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Material can occur in three different forms: gas, solid or liquid. Each of these forms is called a state of aggregation. For example, water (H≈O) can exist as vapour, ice or water. The transformation from one state to another occurs at a stationary point, and at this point the heat content changes, while the temperature does not. The hidden amount of heat is called latent heat.
The stationary point where ice becomes water is called the melting point, with a temperature of 0 °C, and the amount of heat needed to melt 1 kg of ice is 93 Watt. The temperature at which water becomes vapour is called boiling point, or 100 °C at 1 Bar, while the latent heat is 268 Watt. It should be noted that pressure only influences the boiling point, and not the melting point.
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State of aggregation of material.
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- Condensing
- Evaporating
- Melting
- Freezing
- Condensing
- Sublimation
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Many cooling processes involve the evaporation heat of a liquid. If a liquid evaporates, it needs heat. This heat is taken from the surroundings of the evaporating liquid.
An early example of cooling by evaporation can be found in ancient Egypt. Stone bottles, called Gandis, were filled with water. Because of the porous material, some of the water seeped through to the outside of the bottle wall and evaporated. This evaporation took the heat out of the bottle and therefore out of the water inside.
If the milk must be stored on the farm for long periods of time, any cooling method is better than no cooling. However, if cooling facilities are basic and the time required to transport milk to the collection centre or dairy plant is comparatively short, it is advisable to deliver milk as soon as possible to the nearest milk collection centre.
Several systems are available for cooling milk. The simplest systems use water from a main or well. If abundant quantities of well water are available, the milk cans can be immersed in the well. This method, however, is not advisable if the well water is also used for drinking, because the immersion of cans easily leads to contamination of the well. Simple systems of cooling that use water will bring the milk to a temperature only 3 – 5 °C above that of the water. This means that water at a temperature of 11 – 12 °C is able to cool milk to about 15 °C (at the lowest). Apart from the fact that this temperature is still high, water of 11 – 12 °C will generally not be available in warm tropical conditions. Such conditions require artificial cooling with special equipment.
Whenever running water is available, milk can be cooled by putting a perforated tubular ring around the neck of the can of warm milk. After the ring has been connected to the mains, water will spray onto the can and flow over its surface. If ice water from a cold water tank is used, the water should be collected under the can and recirculated, for example, by standing the can on a rack over the cold water tank.
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Surface coolers consist of a series of small-diameter horizontally arranged tubes. Mounted on top of each other, these tubes terminate at each end in a header. The headers connect the tubes, thus allowing the cooling agent to circulate through them.
The warm milk is distributed over the surface of the cooler, i.e. over the set of horizontal tubes, by means of a spreader-pipe or a tray with small openings fitted on the top of the upper tube. Surface coolers may consist of two independent sections on top of each other. The upper section is cooled with water from the mains or from a well, whilst iced water or direct cooling is applied in the lower section. The surface cooling system, also called ‘open cooling system’, is simple, but requires a proper sanitisation programme. Special care must be taken to prevent airborne contamination.
If small amounts of milk have to be collected and transported over long distances, and it is not technically or economically feasible to cool the milk in advance, metal ice-cones may be used. These cones are inserted in the milk cans, so that the rim of the cone rests on the collar of the can and fits sufficiently tightly to prevent milk splashing out during handling and transport. The cone takes up about one-third of the volume of the can. If the cones are filled with crushed ice, the milk can be cooled from 30 °C to 5 – 10 °C during transport. The cones and the ice can be taken to the farms or collection centres by the milk transport truck. The ice should be transported in an insulated box, and the cones must be properly sanitised after they have been used; preferably at the chilling centre or dairy plant.
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Ice-cones |
Water tanks |
Cooling with water and ice- simple and almost always appropriate techniques.
The simplest cooling system involves an open tank with cold water. Milk cans have be inserted into the tank, where they are immersed in the water up to their ‘neck’. The water must be refreshed continuously or at regular intervals.
To allow de-aeration of the milk during cooling, the lids of the cans should be loosened. The tank may be covered with a lid to protect the milk from flies and dust. If well water or water from a main supply is used, this system only enables slow cooling to comparatively high temperatures. Better results are obtained by using iced water, and the cooling rate can be improved further by forced circulation of the iced water in the tank. To limit losses of cold by radiation, the tank and its cover must be insulated.
