Dairy cooling

BUCO ice water cooling in the dairy industry

Ice water of 0.5ºC has a very high cooling capacity, which means that heat can be transported at relatively low flow rates compared to other coolants. Very high heat transfer coefficients (up to 2000 W/m² K) are achieved in our ice water systems, while ice water temperatures close to zero degrees (0.5ºC) can be reached without compromising the physical integrity of the equipment and without the need for sophisticated regulation and control methods to prevent the water from freezing, as is required in other systems.

Ice water cooling in the dairy industry is the most hygienic, effective and safest method of lowering the temperature of products under the hygienic conditions required by the food industry. Ice water has a very high cooling capacity, which means that heat can be transported at relatively low flow rates compared to other cooling media. The thermodynamic parameters and the technical properties of the water cycle are favourable, so that very high heat transfer coefficients are achieved. The production of cold water and the cooling with this water have in any case the physical limit of the freezing point of water. On the one hand, one wants to work with water at temperatures as close as technically possible to its freezing point in order to lower the temperature of the product to be cooled as much as possible, but on the other hand, problems with ice formation increase as they approach zero. The well-known anomaly that water has the lowest specific volume at +4ºC causes its volume to increase when it freezes, which in turn can have destructive effects on the components of the installation (breakage of pipes, heat exchangers, pumps, etc.). In addition, ice formation is associated with increased energy consumption, as the ice acts as an insulating layer and reduces the heat transfer coefficients. For the production of ice water with a BUCO falling film chiller, it allows to reach temperatures as close as possible to zero degrees Celsius, but insensitive to ice formation.

Falling Film Chiller for direct cooling

The BUCO falling film chiller fullfills the requirements mentioned in the previous point. Very high heat transfer coefficients (up to 2000 W/m2 K) are achieved. It is resistent to possible ice formation. Since the water flows around the outside of the heat exchanger plates, accidental icing will not cause any damage. Temperatures close to zero degrees (0.5ºC) can be reached without compromising the physical integrity of the equipment and without the need for sophisticated regulation and control methods to prevent the water from freezing, as required by other systems. It is very easy to clean. All surfaces to be cleaned are easily accessible, even during operation. For example, high-pressure equipment can be used. It is very sensitive to contamination. It is therefore very suitable for cooling process water in circuits. This falling film cooler has the advantage of eliminating the cost of regulation and control. In order to achieve a certain degree of safety, plate heat exchangers in practice only work at temperatures below +2°C with a considerable amount of control and regulation technology, whereas with the BUCO falling film cooler +0.5°C can be achieved without any problems.

A perennial problem with plate heat exchangers is when water contamination occurs. Internationally, this problem poses a tricky task in the process flow in the case of process water contamination. In practice, it is very advantageous and advisable to draw up a diagram of the cold water demand and plan the power generation plant on the basis of this diagram, taking into account the following points: The minimum consumption (basic consumption) increases. The higher the percentage of this basic consumption in the total consumption, the less sense it makes to accumulate ice. Accumulation reduces the installed capacity of the cooling unit. The components required for ice storage significantly increase the value of the investment (tanks, pumps, evaporator, etc.). The production and accumulation of ice requires lower evaporation temperatures than the direct production of ice water, which has a strong impact on the power consumption. The BUCO ice bank silos offer an excellent opportunity to meet modern market requirements in all aspects of ice water production. In these systems, the plate evaporator is on top and the ice water tank is below. In typical plate evaporators, the evaporating temperature must be kept above 0°C for safety reasons; in the BUCO falling film chiller, the evaporating temperature can be kept at -3/-4°C. The transfer area can therefore be much smaller.

