12.3 Developments in heatexchangerdesign
Although heat transferis a verymaturetechnology therehavebeena number of
significantdevelopments withinrecentyears.
12.3.1 Incrementalimprovements in heattransfer performance
Overrecentyearsthe metalthicknessof the platesin plateheatexchangershas
beenreducedfromtypically0.8 mm to 0.4±0.5mm.Thishas reducedthe weightof
the materialrequiredby 40±50%and hencethe cost.Therewill also be an improve-
mentin heat transferas the thinnermetalresultsin a reducedthermalresistance.
The improvementwill be significantonlyfor applicationswherebothfluidsare of
low viscosityand in turbulentflow.For a typicalapplicationwithequalthermal
resistanceson the productand servicesidesof 10ˇ^4 m^2 K/W, a reductionin
thicknessfrom0.8 to 0.5 mm will improveheat transferperformanceby 8%.
In tubularheat exchangers, improvements in heat transfer performance have
beenachievedby usingcorrugated tubesin place of conventionalplaintubes.
Thecorrugations are claimedto enhance heattransferby disruption of the
laminarboundarylayeras the fluidflowsacross it. It is, however, probable that
suchenhancement wouldbe onlyminimal for higher-viscosity fluidsas the
corrugations are unlikely to havea majoreffect on the fluid dynamicscloseto
the heattransfersurface.
12.3.2 Alternativegeometries to addressthe technicallimitationsof
existing designs
Coiledtubes
Coiledtubedesignshavebeenusedfor a numberof years to provide a more
compactdesignof exchanger. Recentworkhas indicatedthat the movementof
the fluidin a continuous spiralprovides an enhanced mixing and heattransfer
performancethanwouldbe predictedfromconventionalheattransferdesign
correlationsfor linearsystems.
Dualplateswithair gap
Oneof the mainhygieneconcerns withconventional plateheatexchangers is
that the two fluidstreamsare separated fromeachotherby a single, relatively
thinmetal surface.If this surface becomesdamagedthrough,for example,
corrosion or flow-induced vibration,thereis potentialfor cross-contaminationto
occurbetween the two streams.If heatrecoveryis usedto heatincomingcold
product with hot product, it is possible to contaminate the heat treated
(pasteurised/sterilised) product withraw,untreatedproduct.
Current ways of minimising this risk are to maintain a higher pressureon the
pasteurised/sterilisedside to ensure any flowis fromprocessed to raw product.
Thisdoesnot, however, providecompleteassurance as microorganismscan
moveagainst a pressure gradient. Anotherapproach is to use a secondarywater
circuitwitha recirculationpumpsuchthat direct product/product contact is
Improvingthe hygienicdesign of heatingequipment 215