TECHNICAL SUMMARY

26June 2002 The BREWER International www.igb.org.uk

The reasons for wort boilingwere covered in TechnicalSummary No 2 published in theFebruary edition of The BrewerInternational. This featurecovers the methods used toachieve wort boiling.

Wort boiling has the highest energyrequirement of any of the brewingprocesses. It can account for as much as 60%of the total steam demand of the brewery(depending on the type of packagingoperations). It is therefore hardly surprisingthat a great deal of effort has gone in toreducing energy consumption and recoveringenergy from boiling.

Wort Boiling Plant Fig. 1

Traditional direct fired kettles Fig 1Traditionally, wort was boiled in direct-firedkettles, often made of copper, since this metalhas particularly good heat transfer properties.

Because the heat source was localised atthe bottom of the kettle, it restricts the volumeof wort which could be boiled at any one timeto a maximum of 200 barrels (330 hectolitres)which probably explains why traditionalbreweries with larger brew-lengths usednumber of separate smaller size kettles.

The principal disadvantage of traditionaldirect peat or coal fired kettles are that theyare relatively inefficient in heat transfer andtend to be labour-intensive. The heatingsurface of the copper becomes very hot andtends to promote caramelisation and burningof the wort, requiring frequent cleaning usuallyevery 2 to 5 brews to ensure effective heattransfer is maintained. High evaporation rateswere required to produce sufficient vigour orturbulence in the boil and typical boils wouldtake over 90 minutes with an evaporation rateover 10% per hour.

Kettles with Internal Heating Systems Fig. 2The advent of steam coils and internal heatingsystems allowed the production of largerkettles, as it enabled the designers to provide

a larger heating area and, because it wassurrounded by the wort, the heat transfer wasmore efficient.

In many designs the heaters were uprightand located in the centre of the kettle to give aturbulent boil. Some of the kettles also includebase steam coils for preheating the incomingwort, and to avoid the creation of dead spotswithin the kettles.

The disadvantage of internally heatedkettles is that the heaters tend to be difficult toclean with conventional CIP, and were oftenmanufactured from copper, which is dissolvedby caustic cleaning.

The internal coils in particular are prone tocorrosion, which can result in steam leaks in tothe boiling wort which are difficult to detectand repair. Because wort circulation relies onthermal currents within the kettle theturbulence over the heating surfaces issometimes limited, resulting in wortcaramelisation, which requires more frequentcleaning to ensure effective heat transfer ismaintained.

Kettles with external heating jackets Fig. 3To overcome the difficulties with cleaninginternal heaters, kettles with external heatingjackets were designed. One of the mostprolific designs was the SteineckerAsymmetric Kettle. They are generally made ofstainless steel and achieve a rolling boilthrough the location of the heating jackets onone surface.

They suffer from similar problems to thedirect fired kettles in achieving effective heattransfer, with the higher volume kettles beingrather long and thin. They require mechanicalpaddles to achieve the necessary agitation for

a satisfactory boil. This design overcomes thecleaning problems of the kettles with internalheaters, and has a lower tendency to foul, butit still requires cleaning every 6 to 12 brews toensure effective heat transfer is maintained.

These kettles are also prone to fobformation during boiling and often use a coldair draught over the wort surface and anextractor fan to keep fob under control.

Kettles with External Wort Boilers Fig. 4A more modern design uses an external heater(external wort boiler) which takes the wort outof the kettle and passes it through a shell andtube or plate heat exchanger for heating.

These wort boilers achieve high heattransfer through two phase flow and nucleateboiling, and operate at low steam pressure (at3.0 to 3.5 bar) to heat the boiler.

In these kettles vigour can be introducedmechanically, by wort circulation, and theclassical 10% evaporation/hour with a 90-minute boil, can be reduced to 5% to 6%evaporation/hour with a 60-minute boil withoutloss of wort/beer quality. This represents aconsiderable saving in energy.

These kettles have other advantages overinternal heaters since pre-heating can startonce 15% of the total kettle contents havebeen collected, allowing the kettle to boilimmediately it is full, thus improving vesselutilization.

Since low pressure steam is used, the rateof fouling is decreased, allowing more brewsto be processed between cleans. Typicalinstallations can process 16 brews betweencleans and this number can increase up to 32brews. This decreases brew house downtimethus improving throughput.

There are some hybrid kettles which use aninternal heater but also recirculate the wortthrough an external pumped loop to improvemixing and increase the vigour of the boil.

One of the negative aspects of external wortboiling involves having to pump the wort,where shear forces may damage the flocformation (trub or hot break particles). Inappropriately designed installations thisproblem can be resolved by using the naturalcirculation of the thermosyphon effect. Theboiler has to be primed during the pre-boilstage using a small circulation pump.

Once boiling is achieved the circulationpump can be by-passed and the wort willcirculate due to the energy and change ofstate resulting from the density changebetween incoming wort to the boiler at 98Cand the outlet wort and vapour from the boilerat around 105C.

Overpressure Wort BoilingBoth the internal and external boilers can beoperated with an increased over pressureduring the boil usually up to 1 bar.

