Handbook of Hygiene Control in the Food Industry

turbidity monitoringto confirmwhen rinsing is completeand the fluidflowing
downa pipeis clear.Somemonitors,suchas the `Monitex'program(Mu»ller-
Steinhagen, 2000),analyseinformation,to allowschedulingof cleaning. Key
parameters are givenbelow. The pressure dropacrossa system (
P) is defined
as:

PàPIˇPO Ö 29 : 1 Ü

wherePIandPOare the inletand outletpressure,andPOincreases withtime
duringfouling. Thechangein HTCduringfoulingcan be accounted for by
including a fouling resistance,Rf, in the equation relatingthe initialcleanHTC,
(U 0 ) to that at timet, (Ut):

1

Ut

à

1

U 0

áRf Ö 29 : 2 Ü

The extentof foulingmaybe expressed by a Biotnumber(Bi), whichaccounts
for deposit thickness (x) and thermal conductivity (): BiàRfU 0 where
Rfàx=for the deposit. Bothheattransferand pressuredropmeasurements
can be insensitiveto smallamountsof foulingand do not give goodinformation
aboutthe levelof depositleft on the heattransfer surface.Tuladhar (2001)
definesa depositresistanceduringcleaning(Rd):

Rdà

1

Ut

ˇ

1

Uc

Ö 29 : 3 Ü

whereUtandUcare the HTCat timetand the HTCof the finalcleansystem.A
combination of this approachwithin-line turbidity sensors that showwhena line
is cleanof product, and thusthe end of rinsestages,can give reasonable
monitoringof processes.

29.3 Investigationsinto cleaning process parameters

29.3.1 Temperatureeffect
In general, as temperature increasesthe cleaningrate increases (for example,
AlfaLavalAB, 1995;Hankinson and Carver,1968;De Goederenet al., 1989;
Fryerand Bird,1994).Timperleyand Smeulders(1988)investigatedthe effect
of temperature (60±90ÎC) cleaningof a PHE,using a single-stage cleaner.They
observeda 60%decreasein cleaningtimewhen the temperatureof the cleaning
fluidwas increasedfrom60 to 90 ÎC, 40%of whichoccurredbetween 60 and
75 ÎC. The increase or decreasein the rate of a process(e.g.of cleaning) for each
10 ÎC increase in temperaturehas beenreported for cleaningof milkdeposits
usingsodiumhydroxide solutionsat temperaturesup toca.90 ÎC; values of
between 1.6 and 2 havebeenreportedby a number of authors (Jennings, 1959;
Gallot-Lavalleeet al., 1984;Grasshoff,1989).
Optimaltemperatures havebeenreported. Hankinsonand Carver(1968)
foundan optimum temperature of 55 ÎC for cleaningmilkdeposits by water

Improvingthe cleaning of heatexchangers 479