cleaning trails, the conclusionis that the areasthat provemost difficult to clean
in a standard cleaning test are in fact the areaswhere fluidexchange and wall
shearstressare fairlylow.
The swirlin the valvehouseis generated because of a singledesignfeature
(probably not by intention). Certainpartsof a pipemounted on a spherical
surfaceintersectwith the valvehouse slightly beforethe rest of the pipe
intersects withit. This applies especiallyto part of the inletpipelocatedon the
equatorof the valve house.The same happens to the flowin the pipe.Liquid
first enters the valve houseat the equator.This liquidexperiencesemptyspace
to the sidesand aboveand belowand it triesto fill the space. Normallythis will
createa recirculation zone(e.g.flowovera backward-facing step ± Durst and
Tropea, 1981).Thisis not the casein the mix-proof valveas the liquid,after
turningtowards the emptyspace,hits the spherical-shaped walls and is reflected
alongthe walls untilthe liquidmeets the liquidfromthe otherswirlzone
generated simultaneously. Becauseof the momentumfromthe inletpipe, this
movement goesalong the mainflowdirectiongenerating the swirl.
Thisexample and the example on expansions illustratethat thinkingswirling
flowin a designof a pieceof closedequipment will promote cleaning of the
surfaces located in the swirling zones. Swirl alsogeneratespressureloss and
often swirl-generating devices (e.g. winglets and obstacles) present other
problems related to cleaning, which are not be discussed here.
11.5 Future trends
It is nowpossible to manufacture closedprocessingequipmentto optimise the
designof equipment withrespectto the cleaningeffectof fluidflow.Thisshould
Fig. 11.6 (a) Vectorplot showingthe flowpatternin a cross-sectionin the mix-proof
valve.(b) The locationof the cross-section.
Improvingthe hygienicdesignof closedequipment 207