0960–3085/03/$23.50+0.00# Institution of Chemical Engineers

www.ingentaselect.com=titles=09603085.htm Trans IChemE, Vol 81, Part C, March 2003

THE RATE AND EXTENT OF FOULING IN ASINGLE-TUBE WORT BOILING SYSTEM

K. L. TSE, A. M. PRITCHARD and P. J. FRYERCentre for Formulation Engineering, University of Birmingham, Birmingham, UK

W ort is the fermentation feedstock that provides all the necessary sugars and nutrientsrequired for successful brewing. It is boiled to stabilize its composition prior tofermentation, a process that leads to the coagulation and subsequent � occulation of

proteins and tannins, and fouling of the heat transfer surfaces. Fouling by commercially-supplied wort was studied in a single-tube (15 mm £ 0.2 mm wall thickness, with walltemperature control) model wort boiling system, at wall temperatures of 130, 150 and 170¯Cand � ow velocities of 0.07, 0.14 and 0.18m s¡1, with the overall objective being to identifymodes of operation which prolong operating times. Heat transfer was quanti� ed in terms of theamounts of heat transferred to the liquid and vapour phases. The in� uence of operatingconditions on the fouling process was determined by calculating the decrease in heat transfercoef� cient with the increasing number of batches of wort processed. The work quanti� edthe in� uence of wall temperature, wort velocity (and hence circulation time) and the duration ofthe boil. Fouling showed little temperature dependence as for all wall temperatures, aconstant rate of fouling (dRf=dt) was determined; dRf=dt ˆ 1 £ 10¡5m2kW¡1h¡1, in theabsence of vapour condensation and dRf=dt ˆ 2 £ 10¡5m2kW¡1h¡1, with vapour condensa-tion. The rate of fouling in the system was more affected by wort velocity, particularly underconditions where vapour was condensed. Under these conditions, curves obtained forvelocities 0.07 and 0.14 m s¡1 showed an initial, more rapid rate of fouling, which thencontinued at a reduced rate. The initial rate of fouling doubled as the wort velocity halved(at wort velocity 0.07m s¡1 initially dRf=dt ˆ 4 £ 10¡5m2kW¡1h¡1 compared to dRf=dt ˆ2 £ 10¡5m2kW¡1h¡1 at velocity 0.14 m s¡1), whereas the � nal rate was the same at bothvelocities (dRf=dt ˆ 8 £ 10¡6m2kW¡1h¡1).

Keywords: wort boiling; fouling; vapour condensation; fouling resistance; nucleate boiling.

INTRODUCTION

In the brewing process, wort is the fermentation feedstockwhich provides all the necessary sugars and nutrientsrequired by the yeast for successful fermentation. Wortboiling is the unit operation during which the compositionof the wort extracted by mashing and recovered duringlatering (or � ltration) is stabilized prior to fermentation. Itis the most energy-intensive of the brewhouse operations,utilizing up to 40% of the total brewhouse energy require-ments (Kollnberger, 1984). The boiling process isresponsible for the principal colour and � avour developmentof the � nal product (Briggs et al., 1982; Lewis and Young,1995), although it has multiple objectives (Briggs et al.,1982), including the coagulation and subsequent � occula-tion of proteins and tannins, as hot or cold break (formedduring wort boiling and on wort cooling, respectively). Thecoagulation of proteins and formation of protein–tannin complexes during wort boiling is important as itpromotes colloidal stability of the � nal product and preventsundesirable haze formation (Briggs et al., 1982). Although

by far the greatest amounts of insoluble substances areremoved as hot and cold break (or trub), fouling of heattransfer surfaces due to the deposition of insoluble materialis an inherent and problematic aspect of the process(Andrews, 1992; Reed, 1991).

Wort boiler fouling is the principal cause for the rigorouscleaning-in-place (CIP) practices necessary in the brew-house and which both shorten production run-times andoccupy valuable process time. Additionally, effective clean-ing of the fouled surfaces requires the use of cleaningagents, with associated cost and waste management consi-derations (Renfrew, 1999). Although wort boiler fouling is awell-recognized problem, with implications for overallprocess ef� ciency (Reed, 1991), few studies have investi-gated either the deposition process or the nature of thefouling itself. Increased understanding of wort boiler foulingwould provide signi� cant bene� ts to the brewing industry.Identi� cation of the in� uential variables would allow opera-tions to be run in such a way as to mitigate fouling, thusincreasing process ef� ciency. Investigations into thecleaning of fouled plant provide information on cleaning

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mechanisms and regimes which could allow the ef� cientuse of more effective cleaning agents. This knowledgeshould, in turn, translate into longer process runtimes anddiminished brewhouse downtime for CIP. Ultimately such astudy would aim to provide useful information to enabledesign improvements in the speci� cations of processequipment.