Ice water production through ice storage

A very widespread method of ice water production is ice storage; particularly interesting when the consumption of ice water is subject to strong fluctuations. Peaks in consumption can be compensated for by the cooling units by accumulating ice during periods of low consumption. The traditionally applied system of ice storage during night hours has limited applicability nowadays for the following reasons: Increasing sensitivity of energy costs. Increasingly complex technologies in production processes. The trend towards more efficient use of capital equipment. Increasing environmental and safety requirements. Today‘s dairy processing plants require highly flexible cold water production, temperatures generally below +1°C, low production costs and minimal refrigerant (ammonia) content. The practice of producing and storing ice to obtain ice water by taking advantage of reduced night tariffs is no longer valid. It is now necessary to adapt to complicated production processes, avoid consumption peaks, take advantage of off-peak periods and, above all, optimise investment and operating costs. In plain language, this means that in addition night storage, day storage and direct cooling at night and during the day must also be taken into account, i.e. the system must be optimised by finding the best solution between direct cooling and ice making and consumption during the day and at night. To find the most favourable solution, the cold water consumption must be analysed in detail according to the time of day and day of the week. Based on our practical experience, we can make the following observations: There are usually large differences between the hours of the same day. The differences between daytime and nighttime consumption decrease in modern dairies. There is a certain minimum consumption during the 24 hours of the day. There can be considerable differences between the individual days of the week. It is advisable to plan and build the cold water production with sufficient reserves, as consumption generally increases rapidly as soon as the plant is put into operation. In our BUCO ice bank silos, ice water can alternatively be produced or flake ice can be generated. To produce ice, water is pumped from the bottom of the ice bank silo to the water distributor above the evaporator; the water runs down the outside of the evaporator‘s vertical plates in a film and forms ice. Once the predetermined thickness (6-8 mm) is reached, the hot gas defrost system is started (the cooling circuit is divided into three zones for this purpose). The ice is released, falls to the bottom of the silo and floats in the water. This process is maintained until the programmed water-ice mixture and fill level are reached. The ice silo would then be full and ready to start the ice melting process to produce ice water.

During the melting period, the ice bank silo is connected to the industry‘s production process. The inlet to the consumption is above the ice level, while the return is led to the bottom of the silo by connecting to a distributor. In this way, forced circulation is achieved from bottom to top through the entire area of suspended ice. The return water is cooled to approx. 0°C on contact with the chip ice pieces. Compressed air is blown into the silo for better distribution of the return water. This ice in suspension has a much larger melting surface than that of storage coil-in-tanks, which means that the maximum melting capacity, i.e. the instantaneous ice water production capacity, is very high. This is of great importance as it leads to a high flexibility of the system to absorb sudden fluctuations in the return temperature of the consumers. The main advantages of these silos can be summarised as follows: Due to the large ice surface in the melting period, a high flexibility in operation is achieved. The required Nh3 content is very low. It is only about 1/8 of the amount required for conventional coil-in-tank ice storage. Since these BUCO ice bank silos offer the possibility of producing and storing ice, defrosting at high speed and direct cooling, a control system can be programmed to supply the consumer lines exactly as required. By using direct cooling, the capacity of the system can be expanded with little investment. This is not possible with any other storage system. The evaporation temperature during the icing period is almost constant with these systems (-6°C and -10°C), whereas with conventional coil-in-tank ice storage the temperatures must decrease with increasing ice thickness (-15/-17°C with ice thicknesses between 35 and 50 mm). The consequences of using a BUCO ice bank silos are smaller cooling units and lower consumptions compared using conventional coil-in-tank ice storage. Cleaning our BUCO ice bank silos is also very easy. These silos are characterised by simple assembly and disassembly and can therefore be moved without great effort.

Combined ice making features

The combined functions of ice production, accumulation and direct cooling mentioned above are not the only ones in these BUCO ice silos. Other variants can also be set up.

Using existing tanks

In other cases, existing tanks have been used. For example, when the conventional coil-in-tank-systems are in poor condition, but the tanks are still in good condition. The conventional coils are removed so that the entire tank is available for ice and ice water storage. This has been done in several Nestlé factories in France. For this purpose, BUCO falling film chillers and BUCO ice makers were placed on top oft he old coil-in-tank ice storage  tank. The return water is first passed through the falling film chiller and its temperature is lowered to approx. 1°C. The return water is then fed into the ice storage tank. Immediately afterwards, it is fed into the BUCO ice makers where the ice is produced. After defrosting, the ice falls into the tank and is distributed in the tank by forced circulation. The ice remains suspended in the water in the tank until it is melted off in the form of ice water at 0.5 °C. In several Gervais-Danone factories in France, these ice water and ice production combinations are also used to increase flexibility in todays‘ process flow.

Conclusion

With the above descriptions, we have tried to adapt ice production and direct cooling using the falling film system to the current requirements of modern dairies. The production of water at a temperature of 0.5 °C is possible without any problems. The falling film cooling with plates is highly efficient, robust, easy to regulate and inexpensive. As described above, the BUCO ice bank silo offers great flexibility and is characterised by the fact that it requires a very small volume of refrigerant. A combination of ice storage and direct cooling is economically very attractive, as we have shown with the various options.

We have always assimilated engineering science and thermodynamics optimally in the various manufacturing processes.

Thermodynamicists,mechanical engineers and welding engineers define the dimensioning, design and construction of customised heat exchanger panels and systems in materials ranging from mild and austenitic steels through to titanium, and ensure successful distribution of their work worldwide.

In doing so they fall back on production engineering expertise and calculations developed in the course of the past hundred years that are still being continuously optimised in an ongoing process.

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