Exclusive interview with Paul Kidger on refrigeration needs of breweries

ammonia21.com: What share of a brewery’s electricity bill does the refrigeration plant typically account for in your experience?

Paul Kidger: Refrigeration plays an important part of beer processing and is the major user of electricity, accounting for approximately 50% of the total use. However this does depend on the brewery, the season and its geographic location. For a typical brewery having an annual production of 1 million hectolitres of beer per year, 3.5 million kWh of electrical energy will be needed for the refrigeration plant.

ammonia21.com: How would a typical refrigeration plant in a British brewery look like? Where does ammonia come into play?

Kidger: The majority of breweries have a centralised refrigeration plant, distributing refrigeration around the site in the form of a secondary refrigerant at -5°C. Ammonia still finds widespread use as the primary refrigerant, probably for historical reasons. The breweries were originally equipped with ammonia plants, therefore when renewals and expansions were made, it was easier to retain ammonia, not least because the operator and maintenance skills were familiar with the material. Furthermore ammonia is a particularly effective refrigerant at the conditions found in most breweries and from this aspect, there was little reason to change. Typically the plants would comprise of a compressor, often reciprocating but later the twin and single screw machines were installed in larger plants, evaporative condenser, expansion valve and shell and tube evaporator, which held the majority of the ammonia inventory. In large plants, this inventory could be as much as 2 tonnes per refrigeration unit, of which there could be 8 such units in each plant room. More recently plate evaporators and in some cases plate condensers (in conjunction with a conventional evaporative cooling tower) have been used thereby drastically reducing the ammonia inventory and improving efficiencies by reducing approach temperatures.

ammonia21.com: What are the standard and most effective measures to reduce energy consumption of a brewery’s refrigeration plant?

Kidger: A typical plant would operate with an evaporation temperature of -10°C and a condensing temperature of 30°C. However, in the energy conscious environment, it has been possible to reduce this range. Evaporation temperatures can be increased, by changes to the process, for example the use of plate evaporators to reduce approach temperatures and by increasing the temperature of the secondary refrigerant service. Reductions in the condensing temperature have been possible by increasing the water treatment/descaling regime in existing condensers and by installing condensers of increased size when replacements become due.

As with all ammonia plants, the removal of oil from the system ‘low-points’, is vital. Since the ‘low-point’ is the evaporative condenser, there is a real risk of obscuring part of the heat transfer surface with oil thus reducing the effective available heat transfer area.

As with most refrigeration plants, these improvements are very effective since they represent a ‘Double Gain’ situation in that more refrigeration becomes available and the Coefficient of Performance is increased.

Another major area of saving has been to reduce the level of energy needed to circulate the secondary refrigerant. This energy is termed a ‘parasitic load’ on the refrigeration plant because it reduces the plant’s output and needs additional energy for its removal. The use of variable speed pumps can have a remarkable effect at reducing such loads. As a general guide, a variable speed pump will save about 20% of the energy used by its constant speed equivalent.

ammonia21.com: Breweries necessitate different temperature levels throughout the brewing process. Could you take us through the various steps involved?

Kidger: Within the brewery, refrigeration is needed for the following duties:

Firstly, refrigeration is required to reduce the temperature of the wort (unfermented beer) from the previous boiling stage to between 7°C and 18°C (depending on the beer produced) in readiness to accept the yeast. The majority of this cooling process is achieved by heat exchange in a plate heat exchanger, with incoming cold water. A well designed wort cooling system is at the heart of an energy efficient brewery since it reduces refrigeration needs and provides a vital source of hot water for the brewing process. However, the limitations of heat exchangers mean that water will only provide cooling to within about 3°C of the incoming water temperature. In turn the water temperature will vary according to the source. Ground water is at an almost constant temperature of about 10°C but surface sources will vary seasonally. Refrigeration is therefore needed to ‘trim’ cool the wort to the required temperature. The production of traditional ales will need little ‘trim’ cooling but lagers generally, need the lower wort temperature, This refrigeration load tends to be a large demand of short duration. Some breweries have successfully installed storage systems, based on either chilled water storage or ice banks storage, to convert this load to a more constant profile which in turn has lead to better plant utilisation and reduced operating costs.

Fermentation is an exothermic process. Without cooling, the temperature of the beer would rise at a rate of approximately 0.5°C per hour. The final character of the beer is determined by the fermentation conditions therefore accurate temperature control is necessary. Typically this would be in the range of 10°C to 18°C but does depend on the style of beer. Traditional ales were fermented in open square vessels and cooled by coils immersed in the beer. Cooling was provided by well-water which passed through the coils and then to the drains or local river: not very good for water conservation, but excellent at minimising refrigeration needs. In modern breweries, fermentation vessels are vertical cylindro-conical vessels. Cooling jackets, through which the secondary refrigerant solution flows, are incorporated into the vessel shell so as to preserve the smooth, hygienic internal surface. Although not practiced in UK breweries, some fermentation units employ direct expansion cooling with ammonia at approximately -5°C circulating between the vessel jackets and a local liquid separation vessel. However, this does have safety implications since the ammonia is no longer confined to the plant room and it means that the jackets must be designed for pressures in the order of 16 bar thereby increasing construction and statutory inspection costs.

After fermentation, most beers are conditioned or matured. Exceptions to this are some traditional English Ales which are filled into the casks directly from the fermentation vessels. Maturation involves cooling the beer to approximately 0°C (the freezing point is approximately -4°C) and maintaining this temperature for a minimum of 3 days, often longer. The traditional ‘lagering’ process involved storing the beer for weeks or even months at a low temperature during which time, the characteristic flavours developed. Initial cooling is achieved in the fermentation vessel but since the inversion point is approached, the rate of cooling becomes very slow. The final 5° C are usually provided by transferring the beer to a dedicated Conditioning or Maturation vessel through a plate heat exchanger supplied with secondary refrigerant.

Following maturation, refrigeration needs are relatively small. Prior to filtration, beer is ‘trim-cooled’ to approximately -2°C in order to precipitate as much as possible of the potential haze forming solid matter. Again this is achieved by a plate heat exchanger against the secondary refrigerant.

Beers, which are to be filled into kegs and sometimes sterile-filled bottles, are bulk pasteurised. This is a process in which the beer is heated to approximately 80°C, held then cooled in readiness for packaging. The process takes place in a 3 section plate heat exchanger in which over 95% of the heating and cooling is provided by heat exchange between the incoming and outgoing streams. Incoming beer is pre-heated by the outgoing beer then given additional heating by steam or hot water before entering the holding tube. After 30 seconds, the beer is returned to the plate heat exchanger to be cooled by incoming beer then secondary refrigerant in the final section. This final cooling is required in order to achieve a sufficiently low temperature for packaging. Without this cooling stage, the dissolved carbon dioxide would cause excessive foaming and make filling impossible.

About Paul Kidger

Paul Kidger has been associated with the UK brewing and malting industries for 40 years, mainly in connection with the use of energy and especially refrigeration systems. In 1985 he formed Newton Energy Management, of which he is the Principal Consultant. This consultancy specialises in helping companies achieve considerable cost savings by better management of their energy use.