Design and performance of a low-frequency non-intrusiveacoustic technique for monitoring fouling in plate heat exchangers

B. Merheb a,*, G. Nassar a, B. Nongaillard a, G. Delaplace b, J.C. Leuliet b

a I.E.M.N. – U.M.R. C.N.R.S. 8520 – D.O.A.E., Universite de Valenciennes, B.P. 311, 59313 Valenciennes Cedex 09, Franceb INRA-LGPTA – 369, rue Jules Guesde, B.P. 39, 59651 Villeneuve d’Ascq, France

Abstract

The fouling of plate heat exchangers (PHE) is a serious problem in the food industry. Fouling reduces both PHE efficiency and foodquality and can also give rise to microbiological problems. In order to monitor fouling inside PHE in real time, a new acoustic technique,using multiple optimized non-intrusive sensors, was developed for experimental on-line trials. Low-frequency acoustic waves were prop-agated through the plates, and these waves were analyzed to measure their ability to detect fouling inside the PHE. By comparing theevolution of the acoustic wave parameters (e.g. power and delay), an indicator of the fouling rate is obtained for each zone inside theexchanger. The results of this analysis show that low-frequency acoustic waves are sensitive to PHE fouling.

Keywords: Fouling; Plate heat exchangers; Acoustic; Sensors

1. Fouling in plate heat exchangers

At one time or another during their transformation,many food products undergo heat treatment for finalitiesof texture (e.g. cooking) and/or microbiological stability(e.g. pasteurization, sterilization). Plate heat exchangers(PHE) are usually used to perform these heat treatmentsbecause they offer several advantages over tubularexchangers: compactness, flexibility and better thermalperformance.

The fouling of plate heat exchangers is a serious prob-lem for the food industry. This fouling mainly consists ofa film of denatured whey proteins and calcium depositsthat builds up on the plates. This build-up can:

� decrease the flow rate of the food product through thePHE,� reduce the overall heat transfer coefficient of the PHE,

� reduce product quality, and also lead to potential micro-biological problems.

To enforce compliance with critical pressure andhygiene criteria, exchangers must be cleaned often, accord-ing to a regular maintenance schedule (Bott, 1995). How-ever, unnecessary cleaning leads to system downtime andwaste of water and chemicals, which increases costs andcauses ecological problems.

Monitoring fouling and the consequent cleaning pro-cesses can provide useful information for operational deci-sion-makers in food processing plants. Fouling is usuallynot visible from outside the industrial processing equip-ment, and thus can only be ascertained from its effects,such as by measuring heat transfers (Bott, 1995; Lalande,Rene, & Tissier, 1989) or pressure drops (Burton, 1966;Delplace, 1995; Fryer, Pyle, & Reilly, 1989), which in thecase of small, local deposits may not be significant enoughto allow an operational decision to be made. In addition,such measurement-based methods can provide differentresults for trials performed under the same conditions

because they assume that the fouling build-up is uniformlydistributed in the exchanger, or because a sizeable depositcan accumulate at the entrance and exits of each channel,thus providing erroneous information on the thickness ofdeposit in the exchanger.

Recently, Fillaudeau, Debreyne, Vardenas, and Korol-zuck (2005) proposed a method based on the hot wiretechnique to detect the appearance of fouling on-line, ina tubular heat exchanger (THE). This sensor is notadapted to the PHE context. Other sophisticated methodshave been developed to monitor fouling such as siliconsensors (Stenberg, Stemme, & Kittilsland, 1988), micro-strip monitoring technique (Root & Kaufman, 1992),photothermal deflection method (Fujimori, Asakura, &Suzuki, 1987), optical techniques (Withers, 1996) or flux-meters (Davies, Henstridge, Gillham, & Wilson, 1997).Unfortunately, most of them (i) require important instru-mentation; (ii) are not always adapted to non-transparentequipment; (iii) are not compatible with an industrial envi-ronment since they are often restricted to laboratory usewhich limits the implementation in a PHE and (iv) arenot adapted to cleaning requirements encountered in thefood industry.

