cataloguedata froma leadingequipmentmanufacturer. Flowat the inletis fully
developedturbulent flowat an average velocity of 1.5 m/s. The simulationresults
are evaluated withrespect to the lengthand volumeof the recirculationzone
generated in the expansion part.The measure of the volumeprovidesan indirect
measure of the surfacearea of the expansion sweptby the recirculation zone.
Looking at the geometries one wouldimagine a recirculationzonein the
expansion in boththe eccentricand the concentric expansions.However, 3D
visualisationof the flow using CFD simulations of the flow inside both
expansions shows a differencein the size and positionof the recirculation zones.
In the eccentricexpansion a recirculationzoneis established in the expansion as
expected (in the part of the flowdomain just next to the slopedsurface).The size
of this recirculationzonedepends on the slopeof the expansion. The higherthe
slope,the largerthe recirculationzone(Fig. 11.4).Hence,for cleaningpurposes
a verylongexpansion (lowslope)is preferred,as this reduces the slope of the
expansion and the size of the recirculationzone.
The concentric expansion, on contrast, does not show a single, large
recirculationzonelocated 360Îaround the centreaxisof the pipe alongthe
walls.Instead, severalsmallerrecirculationzonesbuild,mergeor disappearover
time.Thisis a consequenceof the Coanda effect (English, 1999). The fact that
no steadyrecirculationzoneis observed means that fluidexchange is relatively
highin all areasof a concentricexpansion. Furthermore, the rotatingflowalso
disturbs the boundarylayer,makingthe diffusionpath of heat and mass smaller;
hence, masstransfer to the soil is madeeasier(referto the beginningof this
chapter). In relation to hygienic design,this meansthat a concentricexpansion is
preferable to an eccentricone if, andonlyif, it is mounted in a position where it
is drainable.
11.4.4 Example3: Goodcleaningof a sphericalvalve house
Spherical-shaped valvehouseshavean interesting geometry withrespect to flow
patterns.The use of thesehas explodedwiththe introductionof mix-proof valve
types(Fig.11.5).Cleaning testshavebeencarried out on spherical-shaped valve
housesand many of thesehaveshown that this typeof valvehouseis a very
goodhygienicdesign(Jensen2003). Whyis that?The wall shearstressis low in
largepartsof the valve housebecauseof the relativelylargecross-sectionalarea
compared withthe inletand outletpipes(Jensenand Friis,2004b) and the wall
shearstresscan onlyroughlyexplainthe differencein cleanability shown in
cleaning tests(Jensenand Friis,2005).However, a goodfluid exchange in the
valvehousepromotes loosening of the bondsbetweensoil and surface,hence,
the wall shearstressneeded to removethe soil fromthe surfaceis smallerthan in
areasof poor fluidexchange(Sinner'scircle).The reason for the goodfluid
exchange in largepartsof a spherical valvehouseis discussedbelow(flow
patternsare discussed in detailin Jensenand Friis,2004b).
As mentioned,the reason for goodcleaningis foundin goodfluidexchange.
Looking at construction drawings of a sphericalvalvehousedoesnot providean
Improvingthe hygienicdesignof closedequipment 205