This elevates the boiling temperature to

Technical Summary 6

By Tim ORourke

The sixth in this series oftechnical summaries for

the Institute & Guilds AME candidates.

The process of wort boiling

Figure 1:Direct fired

copper

Figure 2: Steam heated kettle

27The BREWER International www.igb.org.uk June 2002

around 106 to 110C, which has the effect ofaccelerating the various wort reactions, andallows the boiling time to be reduced. At theend of boil the excess pressure is releasedallowing the escape of the volatilecompounds.

Over pressure kettles are often operatedwith some form of vapour recovery energysystems. The advantage claimed from thissystem is that it allows a shorter boiling timeand lower evaporation rates than might beconsidered necessary in a conventionalboiling system.

Combined wort boiling and stripping(Merlin) Fig.5Merlin is a more recent development whichuses a form of external wort boiling to boil thewort and then to strip out the volatiles after thewhirlpool stand.

Wort is pumped from the collection vesselacross a conical heating surface, which is fedwith live steam at 0.6 to 1.5 bar, thus giving asteam temperatures of the order of 110C. Theboiler is supplied with a large heating surfacearea about 7.5 sq.m per 100 hl of wort.

The heater operates by providing a largeheating surface covered by a thin film of wortallowing gentle boiling and rapid elimination ofaroma compounds. The system is able toproduce good quality worts with 4%evaporation in 40 minutes.

To strip any addition unwanted aromacompounds formed during the whirlpool standthe clarified wort from the whirlpool is passedover the heating cone on the way to wortcooling. This provides approximately anadditional 1% evaporation. See Table 1.

Continuous high temperature boiling Fig. 6An efficient way of reducing energy demands isto use continuous wort boiling, where theenergy used for boiling is used for heating upthe incoming wort in a multistage process. Theprocess operates as follows: The wort from the lauter tun, feeds into a

holding vessel where hop additions can be

made. The wort runs through a specially developed

three stage, reverse flow heat exchanger andis heated to approximately 135C

The temperature is held for approximately 1.5to 2.0 minutes in holding tubes.

The wort is held constant at 135C byregulating the flow rate at the inlet to the firstof two adjoining separators.

As the wort flows into the separator, thepressure is lowered to a set level. Thisenables the wort to boil and evaporate.

The latent heat (enthalpy) in the vapour isgiven up in the separators and is reused inheat exchangers I and II. Only heat exchangerIII is heated with fresh steam (or hot water).

The wort from separator II runs through abooster pump to one of three whirlpool-casting vessels. The effective volume of thewhirlpools should be approximatelyequivalent to the capacity of one hourthroughput from the boiler.

Each pair of whirlpool vessels are filledalternately. As one is emptied and cleanedthe other is available to receive the wort.

The higher boiling temperature of 135Caccelerates the chemical processes of: Isomerisation of the hop alpha acids Coagulation of the high molecular weight

nitrogen compounds which are temperaturedependent and are completed in 1.5 to 2minutes.

An effective evaporation of around 7% isrequired to remove the undesired aromacomponents. Continuous wort boiling allowsthe steam demand of the brewhouse to bemaintained at a constant level, thus avoidingthe peaks resulting from batch heating orboiling of the wort.

Heat recovery is very efficient, requiring onlyprime energy input to compensate for thedifference between the wort inlet and outlettemperatures and minor heat losses from theheat exchangers.

However, continuous wort boiling is difficultto manage with a number of different wort

streams, and a number of brewers stillreservations over quality.

Wort strippingOne of the principle functions of wort boiling isto remove unwanted volatiles such as hop oilsand DMS (dimethyl sulphide) which come fromthe raw materials. Stripping of volatiles canoften be the rate determining step for wortboiling and any reduction in boiling time orevaporation rate will have an adverse effect onthe level of volatiles remaining in the beer.

Similarly some volatiles, DMS in particular,continue to be formed in the hot wort afterboiling is completed and the levels build up inthe wort prior to cooling.

The removal of unwanted volatiles afterboiling can be split into two stages: The first stage takes place in a conventional

wort kettle, where the wort is boiled or heatedto boiling point and the volatiles are removedwith the water vapour evaporated,

The second stage occurs after wortclarification and before wort cooling, whenthe volatiles are stripped from the wort in astripping column. The wort leaving thestripping column has the same or even alower level of undesired wort aromacompounds compared to a conventionallyboiled wort.

Wort stripping should take place after (hot)wort clarification (e.g. whirlpool) and wortcooling. In the normal process wort volatilescontinue to be formed after the end of wortboiling during the hot wort stand (clarificationand cooling). However, in the absence ofevaporation, they are no longer eliminated.Wort stripping is an effective method ofremoving some of these volatile substances.

The Merlin wort boiling system offers a wayof stripping the volatiles after the whirlpoolstand.

Factors affecting boiling efficiencyWort boiling relies on the efficient transfer ofenergy from the heating source in to the wort.The efficiency is influenced by a variety ofdesign characteristics such as: heating area material of construction steam pressure (which directly relates to

temperature).Traditional kettles were made from copper(hence their name) and copper has a muchbetter heat conductivity than stainless steel(the current preferred material of construction)see Table 2.