Recently, many authors have studied changes in acous-tic parameters to detect fouling in THEs. Though signifi-cant work has been done on ultrasonic/acoustictechniques for pipe fouling detection guided waves byHay and Rose (2003), guided waves and acoustic impactby Lohr and Rose (2003), Lohr, Rose, and Tavossi(1999), normal beam and pulse-echo by Withers (1996)no application has yet been reported for PHE.

Hay and Rose (2003), designed a new piezopolymerguided wave sensor aimed at detecting fouling on the innersurface of piping commonly used in the processing of dairyproducts. On other hand, Lohr and Rose (2003), investi-gated guided waves and acoustic impact for pipe foulingdetection. In both cases simulated fouling was used by add-ing a viscous layer on the inner surface of the pipe. Trialswere performed under a constant ambient temperature.

Wither’s research (Withers, 1996) concentrated on theuse of normal-beam ultrasonic through-transmission andpulse-echo velocity measurements to detect and quantifythe extent of fouling inside tubular exchangers. Othermethods commented upon by Withers include an acousti-cal-vibrational method in which the change in the natureof the frequency of vibration is related to the extent of foul-ing, and an optical method in which the reflection from theinner pipe wall is correlated to the extent of fouling.

These investigations were carried out using THEs, acontext in which acoustics’ sensors can be more easilyadapted than in plate heat exchangers. Thus, a real break-through in fouling detection inside a PHE has not yet beenreached, mainly due to the complex structure of the PHEand a lack of understanding of the fouling mechanism.An optimal monitoring method would accurately indicatethe location and extent of the deposit inside the PHE.For this to be done in industry, this information would

have to be acquired non-destructively, reproducibly andautomatically as well as on-line, in situ, and in real time.In addition, the sensor’s mechanical design would have totake several criteria into account so that any deviceequipped with these sensors would be robust and easy touse.

The objective of this research is to define and implementoptimized acoustic instrumentation to monitor and mea-sure the fouling phenomenon inside PHE under criticalthermal, hygienic and pressure conditions. The developedtechnique was studied for a given product under a varietyof experimental conditions.

2. Study and design of a new low-frequency acoustic device

2.1. PHE limitations versus conventional ultrasonic

techniques

Ultrasonics is the study of stress waves in solid and fluidmedia with a frequency content greater than 20 kHz (Rose,1999). Two important parameters in the study of ultrason-ics are velocity and attenuation.

Most ultrasonic sensors are constructed around a piezo-electric ceramic element, which is used for the generationand reception of stress waves in solid or liquid materials.The piezoelectric element is deformed when an electricpotential is applied across it. Similarly, when the crystalis mechanically deformed, it emits an electrical charge. Pie-zoelectric crystals are thus ideal for use as ultrasonic sen-sors, both for outputting (pulsing) and for receiving thewaveform. Even though ultrasonic transducers can bemade using other principles, only piezoelectric devices wereused for this research.

Experimentally, one may use either through transmis-sion or pulse echo. Through transmission uses one trans-ducer as a transmitter, or pulser, and a second as areceiver. The pulser outputs a pressure wave, which isreceived and converted back into electrical energy by thereceiver. Pulse echo utilizes one transducer to do both thepulsing and the receiving. More resources on through-transmission and pulse echo techniques are available(Graff, 1975; Rose, 1999).

Guided waves are created from the interference betweenlongitudinal and shear waves. This occurs when the thick-ness of the specimen is similar to or less than the wave-length of the ultrasonic wave.

Normal beam ultrasonic is limited by its ability toinspect on a point by point basis only. Guided waves, onthe other hand, can be used to inspect large regions of aspecimen at once. The background behind guided wavesis not presented here, although more in depth resourcesare available (Graff, 1975; Rose, 1999). In general, normalbeam ultrasound is concerned with the study of the longi-tudinal wave, while oblique incidence is concerned withthe behavior of longitudinal, shear, and Rayleigh waves.A discussion of guided waves should include an overviewof oblique incidence.

