results,wall shearstress levelsand relative fluidexchange timesfor the spherical
valvehouseof a mix-proofvalve(shownlaterin Fig. 11.5)and the similarvalues
for the upstand producethe prediction given above.Hence,a referenceof fluid
exchange is still needed to performa soundprediction of cleaninglevel.
Comparisonof the predictedcleanability with the cleaningresults showsthat a
goodprediction is possible.The interestingpart is that the reasonfor the cleaning
difficulties in the downstreampart of the geometry can be identified. Hereslow
fluidexchangeis present, whichis a consequenceof the disturbancefromthe
shortupstandinto the mainflowbecause of the geometry. Cleaningis difficult
eventhough wall shearstressis similarto the wall shearstressin regionsthat are
cleaned.FromCFDsimulationsof the shortand a longupstand,this disturbance
was shownto be non-existent in the long upstand.Investigationsare continuing to
find a lengthof the upstand that produces a flowthat doesnot disturb the main
flow.The disturbanceof the mainflowmakesthe problemout of control,whilea
longerupstandisolatesthe problemarea to be in the upstandonly.
11.4.3 Example2: Flowin expansions
Expansions are appliedin differentpartsof foodproduction systemsfromthe
simpletransitionfromone pipediameterto anotherdiameter, e.g. to control the
velocityof the product(residencetime),to the connectionsbetween equipment
and the pipesystem(e.g.froma pumpwithan off-the-shelfoutletdiameterto
Fig. 11.3 Illustrationof the areasin the shortupstandexposedto the differenttypesof
cleaningconditiontype definedin Section11.4.2.Predictionof cleaningcharacteristicsis
basedon wallshearstressand fluidexchangedataavailablefromCFDsimulations.
Improvingthe hygienicdesignof closedequipment 203