Evaporation in beer production
- 1 Wort Boiling
- 2 General Description
- 3 Types of Wort Boilers
- 4 Reducing the energy consumption during wort boiling
- 5 Other Energy Saving Technologies
Wort boiling has the highest energy requirement of any of the brewing processes. It can account for as much as 60% of the total steam demand of the brewery (depending on the type of packaging operations). It is therefore hardly surprising that a great deal of effort has gone in to reducing energy consumption and recovering energy from boiling.
Wort stabilisation involves the boiling and evaporation of the wort (about a 4-8% evaporation rate) over a 1 to 1.5 hour period. Wort boiling has a number of aims: to sterilize and stabilise the wort, enzyme inactivation, flavour and colour formation, removal of unwanted volatiles, acidification of the wort, wort concentration and, in the case of most beer production, to extract the bittering, flavour and aroma from hops. At the end of boiling, the hot wort is quickly cooled to a temperature favourable to the yeast. Once sufficiently cooled, the yeast is added, or "pitched", to begin the fermentation process.
Types of Wort Boilers
Traditionally, wort was boiled in direct-fired kettles, often made of copper, since this metal has particularly good heat transfer properties. Because the heat source was localised at the bottom of the kettle, it restricts the volume of wort which could be boiled at any one time to a maximum of 200 barrels (330 hectolitres) which probably explains why traditional breweries with larger brew-lengths used number of separate smaller size kettles. The principal disadvantage of traditional direct peat or coal fired kettles are that they are relatively inefficient in heat transfer and tend to be labour-intensive. The heating surface of the copper becomes very hot and tends to promote caramelisation and burning of the wort, requiring frequent cleaning usually every 2 to 5 brews to ensure effective heat transfer is maintained. High evaporation rates were required to produce sufficient vigour or turbulence in the boil and typical boils would take over 90 minutes with an evaporation rate over 10% per hour.
Kettles with Internal Heating Systems
The advent of steam coils and internal heating systems allowed the production of larger kettles, as it enabled the designers to provide a larger heating area and, because it was surrounded by the wort, the heat transfer was more efficient. In many designs the heaters were upright and located in the centre of the kettle to give a turbulent boil. Some of the kettles also include base steam coils for preheating the incoming wort, and to avoid the creation of dead spots within the kettles. The disadvantage of internally heated kettles is that the heaters tend to be difficult to clean with conventional CIP, and were often manufactured from copper, which is dissolved by caustic cleaning. The internal coils in particular are prone to corrosion, which can result in steam leaks in to the boiling wort which are difficult to detect and repair. Because wort circulation relies on thermal currents within the kettle the turbulence over the heating surfaces is sometimes limited, resulting in wort caramelisation, which requires more frequent cleaning to ensure effective heat transfer is maintained.
To overcome the difficulties with cleaning internal heaters, kettles with external heating jackets were designed. One of the most prolific designs was the Steinecker Asymmetric Kettle. They are generally made of stainless steel and achieve a rolling boil through the location of the heating jackets on one surface. They suffer from similar problems to the direct fired kettles in achieving effective heat transfer, with the higher volume kettles being rather long and thin. They require mechanical paddles to achieve the necessary agitation for a satisfactory boil. This design overcomes the cleaning problems of the kettles with internal heaters, and has a lower tendency to foul, but it still requires cleaning every 6 to 12 brews to ensure effective heat transfer is maintained. These kettles are also prone to fob formation during boiling and often use a cold air draught over the wort surface and an extractor fan to keep fob under control.
Kettles with External Wort Boilers
A more modern design uses an external heater (external wort boiler) which takes the wort out of the kettle and passes it through a shell and tube or plate heat exchanger for heating. These wort boilers achieve high heat transfer through two phase flow and nucleate boiling, and operate at low steam pressure (at 3.0 to 3.5 bar) to heat the boiler. In these kettles vigour can be introduced mechanically, by wort circulation, and the classical 10% evaporation/hour with a 90- minute boil, can be reduced to 5% to 6% evaporation/hour with a 60-minute boil without loss of wort/beer quality. This represents a considerable saving in energy. These kettles have other advantages over internal heaters since pre-heating can start once 15% of the total kettle contents have been collected, allowing the kettle to boil immediately it is full, thus improving vessel utilization. Since low pressure steam is used, the rate of fouling is decreased, allowing more brews to be processed between cleans. One of the negative aspects of external wort boiling involves having to pump the wort, where shear forces may damage the floc formation (trub or hot break particles).