Table 1Eigenfrequencies for a simply-supported uniform steel plate

Mode (m,n) Analytical eigenfrequencies (Hz)

1–1 221.82–1 278.63–1 373.34–1 505.95–1 666.31–2 830.46–1 884.72–2 887.23–2 981.94–2 1114.57–1 1130.95–2 1285

In the context of non-destructive testing, the build-upon the exchanger plates can be seen as a defect, or aloading condition, in the exchanger structure. Thedescribed conventional non-destructive ultrasound tech-niques are limited by several unavoidable constraintswhen meant to monitor fouling inside PHE. These con-straints are:

� The exchanger plates are accessible only on the edges.PHE plates are stacked up one on top of the other, witha gasket separating each pair, which makes it difficult toposition and couple a traditional ultrasonic sensor onthe surface of an exchanger plate. This limits the gener-ation of normal beam or guided waves inside theexchanger plates. Moreover, as the resonant frequencyof a transducer is inversely proportional to its size, verylow-frequency ultrasonic techniques face sensor size lim-itations when coupled to the PHE plates.� According to the food industry’s hygiene constraints,

the sensor should not come in contact with the productto be treated in the exchanger.� The temperature in heat exchangers can reach 150 �C,

which can irreversibly degrade any output signals dueto direct contact of traditional transducers with thehot plates.� According to the results of preliminary trials, high-fre-

quency ultrasonic waves (over 100 kHz) propagatedthrough the exchanger plates are highly attenuated bythe PHE structure (corrugated plates separated bygaskets).

In brief, the generation of normal beam or guided wavesis associated with the position and the incidence angle ofthe sensor coupled on the specimen (e.g. exchanger plate)surface. In the context of monitoring PHE, most ultrasonicsensors face the above listed limitations when coupled toexchanger plates and thus generation or even the receptionof ultrasonic waves through the exchanger will be very dif-ficult to setup.

On the other hand, investigation of the plate’s modeshapes theory permits to evaluate the influence of afluid loading condition on the vibration of the platestructure. An overview of frequency response analysisof a simply supported plate is presented in the nextparagraph.

2.2. The plate mode shapes: mechanical equations

The frequency response analysis (modal analysis) aimsto determine the eigenfrequencies and the associated dis-placements of a given structure. These modes (frequencies)depend on the geometry and mechanical properties of thestructure. Their determination is important in the designof structures subjected to excitations.

Investigating vibrating plate’s modes helps understand-ing the influence of a mechanical load on the vibrationsof these plates. The governing equations for a flat vibrating

plate and the motion of this plate can be found in Rayleigh(1945). The fluid–structure interaction model can bederived from these basic equations and describes the struc-tural motion in the presence of surrounding fluid. How-ever, these equations will not be elaborated here and thisstudy is limited to the determination of the eigenfrequen-cies for a flat vibrating plate. This is the first step to designthe sensor used in this study.

The eigenfrequencies (fmn) of the simply-supported uni-form plate are analytically given by Blevins (1984) and onlydepend on the material properties: Young’s modulus E,Poisson’s ratio m, and mass density q, and the geometricdimensions of the plate a, b, and h.

xmn ¼Lmn

a

� �2ffiffiffiffiffiffiffiDq:h

sð1Þ

with:

D ¼ E:h3

12ð1� m2Þ ; ð2Þ

xmn ¼ 2pfmn ð3Þ

and

L2mn ¼ p2 m2 þ n2 a

b

� �2� �

; ð4Þ

m and n define the mode of resonance.After simplification the eigenfrequencies are given by

Eq. (5):

fmn ¼ph2

m2

a2þ n2

b2

� �E

12qð1� m2Þ

� �1=2

ð5Þ

Table 1 lists the first 12 eigenfrequencies for a simply-sup-ported uniform steel plate calculated according to Eq. (5).The geometrical and physical properties of this plate are gi-ven below:

a = 360 mm; b = 110 mm; h = 1 mm; q = 7500 kg/m3;m = 0.3; E = 200 GPa.

It can be noticed, by referring to Table 1, that the first 12modal frequencies for the simply-supported flat plate are inthe low-frequencies band.

