Introduction to Heat ExchangersCourse objectivesWhat are exchangers for?Exchanger typesHow are they specified?The design task

ObjectivesBy the end of the course you willbe familiar with the main exchanger typesknow which is likely to be the best type for a given applicationunderstand what are the key factors in exchanger designbe able to estimate the size and cost of key exchanger typeshave the background necessary to start using commercial exchanger design softwarebe an informed purchaser of heat exchangers

Lecture seriesIntroduction to heat exchangers Selection of the best type for a given applicationSelection of right shell and tubeDesign of shell and tube

Q = U A T

ContentsWhy we need heat exchangersThe basics of their designSome general features of exchangersDifferent types of exchangerThe design process

Example of an exchangerBundle for shell-and-tube exchanger

What are heat exchangers for?To get fluid streams to the right temperature for the next processreactions often require feeds at high temp. To condense vapoursTo evaporate liquidsTo recover heat to use elsewhereTo reject low-grade heatTo drive a power cycle

Feed-effluent exchangerFeed-effluentexchangerExothermic reactionHeat recovery

DistillationBottom product

Typical crude oil distillationE2E1E3E4E5E6E2E5StorageKeroseneDesalterTop pumparoundTop pumparoundNaphthaand gasesKeroseneReduced crudeLightgas oilHeavygas oilReducedcrudeHeavy gas oilLight gas oilBottom pumparoundDistillation towerBottompumparound

Power cycleBoilerCondenserSteam turbineFeedwaterheater

Q = U A DTWe have thermal resistances in seriesThotTcoldyw

Heat utilitiesHot utilitiesBoiler generating service steam (maybe a combined heat and power plant)Direct fired heaters (furnace)Electric heatersCold utilitiesCooling tower (wet or dry) providing service cooling waterDirect air-cooled heat exchanger

Thermal integrationor process integrationReducing the hot and cold utility needs by interchanging heat between process streamsIf the plant needs are primarily heat, thermal integration is usually by pinch technology - Software HX-NetIf the plant is concerned with heat and work, pinch technology is supplemented with exergy analysis

Local and mean valuesOverall means from the hot side to the cold side including all resistancesHowever it is still at a particular point in the exchanger: i.e. it is localHence you can have a local, overall coefficientLOCALLY

FOR WHOLE EXCHANGER

Integrating over the exchanger areaLocal equation

Rearranging

and integratingdQdATotal area AT

Definitions of mean values From previous slides

Comparing the two sides

Special case where Ts are linear with QEqn. integrates to give log. mean temperature difference - LMTDTa

Multipass exchangersFor single-phase duties, theoretical correction factors, FT, have been derivedFT values are less than 1Do not design for FT less than 0.8QTemp.T1T2t1t2

Typical FT correction factor curvesFor shell and tube with 2 or more tube-side passes T, t = Shell / tube side 1, 2 = inlet / outletCurves are for different values of R

Thermal effectivenessStream temperature rise divided by the theoretically maximum possible temperature rise T1,inT1,outT2,outT2,in

CompactnessCan be measured by the heat-transfer area per unit volume or by channel sizeConventional exchangers (shell and tube) have channel size of 10 to 30 mm giving about 100m2/m3Plate-type exchangers have typically 5mm channel size with more than 200m2/m3More compact types available

Compactnessm2/m3100100010 000Hydraulic diameter, mm601010.1Shell-&-tubePlatePlate finCar radiatorSpecialHuman lungs

Main categories of exchangerMost heat exchangers have two streams, hot and cold, but some have more than twoHeat exchangersRecuperatorsRegeneratorsWall separating streamsDirect contact

Recuperators/regeneratorsRecuperativeHas separate flow paths for each fluid which flow simultaneously through the exchanger transferring heat between the streamsRegenerativeHas a single flow path which the hot and cold fluids alternately pass through.Rotating wheel

Double PipeSimplest type has one tube inside another - inner tube may have longitudinal fins on the outside

However, most have a number of tubes in the outer tube - can have very many tubes thus becoming a shell-and-tube

Shell and TubeTypical shell and tube exchanger as used in the process industry

Shell-side flow

Complete shell-and-tube

Plate and framePlates hung vertically and clamped in a press or frame.Gaskets direct the streams between alternate plates and prevent external leakagePlates made of stainless steel or higher quality materialPlates corrugated to give points of support and increase heat transfer

Plate typesChevronWashboardCorrugations on plateimprove heart transfergive rigidity

Many points ofcontact and atortuous flow path

General view of plate exchangerPlate exchanger normally refers to a gasketted plate- and-frame exchanger

Flow Arrangement within a PHEAlternate plates (often same plate types inverted)Gasketsarranged foreach stream toflow betweenalternate plates

Air-cooled exchangerAir blown across finned tubes (forced draught type)Can suck air across (induced draught)Finned tubes

ACHE bundle

Plate-fin exchangerMade up of flat plates (parting sheets) and corrugated sheets which form finsBrazed by heating in vacuum furnace

Can have many streams7 or more streams are typical

Typical plate-fin

Spiral (plate)Good for streams with large solids

Cooling TowersLarge shell with packing at the bottom over which water is sprayedCooling by air flow and evaporationAir flow driven by forced or natural convectionNeed to continuously make up the cooling water lost by evaporation