Wort boiling equipment can be broadly divided into twocategories: kettles with internal boilers and those � tted withexternal boilers or calandrias. The type of kettle useddepends on several factors including the size of the brewingoperation, geographical location and the age of the brew-house. Traditionally kettles were � tted with steam coils inthe base such that boiling occurred in the kettle. However,due to the low surface area available for heat transfer and thecomparatively large volume of wort to be boiled, thiscon� guration tends to result in high wall temperatures,which brewers tend to associate with increased rates ofboiler fouling. For external boilers, kettles are � tted withexternal shell and tube heat exchangers through which thekettle contents are circulated, initially under forced convec-tion and then, as the temperature of the wort increases toallow boiling, under thermosyphon action. The increasedsurface area available in external heat exchangers enableslower steam pressures to be used, thus reducing walltemperatures and potentially mitigating the rate of boilerfouling.

In a typical brewing operation in the United Kingdom,batches of wort are each boiled for about 90 min to produceevaporation rates between 8 and 10% per hour (Briggs et al.,1982), although rates as low as 5% are used for some brews.Measurements of speci� c gravity are used to follow theprogress of the boil and to indicate when suf� cient evapora-tion has occurred. The number of batches that can beprocessed before CIP is required varies widely betweenbreweries, depending on the type of equipment (includingthat upstream in the brewhouse), the size of the brewingoperation, the raw materials, the water used, the productand the control measures for steam heating. In someinstances (generally small breweries) cleaning is carriedout after each boil, although cleaning is more commonlyundertaken after 6–12 batches have been processed (Briggset al., 1982). In some thermosyphon units up to 24 batchescan be processed before cleaning is necessary (Hancock andAndrews, 1996). In almost all cases, by the time cleaning ofthe heat transfer surfaces takes place, operating ef� ciency isgenerally signi� cantly lower than in a clean system(Andrews, 1992).

To attempt to resolve the problem of wort boiler fouling, amodel system was constructed, based on an electricallyheated single tube evaporator. The wort was constantlyrecirculated from a holding tank through the test section,the wall of which was maintained at a constant temperature.Heat transfer was quanti� ed in terms of both the sensibleheat transferred and the amount of latent heat removed byevaporation. Once boiling commenced, the wort was boiledfor a given period during which the evaporated material wascondensed and recovered. To simulate brewery practice, aseries of 10–12 batches of wort was boiled, each for a � xedtime, at each operating condition. The in� uence of operatingconditions on the fouling process was determined bymeasuring the decrease in heat transfer coef� cient with theincreasing number of batches processed.

The study aimed to determine the effects of three processparameters, i.e. boil time, wort velocity (and hencecirculation time) and wall temperature on the overall heattransfer coef� cient and fouling process. The model systemwas characterized using water and the data so obtained usedto determine the controlling in� uences on the rate offouling.

MATERIALS AND METHODS

Single-tube Wort Boi ling System

The model wort boiling system was designed to reproducethe salient features of process plant and is shown schemati-cally in Figure 1. The working � uid was � rst pumped into theholding tank through the inlet piping. The valves were thenadjusted to allow wort to be circulated from the holding tankinto the heated test section by a rotary diaphragm pump(RD5S, Charles Austen Ltd, UK). The � uid passed from thetest section into a disengagement chamber, from which theliquid drained back into the holding tank, for recirculation.The holding tank, disengagement chamber, valves and allpipework (o.d. 15 mm) were all constructed from type316 stainless steel: the test section was made of thin wall3=8 inches o.d.£ 0.2 mm wall thickness type 316 stainless

Figure 1. A schematic diagram of the model wort boiling rig. Components:(1) inlet, (2) rotary diaphragm pump, (3) electromagnetic � owmeter, (4) testsection, (5) heating blocks, (6) disengagement chamber, (7) recirculationline, (8) holding tank, (9) holding tank vent, (10) recirculation line, (11)bypass loop, (12) vapour transfer line, (13) condenser, (14) collectionvessel, (15) balance. Notation: TCC, control thermocouple; TCM, measuringthermocouple.

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steel to reduce the wall heat transfer resistance and to allowthe fouled test section to be sampled with minimal distur-bance of the deposit. Flow rates were measured using anelectromagnetic � owmeter (Model IFM5080, Krohne Ltd,UK). Vapour was transferred from the top of the disengage-ment chamber to a shell and tube condenser and the conden-sate collected in a vessel placed on a balance. To reduce heatlosses, the apparatus was insulated with mineral wool.

