influence of pulsing in tubularsystems on heattransfer can show:an increase
(Karamercanand Gainer, 1979),no enhancement(Martinelliet al., 1943)or a
decrease(Lemlich,1961)in the heat transfer coefficient.Low-frequency/large-
amplitude waves generated by a solenoidvalvearrangementenhanced removal
of whey protein deposits (Farries and Patel, 1993).Gillhamet al. (2000)
concludedthat pulsing had negligible effecton the initialswelling'phase,but had significanteffecton bothsubsequentphases. Cleaning behaviourin the uniform' stage was more sensitive to pulse amplitude than frequency.
Enhancementof cleaningis evidently controlled by a combination of surface
shearstress(amplitude)and frequencyeffects, it doesnot solelydependon the
shearstressimposedon the surface. Flowreversal seemsto be especiallyuseful
in speeding cleaning ± but this might be difficult to do in practice.
29.4.2 Reductionof environmentalimpact:noveltechnologies
Initialissueswithrespect to the use of enzymes to cleandairyequipment
included highcosts and low cleaningefficiency (Grasshoff,1997).However,
withincreasing environmental concern, enzymatic cleaners are a promising
alternative to traditional chemicals(Grasshoff, 2002).The textile industryhas
employed suchmethods, resultingin reductionin the chemicalsrequiredand
reducedheating, henceenergysaving.Enzymes havebeen successively usedfor
the cleaningof coldmilkprocessingequipment(Potthoffet al., 1997)and
membrane cleaning. A number of investigations on the use of enzymesto clean
milkheatershavebeenreported (Grasshoff,1997).
Theuse of ozonatedcoldwater(10 ÎC) priorto cleaningof a dairysoil
(reconstitutednon-fat dry milk)fromstainlesssteelplateshas beeninvestigated
(Guzel-Seydimet al., 2000).Results showed that the ozonatedwaterpre-
treatment removed33%moreof the depositthan pre-treatmentwithwarmwater
at 40 ÎC. Trials on a novel`pigging' system, whichusesa mixtureof waterand
ice combined witha freezing pointdepressant, to removedeposits suchas jam,
margarine, saladcreamand toothpaste havebeenreported (Quarini,2002).The
advantages of this techniqueincludelow environmentalimpactand the ability to
separate and recover products. Results showa significant improvement on
cleaning behaviourcompared withwaterat 20 ÎC; however, the results are not
compared withcleaningwithchemicals.It is also not clear yet to whatextent
piggingremovesverythin layersof deposit, for example ± but the methodmay
significantlyreducerinsingtimes.
29.4.3 Effect of surface modification
Thephysical properties of the depositand the surfacewillaffectremoval.
Foulingdeposits formas a result of adhesion of speciesto the surface and
cohesion between elements of the material.The forces between elements of the
depositdependon the nature of the material; deposits may be covalentlybonded
(for example, reactedegg or milkproteins)or heldtogether physically(such as
Improvingthe cleaning of heatexchangers 487