell into the gas side of the APH. • 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 . .Bars are bolted to these angles. • The angles are bolted to the top & bottom periphery of the rotor shell and T.BYPASS SEALS • Minimizes the air & flue gas from bypassing the heat transfer elements through the gap between the rotor & the housing of the APH. • 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 . • During rotation of rotor.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. . 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 . Water Wash/Deluge system for cleaning & fire fighting purpose. • Service air line connection for soot blowing and air motor operation. • Steam connection for soot blowing. • Instrument air connection for solenoid valve operation. Fire sensing device.Other Systems of APH • • • • • • Oil carry over mechanical detection probe. . Rotor stoppage alarm Steam Coiled Air Pre-heater (SCAPH) APH Bypass Secondary Air Duct. • SCPAH used steam as a heating medium from the Auxiliary PRDS for heating the 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.Steam Coiled Air Pre-Heater (SCAPH) • During cold boiler light up. 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. the temp. . • In some units of 250/210 MW (New Parli-6. . in APH.Steam Coiled Air Pre-Heater (SCAPH) Contd. SCAPH is used to heat only the secondary air and not the primary air. 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 the Tri-sector APH. 250 MW).. 330 >40 >40 >40 >40 315 40 40 10 Service Air line 11 Water Wash Line .APH Operating Parameters (210 MW) S.150 40 . N Description Medium Operating parameters Pressure (ksc) 1 2 3 4 5 6 7 8 9 Flue Gas Inlet Flue Gas Outlet Primary/Sec. Air Outlet Support Bearing Guide Bearing Support Bearing LOP O/L Guide Bearing LOP O/L Soot Blowing Line Gas Gas Air Air Lub Oil Lub Oil Lub Oil Lub Oil Steam Air Water 1 1 1-5 1-5 14 5-7 5 Temp.45 300 .400 130 . (0C) 350 . Air Inlet Primary/Sec.