The holding tank and test section were heated electrically.The one metre length of the test section was heated using 10individual heating blocks. Each heating block consisted of amica band heater (each 100 mm in length, 140W), � ttedwith a 100 mm long aluminium sleeve (wall thickness5 mm) to ensure good contact between the heater and thetest section. The heaters were connected in parallel in threegroups, consisting of the bottom two, the central six and thetop two, each group being separately controlled electrically.Three of the heating blocks, the upper (no. 1), central (no. 5)and lowest (no. 10) were � tted with K-type thermocouplesplaced between the test section and the aluminium sleeve, tomeasure the wall temperature and provide data for control ofthe groups of heater blocks. A 900 W silicone rubberheating mat (height 400mm) surrounded the holding tankand was used only to maintain the contents at temperatureand to reduce heat losses (thermocouple controlled at a set-point of 100¯C). Set-points for the wall temperatures for theelectrical test section heaters and holding tank heater werede� ned and controlled using a LabviewTM program. Thesame program logged the values of the variables, shownschematically in Figure 2: the wort temperatures at the inlet

and outlet of the test section, the wort input � ow-rate, themass of condensate collected on the balance, the powersupplied to the three heating blocks and the temperaturesfrom the controlling thermocouples.

Typical operation of the model system involved circulat-ing the wort at a constant mass rate through the test section,with a consequent increase in wort temperature with time.Once boiling commenced the evaporated material wascondensed and the mass recovered was recorded as afunction of time.

Calculations and Modelling

The heat transferred was measured as the sum of thesensible heat transferred, from the inlet and outlet tempera-tures and the mass of condensate recovered.

Simple mass and heat balances carried out over thesystem were used to calculate the rate of heat input, Qwall,through the wall.

Qwall ˆ minCp,l(Tout ¡ Tin) ‡ mevap(l ¡ Cp,gTout) (1)

where the measured variables are the mass � ow rate into thetest section, min, the mass � ow rate of condensate, mevap andthe inlet and outlet temperatures from the test section, Tin

and Tout. Values for water were used for the speci� c heatcapacity of the liquid, Cp,l and gas, Cp,g and for the latentheat of evaporation, l. The � rst term describes the sensibleheat transferred, and the second term the latent heat ofevaporation.

Having determined the rate of heat input across the testsection, the overall heat transfer coef� cient, U can be foundfrom:

Qwall ˆ UADTLM (2)

By determining an average value for the overall heat transfercoef� cient for each batch processed in a series of experi-mental runs, a value can then be obtained for the foulingresistance (Fryer et al., 1997):

1U

ˆ 1Uo

‡ Rf (3)

where Uo is the clean heat transfer coef� cient. Averagevalues of U and hence of Rf were calculated from measure-ments with Tout between 90 and 95¯C (referred to below as‘pre-boiling conditions’) and after the start of boiling. Thelatter contain contributions from both the sensible heattransferred and the latent heat removed. With the increasingnumber of batches processed, the fouling curve can bedetermined by plotting values of the fouling resistancesagainst the elapsed time. The slope of this curve gives ameasure of the rate of fouling in the system. Using thefouling resistance curves generated under a range of condi-tions, it is then possible to determine the in� uence of thevarious operating parameters on the rate of fouling.

Figure 2. Test section showing variables measured for determination of theoverall heat transfer coef� cient: min, mass � ow rate at inlet; Tin, temperatureat inlet; Qwall, rate of heat input across wall of test sections; Tout,temperature at outlet; mevap, mass of evaporate collected. TCC, controlthermocouple.

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Experimental Procedure

Experiments were designed to mimic standard breweryoperation. Consequently, each batch was boiled for a � xedperiod of time (usually 90 min) from the start of boiling, asindicated by the onset of condensate recovery. At the end ofthe experiment, the boiled wort was discarded, a freshvolume introduced into the rig and the experiment repeated.Unlike fouling in dairy systems, for example, the build-upof deposit occurred over a relatively long time-scale, with aseries of typically 12 batch volumes required at eachoperating condition (i.e. in excess of 18 h of fouling)before a signi� cant difference in the overall heat transfercoef� cient could be observed. The total time taken for a runvaried between 2 and 5 h depending on the wall temperaturefor the test, the initial temperatures of the working � uid andthe rig, and the ambient temperature.

The wort used in the experiments was obtained from acommercial brewery and supplied with hops and sugaradded. Although all the wort was prepared in the breweryusing the same protocol, there is naturally some betweenbatch variation. A typicalwort contains ‘simple sugars, morecomplex polysaccharides, amino acids, peptides, proteins,other nitrogenous materials, vitamins, organic and inorganicphosphates, mineral salts, polyphenols (including tanninprecursors and tannins) and small quantities of lipids’(Lewis and Young, 1995) in addition to other minor com-ponents, some as yet unidenti� ed (Briggs et al., 1982).Elemental analysis of wort used in this study has revealedthe following composition (Tse et al., 2001): 39.37% carbon,6.37% hydrogen, 0.71% nitrogen (equivalent to 4.44%protein), 0.09% calcium and 0.15% phosphorus. Speci� cgravities of the unboiled wort (measured at 25¯C using adensity meter; Mettler-Toledo Ltd, UK) were typically about1.073 g cm¡3. Following 90 min boiling, values varied from1.074 to 1.088 g cm¡3, dependingon both the initialwort andthe number of preceding boils.

