Wort boiling — initial state and energy efficient alternatives

The wort boiling is the key process within a brewery. Because of the high thermal energy consumption and the high temperature level, two heat recovery systems that can cover the hot water demand of the whole brewhouse or even the whole brewery are usually installed.

Within the last 100 years, there has been a continuous development of the wort boiling process by the brewing industry. The main objective was to reduce the amount of water that has to be evaporated, which results in shorter boiling time and reduced energy demand. So it was possible to reduce the amount of evaporated water from more than 16% (equals a boiling time of more than 120 min) to 3..5% (35..50 min), while increasing the quality of the produced wort. At present, there are lots of boiling systems available that are offered by different companies. These systems differ mainly in the used boiling copper, its position within the brewhouse, the process control (continuous or batch), heat exchangers, pressure and temperature profiles as well as used heat recovery installations [9].

The wort boiling at the Hutt brewery takes place by so called classical internal boilers at atmospheric pressure. In the beginning, the wort is heated from 74°C to boiling temperature by indirect heating of the outer surface of the wort copper. After reaching the boiling temperature, the wort flows several times through an internal boiler that is placed directly in the copper. While passing the tube bundles of this boiler, the wort is heated under pressure to about 101..105°C.

While leaving these tubes, the wort starts to evaporate. Compared to the state of the art within wort boiling, this boiling system is relatively simple and old. It consumes a high amount of thermal energy, due to the required time and temperature for atmospheric boiling, the relatively high amount of water that has to be evaporated and the missing insulation of the boiling copper [10].

To clarify the operating mode of heat recovery installations within a brewery, figure 2 displays the initial state of the wort production at the Hutt brewery in a simplified way. Two hot water storage tanks, each 50 m3, are installed to cover the hot water demand of the whole production process, including filtration and bottle filling hall. These tanks are connected in serial and charged by two heat recovery installations. The first storage tank is fed by the heat exchanger within the process step of wort cooling and has a temperature level of maximally 80°C. The second store is fed by a tube bundle heat exchanger that condenses the vapours which occur during wort boiling. Two modes of operation can be chosen: heating cold brewing water to 80°C, or increasing the temperature of the already stored hot water. All consumers of the brewery are fed by the second storage tank with the higher temperature level. The main hot water load is caused by mashing (58°C) and lautering (78°C). Other consumers with lower hot water consumption are cleaning processes, sterilisation and keg filling. If process steps require lower temperatures than the storage tank temperature, the stored water is mixed with cold water. The consumed amount of water from store number two is settled by the first storage tank. An additional heating device that runs with steam ensures a set temperature of minimum 80°C in the upper part of storage tank number two during weekends or longer periods with no heat recovery.

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Fig. 2. Initial state in the brewhouse at Hutt brewery.

As mentioned before, there are multitude wort boiling systems available. These systems differ in the way of heating lauter wort and the used boiler. The overall objectives for this process step are to assure a gentle and rapid heating of lauter wort and gentle wort boiling with low shear forces. Further on, large evaporator surfaces, good circulation and mixing of wort and a limited but sufficient boiling time is requested [11]. Based on these and even more requirements, every brewhouse manufacturer has its own solution for the ideal wort boiling system. These can be classical internal or external boilers, thin-film evaporators, dynamic low-pressure boiling, high — temperature wort boiling, secondary evaporation under vacuum or downstream thin-film evaporation [10]. All of these systems vary a lot in the overall energy consumption, due to the boiling process as well as the heat recovery installations, which influences the water balance of the whole brewery.

Based on the relatively old and inefficient atmospheric wort boiling at the Hutt brewery, the technical management planned to implement one of these new boiling systems. During the planning phase, two different boiling systems were under consideration: a vacuum boiling system and dynamic low-pressure boiling. The vacuum boiling is characterised by a special geometry of installations and two boiling phases (atmospheric and vacuum) that are adjustable at will and cause a reduced evaporation. The whole boiling procedure takes place in a cycle consisting of a storage vessel, an external boiler (calandria) and an expansion evaporator in which a vacuum can be applied depending on the respective boiling mode. A tube bundle heat exchanger will be used to condense the vapours and pre-heat cold water. The main advantage of this boiling technology is the reduced effort for implement all installations in the existing brewing process. Besides savings of thermal energy, this boiling system shows an increased electricity demand for generation of
vacuum. The dynamic low-pressure boiling represents boiling at slight positive pressure with periodically increased and subsequently reduced pressure. An internal boiler is mostly used for heating and boiling. This boiling system is combined with a special heat recovery system that uses the condensation enthalpy of vapours to pre-heat the lauter wort. This heat recovery system consists of a common tube bundle condenser, special high temperature storage and an additional heat exchanger for the lauter wort [12].