avoided. Thisresults in the heat transferarea beingconsiderablyincreased,and
careful maintenance of the recirculationcircuitis required.
Theuse of a dualplatewithan air gap has beendevelopedto provide
additionalassurance againstcross-contamination. The principleis basedon that
of the double seat valve,withtwo platesin placeof the valveseatsand an air
gap to atmospherebetweenthe two plates.Anydefectin one of the plateswill
result in fluidpassinginto the air gap and out to atmosphere.It is, however,
counter-intuitive to deliberately createan air gap within the heattransferpath
and in so doing provide an additional heattransfer resistance.Equipment
manufacturers haveminimisedthe loss of thermal efficiency by usingan air gap
of 3±5m and thinnerplatematerials of 3 mm. The loss of performancewill be
greater the lowerthe overall heattransfer resistances on the fluidside,for
example an applicationwithtwo low viscosityfluids, ratherthanwhereone or
more of the fluidsis viscous.Table12.4 shows the effectof the air gap on heat
transfer performancefor differentapplications.
Although the reductionin heattransfer performance can be minimised by
reducing the air gap,thereare a number of potentialhygieneissues:
∑ If a fluid entersthe narrowair gap betweenthe two platesdue to a defect,
surface tensioneffects may prevent fluid draining out of the system by
gravity.
∑ If a defectdoesoccur, it will be difficult to ensure that any productin the gap
can be cleanedeffectively due to the minimalflowsof cleaningfluidthat can
be deliveredinto the gap throughthe defect.
∑ Any residual product within the air gap could provide a source for
recontamination of product duringsubsequentproduction.
Linear scraped surfaceheatexchanger
Scraped surface heat exchangersare used for processing fluidsthat other
geometriescannothandle, suchas largeparticles. Conventionaldesignsare
Table12.4 Effectof air gap on heattransferperformance
Air Duty 1 Duty 2
gap Dualplateseach0.4 mm thick Dualplateseach0.4 mm thick
(mm) Productviscosity:1 N S mˇ^2 10 ˇ^3 Productviscosity:10 N S mˇ^2 10 ˇ^3
Servicefluidviscosity: Servicefluidviscosity:
1 N S mˇ^2 10 ˇ^3 1 N S mˇ^2 10 ˇ^3
Reductionin performance(%) Reductionin performance(%)
0.000 0.0 0.0
0.001 8.2 5.0
0.002 15.1 9.5
0.003 21.1 13.6
0.004 26.2 17.3
0.005 30.8 20.8
216 Handbookof hygiene controlin the foodindustry