Handbook of Hygiene Control in the Food Industry

and masstransfer. Transport of detergent throughoutthe entireprocessingline is
necessary.In closedequipment,the obviousand cheapest transport mediumis
water(Holah,2003)heatedto a specifictemperature withthe rightamount of
chemicalsdissolved in it.
The influenceof flow on contacttimeis best illustratedby comparingtwo
commonlyencountered situations:

1. In a straight pipedetergentflowsparallel to the walland fresh'detergent (temp.and chem.) is continuously transported across the soil, resultingin a contacttimeat ideal cleaningconditionscorrespondingto the cleaningtime (illustratedby Sinner'scircle in Fig. 11.1(a)assuming the idealcleaning conditionsare equalamountofenergy'fromall fourcontributions).


2. In a dead-end or a suddenchangeof geometry, recirculationzonesare
   present. In suchrecirculationzones,the detergentis not replacedat the
   samerate as in the straightpipe(heatand masstransfer are low)and the
   temperature and strengthof the detergentdecreaseslightlyas a function of
   time.Hence, the resultingcontacttimeat idealcleaningconditions(here
   assumedto be the conditionspresent at the surfaceof a straightpipewith
   fullydeveloped turbulent flow)is reduced comparedwiththe contact time
   in the straight pipe,and the totalcleaning timehas to be increased to clean
   the surfaces locatedin the recirculation zone(illustratedin Fig. 11.1(b) and
   (c)).

An additionaleffectof the flowis the transport of detachedsoil out of the
equipmentto preventrecontamination.
In Section11.4information on local wallshearstressand fluidexchange at
the surfaceis exploitedto illustrate goodand bad flowpatternsin relation to
cleaningof closedequipment.

11.2.2 Guidelineson flow conditions during CIPcleaning
A meanvelocity of at least 1.5 m/s for cleaning-in-place(CIP) of closedequip-
ment is suggested as a minimum.However,verylittle,if any, hard evidencehas
beenpublishedstatingthat 1.5 m/s is a universal value (Timperley and Lawson,
1980),but it shouldbe remembered that 1.5 m/s is usedwithsuccess for cleaning
at presenttime.In this section,the validityof specifyinga minimummean
velocity is discussedwitha specialemphasison closedequipment to showthat
more focuson localflowphenomena is neededto improve the overall hygienic
design of processingequipment.
Volume flowscorrespondingto a mean velocityof 1.5 m/s havebeenused
withsuccess for CIPin many food-processing facilities. At this velocity
turbulentflowis guaranteedfor straightpipeswithan inner diameter above
0.01 m. Turbulent flowis neededto improve cleaning (Majoor,2003)as it
enhances the transport of detergent(massand heat) fromthe bulkto the surface
comparedwithlaminar flow.Furthermore, the thickness of the so-calledviscous
(or laminar) sublayer covering the wallreachesan asymptote at a meanvelocity

194 Handbookof hygiene controlin the foodindustry