Characterizing the Model System

Characterization of the system was carried out withdeionised water to which 1% KNO3 had been added toprovide suf� cient conductivity for the � owmeter to operate.Fluid velocities were varied from 0.07 to 0.18 m s¡1 andwall temperatures from 110 to 170¯C. This gave informationon system behaviour and values for the initial overall heattransfer coef� cient in the absence of deposit. As no conden-sate was recovered at measured wall temperatures of 110¯C,the experimental matrix chosen for the wort fouling experi-ments used standard wall temperatures of 130¯C (equivalentto a steam pressure of 3 bar gauge, which is cited byoperators as a typical industrial value for kettles withexternal calandrias), 150 and 170¯C. The last condition ismuch higher than would be desired in industry but waschosen to exaggerate the possible in� uence of this operatingparameter on the rate of boiler fouling. Most experimentswere carried out at a � ow rate of 0.14 m s¡1 (Re 7600 at100¯C), with additional measurements between 0.07 m s¡1

(Re 3800) and 0.18 m s¡1 (Re 9900). In industry wortvelocities may vary widely during a single run, dependingon whether forced convection or thermosyphon modes areused, although for external calandria systems, the range of

Reynolds numbers falls between 17,000 and 340,000. Themodel Re are lower, although still mostly in turbulent � ow,due to the high pipe diameters used in process plant.

RESULTS AND DISCUSSION

The initial set of experiments with water provided infor-mation on how the heat transfer coef� cient varied with � owrate and wall temperature. Figure 3 shows the percentages ofsensible and latent heat transferred during the course of asingle batch boil. At the onset of vapour production andrecovery, the percentage of convective heat transfer changesfairly rapidly to around 50%, a value which is maintainedthroughout the remainder of the experiment.

All heat transfer coef� cients calculated are the overallheat transfer coef� cients U for the system. The changes in Uwith wall temperature are shown in Figure 4. There is aconsistent increase in the value of U with increasing walltemperatures both before and after vapour production andcondensation, which can be explained by an increase innucleate boiling and the amount of vapour produced, asshown on the � ow pattern map of Hewitt and Roberts(1969) (Figure 5). Figure 6 shows the changes in U withvelocity. The average pre-boiling values decrease consis-tently with increasing velocity, presumably because theamount of any sub-cooled nucleate boiling decreases morerapidly than any increase in the convective component due tothe increased degree of turbulence at higher velocities.However, values for the average boiling coef� cient and forthe maximum value of the boiling coef� cient for a givenwall temperature do not show a similar decrease, but aresimilar, with average values of 500 and 700 W m¡2K¡1,respectively. This suggests that the main mechanism for heattransfer under these conditions is nucleate boiling, which isrelatively insensitive to � ow conditions until annular � lmboiling occurs.

Calculation of the mass � ows of liquid and vapour at theoutlet from the test section allowed deductions to be made

Figure 3. A typical time course for a single batch boil, showing the changein sensible and latent heat contributions as a percentage of the total heattransferred. Batch number 3 processed in series of 20 wort boils; Twall,130¯C; velocity, 0.14m s¡1 (Re 7600).

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about the likely two-phase � ow patterns within the testsection, as shown on the two-phase � ow pattern map(Figure 5). With increasing wall temperature, increasingvolumes of vapour are formed and the � ow pattern in thesystem becomes churn � ow. The pulsating, highly unstablenature of churn � ow was re� ected in the behaviour ofthe system at high wall temperatures; towards and afterthe onset of boiling, the test section was observed to shake,presumably due to the unstable pulsing of the liquid near thewall and the passage of large unstable vapour bursts up thetube. Figure 5 shows that as the liquid � ow rate, as measuredby the Reynolds number, increases, the mass � ow of vapourin the system decreases.

Wort Boi ling Experiments

Heat transfer

A typical pro� le for the change in U over the duration of asingle batch run at a wall temperature of 130¯C and a wortvelocity of 0.14 m s¡1 is shown in Figure 7, together withthe outlet temperatures and log mean temperature differ-ences over the test section. This illustrates how the heattransfer changes according to whether vapour is beingcondensed or not. U increased with run time, with a meanvalue of 850 W m¡2K¡1 before vapour condensationstarted, to a maximum value of 1120 W m¡2K¡1 at thepoint at which the temperature of the wort at the outlet of the

Figure 4. The in� uence of test section wall temperature on the average heattransfer coef� cients, in the absence of and with vapour condensation,measured with water. Wall temperature varied from 110 to 170¯C; � uidvelocity 0.14m s¡1.

