explanation. At firstglance,recirculationzoneswillbe presentin the valve
housebecause of shadowareas. However, performing3D CFDsimulationsof
the flowin a spherical valvehousewiththe stemin the closed positionshows
howgoodfluidexchangeis achieved(Jensenand Friis,2004b). The key to the
goodfluidexchange is that onlya few stationary recirculationzonesare present
in the valvehouse.Whatappear to be shadowareaslookingat construction
drawings are, in fact,areas where the recirculationzonesrotatearoundaxes
parallel to the axialaxis of the inletand outletpipes.In thesezonesthe fluid
moves along theseaxesfromjust downstreamof the inletto the valvehouseand
towards the outletof the valve house.Moving recirculationzonesare also
known as swirl. The advantage of a swirlingflowis that the part of the detergent
enteringthrough the inlet of the pipe that goes into the swirl is moved
downstreamin a circling motion.In the casewherethis twistingmotionhas one
side of the flowpathlocatedneara wall,detergentis moved to the walland the
soil is loosened.
In the spherical valve houseinvestigated, two swirlzonesin eachside of the
valve, on top of one other,movedownthroughthe valve housegeneratinggood
fluid exchange in the upperand lowerpart of the valvehouse(Fig.11.6).The
partof the walllocated betweenthe two swirlzonesis exposed to flow
conditionsof a stagnationzonenaturewherebothfluidexchangeand wall shear
stressare low.Such a zoneis presentfromthe inletto approximately three-
quarters of the waydownstreamin the valve house.Hence,this is an areaof
potentialhygieneproblems.Comparing the flowpatternswithdata fromactual
Fig. 11.5 Designof a mix-proofvalve:(a) showsthe outersurfacesof the valvehouse
onlyand (b) is a cross-sectionalcut downthroughthe valvefullyequippedfor operation.
206 Handbookof hygiene controlin the foodindustry