aph bid assessment - howden

(New)Air Preheater bid assessment.doc - 24/10/2000

Howden Power LtdCustomers Manual

Product InformationAir preheater bid assessment

01/07/00

Howden Power LtdOld Govan RoadRenfrew, UKPA4 8XJ

Tel: +44 141 885 7300Fax: +44 141 885 2887Web: www.howden.com

Customers ManualProduct Information - Air preheater bid assessment


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Introduction

To ensure that a air preheater meets the customers basic requirements, in terms of performanceand reliability, it is important that it is specified and tendered correctly. In addition the purchasermust be confident that the air preheater has been designed to the high standards demanded bymodern day power plant.

There are three stages in the identification of a suitable air preheater. Firstly basic qualification ofa vendor and his products. Secondly in the production of a technical specification. Thirdly theassessment of the bid. The purpose of this discussion is to highlight areas which a purchasershould consider when carrying this process out.

Broadly these points can be grouped in six technical categories; layout, duty, selection,performance, scope and engineering.

Reference is made to Draft European Standard prEN 45524 'Guides for Procurement - BoilerAuxiliaries - Gas-Air, Steam Air and Gas-Gas Heaters,

Layout

Temperature stratificationThe temperature of each pack of heating elements varies cyclically during each revolution. Thusthe temperature of the flue gas leaving the preheater will vary across the duct perhaps by asmuch as 20K from the average gas temperature. If the gas does not mix properly before it entersthe precipitator the higher velocities and temperatures could affect precipitator performance. Thedirection of rotation and orientation of the preheater can assist the gas in mixing whilst it travelsalong the downstream ducting.

Inlet flow profileThe inlet flow and temperature profiles must be reasonably even to ensure that the preheater willperform efficiently. It is worthwhile allowing the manufacturer to comment, before the final layoutis decided.

Horizontal shaftThe vast majority of air preheaters are designed with a vertical shaft arrangement. With thehorizontal shaft option there are fluctuations in gravitational loading and as such the elementbaskets must be fixed securely in position to prevent them moving. Construction must be heavierto cope with the cyclic fatigue stresses and thus the preheater will be slightly larger than itsvertical shaft equivalent.

The horizontal shaft heater is not self-draining in the same manner as a vertical shaft heater.

Ducting arrangementThere are four main preheater arrangements, bisector, tri-sector, quad-sector and concentric,with each having its own pros & cons for a particular duct arrangement.

Element removalConsideration should be given to the fact that cold end elements will have to be replaced moreoften than those in the hot and intermediate tiers. As an option to removing the cold endelements vertically upward and therefore having to remove and replace the other tiers, it ispossible to withdraw them radially.



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Duty

EffectivenessThere must always be a reasonable temperature difference between hot and cold streams forheat transfer to take place, thus the maximum temperature of the air leaving the preheater islimited by the temperatures of the air and gas entering it. Air side effectiveness, εa, is a usefulmeasure of how hard the heat exchanger is working.

εa = Tao - Tai Tgi - Tai

It is unlikely that air preheaters will exchange heat at effectiveness levels much greater than 92-93%.

In addition, a rough rule of thumb is that there should be about 20-25K difference between gas inand air out temperatures.

LeakageLeakage levels of air to gas can be defined in a number of ways. The most common is to expressthe mass flow of air leakage as a percentage of the gas flow entering the air preheater.Alternatively as excess air rise.

L = mal x100% L = A ( E" - E' ) x 100% mgi A

Other methods are to express leakage relative to

a) the air flow entering or leaving the preheater

L = mal x100% or L = mal x100% mai mao

b) the O2 rise across the gas side of the preheater

L = 0.89 (O2o - O2i) x100% 21 - O2o

c) the CO2 drop across the preheater

L = 0.89 (CO2i - CO2o ) x100% CO2o

Gas velocityIn addition to affecting pressure drop, the velocity of the gas entering the preheater will influencethe rate of erosion of the elements. It is important to understand where the velocity is measuredto ensure consistency of assessment between suppliers. Howden specifies velocity between theelements, at the point where the gas stream enters the elements.

Hot end foulingFouling at the hot end of an air-preheater is generally a reflection of slagging within the boilerwhich in turn is dependent on fuel properties and firing conditions. If there is a chance that thiscondition could exist then the air preheater can be fitted with a more open profile and/or elementswhich are loosely packed.



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Cold end fouling/corrosionThe cold end of an air preheater operates at temperatures below the acid (H2SO4) dew-point.Generally, with coal firing, there is sufficient alkaline fly ash in the fuel to adsorb the sulphuricacid and prevent fouling and corrosion of the heater and ducting. As a rule of thumb, if the ash tosulphur ratio in the fuel is 7:1 or greater, then a corrosion problem should not occur, even attemperatures below 120oC.