Figure 5. The in� uence of � uid velocity, on the average heat transfercoef� cients, in the absence and with vapour condensation, measured withwater. Velocities varied from 0.07 to 0.18m s¡1; wall temperature 130¯C.

Figure 6. The in� uence of test section wall temperature and � uid velocityon the two-phase � ow composition at the outlet from the test section,plotted on the Hewitt and Roberts (1969) � ow pattern map for vertical � owin a tube. G2=r is the momentum � ux, where Gg mass � ux of vapour, i.e.vapour mass � ow rate=tube cross sectional area (kg s¡1m¡2), rg vapourdensity (kg m¡3), Gl mass � ux of liquid, i.e. liquid mass � ow rate=tubecross sectional area (kg s¡1m¡2) and rl liquid density (kg m¡3).

Figure 7. Changes in the heat transfer coef� cient, outlet temperature and logmean temperature difference over the test section during the processing of asingle wort batch. Batch number 2 in a series of 20 batch boils; Twall,130¯C; velocity, 0.14m s¡1.

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test section reached approximately 100¯C. Thereafter,during the boiling phase and particularly towards the endof the processing time, there was a considerable degree ofscatter in the values of U, with a mean value of780 W m¡2K¡1, which makes it dif� cult to determineparticular trends in behaviour. These values are smallerthan other quoted values for nucleate boiling (1.2–1.6 kW m2K¡1; Briggs et al., 1982), suggesting that theremay be some additional heat transfer resistance in thesystem, possibly of the test section wall. The decreasefrom the maximum value suggests that there may be somesteam blanketing occurring at the heat transfer surface.However, nucleate boiling on surfaces is strongly dependenton surface wettability and roughness, so changes in theseparameters during the run may explain the variations. Thepicture is further complicated by the variability in composi-tion and amount of any deposits formed along the tube (Tseet al., 2003).

As the number of batches processed increased, therecorded maximum value of the heat transfer coef� cientdecreased (from 1120 to 760 W m¡2K¡1) and there was aconcomitant � attening in the curve after the onset of vapourcondensation. This indicates that the fouling process in� u-ences the heat transfer through both the convective andnucleate boiling modes. Over the duration of 20 batch boils(each 90 min in duration, with a total process time ofapproximately 76 h), the value of the average heat transfercoef� cient before vapour condensation was observed wasobserved to fall from 790 to 470 W m¡2K¡1 and that withvapour condensation from 740 to 480 W m¡2K¡1.

Using the average values of U for each batch, the foulingcurve for a series of runs can be determined for theparticular operating conditions and � tted to an appropriateequation, as for example in Figure 8 for a series of runs withTwall set to 130¯C and a velocity of 0.14 m s¡1. Values of Rf

both before and after the onset of vapour condensation areshown. In determining values of Rf for these experiments,the values of Uo taken were those obtained from the � rstbatch of each fouling run, as this was considered to providethe best indication of a clean system, prior to the formationof further fouling deposit.

Up to a total processing time of 42 h, both series ofdata could be � tted to equations showing the same linearrate of increase of Rf with time, with dRf=dt ˆ2 £ 10¡5m2kW¡1h¡1. For processing times from 42 to76 h a reduced rate was observed, given by dRf=dt ˆ6 £ 10¡6m2kW¡1h¡1 before vapour condensation, anddRf=dt ˆ 8 £ 10¡6m2kW¡1h¡1 after condensation starts.The existence of two distinct regions of different slope inthe fouling resistance curves is not unexpected, similarcurves are well known (Fryer et al., 1997; Hewitt et al.,1994), where the initial rapid rate of fouling slows toproduce a � nal equilibrium deposit.

In� uence of Total Boiling Time

In a separate series of experiments under the sameconditions, individual batch boils were carried out for only45 min instead of the normal value of 90 min. The absenceof any signi� cant differences in the heat transfer for a giventotal process time in the two series, and the steady rates ofdecrease for both coef� cients, shown in Figure 9, suggests

that the amount of deposit formed was not limited bydepletion of the fouling precursor species up to at least90 min boiling. In accordance with brewery practicetherefore, all further experiments were carried out at astandard batch boiling time of 90 min.