Table 2Eigenfrequencies for the developed sensor according to numerical andexperimental simulations

Mode Numericaleigenfrequencies (Hz)

Experimentaleigenfrequencies (Hz)

1 204 –2 515 –3 899 –4 1683 15865 2387 23946 3557 36397 5422 53308 5734 56399 7417 7390

10 8849 8750

2.3. Design of the acoustic sensor

During preliminary trials, it was determined that for anacoustic wave to propagate through the PHE with a goodsignal-to-noise ratio, it must be of very low frequency witha strong amplitude at the emission source. On-site experi-ments by Lohr et al. (1999) have already shown that acous-tic waves generated by acoustic impact can travel through50 ft of pipe with a very good signal-to-noise ratio. More-over, it’s possible to maintain a good signal-to-noise ratiowhile penetrating pipe joints. For this reason a mechanicalpulse was used in this study (acoustic impact) to generatepropagating acoustic waves on a frequency band rangingfrom around 0 kHz to 20 kHz. This mechanical pulse wasgenerated by an electromagnet pushrod striking a givensurface, and the intensity and periodicity of the shock werecontrolled by a computer.

After choosing the source of the acoustic vibrations, sev-eral techniques were compared using acoustic sensors forreceiving the transmitted wave. Making use of appropriatesensors to continuously monitor the state of fouling insidethe PHE presents a real problem that needed to be solved.The sensors had to be both sensitive to low-frequencyacoustic waves and unaffected by temperature fluctuations.These constraints led to the development of a specific sen-sor using composite materials with an embedded ferroelec-tric disc (D = 14 mm, e = 1 mm) (Fig. 1). The chosen

Fig. 1. Schematic representation of two sensors with a mechanical pulsegenerator fixed on an exchanger plate.

composite material (FR4 epoxy laminate) has a low ther-mal conductivity (0.11 W/m K at 300 K), which rendersthe entire structure resistant to small temperature fluctua-tions. This technique is based on the principle of settingin resonance all the mechanical structure of a reduced sizesensor which is in contact with the exchanger plate. Someresearchers like Nassar and Nongaillard (2004), Degertekinand Khuri-Yakub (1996), Shuyu (1997) and Nikolovskiand Royer (1997) used a similar physical principle, butassociated a tapered volume with the ceramic pieces, toconcentrate the mechanical energy.

The eigenfrequencies of the developed sensor are given(Table 2) according to numerical simulations (with Fem-Lab 3.2) and experimental measurements. As it is designed,the sensor has proved to have a good sensitivity to low-fre-quency (0–20 kHz) acoustic waves. In addition, theseadjustable sensors were specifically adapted to the geome-try of the heat exchanger plates and were coupled to thePHE (Fig. 1).

3. Materials and methods

3.1. Model fluid

The fouling experiments were carried out in situ, using amodel fluid (Whey protein concentrate (WPC) powder(Protarmor 750, Armor Proteines, France) dissolved inwater (1% w/w)). The composition of WPC powder wasessentially proteins (75% w/w) in which b-lactoglobulinand a-lactalbumin represented 63% and 11%, respectively,and lactose (10% w/w). Minerals represented less than 5%of the total dry weight. The model fluid, obtained at a con-stant pH value (pH 7), was kept in a storage tank forapproximately 12 h at 4 �C in order to prevent bacterialproliferation. As reported by Delplace, Leuliet, and Tissier(1994), such a model fluid makes reproducible foulingexperiments possible.

3.2. Experimental pilot plant

The experimental set-up at a pilot plant scale is shown inFig. 2. The model fluid was pre-heated from 20 �C to 70 �C

Fig. 2. Schematic representation of the configuration used in our experimental trials.

in a V7 PHE (Vicarb Alfa-Laval, France) and from 70 �Cto 95 �C in an electric tubular heat exchanger. The focusedholding zone was a V2 PHE (Vicarb Alfa-Laval, France)which consisted of 10 plates forming 5 passes of one chan-nel. The WPC flow rate was maintained constant at 300 l/h. The mean Reynolds number was 6950. During all foul-ing runs, a constant back pressure of 2.0 bars was appliedto the PHE. One of the advantages of using a PHE withonly 10 plates is the possibility of controlling the flowsand the temperatures better, while still preserving a tolera-ble drop in pressure within the exchanger.

A single stream of hot water was used to pre-heat theproduct in the V7-PHE. After achieving steady state condi-tions, the WPC flow rate and inlet and outlet temperatureswere maintained constant. To maintain a constant producttemperature at the outlet of the V7, the hot-water inlet tem-perature was adjusted as fouling took place. In this config-uration, no hot water circulates in the V2 exchanger, whichmaintains a uniform temperature profile (95–93 �C). Theproduct in the tubular exchanger, which is used to ensurea temperature of 95 �C at the entrance of the V2 exchanger,was heated via the Joule effect where the metallic tube issubmitted to a potential difference (Terral, Bandelier, Mar-villet, & Vidil, 1998).