However as each brew is boiled smalldeposits of caramelised wort along withprecipitated mineral from the hardness in thewater are deposited on the heating surfacebuilding up a fouling layer, which acts as abarrier to heat transfer. This fouling layer has amuch greater effect on heat transfer than anymaterial of construction and is the principalresistance to heat flow. The formation offouling on the wort side of the heater results ina steady fall off in heat transfer with each brewwhich can be followed by a decrease inevaporation. See Figure 7.

Figure 3: Jacketed Asymmetric Kettle

Wort boiling Time (mins) Flow rate (hl/h) Steam Pressure (bar) Evaporation rate (%)

Heating up 40 650 1.5 1 Boiling 40 500 1.1 2 Whirlpool rest 15 - - - Stripping 50 120 1.2 1

TABLE 1: COMBINED WORT BOILING & STRIPPING TYPICAL OPERATING CONDITIONS

Figure 4: Externalwort boiling withThermosyphon

TECHNICAL SUMMARY

28June 2002 The BREWER International www.igb.org.uk

The key factor in reducing fouling include: Soft water (ie; low hardness/ carbonates) Whole hops (rather than pellets or extract) Lower wort original gravity Low differential heating temperature (hence

moderate heat flux) Avoiding excessive energy input, especially

short term peaks Thorough mixing of liquid adjuncts prior to

entering the heater Turbulent nucleate boiling (rather than film

boiling).It follows for any kettle processing more thanone brew between cleaning, and boiling to aconstant time, there will be a difference inevaporation rate between the first and the finalbrews.

To ensure a constant evaporation isachieved, some systems control wort boilingby the mass of steam delivered. This can beintegrated so that it is evenly supplied throughthe allotted boiling span by means ofproportional steam control value, thusensuring that the evaporation rate is constantregardless of copper volume.

Other systems control evaporation rate bythe increase in original gravity or decrease inwort volume, or a combination of bothsystems.

Reducing the energy consumptionduring wort boilingAll the sensible heat supplied to heat theincoming wort from lauter transfer (around78C) to boiling (at just over 100C) will be

recovered fromwort coolingthrough the heatexchanger orparaflow.

It is generally theenergy supplied toevaporate thewater (plusvolatiles) from thewort which is notso easily recovered. The best way to reducethis energy demand is not to use it in the firstplace, and there has been a gradual reductionin evaporation rates from 10 to 12% per hourfor a 90 minute boil (amounting to a total of 15to 16% evaporation per hour) to 5 to 6%evaporation for 60 minutes. This has beenbrought about by designs and process controlchanges detailed above.

There are a number of ways in which thebrewer can recover or re-use the energy usedduring evaporation.

A number of heat recovery systems producehot water and the effectiveness of the systemdepends on the brewery being able efficientlyto utilise the low grade hot water recovered.

The typical schemes used recover the latentheat of evaporation from the wort boilingprocess may be grouped into three types:

1. Recovery of energy for use outside thebrewhouse, e.g., either by a simple condensersystem exporting hot water or usingabsorption refrigeration;

2. Recovery ofenergy for use in thebrewhouse, e.g.,using hot water froma vapourcondenser/energystore system for wortpreheating prior towort kettle;3. Recycling energywithin the wort boiling

process using either mechanical vapourrecompression (MVR) or thermal vapourrecompression (TVR).

SummaryWort boiling has the highest energy demand ofall brewing operations, and hence has beensubject to considerable research into ways ofreducing its energy consumption. The primeenergy used to heat the wort to boiling point isrecovered through the wort coolers for re-usein brewing.

It is the energy used to evaporate the waterwhich is more difficult to conserve. Over thelast three decades evaporation rates havefallen by a factor of three, through betterprocess operations and improved kettledesign. The opportunity for further decreasesno longer exists and brewers are looking atways of recovering the energy used inevaporation and either recycling it in theboiling process or using it as a source ofenergy for other processes in the brewery.

Further Reading1. Moll Beers and Coolers2. Hough, Briggs and Stephen Malting and

Brewing Science3. O`Rourke The Brewer 19944. Wilkinson R. Ferment p 397 Vol 4 No6 Dec 19915. European Brewery Convention Manual of Good

Practice Wort Boiling and Clarification.

Figure 5: Wort boiling combinedwith wort stripping ( Merlin system)

Figure 6: Continuous wort boiling system.

20C 100C 200C 300C

Copper (pure) 396 379 374 369 Ferritic Stainless Steel 25 25.5 - 27.5 Austenitic Stainless Steel 16.3 17 17 19

TABLE 2: THERMAL CONDUCTIVITY k FOR DIFFERENTMATERIALS OF CONSTRUCTION (W/mC)

FIGURE 7: FALL OFF IN EVAPORATION WITH SUCCESSIVE BREWSBETWEEN A CLEAN

% evaporation from a standard boil for each brew as measured from theweight of steam supplied. Source: ORourke The Brewer 1984.