In� uence of Wall Temperature

Anecdotal evidence from brewery operators and others(Reed, 1991; Hancock and Andrews, 1996) suggests thatwall temperatures have a strong in� uence on the rate offouling; minimizing the temperature driving force reducesthe degree of fouling and subsequent need for cleaning.Operational constraints prevented experiments being

Figure 8. Changes in the fouling resistance curves, before and after vapourcondensation for a series of 20 batch boils (total processing time 76 h).Processing conditions: Twall, 130¯C; velocity, 0.14m s¡1. Lines show thecurves used to calculate the rate of fouling for each condition: solid line, novapour condensation, dRf=dt ˆ 2 £ 10¡5m2kW¡1h¡1 (0–42 h) anddRf=dt ˆ 6£ 10¡6m2kW¡1h¡1 (42–76 h); dashed line, with vapourcondensation, dRf=dt ˆ 2 £ 10¡5m2kW¡1h¡1 (0–42 h) anddRf=dt ˆ 8£ 10¡6m2kW¡1h¡1 (42–76 h).

Figure 9. The in� uence of batch processing time on the measured averageheat transfer coef� cients with and without vapour condensations, as afunction of total processing time. For all experiments: Twall, 130¯C; velocity,0.14m s¡1 (Re ˆ 7600).

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carried out at lower wall temperatures, although the highertemperatures used in comparison with brewery practicewould exaggerate the effect of temperature driving forceon the rate of fouling.

Figures 10 and 11 show the in� uence of test section walltemperature on the development of the fouling resistancecurves, before and after vapour condensation, respectively.Within each set of data (before and after vapourcondensation)the values of Rf appear independent of wall temperature overthe total processing time investigated (equivalent to 12 batchboils, i.e. up to 42 h processing for a wall temperature of130¯C), and increase linearly with time, with the equationsdRf=dt ˆ 1 £ 10¡5m2kW¡1h¡1 (Figure 10) and dRf=dt ˆ2 £ 10¡5m2kW¡1h¡1 (Figure 11), respectively.

The lack of any temperature dependence in Rf appears tocontradict brewing industry experience. However, the resultsmay re� ect the fact that the measured values of Rf arederived from averaged heat transfer measurements over thewhole test section and may be complicated by the differentamounts and types of deposit observed at different positionsalong the test section and in industrial plant. It may also bethat in thermosyphon systems the velocity of � ow dependson the temperature driving forces in the system. The lack ofany signi� cant effect of temperature suggests that thedeposition process does not have a thermal activationenergy, such as might be expected for a chemical reactionprocess, or a decrease in solubility of the material withincreasing temperature. The lower rate of deposition underconditions before vapour condensation compared with thatafterwards suggests that the deposition is closely connectedwith evaporation, since nucleate boiling can lead to veryhigh solute concentration factors.

In� uence of Wort Velocity

Curves for Rf without and with condensation of vapour atwort velocities of 0.07, 0.14 and 0.18 m s¡1 (correspondingto Re ˆ 3800, 7600 and 9900) at a wall temperature of 130¯Care shown in Figures 12 and 13, respectively, for processingtimes up to 52 h (equivalent to 12 batch boils at velocity0.18 m s¡1; Re 9900). For the case with no vapour con-densing (Figure 12), there is very little difference betweenthe curves obtained at 0.07 and 0.14 m s¡1. Both curves showa constant rate of fouling, expressed by the equationdRf=dt ˆ 1 £ 10¡5m2kW¡1h¡1 up to a processing timeof 42 h. At 0.18 m s¡1, however, the curve suggests anincrease in the heat transfer coef� cient (negative Rf) withincreasing process time (dRf=dt ˆ ¡2 £ 10¡5m2kW¡1h¡1).

Figure 10. The in� uence of the test section wall temperature on the foulingresistance curves, in the absence of vapour condensation. For allexperiments, wort velocity was 0.14m s¡1. The solid line shows thecurve used to calculate the rate of fouling at all three conditions:dRf=dt ˆ 1 £ 10¡5m2kW¡1h¡1.

Figure 11. The in� uence of the test section wall temperature on the foulingresistance curves, with vapour condensation. For all experiments, wortvelocity was 0.14m s¡1. The dashed line shows the curve used to calculatethe rate of fouling at all three conditions: dRf=dt ˆ 2 £ 10¡5m2kW¡1h¡1.

Figure 12. The in� uence of wort velocity on the fouling resistance curves,in the absence of vapour condensation. For all experiments, Twall was130¯C. Lines show the curves used to calculate the rate of fouling at eachcondition: solid line, velocity 0.07m s¡1; dotted line, velocity 0.14m s¡1—dRf=dt ˆ 1£ 10¡5m2kW¡1h¡1 for both; dashed line, velocity0.18m s¡1—dRf=dt ˆ ¡2 £ 10¡5m2kW¡1h¡1.

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Examination of the test section after completion of thisexperimental series showed a continuous layer of powderydeposit, strongly adhering to the wall over the entire length ofthe test section, in comparison with the multi-layered depos-its observed at the inlets to the test sections at the lower wortvelocities. If the homogeneous deposition promoted theonset of nucleate boiling, possibly in a manner similar tothat described by Macbeth (1977), this could explain thedecrease of Rf below zero, since it would increase theoverall heat transfer coef� cient. At the lower velocities itwould appear that the amount or morphology of the depositis more effective in reducing heat transfer than any increasein nucleate boiling that may take place as a result ofdeposition.

