significantpenaltiesin termsof heattransferand werereplaced withcopper
discs, whichwereplatedwithstainlesssteel. Problems wereencounteredwith
the deposition of fouling causing localisedblockage in the flow celland
distortion of flowwithin the cell.
35.4.2 Taperedtubesystem
The taperedtube was developed(Fryerand Slater 1987)to overcome the
limitationsof the radialflow cell concept. It consisted of a stainless steel tapered
tube, 40 cm in length, tapering uniformly from19.2 mm at the inlet to 12.7 mm
at the outlet, thuswide enoughto avoid blockingby deposit. Thetubewas
constructedin two halves, witha gasket betweento allowexaminationafter each
experiment.Shear stresses in the range0.5 to 8.1 N/m^2 weregeneratedunder
turbulentflowconditions.The taperedtube could be enclosedin a steamchestto
provide the temperaturedrivingforcefor fouling and a recirculationsystem used
to circulateproductthroughthe tubefor the requiredtime.
35.4.3 Robbinsdevice
Food processequipment often has surfaceswheremicrobes mayattach, grow
and develop into biofilms, whichcan havean adverse effect on the microbial
quality of the food.Growthof biofilms can be monitoredusingstainlesssteel
couponsattachedto surfaces withinthe factoryenvironment. In closed,flowing
systems suchsamplesare more difficult to obtain.The Robbinsdeviceallowed
biofilm development to be monitored at a laboratory scale but under
representative conditions.The tubularunit was constructedof 1.42 cm internal
diameter, 92.5 cm long,admiraltybrass and had replaceable plugsfittedat
samplingportsalongits length.Individualplugswitha surfacearea of 0.5 cm^2
could be removedasepticallyfrom the device without needing to drainthe
system. The buildup of biofilms couldbe monitoredby measuring the pressure
dropacrossthe unit and the structurecould be analysedusing scanning electron
microscopy.Flowvelocitiesof 1.39to 2.65 m/s wereachievedequivalent to
Reynoldsnumbers of 24 618 to 47 175.Various modificationshavebeenmade
to the Robbinsdevice (Blanchard et al. 1997) to overcome some of the
limitationsof the original suchas the edgeeffects resultingfromlocatinga flat
sample disc at the wall of a cylindrical pipe and its impacton the fluiddynamics.
These includeusinga rectangularflowchannelconstructed frompoly(tetra-
fluoroethene) (PTFE),whichcould be autoclavedpriorto use.
35.4.4 Pilot-scale heatexchangers
Small-scaleheatexchangerscan havethe benefitof being ableto simulate the
commercialsize heat exchangersin termsof the geometry of the exchangerand
the product temperatureprofile withinthe exchanger.Suchsystems tend to have
capacitiesin the regionof 50±150L/h and hence extendedrunsto observe
582 Handbookof hygiene controlin the foodindustry