During the experiment, fouling was monitored globally inthe tubular exchanger and in the V2-PHE through pressuredrop measurements. At the end of the experiment, the V2-PHE was dismantled, and the wet weight of the deposit oneach plate was measured (Fig. 3). This step was the localapproach for determining the quantity of deposit. Analysis

Fig. 3. Deposit on heat exchange plate in the first channel of the PHEafter 3 h.

of the deposit showed that it was a type ‘A’ deposit, thus con-firming the results already published in the literature (Del-place, 1995). Type ‘A’ deposit builds up at temperaturesranging from 70 �C to 110 �C, it’s a whitish color and appearsto be bulky and spongy. It consists of protein matrices (50–60%) and contains minerals (30–35%) and fat cells (4–8%).

After the weighing, the PHE was remounted andcleaned using a single stage cleaner which consists of a1% (w/w) caustic soda solution at 85 �C. The system con-figuration and operating conditions were preserved duringcleaning process.

3.3. Methodology

An acoustic sensor on the upper section of the V2exchanger triggers the data acquisition device, while thethree sensors in the lower part receive the ultrasonic oracoustic energy and relay the signals to the acquisition-pro-cessing device (Fig. 4). When the acoustic impact encoun-ters the trigger sensor, the acquisition device is activated,and the recording window is opened. At the same time,the other sensors are ‘‘listening” for the waveform thattravels through the exchanger. When the waveform reachesthe receiving sensor, it is recorded and analyzed by the pro-cessing device and compared to the response of the triggersensor. The processing device then provides a real-timevalue for the power and speed of the waveform caughtby the receiving sensor. Up to three different responses(channels) can be obtained by shifting the position of thereceiving sensor. The different acoustic channels are num-bered from right (Product inlet) to left (Product outlet) ofthe PHE (Fig. 7).

For example, Figs. 5 and 6 depict the waveform signalsover time and the Power spectral densities for the triggersensor (St) and central sensor (S1).

The two main acoustic parameters that are studied andpresented in the graphs of Figs. 6–9 are the acoustic powerand the delay of the acoustic waves received by a given sen-sor Si (i:0-2).

The acoustic power of the waveform Xi received by sen-sor ‘i’ is computed by integrating the sensor response overtime (Eq. (6)).

P i ¼1

T

Z T

0

X 2i dt where T ¼ 4 ms ðT is length of the signalÞ

ð6Þ

Fig. 4. Schematic representation of the system used to monitor PHE fouling.

Fig. 5. Response of sensors S1 and St to the mechanical impact.

The delay DTi, k can be defined as the difference between thetime of flight ti,0 of the acoustic wave received at the begin-ning of the experiment (reference time) and the time offlight ti, k for the same acoustic wave received at excitation‘‘k” (Fig. 5).

DT i;k ¼ ti;0 � ti;k ð7Þ

In order to calculate the delay DTi, k, the cross-correlationCi, k(t) between the waveform Xi, k(t) received by sensor‘‘i” due to excitation ‘‘k” and the reference waveformXi,0(t) received by the same sensor at a given reference timeis computed first.

Ci;kðtÞ ¼Z

X i;0ðsÞX i;kðt þ sÞds ð8Þ

Then the time tMi, k corresponding to the position of themaximum value of the cross-correlation function Ci, k(t) isdetermined. Finally, the delay of the acoustic signal canbe presented as:

DT i;k ¼ T � tMi;k ð9ÞThe parameters used in all calculations and experimentsare presented in Table 3.

