Pemilihan Heat ExchangerChoosing the best exchanger for a given process applicationCopyrightHyprotech UK Ltd holds the copyright to these lectures. Lecturers have permission to use the slides and other documents in their lectures and in handouts to students provided that they give full acknowledgement to Hyprotech. The information must not be incorporated into any publication without the written permission of Hyprotech.

Langkah-langkahCoarse filterBuang Jenis Alat Penukar Kalor yang tidak memenuhi ketentuan tekanan dan temperatur operasi, fluid-material compatibilitas, kondisi termal yang extremFine filterEstimasi HargaCoarse filterUse information on next few slides to reject those exchangers which are clearly out of range or are otherwise unsuitableThe information is summarised in the tableAt this stage, if in doubt, include the exchanger (poor choices are likely to turn out expensive at the fine filter stage)The table in the accompanying Lecturer Pack should be copied for students use in the examples.Point-point utamaTube /pipa dan cylinders dapat menahan tekanan yang lebih besar dibanding dengan platesJika APK dapat dibangun dengan material yang bervariasi, berarti anda dapat menentukan metal yang dapat tahan terhadap temperatur yang extrem dan fluida-fluida yang korosifAPK yang khusus hanya memiliki supplier yang sangat sedikit, waktu pengiriman barang yang lebih lama dan harus diperbaiki oleh orang yang sangat ahli.The last point means that specialist exchangers are not favoured in less developed parts of the worldThermal effectiveness

Stream temperature rise divided by the theoretically maximum possible temperature rise T1,inT1,outT2,outT2,inThe effectiveness can be calculated for each stream. The higher of the two is the one that is important. Typically, exchangers are designed with an effectiveness of 60 - 80 per cent. All exchanger types can handle this. However, more specialist exchangers are required for an effectiveness above about 90 per cent, as will be seen.Double PipeTipe APK ini adalah yang paling simpel, memiliki satu tube di dalam dan satu tube pada bagian luar, Tube paling dalam bisa memiliki sirip secara longitudinal pada bagian luarnya

Walaupun demikian terdapat pula jenis APK ini yang memeiliki beberpa tube didalam tube luarnya.Double pipeUkuran Normal 0.25 to 200m2 (2.5 to 2000 ft2) per unitNote multiple units are often usedBuilt of carbon steel where possible

Advantages/disadvantages of double-pipeAdvantagesEasy to obtain counter-current flowCan handle high pressureModular constructionEasy to maintain and repairMany suppliersDisadvantageBecome expensive for large duties (above 1MW)

Scope of double pipe

Maximum pressure 300 bar(abs) (4500 psia) on shell side1400 bar(abs) (21000 psia) on tubesideTemperature range-100 to 600oC (-150 to 1100oF)possibly wider with special materials Fluid limitationsFew since can be built of many metalsMaximum e = 0.9Minimum DT = 5 KIt should be noted that the ranges and limits quoted above are a guide as to what is normal today. This limits are being extended. Also, with care in design and with specialist manufacture, it is possible to extend the limits, although this may be at additional cost.Shell and tubeSize per unit 100 - 10000 ft2 (10 - 1000 m2)Easy to build multiple unitsMade of carbon steel where possible

Advantages/disadvantages of S&TAdvantagesExtremely flexible and robust designEasy to maintain and repairCan be designed to be dismantled for cleaningVery many suppliers world-wideDisadvantagesRequire large plot (footprint) area - often need extra space to remove the bundlePlate may be cheaper for pressure below 16 bar (240 psia) and temps. below 200oC (400oF)Scope of shell and tubeEssentially the same as a double pipeMaximum pressure 300 bar(abs) (4500 psia) on shell side1400 bar(abs) (21000 psia) on tubesideTemperature range-100 to 600oC (-150 to 1100oF)possibly wider with special materials Fluid limitationsFew since can be built of many metalsMaximum e = 0.9 (less with multipass)Minimum DT = 5 KPlate and framePlates pressed from stainless steel or higher grade materialtitaniumincoloyhastalloyGaskets are the weak point. Made ofnitrile rubberhypalonvitonneoprene

Advantages of plate and frameHigh heat transfer - turbulence on both sidesHigh thermal effectiveness - 0.9 - 0.95 possibleLow T - down to 1KCompact - compared with a S&TCost - low because plates are thin Accessibility - can easily be opened up for inspection and cleaningFlexibility - Extra plates can be addedShort retention time with low liquid inventory hence good for heat sensitive or expensive liquidsLess fouling - low r values often possible

Disadvantages of plate & framePressure - maximum value limited by the sealing of the gaskets and the construction of the frame.Temperature - limited by the gasket material.Capacity - limited by the size of the ports Block easily when solids in suspension unless special wide gap plates are usedCorrosion - Plates good but the gaskets may not be suitable for organic solventsLeakage - Gaskets always increase the riskFire resistance - Cannot withstand prolonged fire (usually not considered for refinery duties)

Scope of plate-frameMaximum pressure25 bar (abs) normal (375 psia)40 bar (abs) with special designs (600 psia)Temperature range-25 to +1750C normal (-13 to +3500F)-40 t0 +2000C special (-40 to +3900F)Fluid limitationsMainly limited by gasketMaximum e = 0.95Minimum DT = 1 K

Welded platesWide variety of proprietary types each with one or two manufacturesOvercomes the gasket problem but then cannot be opened upPairs of plates can be welded and stacked in conventional frameConventional plate and frame types with all-welded (using lasers) construction have been developed Many other proprietary types have been developedTend to be used in niche markets as replacement to shell-and-tubeAir-cooled exchangers

