basedon a rotatingapproach wherebyscrapers on a rotatingshaftcontinually
removefluidfromthe heat transfer surfaceand mix it backinto the bulk,while
allowingfreshmaterial to reach the surface.A recentdevelopmenthas used
linearratherthan rotarymotion to providethe scraping action, witha number of
speciallydesignedbaffles attachedto the reciprocatingshaft. The design of the
baffles can be variedto suit the product beingprocessed.
Fluidbed heatexchanger
For severelyfoulingliquidsa fluidbed exchangerhas beendevelopedovera
number of yearsfor industrialapplicationsin whichsmall particles (1±4mm
diameter) of glass,ceramicor metalare fluidisedinsidevertical paralleltubes by
the upward flow of liquid.The solidparticles disrupt the laminar boundary layer
to improveheat transferand in addition the particles havean abrasive effecton
the wall of the heat exchangertubes,helpingto minimisethe buildup of fouling
deposits (Klaren2001).It is claimedsuch techniquesare suitable for food
industry applicationssuchas raw juiceheating.
Improved workingpressurecapabilities
In additionto reducing metalthickness, considerableeffortshavegoneinto
increasing the rangeof operatingpressures that the plateheatexchangercan
operate within.Modifieddesignsare nowcapableof operatingat working
pressures of 20 bar,whichalthoughlowerthan tubularsystems are stilla
significantimprovement.
Improvementsin defect detection
Heatexchangersurfaces can becomedegradedovertimeowingto bothphysical
and chemicalstresses resultingin cracksor pinholesin the heat transfer surface
leading to potential chemical or microbiologicalcontamination of product.
Typicallyplate heat exchangers haveto be dismantledbeforeeitherusing a dye
penetranttechniqueor sendingthe platesawayfor inspectionby the supplier.A
techniquehas beendeveloped(Bowling1995)wherebyan electrolyteis circulated
underpressurethroughthe productside of the heat exchanger.On the surfaceside,
wateris circulatedand the conductivityis monitored.If a defectis present,flow of
electrolytethrough the defectunderthe influenceof a pressure differentialwill
resultin a detectablechangein conductivity.It is claimedthat this techniquehas a
similarsensitivityto that of traditionaldye penetrantmethods.
12.4 Future trends
Thereare number of areas for futureimprovement in heatexchangerdesign.
12.4.1 Modified surfacesto reducefouling/enhancecleaning
Foulingis still one of the majorunresolvedproblemsin heat transfer,resulting
in reducedperformance, the needfor regularcleaning and the cost incurreddue
Improvingthe hygienicdesign of heatingequipment 217