Reducing the energy consumption during wort boiling
There are a number of ways in which the brewer can recover or re-use the energy used during evaporation. A number of heat recovery systems produce hot water and the effectiveness of the system depends on the brewery being able efficiently to utilise the low grade hot water recovered. The typical schemes used recover the latent heat of evaporation from the wort boiling process may be grouped into three types:
- Recovery of energy for use outside the brewhouse, e.g., either by a simple condenser system exporting hot water or using absorption refrigeration;
- Recovery of energy for use in the brewhouse, e.g., using hot water from a vapour condenser/energy store system for wort preheating prior to wort kettle;
- Recycling energy within the wort boiling process using either mechanical vapour recompression (MVR) or thermal vapour recompression (TVR).
Other Energy Saving Technologies
= Wort Boiling in combination with Steam Stripping
The heat treatment of the wort and elimination of volatiles is separated into two steps (in a classic brewhouse the wort kettle performs these processes at the same time). In a first vessel, called the formation vessel, the wort is kept at 100°C with almost no evaporation (less than 1%). In this step all processes that involve heat treatment are performed (formation of DMS, sterilization, enzyme deactivation, hop isomerisation etc.) After this formation step the trub is eliminated by a whirlpool. The final step, in-line with the wort cooling, is the wort stripping technology. Wort is pumped on top of the stripper and in counter-flow 0.5% steam is injected to eliminate the unwanted volatiles. The overall evaporation rate is therefore significantly lower than in classic wort boiling. The energy from the steam injected into the wortstripper is partially recovered and reused.
One such system developed by Meura is their “Meurastream”.
The following table compares the Meurastream with a brewhouse without energy recovery and one with the conventional vapour condensation technology (often called the “pfaduko” system). The calculations are made under the same conditions and recalculated to 15°P cold wort.
|CLASSIC BREWHOUSE||PFADUKO BREWHOUSE||MEURASTREAM|
|Thermal energy||26,0 MJ/hl||19,4 MJ/hl||12,6 MJ/hl|
|Excess in hot water||37,8 l/hl||37,8 l/hl||15,6 l/hl|
The table shows that the Meurastream reduces by 52% the thermal energy of a brewhouse without energy recovery and 35% for a brewhouse with pfaduko. Excess hot water is reduced by 60%.
Dynamic Wort Boiling
‘Dynamic’ or ‘low pressure’ boiling involves heating wort under pressure of 150 mbar, equivalent to a boiling temperature of 103°C. When this pressure is reached, it is rapidly reduced to 50mbar and the temperature drops back to 101°C. This takes place at least six times during each boil and the effect produces a flash evaporation with the formation of foam and bubbles within the wort kettle which strips unwanted volatiles and aids coagulation of hot break particles. In order to accommodate the flash evaporation, the copper volume needs to be 30% greater than for a standard system and the wort is circulated 20–30 times per hour.
Both the internal and external boilers can be operated with an increased over pressure during the boil usually up to 1 bar. This elevates the boiling temperature to around 106° to 110°C, which has the effect of accelerating the various wort reactions, and allows the boiling time to be reduced. At the end of boil the excess pressure is released allowing the escape of the volatile compounds. Over pressure kettles are often operated with some form of vapour recovery energy systems. The advantage claimed from this system is that it allows a shorter boiling time and lower evaporation rates than might be considered necessary in a conventional boiling system.
Direct Steam Injection
Steam condenses directly into wort. Advantages include no heater requirements and no fouling. Disadvantage is that the condensate will dilute the wort, offsetting heat input plus steam must be of good process quality.
Wort Boiling in combination with Vacuum Stripping
The combination of a relatively short boiling phase with a low evaporation rate and the vacuum evaporation gives a boiling system with a low energy cost, a reduced thermal load and sufficient stripping of unwanted volatiles. The wort is first boiled for 40–50 min using the existing boiling system. An evaporation of approximately 4% is achieved. Next, the whirlpool is employed as usual. After the rest period in the whirlpool, the wort is led tangentially as a thin film through the by-pass of the existing wort pipe into the vacuum vessel. The necessary vacuum of approximately 0.4 bar underpressure for an evaporation of 2% is produced by means of a liquid ring vacuum pump. After the start and during the flash evaporation process, the vacuum is maintained by vapour condensation. Undesired flavour compaound (e.g. free DMS, Strecker aldehydes, ...) are driven off with the vapour produced. The vapour that is formed during this process is condensed in the vapour condenser with the production of hot water at 80°C. The resulting condensate from the cooler is also used for heating brewing water before it goes down the drain.
- Case Studies
- Murauer (Austria)
- Neuwirth (Austria)
- Gwangiu (Korea)
- San Antonio (USA)
- Gösser Brauerei