surface are more difficult to cleanthan other areas (Jensen, 2003).Such
considerationswouldaid designers of closedequipment to identify areas that are
difficultto clean in the earlystagesof the designphase. In the longer term,this
should result in an improved understanding of guidelines for hygienicdesign of
closed equipment.Guidelines should cover advantageousflow patterns that
promote combinations of wallshearstress and fluidexchange/mass transfer
favourablefor cleaning. In Section11.4,examples are givenon howCFDhas
beenusedfor identification of areas potentialbeing a hygienic problem. The
CFDmethodis introducedin the nextsection.
11.2.3 Flowvisualisationmethods
Visualisation of flow patterns and parameters is the key parameter for
evaluating,and gaining a higherunderstandingof, hygienicdesignrelated to
flow of detergents. Visualisation can be done experimentally (EFD ±
experimental fluid dynamics) or numerically (CFD ± computational fluid
dynamics).The CFDtools, when validated,havecertainadvantagescompared
withEFD.It should be remembered that whenusing EFD,usershaveto make
certain that they measure what they thinkthey are measuring. Themain
advantage of using CFDis the fact that dataare availablein all the control
volumes (thesame as a hugenumberof measuringprobes). Thisis hardly
possible using EFD techniques such as laser Doppler anemometry, mass
transfer techniques and thermal velocity probes. CFDresults on the surfaces are
of specialinterestto hygienicdesign.In orderto make a completeevaluationof
the hygienicdesignof a pieceof equipment,resultsmust be known overas
many portions of the surfacesof the equipmentas possible and ideallyoverthe
Fig. 11.2 Localvelocityestimatedby use of CFDcalculations.The averagevelocityis
2 m/s and the liquidis waterat 20 ÎC.
196 Handbookof hygiene controlin the foodindustry