The Rf curves for conditions with condensation of vapour(Figure 13) for all three wort velocities are signi� cantlydifferent. At 0.07 m s¡1 the rate of fouling initially followsthe equation dRf=dt ˆ 4 £ 10¡5m2kW¡1h¡1 for the � rst15 h, and then decreases signi� cantly to follow theexpression dRf=dt ˆ 8 £ 10¡6m2kW¡1h¡1. It is not clearwhether this represents a change in the rate of deposition,or in the nature of the deposit; a possible explanationmight be an extension of nucleate boiling to cover almostthe whole of the test section. In comparison, the initialrate of fouling at 0.14 m s¡1 is half the initial rate at0.07 m s¡1, with dRf=dt ˆ 2 £ 10¡5m2kW¡1h¡1. No reduc-tion in the rate of fouling is observed until after about 42 hof processing, when the rate reduces to the same value asfor velocity 0.07 m s¡1, dRf=dt ˆ 8 £ 10¡6m2kW¡1h¡1

(Figure 8). The rate of fouling is lowest at the highestwort velocity (0.18 m s¡1), although due to the degree ofscatter it is not possible to obtain a valid � t for the curve. Atall three velocities Rf increases faster than when no vapour isbeing condensed, supporting the view that the rate ofdeposition is controlled by nucleate boiling. The highermass � ows of vapour at the lower velocities (Figure 6),

suggest that the rate of deposition is likely to be higher atlower velocities, thus giving the observed higher values ofdRf=dt.

The results indicate that in terms of operational para-meters, the rate of wort fouling is more strongly in� uencedby the � uid velocity than the wall temperature of the testsection. Mechanistically, the crucial factor appears to be theextent of nucleate boiling, whether or not this leads to theproduction of condensable vapour at the outlet from the testsection. However, there will be a degree of interactionbetween the two operating parameters investigated. Anincrease in the wall temperature, with its consequentin� uence on the amount of vapour produced, will increasethe degree of thermosyphon action, thus affecting the wortvelocity through the test section. Consequently, it may bethat any effect of wall temperature in these experiments hasbeen mediated by the associated increase in wort velocitythrough the test section.

Nature of the Deposition

Following each series of experimental runs, the testsection was removed and samples of the deposit taken foranalysis. Visual assessment suggested that there were signi-� cantly larger amounts of deposit on the lower sections,although the variations along the test sections dependedheavily on the processing conditions (Tse et al., 2003). Theheaviest deposition overall was at the bottom of the testsection and was obtained at the lowest � ow-rate and thelightest at the highest wall temperature, near the top of thetest section. Variations were also observed in the nature ofthe deposit over the length of the section. Towards thebottom of the test section the deposit was continuous,multi-layered, quite granular in appearance and darkbrown in colour (Figure 14a and c). Near the top of thesection, it formed a very thin, smooth layer, lighter in colourand interrupted at intervals either by slightly heavier patchesof deposit or by apparently clean areas of test section(Figure 14b and d). As the wall temperature was increased,the transition from the heavy multi-layered type to smoother,more interrupted deposit occurred lower down the testsection. Chemical analyses of the two types of deposit(Tse et al., 2003) showed signi� cant differences in theircomposition. This suggests that at least two differentfouling mechanisms may be involved, possibly chemicalreaction of species in the wort to form polymers or othernew chemical species, and crystallisation of species as aresult of evaporation of the wort at bubble nucleation sites innucleate boiling regions. A study has been conducted intothe composition and morphology of the deposit and resultsare reported elsewhere (Tse et al., 2003).

CONCLUSIONS

A series of experiments have been undertaken in asingle-tube model system to resolve the problem of wortboiling fouling in brewery operations. The results of thestudy show that material is deposited over the duration of

Figure 13. The in� uence of wort velocity on the fouling resistance curves,with vapour condensation. For all experiments, Twall was 130¯C. Linesshow the curves used to calculate the rate of fouling at each condition:solid line, velocity 0.07ms¡1, dRf=dt ˆ4 £ 10¡5m2kW¡1h¡1 (0–15 h)and dRf=dt ˆ 6 £ 10¡6m2kW¡1h¡1 (15–41 h); dotted line, velocity0.14m s¡1, dRf=dt ˆ 2 £ 10¡5m2kW¡1h¡1 (0–35 h). No valid � t couldbe obtained for velocity 0.18m s¡1.