Agitated VesselUsed for batch heating or cooling of fluidsAn agitator and baffles promote mixingA range of agitators are usedOften used for batch chemical reaction

Proprietary typesTypes described so far are generic typesThese can be made by any company with necessary skills (no real patent protection)There are now many special, proprietary exchangers made by one company or a small number of companies under licenceOne example is the printed circuit exchanger by Heatric

Printed circuit heat exchangerPlates are etched to give flow channelsStacked to form exchanger blockBlock diffusion welded under high pressure and temperatureBond formed is as strong as the metal itself

Printed circuit exchangerNote that compact does notmean small but means largesurface area per unit volume

Distribution of typesin terms of market value in Europe

Preliminary points on selectionTubes and cylinders can withstand higher pressures than platesIf exchangers can be built with a variety of materials, then it is more likely that you can find a metal which will cope with extreme temperatures or corrosive fluidsMore specialist exchangers have fewer suppliers, longer delivery times and must be repaired by expertsS&Ts cannot normally give high thermal effectiveness, e

Design sequenceDesign the process flow flow-sheetSpecify the heat exchanger requirementsSelect the best exchanger type for the jobThermal design of exchangerMechanical design of exchangerLooping back may be necessary at any stage but can be difficult because of the project timetable

Who does what?Design the process flow flow-sheetSpecify the heat exchanger requirementsSelect the best exchanger type for the jobThermal design of exchangerMechanical design of exchangerProcessor/end userContractorManufacturer

Exchanger specificationHeat load (duty) along with the terminal temperatures of the streamsMaximum pressure drop each streamsliquids - 0.5 bargases/vapours below 2bar - 10% of inlet pressureDesign pressures and temperaturesSize/weight constraintsStandards to applyGeneral standards like ISO, TEMA, ASME etcCompanies own standardsOther requirements

The designer must supply an exchanger whichMeets the stated specificationHas reasonable initial costs and operating costs (most exchangers are bought on the basis of the cheapest tender)Has a reasonable lifetimeno damaging vibrationno thermal fatigueno unexpected fouling or corrosion

This first lecture is mainly qualitative as a lead in to the later lectures.

It is very helpful to obtain samples from exchanger manufacturers to show the students. Possibilities areA small, chevron-type plate for a plate and frame exchangerVarious fin types for a plate-fin exchangerEtched plates and a sample block for a plate of a printed-circuit exchangerHigh-finned tube of the type used in air-cooled heat exchangersSamples of fins used in plate-fin exchangers

CopyrightHyprotech 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.

Exchanger from Motherwell Bridge Thermal, Scotland

Picture just to introduce a real exchanger early on.Worth emphasising on this case that the feed-effluent exchanger needs a temperature difference to drive it, so there is a limit to what can be removed by the heat recovery exchanger exchanger. Typically. Feed-effluent exchangers involve a number of exchangers in series so that the picture is a simple case.This illustrates that real flow-sheets are much more complicated than the idealised cases shown previously. The many exchangers are to heat up streams to the required temperature for distillation. The main heat input is from the furnace or fired heater shown. Also, as much heat as possible is recovered when the refined streams are cooled down. As if this were not complicated enough, many of the exchangers shown would actually be groups of exchangers.There, in practice, many more heat exchangers in a real plant.The refinery example shown previously, the hot utility is the furnace.It is worth mentioning that the log mean becomes the arithmetic mean when the two end temperature differences become the same.

Students could try the derivation. The starting point is a simple change of variables. Given that DT varies as a straight line with Q, the equation from the last slide may be rewritten as

It should be stressed that modern design software does not use these correction factors because their derivation involves too many assumptions that are not realised in practice. In stead, modern software carries out numerical intigrateions to obtain the results.There are two values of e depending on which stream is taken as stream 1. We are concerned with the higher of the two in this lecture series.The human lung is included to put mans designs into the context of what nature achieves. However, our lungs are mainly mass transfer devices which transfers oxygen to the blood and removes carbon dioxide. Nevertheless, the lungs have an important heat transfer function. The air we breath in is relatively cold and must be heated to blood heat before making close contact with the blood. The air is therefore heated as it is taken in, thus cooling the various passage ways through which it flows. These passage ways are reheated as we breath out the stale air. The is a form of regenerative heat exchanger.The case of recuperators with the wall separating the streams is highlighted. It is the most important and the main subject of these lectures.As has been noted, the human lung acts as a regenerator because the cold stream (the incoming air) passes through the same passages as the hot stream (the outgoing stale air).

The regenerator shown above is a heat wheel.Photograph from APVForced draught most common because it is easier then to service the fan motor and also the fan motor runs coolerPicture from Motherwell Bridge ThermalFigure shows a number of interesting pointsThe way headers are arrangedThe way gaps are left at appropriate places to allow flow between the layer and the headerThe use of low frequency finning to distribute the flow across the channelPhotograph from Chart Heat Exchangers LtdPhotograph from Heatric LtdPhotograph from Heatric LtdThis is for a wide range of industrial heat exchangers. If we look at chemical and refinery applications, the shell and tube type predominates (see lecture 3).