If the ash to sulphur ratio is less than 7:1 then higher back end temperatures, materials moreresistant to acid, or the addition of an alkali to the flue gas should be considered.

Conversion of SO2 to SO3 will occur in a Selective Catalytic Reduction (SCR) DeNOx plant. Thiscan increase the amount of sulphuric acid in the flue gas by a factor of 2-4. Thus whencalculating the ash to (effective) sulphur ratio, if an SCR plant is installed upstream of the airpreheater, the sulphur in the fuel should be multiplied by the SO2 to SO3 conversion ratio.

Ammonia escaping from the SCR plant (ammonia slip) can form ammonium bisulphate which willdeposit on the air preheater elements. It is preferable that this does not occur at the spacebetween two tiers of elements and Howden policy is to design the depth of the cold end tier toconfine the fouling within it.

Selection

Element typeHowden has a large number of element profiles available for use in air preheaters. The two mostcommonly used for new projects are FNC and 2.78DU. Two others which find application, mainlyfor retrofits are 9.5/3CU and NP.

Each element has different characteristics. The main properties of interest are listed below, withthe elements ranked from lowest to highest

Heat transfer per unit lengthNP CU DU FNC

Pressure drop per unit lengthNP CU DU FNC

Heat surface area per unit volumeNP CU DU FNC

Resistance to foulingFNC DU CU NP

CleanabilityFNC DU CU NP

Internal gas velocity (erosion).NP CU DU FNC



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Heater typeFor any duty it is possible to select bisector (main and mill), trisector, quadsector and concentricconfigurations. Each configuration has different characteristics. The main properties of interestare listed below, with the preheaters ranked from lowest to highest. Note that being limited to thechoice of standard rotor diameters and depths may affect the precise order of theseconfigurations on a particular project.

Capital costTri Bi Quad Conc

Heating surface areaBi/Conc Tri Quad

Air to gas leakageQuad Bi/Conc Tri

Fan powerBi/Conc Quad Tri

Number of heatersIt is more economic to use 1x100% air preheater than 2x50% heaters. This not only reducesheater cost but also may save ducting, steelwork, erection and C&I costs. It will be possible todesign a single preheater for boiler sizes up to 600MW (also for larger boilers but this will dependon the actual duty).

Temperature controlGenerally it is necessary to control the primary air temperature to suit varying coals and to controlthe metal temperature at the cold end of the preheater to keep corrosion and fouling withinacceptable limits.

There are a number of methods which can be used to control the temperature of the primary air.

Gas biasingVarying the share of gas flow between the primary and secondary air heaters.

Economiser bleedIncreasing the temperature of the gas entering the primary air heater.

The above two methods are only appropriate to main/mill and concentric heaters. Used on theirown, both result in increased temperature to the stack when operating off-design, which affectsID fan power as well as boiler efficiency.

Primary air bypass

Bypassing primary air past the heater will reduce the temperature of the re-mixed flow ofair to the boiler.

Generally reduction in heat transfer in the primary air section will be balanced by anincrease in the secondary air section of trisector and quadsector preheaters.

Steam air preheating

Increasing the temperature of the air entering the preheater will increase thetemperature of the air exiting and will also raise the cold end metal temperature.Generally used in the secondary air duct.



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Secondary air recirculation

Recirculating part of the air flow from the preheater outlet to the fan inlet will increase theaverage metal temperature at the cold end of the preheater but will have little effect onthe temperature of the air to the boiler. Fan power will rise in proportion to the amount ofair recirculated. In addition the fan will have to be sized for the extra flow thus it will beselected with a lower efficiency at normal operation than it would have been wererecirculation not installed.

Element splitsWherever possible the depth of element tiers should be such that

the number of tiers is minimisedfouling and corrosion is contained within the cold end tierhot and intermediate tiers of the same type of element are the same depth, to permitthem to be exchanged.

Space for extra surfaceSpace can be left at the hot end of the heater to permit the addition of extra elements at a laterstage. Howden Sirocco has sufficient confidence in the performance of its elements such that noallowance need be made for under-performance of the air preheater.

Performance

Air leakageOn a performance data sheet, air leakage can be calculated a) from the difference in gas massflow entering and leaving the preheater and b) from the difference between diluted and undilutedgas temperatures.

Leakage driftA common phenomenon on some designs of air preheater is leakage drift, where air leakageincreases substantially over a period of time. Margins have to be added to the draught fan dutiesto account for this. The Howden VN air preheater does not exhibit leakage drift. If margins for driftare not added then fans will be smaller and MCR duties will be performed at higher efficiencies.

Fan powerFan power is affected by

Pressure drop across the preheaterAir to gas leakageGas outlet temperature andPrimary air to secondary air leakage.

Guarantee marginsThe margins between predicted performance and the guaranteed performance will indicate theconfidence the supplier has in his quoted figures.