4. Results and discussion

4.1. The repeatability of the measurement system

The first stage of the experimental work consisted instudying the repeatability of the measurement system fixed

Fig. 6. Power spectral densities corresponding to the signals shown in Fig. 8.

on a PHE in a real industrial environment. The repeatabil-ity obtained for an empty PHE was compared with therepeatability obtained for a PHE subjected to a constantflow of water (200 l/h) at ambient temperature (20 �C). Itwas demonstrated that as long as the measurement systemhas a fixed configuration, the system has good repeatabilityof the measured acoustics parameters (e.g. the time of flight‘TOF’ and total power measurements). The standard devi-ation of time of flight ranges between 1.18 ls and 1.5 ls(Table 4), and the variation coefficient was lower than1% for the three sensors indicating a good repeatabilityof the TOF measurement The standard deviation obtainedin the fouling experiments is slightly higher than the oneobtained with a steady state exchanger which, in this case,did not exceed 3% of the average time of flight.

Moreover, the repeatability of the ratio of total poweron trigger it power (RTPTP) is reported in Table 4. Thevariation coefficient is slightly higher than the one obtainedfor TOF. This scatter is partially caused by fluctuations inthe excitation system and partially by noises and other par-asite signals present in the work environment.

On the other hand, if the system (PHE and sensors) isdisassembled and reinstalled, measurement repeatabilityis not guaranteed. To resolve this problem, only the relativevariations of the acoustic signal compared to a given refer-ence (i.e., the one at the beginning of the test) given no dis-mantling of the measurement device or the PHE, werestudied.

4.2. Fouling experiments

The evolution of the acoustic waves that propagatedinside the exchanger during the fouling phase and thecleaning phase was studied. Specifically, the changes inthe power and propagation time of the acoustic waves

received by the acoustic sensors were analyzed. As men-tioned above, PHE fouling was also monitored duringthese experiments by recording the evolution of the overalldrop in pressure over time for each exchanger. Fig. 7 showsthe evolution of the acoustic power and the drop in pres-sure over time for the V2-PHE.

Fig. 7a shows the evolution in the power of the acousticwave received by sensor ‘‘1”. Respectively, Fig. 7b showsthe evolution in the power of the acoustic waves receivedby sensors ‘‘0” and ‘‘2”. They also show the evolution ofthe pressure in the exchanger. There is a clear negative evo-lution in the power of the acoustic waves recorded by all sen-sors during the fouling phase. The same signal processingprocedure applied to the acoustic waves during the cleaningphase shows the opposite evolution of the power curves.

The power response of sensor ‘1’ is represented on a sep-arate graph (Fig. 7a) because its power value is muchgreater compared to the response of sensors ‘0’ and ‘2’.This big difference observed in acoustic power between sen-sors is due to the fact that sensor ‘1’ is fixed on the sameplate as the acoustic impact. Thus acoustic waves receivedby sensor ‘1’ undergo less attenuation than those receivedby sensors ‘0’ and ‘2’. In addition, the power curves forthe three sensors also show differences in slope and ampli-tude, mainly due to the difference in the degree of foulinginside the channel where the sensors were located.

At the beginning of fouling runs, very slow decreases inacoustic power are recorded for 30 min, with an initialvalue close to 7.5 and 7 lW, respectively for S0 and S2.This slight decrease can be attributed to a thin layer of irre-versibly adsorbed proteins on clean metal surfaces. Thisindicates that no significant fouling occurred during thefirst period. At the same time, the pressure drop was con-stant, confirming the previous observation. After this per-iod, the acoustic power suddenly decreases in function

Fig. 7. Evolution of the power of the acoustic waves received by sensor ‘1’ (a) and by sensor ‘0’ and ‘2’ (b) during the fouling experiment on a V2 plate heatexchanger.

with time until t = 90 min. This decrease in acoustic powermay be attributed to the growth of fouled layers. Thegrowth of deposit interferes upon the acoustic waves char-acteristics. The slope of the S0-curve (0.090 lW/min) istwice as high as that of the S2-curve (0.058 lW/min). Thismay indicate that the rate of deposit is more important atthe inlet of the PHE. The rise of the pressure drop confirmsthe growing of deposit. Modifications of rates of acousticpower changes may indicate that the formation of fouledlayers is a balance between rates of deposition, rates ofre-entrainment of solids from fouled layers into the bulkand particle breakage, as suggested by Kern and Seaton(1959). Thus after t = 90 min, re-entrainment of solidsmay be predominant upon the deposit of protein aggre-gates. However at the same time, the pressure drop contin-ues to increase with a lower slope than in the first period(30 min < t < 90 min). This continual rise may be attrib-uted to the blockage of the outlet of channels by proteindeposit and may not reflect a continual growth of deposit.

