The production of cooling (= low temperature heat) occurs in the brewery almost exclusively by means of compression refrigeration plants, and more seldom in absorption plants. Compressed liquid ammoniac is thereby evaporated. To evaporate, ammoniac requires heat energy. The ammoniac draws this heat from its surroundings, which are cooled as a result. There are two possibilities for cooling:
- the ammoniac is directly evaporated in the cooling pipes or cooling pockets (direct evaporation cooling) or
- the ammoniac is evaporated in an evaporator, thereby cooling down a cooling agent, usually glycol, which is in turn fed through the cooling pipes or pockets (indirect evaporation cooling).
Cooling with chilled water at 0℃ is not possible because at the end a temperature of 0℃ has to be obtained and for heat exchange to take place a temperature difference of 3 to 4℃ is necessary.
Direct evaporation cooling
In the case of direct evaporation cooling, the cold liquid ammoniac from the NH₃ cutter (3) is introduced into the segmented pipes of the cooling zone from above. While the ammoniac evaporates, it extracts heat from the interior of the tank and cools it. The evaporated ammoniac is passed to the NH₃ compressor (4) via the cutter (3) where, on heating, it is compressed. In the evaporation condenser (1), the (35℃) warm NH₃ is liquefied again through cooling and reintroduced to the cycle via the collector (2).
Indirect cooling using glycol
In the case of indirect cooling, the ammoniac circuit (1-4) is connected to a separate glycol circuit. The glycol, which at -1℃ is relatively warm, is cooled to -4 to -6℃ in the NH₃ cutter and is first put into a glycol tank (6) used as storage. From there it is introduced into the segmented pipes of the cooling zone from below and heats up, whilst the contents of the tank cool down. The heated glycol is then put back into storage and reintroduced into the cycle.
The advantage of direct evaporation lies above all in the low specific electrical energy costs, which results in considerable energy cost savings (up to 40%).
In contrast to indirect cooling by means of glycol, there are a number of advantages:
- the glycol circulation stage is unnecessary,
- it is possible to work with higher evaporation temperatures (-4 to -6℃) instead of -10℃,
- substantially smaller pumps are required since less has to be transported,
- substantially smaller supply pipes are required,
- this results in considerably lower insulating and installation costs,
- temperature control is more accurate and the system is more flexible.
Disadvantages on the other hand are:
- higher operating pressures in the conveyors, resulting in higher costs,
- evaporation temperatures are not constant,
- the plant can barely be used in a stationary state,
- large amounts of refrigerants are necessary, which in some countries require special approval (Germany: << 3t NH₃ requires special approval,
- relatively high fittings expenditure for safety reasons,
- danger of loss of refrigerants,
- hardly any possibility of storing cooling energy.
In the case of indirect cooling, nowadays ethylene or propylene glycol solutions are almost always used, which are set to be frost-proof to -10 to -15℃. This procedure also has advantages and disadvantages.
The advantages are:
- the lower operating pressures in the heat transfer surfaces,
- a uniform burden of the cooling plant when using cold storage,
- constant evaporation temperatures are possible, which are 3 to 4℃ below the set-point value temperature,
- considerably less ammoniac is required.
Disadvantages are:
- the considerably increased energy requirement of the cooling plant,
- the larger pipe system and pumps, as well as
- the other advantages of direct evaporation cooling.
In general, the advantages of direct evaporation prevail so the majority prefer this solution.
