monitoredon-lineas a reduction in the heat transfer coefficientand the cleaning
processis the reversewiththe heattransfer coefficient increasingas soil is
removedfromthe surface.
The accuracyof any technique in providinga quantitativemeasureof the
condition of the surfaces willdepend uponboth the typeand number of
instrumentsusedin the calculation.Taking as an example the measurementof
foulingbasedon heattransfer coefficientoversections of heatexchangerin a
typical highly energy-efficient dairy pasteuriser with 95% heat recovery.
Assume the product is beingheatedfrom5 ÎC to 72 ÎC in two stages,firstly by
heatrecoveryand thenby heatingwithhot water. The typicaltemperaturesin
sucha systemwouldbe as follows:
∑ Heatrecoverysection ± heatingmedium (hot pasteurised milk)
Productinlet 5 ÎC
Productoutlet 68.7 ÎC
Heatingmediuminlet 72 ÎC
Heatingmediumoutlet 8.3 ÎC
t 3.3 ÎC
∑ Finalheatingsection ± heatingmedium (hot water)
Productinlet 68.7 ÎC
Productoutlet 72 ÎC
Hot waterin 75 ÎC
Hot waterout 73.9 ÎC
t 4.0 ÎC
If the temperature probes wereaccurate to0.5 ÎC, the overallrangeof
possible temperature differences in the heatrecoverysection wouldbe 2.3±
4.3 ÎC. Since,when the otherprocessconditionsare constant, the overallheat
transfer coefficient,U, is inverselyproportionalto the temperaturedifference,
the accuracy of the probes limitsthe accuracyofUto approximately30%.If
the probeaccuracyis0.1 ÎC, the accuracyis improved to6%. For the final
heatingsection the accuracyof the calculated heat transfer coefficientwouldbe
25%and 5% for probeaccuraciesof0.5 ÎC and0.1 ÎC respectively.This
indicatesthat sucha sensorwouldbe usedas an indicatorof surface condition
ratherthanan absolute measure. It also tends to be mostsensitive at the startof
the fouling process and the end of the cleaning process (Hasting 2002).
However, sincethe sensorsmaywellbe installed on the heat exchangeras part
of the normal instrumentationand control system,it providesa simpleand
effectivewayof monitoring the fouling and cleaning processes.
35.3.4 Pressure drop
Pressuredropis another relatively simple methodof monitoringthe overall
condition of a heatexchanger.As a depositbuildsup on a surface, the cross-
sectional areafor flow will be reduced,regardless of the geometry of the heat
exchanger,e.g. tube or plate.If the product flowrate is maintainedconstant, the
Monitoringof fouling, cleaning and disinfection in closedprocessplant 579