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the boiling process, therefore the longer the boil, the greaterthe amount of fouling. Contrary to industry indications, nosigni� cant effects of wall temperature on the rate of foulingwere observed and a single, constant value was found for therate of increase of fouling resistance for a given wortvelocity. Under conditions when no vapour was condensedthere was little impact of wort velocity on the foulingcurves, but signi� cant differences were observed underconditions when vapour was produced and condensed.

NOMENCLATURE

Cp,g speci� c heat capacity of vapour, J kg¡1K¡1

Cp,l speci� c heat capacity of liquid, J kg¡1K¡1

Gg mass � ux of vapour, kg s¡1m¡2

Gl mass � ux of liquid, kg s¡1m¡2

mevap mass of evaporate, kgmin mass � ow rate at inlet, kgQwall rate of heat transfer across test section wall, WRf fouling resistance, m2kW¡1

Re Reynolds numbert time, sTin temperature at inlet, KTout temperature at outlet, KTwall wall temperature of test section, KU overall heat transfer coef� cient, W m¡2K¡1

Uo clean heat transfer coef� cient, W m¡2K¡1

DTLM log mean temperature difference, K

Greek symbolsrg vapour density, kg m¡3

rl liquid density, kg m¡3

l latent heat of evaporation, J kg¡1

REFERENCES

Andrews, J.M.H., 1992, External wort boiling—latest developments, ProcConv Institute of Brewing (Australian and New Zealand Section),Melbourne, Vol. 22, pp 65–68.

Briggs, D.E., Hough, J.S., Stevens, R. and Young, T.W., 1982, Methods ofwort boiling and hop extraction, in Malting and Brewing Science, Vol. 2:Hopped Wort and Beer, 2nd edition (Chapman and Hall, London),pp 499–526.

Fryer, P.J., Pyle, D.L. and Reilly, C.D. (eds), 1997, Thermal treatment offoods, in Chemical Engineering for the Food Industry (Chapman & Hall,London), pp 365–382.

Hancock, J.C. and Andrews, J.M.H., 1996, Wort boiling, Ferment, 9:212–217.

Hewitt, G.F. and Roberts, D.N., 1969, Studies of two-phase � ow patterns bysimultaneous � ash and x-ray photography, AERE-M2159.

Hewitt, G.F., Shires, G.L. and Bott, T.R., 1994, Fouling of heatexchangers, in Process Heat Transfer (CRC Press, Boca Raton, FL), pp857–877.

Kollnberger, P., 1984, Wort boiling systems—new developments, MBAATech Q, 21(3): 124–130.

Lewis, M.J. and Young, T.W., 1995, Hop chemistry and wort boiling, inBrewing (Chapman & Hall, London), pp 129–140.

Macbeth, R.V., 1977, Fouling in boiling water systems, in Two-phase Flowand Heat Transfer, Butterworth, D. and Hewitt, G.F. (eds) (OxfordUniversity Press, Oxford).

Reed, R.J.R., 1991, In� uence of mixing, shear and surface temperaturesin wort boilers, in EBC Symposium on Wort Boiling and Clari� ca-tion, Strasbourg (Verlag Hans Carl Getranke-Fachverlag, Nurnberg),pp 58–73.

Renfrew, L.L., 1999, New advancements in cleaning technology, MBAATech Q, 36(1): 85–91.

Tse, K.L., Shao, F., Pritchard, A.M. and Fryer, P.J., 2001, Studies of foulingin wort boiling systems, Proc 6th World Cong Chemical Engineering,Melbourne.

Tse, K.L., Pritchard, A.M. and Fryer, P.J., 2003, Characterisation of wortfouling deposits under � ow and boiling conditions (submitted).

Figure 14. The fouled test sections after exposure to 18 h boiling. (a, c) Lower sections, corresponding to heaters 8–10, showing the darker-coloured, clearlyporous deposit, which forms a heavy, multi-layered deposit on the internal surface. (b, d) Upper sections, corresponding to heaters 1–3, showing the thinlayers of lighter coloured deposit, interspersed with patches of very light fouling. (a, b) Twall 130¯C, Re 3800; and (c, d) Twall 170¯C, Re 7600. For all testsections, diameter is 3=8 inch.

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ACKNOWLEDGEMENT

The authors gratefully acknowledge the � nancial support of theEPSRC. This work was carried out as part of the IMI projectnumber GR=M15446-16146-16139 entitled ‘Process Engineering Model-ling for Brewing and Fermentation’ managed by Brewing ResearchInternational.

ADDRESS

Correspondence concerning this paper should be addressed to ProfessorP.J. Fryer, Centre for Formulation Engineering, University of Birmingham,Edgbaston, Birmingham B15 2TT, UK.E-mail: p.j.fryer@bham.ac.uk

The manuscript was received 23 October 2002 and accepted forpublication after revision 28 February 2003.

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