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Cooling systems transfer the heat of the milk via a cooling agent to either air or water. This transfer goes via a separated wall, so there is never direct contact with the milk. The refrigerant, or cooling agent absorbs the heat of the milk inside the evaporator. Each refrigerant has, by a certain pressure, its own boiling point. The cooling rate depends on the design of the equipment. The final temperature depends on the thermostat setting or milk flow through the plate coolers. Large differences in temperature increase the rate of cooling. High speed and turbulent motion of liquids along the wall will improve the heat transfer rate.
If milk is cooled in a modern way, electricity is needed to generate the temperature required. The electricity runs the condensing unit, which condenses the evaporated liquid and makes the process a continuous cycle.
The cooling cycle can be divided into a low- and high-pressure side.
A simple cooling cycle.
The evaporator is partially filled with refrigerant. When the compressor starts, the gas above the liquid will be sucked away. Due to this, the pressure will decrease. The liquid starts to boil as soon as the pressure sinks below the pressure of the present temperature. Parts belonging to the refrigerant will evaporate and take the heat out of the remaining cooling medium. This makes the remaining part colder. If the temperature reduces below the milk temperature, the heat will flow from the milk to the boiling refrigerant. This heat causes an amount of refrigerant to evaporate. The temperature will remain constant once the quantity of heat, which is transported by the compressor, is in balance with the amount of heat from the milk.
The high-pressure side of the compressor is connected to the condenser. The purpose of the condenser is to remove the condensation heat to the surrounding area. The compressor pumps gas into the condenser. As long as the pressure remains below the pressure belonging to the condensing temperature, only the pressure will rise. As soon as the pressure rises above the pressure belonging to the condensing temperature, a heat transfer will start from the gas to the surrounding area. First the ‘super heat’ is taken away. The super heat is the temperature difference between the heated gas above boiling point and the boiling point. Condensation will start after this. To condensate with a certain capacity, a particular temperature difference is needed. The pressure will be constant as soon as the temperature difference is large enough to condensate all of the gas pumped in by the compressor.
To make this process continuous, the liquid in the condenser must be fed back into the evaporator. Since the pressure in the condenser is always higher than in the evaporator, this can be easily done by establishing a pipe connection from condenser to evaporator. If a valve is mounted in this pipe, the amount of refrigerant can be adjusted. Normally this valve is automatic, and is called the thermostatic expansion valve. This valve measures the pressure of the evaporator and the temperature of the suction pipe. The valve opens more or less according to the super heat.
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The individual parts of a cooling installation |
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Key to the figure above.
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1. Compressor
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A gas pump creating low pressure in the evaporator (low temperature) and high pressure in the condenser (high temperature). |
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2. Pressostat
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Mainly used for protection of the condensing side of the installation. If the pressure gets too high, the pressostat stops the compressor. Also used as protector against low pressure caused by refrigerant leakage and as a switch to stop the compressor at the end of a pump-down cycle. |
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3. Condenser
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The part where the refrigerant condenses. The heat in the gas is released into the air and the gas turns into liquid. |
| 4. Liquid reciver |
Meant to be a storage place for the refrigerant. If the installation is in operation, the receiver is almost empty. If the installation stops and a pumpdown system is installed, the refrigerant will be stored in the receiver. |
| 5. Filter / Dryer |
The filter is used to take all solid parts out of the liquid. The dryer is used to remove moisture present in a very small amount in the refrigerant. |
| 6. Solenoid valve |
In installations with a pump-down system, this valve stops the liquid flow to the evaporator. |
| 7. Sight glass |
Gives the possibility to check if there is sufficient refrigerant in the installation. |
8. Thermostatic
expansion valve |
Gives the same amount of refrigerant, in a liquid form, back to the evaporator as the compressor takes out as a gas. |
9. Evaporator
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Part where the refrigerant evaporates and consequently takes the heat out of the milk. |
| 10.Thermostat |
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Controls the temperature of the cooled milk, switching the compressor on or off depending on the temperature. | |
This is the most common milk cooling system. The bottom of the tank has been designed as an evaporator, while the heat of the milk goes through the stainless steel wall to the refrigerant. The refrigerant evaporates, which takes the heat away from the milk. Since direct expansion tanks do not have a cold buffer, energy must always be available. In this type of system, the milk is cooled directly and agitated after arrival in the tank.
