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Refrigeration of dairy products plays a crucial role in sustainability
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Monday, 31 May, 2021, 13 : 00 PM [IST]
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Sandip Kansara
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Dairy is an indispensable part of the global food system and it plays a crucial role in the sustainability of rural areas in particular.
It is a well-known fact that the dairy industry actively contributes to the economies of a number of countries. An increasing demand worldwide is noticeably emerging at present, and the industry is globalising. Milk and dairy products are very essential for human nutrition and development, especially, in children.
Although milk is a highly nourishing food, raw fresh milk is highly liable to rot and can be easily spoiled by the growth of microorganisms. Fresh milk is collected from the farm, transported to cooling centres to prevent spoilage, then to processing units to produce other dairy products and finally delivered to the consumers in several ways.
The transportation of fresh milk from farms to cooling centres and processing units may take time. Consequently, cooling milk in time becomes major problem associated with raw fresh milk. The milk should be cooled within three to four hours of collecting it from the farm, which otherwise leads to spoilage. Thus, refrigeration plays a vital role in dairy industry.
A variety of products are handled by dairy plants such as butter, ice cream, curd, condensed milk, butter milk, flavoured milk and cheese. The heating and cooling requirements for different dairy products and processes. In order to preserve quality and prevent spoilage, the warm and fresh milk should be cooled immediately after milking. The fresh milk should be rapidly cooled to 10°C within two hours of milking and to 4°C within three to four hours.
In many temperate and tropical countries, where refrigerated cooling systems may not be available at the producer or milk collection point, the simple small scale methods for cooling milk to 10°C and below can be used.
Some methods are as follows:
• Evaporative cooling using a charcoal cooler
• Cooling with natural water systems – mains, well or groundwater immersion cooling methods include placing the milk cans in a stream, river, lake or tank
• Surface milk coolers (open cooling systems)
• Refrigerated immersion cooler or cooling rings
As soon as the fresh warm milk arrives at the milk cooling centres, it should be cooled to 4°C. This cooling requires considerable use of energy, suitable refrigeration equipment and insulated storage tanks designed specifically for milk. The most commonly used refrigeration system used in milk cooling equipment for milk cooling centres is vapour compression refrigeration system.
The basic refrigeration system is made up of a refrigerated bulk tank, a refrigeration compressor unit and an air-cooled condenser unit. A typical bulk milk cooling tank. The bulk milk tanks are double-or-triple walled. The compacted polyurethane foam or expanded polystyrene is used for thorough insulation to keep the milk cool for at least 12 hours with a temperature rise of not more than 1°C at an ambient room temperature of 30°C.
The reciprocating type compressor is the most common, which can be open, semi-hermetically sealed or hermetically sealed. Now-a-days scroll compressor is being increasingly used as it is more efficient and uses up to 20 percent less electricity than reciprocating compressors. Condensers use natural or forced air, water or oil to cool the refrigerant. The naturally air-cooled condenser is the most commonly used condenser. The purpose of the condenser is to condense the refrigerant gas by removing the heat.
Evaporators are commonly made from copper and located close to the source of the heat to be removed. The compressor and condenser assembly are mounted on the same support frame as the milk tank or on a separate frame adjacent to the tank. The room in which it is located must be well ventilated to ensure that the refrigeration system operates efficiently.
In very hot countries, the compressor and condenser assembly can be mounted on an external wall of the milk cooling centre building which improves the efficiency of the system, ensure faster cooling of the milk and reduce energy consumption.
The refrigerants used for refrigeration in dairy industry should be in agreement with recent international agreements related to atmospheric pollution, by the gases that contribute to global warming and ozone layer depletion. Natural refrigerants are recognised to be potential permanent solution to phase out the use of synthetic refrigerants which are being restricted progressively.
However, there are many challenges associated with the implementation of natural refrigerants. In India, ammonia based vapour compression refrigeration systems are the most preferred mode for cooling in milk processing plants.
Ammonia based systems are low pressure systems with very less sophistication. They have good heat transfer properties, low cost and high efficiency. Leakage in ammonia system is easily detected due to pungent odour of ammonia.
Ammonia also has zero ODP and low GWP. CO2 is another natural refrigerant which is gaining prominence in the recent days. Compared to other refrigerants, the most remarkable property of CO2 is its low critical temperature (31.1 °C) and high critical pressure (7.1MPa).
Energy requirement for refrigeration
The dairy industry ranks fifth among the most energy-intensive industries after oil, chemical, pulp and paper mill, and iron and steel making industries. The industrialised countries have modern large-scale milk processing plants with about 100 tonne of milk intake. Even smaller plants with a daily intake of about 30 tonne of milk are generally equipped with modern machinery. The energy requirement in modern milk processing plants.
Also, the monthly electrical energy consumption (kWh) for 22 farms over 12 months (mo) for all major energy-consuming processes in the dairy industry. The energy requirement in the refrigeration sector plays a very significant role in the overall energy requirements of a modern milk plant, often constituting above 40 percent of the total electric power consumption. This makes research on the reduction of energy required for refrigerating the dairy industries interesting and challenging.
Methods to reduce energy consumption
There are several methods identified that can be added to the milk cooling systems to reduce the refrigeration requirement and to capture waste heat:
• The condenser temperature should be as low as possible. For this, the correct size of the condenser should be ensured. A small condenser for the refrigeration indicates a small initial outlay, but running costs increase greatly. A condenser that is oversized, however, can sub-cool the refrigerant and affect the function of the expansion value.
• The evaporator temperature should be as high as possible. To achieve this, the evaporator should be of correct size. The evaporator should be kept clean and defrosted when necessary, especially, when cooling air to below 0°C, as ice can build up on the coil. Hot gas from the outlet of the compressor can be used to defrost freezers, but control must be accurate. The defrost water may then be used elsewhere in the plant.
• The compressor should match with the load. If a compressor is oversized, it will operate at only partial load, and the energy efficiency may be reduced. A sequencing or capacity control system to match the compressor with the load could help to improve efficiency.
• The precooling of warm fresh milk using mains or well water will reduce the energy requirements of the refrigeration system to certain extent, thereby, reducing the cost of cooling. Precooling can be done by using both mains/well and chilled water in one operation. The chilled water alone increases the cooling rate and helps to maintain the milk quality.
• Refrigeration heat recovery (RHR) units are used to make a refrigeration system more efficient by collecting heat which is normally released into the air and using it for heating water which is used for various other purposes in the plant. The pre-coolers and RHR units are competing technologies. It is usually more cost effective to use a RHR than pre-cooler.
• The cleanliness of a farm’s refrigeration system plays an important role in its maintenance. The coils that are dirty will reduce efficiency and increase operating costs. So, most of the systems are provided with a watch glass that can be used to determine if the refrigerant needs to be recharged.
• Leakages of refrigerant can reduce a system’s efficiency by 40% and should be kept to less than 2% of the annual charge.
(The author is owner of Sandip Cream Centre)
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