although chemicals used for single-stage cleaning are more expensive
(Timperley and Smeulders, 1987;De Goederenet al., 1989),overall cost can
be reduced.
29.3.3 Mechanicaleffect
In CIP processes shearforce is providedby fluidflow.It is importantto find the
minimaleffectiveconditions as the energy expenditurefor pumpingincreases as
the square of the flowvelocity.The flowmaybe characterised in terms of mass
flow,Reynolds number(Re), surfaceshearstress( ) or flowvelocity (v). A
given flowrate will havedifferent velocities,Re and surfaceshear stress
depending on the system. The flowrate of the cleaningsolutionalsoaffects
masstransfer of boththe chemical and the reacted deposit. The individualnature
of eachsystem makesit difficult to suggest a minimumor optimum flowrate.
For example, AlfaLavalAB (1995)statethat flow ratesbetween 1.5 and 3.0 m/s
shouldgivegoodscouringeffect on the surfaceof equipment.
Increased flow ratesinducegreatersurface shearon the deposit. However,
this reducesthe contacttime; cleaningsolution mustremainin contactwiththe
soil for longenoughfor it to takeeffect.The underlyingeffectof the cleaning
solutionflowrate on cleaningis difficult to determineas bothmasstransferand
surfaceshearstressare relatedto flow.Sincethe thickness of the boundary
layeris also affectedby fluidflow rate,it has beensuggested that boundary
layerthickness maycontrolcleaning,i.e. as the boundary layerbecomesthinner
moredeposit is protruding into the turbulent flow and is removable (De
Goederenet al., 1989).Thistakesinto account the effectof the deposit on the
boundary layer.Turbulence(in termsofRe) has beenfoundto be the most
importantfactorin the cleaningprocess, althoughthe significance becomesless
as chemical efficiency increases (Jenningset al., 1957).Some workershave
founda thresholdbelowwhich the mechanicaleffectof flowis negligible
(Jenningset al., 1957;Schlu»ssler,1976,in Jackson and Ming Low, 1982)on
tomatodeposits, although Birdand Fryer(1991)noted that there was no
significantchangein cleaningrate whenmoving fromlaminar to turbulent
flow, and Bird (1992) found no minimum flow velocity. Timperley and
Smeulders (1988)found that the cleaningtime of a PHE decreased with
increasing flowrate,the greatestreductionoccurring uponincreasing the flow
velocityfrom0.2 to 0.5 m/s.In general,it is clearthat the higherthe flow rate,
the shorter the cleaningtime± but the cost of pumpingthe cleaningfluid may
becomeexcessive.
29.3.4 Materialsproperties of deposits
The abovehas shown the complexity of cleaning,whichis complicatedas the
materials propertiesof deposits are not wellknown, so that the effectof process
variableswill differbetween differentsystems. We havetriedto gainempirical
understandingof the materials properties,usingmicromanipulation probesthat
Improvingthe cleaning of heatexchangers 483