to loss of productiontimeand cleaning.An increasingamountof work has been
carried out on the modification of the heattransfer surfaceto eitherreduce the
rate of fouling or significantlyreduce the timerequired to cleanthe exchanger.
A number of potentialapproachesare currentlybeing researched.
Applicationof a coatingto the surface
It has beenknownfor some timethat the adhesion of foulingdeposits to surfaces
is reduced, the lowerthe surface energy of the surface. Attemptshavebeen
made to coatsurfaceswithceramics, poly(tetrafluoroethane) (PTFE)or other
non-toxiclayers(Zhaoet al. 2002 ). However, suchcoatings must be thin to
avoid the loss in heat transfer performance because of their low thermal
conductivity.Thislimits the adhesion betweenmetalsurfaceand coatingand
hencethe ability to withstand mechanicalstresses. Recentworkusing composite
coatings of Ni-P-PTFE showed improved mechanical strength and the
attachmentof thermophilic streptococci couldbe reduced by morethan99%
(Zhaoet al.2002).
Modificationof the materialsurface
Novel low-foulingsurfaces havebeen developedby ion implantation,sputtering
or electrolyticdeposition (Muller-Steinhagen and Zhao1997).Thesehavethe
advantage of improved abrasion resistance and strong adhesion.Resultsfor
diamond-like carbon (DLC) and sputtered composite coatings(CrN, CrC,
Cr 2 O 3 ) showedreductions of 80±99%in thermophilicstreptococci (Zhaoand
Muller-Steinhagen1999).
12.4.2 Alternative geometriesto achievehigherheattransferarea/volume
ratios
Plateheatexchangersare generally consideredto be the most compactof
commercialheat exchangerdesigns in termsof heat transfer area to volumeratio,
150±350 m^2 /m^3. These are due to the fundamentaldesignprinciple, whichuses a
narrow gap, 2.5±6.0mm,betweenthe heat transfersurfaces.An extension of this
approach has beenthe laboratorydevelopment of cross-corrugated polymerfilm
heat exchangers withgapsbetween0.3 and 1.5 mm,resultingin volumetric heat
transfer areas of 500±2500m^2 /m^3 (El-Bourawiand Ramshaw 1999).
12.5 Conclusions
Heat transferwill continue to forma key unit operationwithinthe foodindustry.
Increasingcost pressures and demandsfor flexibility will continueto challenge
the ingenuityof the heat exchangerdesigners to explore further ways of
enhancing the process.It is likely that one of the most promisingareasfor
improvement lies in the modificationor coating of surfaces to reducefouling
and enhance cleaning.
218 Handbookof hygiene controlin the foodindustry