Reductions of acoustic power (RAP) for the three sen-sors during the growth of deposit are illustrated in Fig. 8.Reductions of acoustic power were computed as follow:

RAP ¼ 100AP0 �AP1

AP0

ð10Þ

AP0 represents the value of acoustic power for the clean ex-changer and AP1 is the acoustic power value at the end ofthe fouling run.

Masses of wet deposit in the corresponding channels arealso reported. The RAP decreases along the PHE,indicating the greater extent of the deposit at the inlet ofthe PHE. This is confirmed by the initial slope of S0 andS2-curves.

In addition, the delay of the acoustic wave of theacoustic signal decreases during the fouling phase, asshown in Fig. 9. This evolution of the delay is accompa-nied by a decrease in the pressure in the exchanger. Theslope of the curve and its scale differ depending on the

Fig. 8. Reduction of acoustic power from the three sensors and mass of deposit during fouling experiment in a V2 PHE at 95 �C with a WPC solution.

Fig. 9. Evolution of the delay of all acoustic waves during the fouling experiment on a V2 plate heat exchanger.

Table 3The parameters used in all calculations and experiments

Sampling rate of the waveform signal: Dt = 1 ls => (fe = 1 MHz)Waveform signal time: T = 4 msRepetition rate of the acoustic

impact during fouling trials:fc = 1/15 Hz

Repetition rate of the acousticimpact during cleaning trials:

fc = 1/5 Hz

position of the corresponding sensor (Merheb, Nassar,Nongaillard, Delaplace, & Leuliet, 2006a; Merheb, Nas-sar, Nongaillard, Delaplace, & Leuliet, 2006b). For exam-ple, the delay curve of the acoustic wave recorded bysensor ‘0’, which is closest to the product inlet, varies

more than the wave recorded by sensor ‘1’. Furthermore,during the cleaning phase, there is a rapid increase in thedelay curves for all acoustic waves. This differencebetween the levels of the acoustic parameters (power anddelay) before fouling and after cleaning is due to thechanges in operating conditions, specifically the tighteningof the plates and the temperature of the fluid circulating inthe exchanger.

5. Conclusion

This study describes a useful acoustic device based on anon-intrusive technique for monitoring the fouling of aplate heat exchanger in real-time. This technique relies on

Table 4Repeatability of the time of flight (TOF) and the ratio of total power and trigger power (RTPTP) measurements from the three sensors during the flow oftaper water at 20 �C in the V2 PHE

Sensor RTPTP (�) RTPTP standarddeviation (�)

RTPTP variationcoefficient (%)

Mean TOF(ls)

TOF standarddeviation (ls)

TOF variationcoefficient (%)

S0 0.056 0.0037 6.62 177 1.45 0.82S1 0.104 0.0087 8.4 163 1.50 0.92S2 0.059 0.0041 6.9 175 1.18 0.67

RTPTP: ratio of total power on trigger power of the acoustic wave received by sensor Si.TOF: time of flight of the acoustic wave propagating from the trigger sensor to the receiver sensor Si.

a low frequency acoustic approach that uses a mechanicalpulse as a source of acoustic vibrations.

In conclusion, the analysis of a low-frequency acousticwave (0–20 kHz) propagated through the plates demon-strated a good sensitivity that could be exploited to moni-tor fouling. The evolution of the acoustic response (Poweror Delay) depends on where the sensor is located on theexchanger. By comparing the evolution of the propagationtime (i.e. the delay of the acoustic wave) measured for eachchannel (Figs. 7a, b, and 9), an image of the fouling rate ineach zone of the exchanger can be obtained. Rapidincreases in the delay curve were observed for sensorslocated near the product inlet, where, after the experimentswere completed, the PHE dismantled, and the dry weightof the deposit on the plate measured, the degree of foulingwas quite high in comparison to other zones inside theexchanger.

Further investigation with local measurements, compar-ing the evolution of the delay or the power at different loca-tions, could further improve understanding of themechanisms and kinetics of fouling. These results shouldallow the food industry to make important operationaldecisions with regard to PHE fouling.

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