Direct expansion cooling system - the most common choice.
In indirect cooling systems, the evaporator is situated in a reservoir filled with the heat carrier, which is mostly water. The evaporator consists of a system of coils or pipes in which the cooling medium evaporates and cools the heat carrier.
Milk cooling with an icebank system.
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The biggest advantage of an icebank system is that it allows the cooling capacity to be stored in an isolated reservoir with a heat carrier and ‘cold buffer’ or ‘ice buffer’. In areas where there is not sufficient energy, an icebank system provides an efficient cooling solution. The formation of ice around the pipes in the reservoir forms the cold buffer that can be used for cooling the milk. The cold buffer makes it possible for cooling in areas where energy in peak times is more expensive, or where the use of electricity is limited, and means that the cooling system can be turned off to avoid an energy rush during milking. The production of cold can occur in periods when energy is inexpensive, and can be extended over a longer period, enabling a small compressor to be used.
The energy efficiency of the indirect system is lower than that of the direct system, because cooling of the carrier demands extra energy. The energy consumption of an icebank cooler is 23 W/l. There are two types of chilled water equipment. The first is the ice builder, which accumulates ice between milkings using a small condensing unit that runs up to 18 hours a day. The second is the package chiller, which has a large condensing unit that runs only during milking.
Milk comes from the cows to an end unit, from where it is pumped at a constant rate through a filter to the plate cooler. The plate cooler consists of corrugated stainless steel plates. The milk flows over one side of these plates, whilst on the other side tap or well water flows in the opposite direction. When the milk leaves the plate cooler its temperature has been reduced to 2 – 4 °C above the water temperature, prior to final cooling and storage in the cooling tank.
Pre-cooling system.
Milk/water flow in a precooler heat exchanger.
Pre-cooling with cold tap water lowers the total and running costs for the plant by reducing the demand for chilled water. A prerequisite for this is, of course, a supply of inexpensive natural cold water. It is always possible to combine pre-cooling with other cooling systems to reduce the energy costs even more. If tap water has been used for pre-cooling, it is advisable to recycle the cooled or cold water by using it as drinking water for cattle. If tap water is not reused, the costs will annul the energy costs savings, whereas if well water has been used for pre-cooling, this aspect is less important.
Today, farms are becoming larger and larger, meaning more work, more cows and more milk – and less time between milkings. This process provides farmers with potential cooling problems, because all the milk has to be cooled and stored. The sheer quantity of milk, combined with high milk flows and longer milking periods, makes it more difficult for conventional bulk tanks to cope.
Quicker milking means greater milk amount per time. Overloaded cooling systems mean slower cooling and higher bacteria counts, and long cooling times involve prolonged agitation with the risk of buttering. Maintaining taste and quality is made more difficult, which puts the entire milk production at risk. Instant cooling is an in-line system, which cools the milk in a matter of seconds before it reaches the storage tank.
Instant cooling from 35-3°C.
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The milk goes from the cows to the end unit and balance tank, from where it is pumped at a constant rate through a filter to the plate cooler. The plate cooler is the heart of the cooling system and consists of corrugated stainless steel plates on one side of which the milk flows in one direction, while on the other side, chilled water flows in the opposite direction. When the milk leaves the plate cooler, its temperature has been reduced to a temperature 2 – 4 °C above the water temperature. The milk is pumped continuously to the insulated storage tank, where it can be kept, with occasional agitation, until collection.
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The flow of milk/water in a deep cooler heat exchanger/plate cooler. |
Ecombies involve a two-step cooling process. It is very advantageous to combine instant cooling with pre-cooling using chilled water. Pre-cooling with cold tap or well water lowers the total costs, including running costs for the plant by reducing the demand for chilled water.
Milk cooling with an ecombi-cooler.
In pre-cooling, the plate heat exchanger is divided into two sections. In the first section, the milk is cooled with cold tap or well water. In the second section, the milk is cooled down to the final storage temperature using chilled water.
Milk/water flow in an ecombi-cooler heat exchanger.
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