Inset figure is of an induced draught ACHE whereas a forced draught type was shown in the last lecture. Induced draught tends to give better air-flow distribution. However, the fan is working in hotter air and is less efficient. Furthermore, access and maintenance are more difficult with induced draught.Advantages of ACHEsAir is always availableMaintenance costs normally less than for water cooled systemsIn the event of power failure they can still transfer some heat due to natural convectionThe mechanical design is normally simpler due to the pressure on the air side always being closer to atmospheric.The fouling of the air side of can normally be ignored

Disadvantages of ACHEsNoise - low noise fans are reducing this problem but at the cost of fan efficiency and hence higher energy costsMay need special features for cold weather protectionCannot cool to the same low temperature as cooling towerThe evaporative cooling in a cooling tower produces cooler waterScope of Air Cooled ExchangersMaximum pressure- tube(process) side:500 bar (7500psia)Maximum temperature: 600oC (1100o F)Fluids: subject to tube materials Size per unit: 5 - 350m2 (50 - 3500ft2 ) per bundle (based on bare tube)Plate Fin ExchangersFormed by vacuum brazing aluminium plates separated by sheets of finningNoted for small size and weight. Typically, 500 m2/m3 of volume but can be 1800 m2/m3Main use in cryogenic applications (air liquifaction)Also in stainless steel

As a rough guide, a plate fin would be a fifth the size of a shell and tube for the same duty. Of course, a shell and tube exchanger is often not suitable for many plate-fin applications involving many streams and small temperature differences.Scope of plate-fin exchangerMax. Pressure90 bar (size dependent)Temperatures-200 to 150oC in AlUp to 600 with stainlessFluidsLimited by material DutiesSingle and two phaseFlow configurationCross flow, Counter flowMultistreamUp to 12 streams (7 normal)Low DTDown to 0.1oCMaximum DT50oC typicalHigh eUp to 0.98Important to use only with clean fluids

The standards of ALPEMA (Brazed Aluminium Plate-fin Exchanger Manufacturers Association) may be downloaded free of charge from the ALPEMA web site - www.alpema.orgPrinted Circuit ExchangerVery compactVery strong construction from diffusion weldingSmall channels (typically 1 - 2 mm mean hydraulic diameter)Can be made in stainless steel, nickel (and alloys), copper (and alloys) and titanium

Scope of PCHEMaximum Pressure1000bar (difference 200bar)Temperature -200 to +800oC for stainless steel but depends on metalFluidsWide rangebut must be low foulingNormal Size1 to 1000m2Flow configurationCrossflow or counterflowEffectiveness up to 0.98Low TYesThermal cyclingHas caused problemsExampleWhich exchanger types can be used for condensing organic vapour at -60oC and 60 bar by boiling organic at -100oC and 70 bar?Would you modify your choice if the boiling stream were subject to fouling requiring mechanical cleaning?The exchangers which can handle the pressure and temperature are

Double pipeShell-and-tube (with special material)Plate-finSome welded plate designs could be investigated

Fouling would rule out plate-fin and some welded plate designs.Heat exchanger costing - fine filterFull cost made up ofCapital costInstallation costOperating costThe cost estimation method given here is based only on capital cost - which is the way it is often doneNote: installation costs can be as high as capital cost except for compact exchangersInstallation cost considerations can predominate on offshore plantScopingThe cost estimate method given here is for the preliminary plant design stage - scopingNote that we are trying to estimate the cost of an exchanger before we have designed itFull design and cost would be done later $

Quick sizing of heat exchangersWe estimate the area fromTaTb

Where

FT correction factorThis correction accounts for the two streams not following pure counter-current flowAt the estimation stage, we do not know the detailed flow/pass arrangement so we useFT = 1.0 for counter flow which includes most compact and double-pipeFT = 0.7 for pure cross flow which includes air-cooled and other types when operated in pure cross flow (e.g. shell-and-tube)FT = 0.9 for multi-passFT = 1.0 if one stream is isothermal (typically boiling and condensation)Using an FT of 0.9 for multipass exchangers assumes that the designer is going to avoid having a value less than 0.8. It cannot be higher than 1.0 so 0.9 seems a reasonable average within the accuracy of these estimates.Estimating UThis may be estimated for a given exchanger type using the tablesThese tables give U values as a function of Q/T (the significance of this group will become clear later)Example: high pressure gas cooled by treated cooling water in a shell-and-tube, whereQ/T = 30 000 W/Kgives U = 600 W/m2KThis includes typical fouling resistancesThe tables are included in the Lecturer Pack with the required table entry circled.

It is worth also noting the the C value of 0.4 at this stage - the significance will become clear later.Estimating costThis has often been done by multiplying the calculated area, A, by a cost per unit areaBut, when comparing exchangers, U and A vary widely from type to type. It is also difficult to define A if there is a complicated extended surface. Note, from our basic heat transfer equationUA = Q / DTThe costs were obtained from manufacturers who looked a the typical costs of exchangers built for the different applicationsSteps in calculationCalculate Tln and hence estimate TDetermine Q/TLook up C value from tableTo determine C at intermediate Q/T, use logarithmic interpolation - see next slideCalculate exchanger cost from - Cost = C(Q/T)Taking the last shell-and-tube example, C = 0.4. Hence, Cost = 0.4 X 30 000 = 12 000Logarithmic interpolation

ln(C1) ln(C2)ln(C)ln(V1)ln(V)ln(V2)Where the Vs are the values of Q/T. V1 and V2 are the values either side of the required value VIn the example given previously, the Q / DT value happens to be in the table. Usually, however, you must interpolate between entries in the table. This is done effectively by plotting on log-log paper and doing a linear interpolation. The slide gives the formula for this.