Preheater sizeIt is possible to select a number of size/depth combinations for a particular duty. Comparing twopreheaters differing by a half size will show that the larger will have

a) a shallower depth (approx 10%)b) a lower pressure drop (approx 20%)c) a greater leakage (approx 10%)



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Pressure dropIt is worthwhile checking that quoted pressure drop is measured between the inlet and outletflanges of the preheater and not just over the elements.

LeakageLeakage is a function of

a) Configuration (bi- tri- quad- conc-)

b) Size - Generally preheaters on larger boilers have a lower leakage (expressed as apercentage of the mass flow of gas entering the preheater).

c) Pressure differential between the air and gas streams

Sealing systemThere are two main types of sealing system which are incorporated on air preheaters - fixed andsensor controlled. The former eg the Howden VN, is set up during an outage and does notrequire adjustment while the boiler is on-line. It results in a simple design which is not prone toleakage drift. Maintenance and C&I requirements are minimal.

The latter eg earlier Howden designs, generally uses a system of sensors and adjustable sectorplates to minimise the gaps between rotor and casing. Experience has shown that this conceptgenerally exhibits leakage drift unless a very high standard of plant operation and maintenance iscarried out at the station.

A variant of this system is available from Howden. Two position actuation provides sector plateadjustment which is either 'In' or 'Out'. This is less prone to leakage drift and requires less C&Iinterface than full sensor control.

Cleaning cycleOn a large plant it may be possible to optimise soot-blowing cycle to minimise the compressorsize, if air sootblowing is adopted.

Scope of supply

A checklist of potential items which could be included in a preheater vendor's scope of supply isappended.



7

Engineering

MaintenanceAs power stations look to extend the period between major outages the ability to supply airpreheaters which can operate for 3 or 4 years between overhaul is advantageous.

ElementsElements must be thoroughly tested to ensure that they will perform to specification. In additionthe difference in performance between ideal laboratory conditions and practical field conditionsmust be understood. Performance verification by independent organisations is of value. AllHowden elements have been thoroughly tested in the laboratory, independently tested in otherlaboratories and verified at site.

Thermal analysisAn accurate knowledge of the relative movement of casing and rotor under the varyingtemperature conditions that the preheater will encounter is necessary to ensure that significantseal wear does not occur. Techniques such as finite element analysis are of great importance, asare comprehensive measurements taken at site.

Stress analysisTechniques such as finite element analysis are of great assistance in ensuring the technicalintegrity of rotary heat exchangers.


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Appendix

Scope of supply check-list

Rotor c/w

ElementsSpace for extra surfaceSide withdrawal of the cold end tierDownward withdrawal of the cold end tierEnamel cold endHot end sealing stripsCold end sealing strips

CasingTransition ductsDuct flexible connectionsLagging fastenersExternal thermal insulation

Adjustable sector platesSensorsActuatorsLocal control panelWiring from panel to sensors and actuators

Centre drive gearboxRack and pinionMain drive AC/DC/AirStand-by drive AC/DC/AirSlow speed drive AC/DC/AirSlow speed drive doubles as stand-byInternal lubrication systemExternal lubrication systemElectric motor starters

Bottom support bearingRolling elementPlain bearingTemperature detector

Top steady bearingRolling elementPlain bearingCooling water isolation valveTemperature detector

Lubrication systemFirst fill of lubricants

Hot end sootblowerCold end sootblowerOff-line low pressure water washingOn-line high pressure water washingSemi-retractable blowersFully retractable blowersLocal control panel Per heater/per boiler

Wiring from panel to blowersAir isolation valveWater isolation valveAir and water supply piping

Bottom bearing access platformTop bearing access platform

Fire detection probes Infra-red ThermocoupleFire fighting isolation valveFire detection panelWiring from panel to probes

Speed detector probesSpeed detection panelWiring from probes to panel

Runway beams and hoistsPlatforms and ladders

Duct access doorsInspection windowsDraught gauges


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Air preheater configurations

Bi-sector Tri-sector

Quad-sector Concentric

Bi-sector Concentric

Tri-sector Quad-sector


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Air preheater elements

FNC - Flat Notched Crossed

DU - Double Undulated

CU - Corrugated Undulated

NP - Notched Plate


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Leakage Drift

0

2

4

6

8

10

12

14

0 1 2 3 4 5 6 7 8

% L

eaka

ge

AfterOverhaul

1 Year 2 Years 3 Years


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Comparison of Sealing Systems

VN designwith double sealing

and fixed sector plates

VAL designwith single sealingand actuated sectorplates

Original sector plate and rotor

Modified sector plate and rotor Leakage

Leakage

Air preheater size versus effectiveness

0

1

2

3

4

5

6

7

0 2 4 6 8 1 0 1 2

Size

Eff

ecti

ven

ess


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Element removal

aph bid assessment - howden

Documents

customers manualproduct information

finite element analysis

pressure drop

cold end

heat transfer

primary air

flue gas

air leakage