The cooling of milk on the dairy farm has two important reasons:
1.) With cooling down of the milking within 3 hours from ~ 35°C to 4°C the reproduction of bacteria is reduced.
- 2.) Lengthening the time that the milk can be kept on the dairy farm. It is possible to store the milk production of several day. Therefore the milk can be picked up less often and milk transport costs are reduced.
Pre-cooling of the milk through a plate cooler is recommended. The
prerequisite for this is that a sufficient amount of cold water (10°C to 15°C)
is available. This water cools the milk in a countercurrent flow process to
about 16°C - 20°C. Therefore a cooling unit with a lower power can be
used and the operating and acquisition costs can be reduced significantly.
The warmed water from the plate cooler is usually fed into a cattle drinking trough.
There are several methods to cool
the milk. Smaller quantities of milk
can be cooled using well water or
ice water directly e.g. with a milk
can. However, a better solution
for storage of small quantities of
milk is the milkcontainer, in which
the milk is cooled via an
immersion cooler to the
prescribed temperature. In
modern milk cooling systems, the
milk is usually delivered with a
milk discharge line to a milk
cooling tank and cooled in the tank.
The milk cooling tank together with the refrigeration unit / chiller / cooling machine, forms the milk cooling plant, where electricity is used for the necessary supply of energy.
The principle of cooling is based on some physical laws:
- When two materials with different temperatures come into contact with each other, there is a temperature compensation.
- When a liquid is heated, it evaporates and comes to a gaseous state.
- This gas liquifies again as soon as it is cooled.
- By increasing of the pressure in the refrigeration circuit, the temperature of the gas increases, or by pressure reduction, the temperature is lowered.
Technically, the milk cooling tank and the cooling machine form what is called a refrigerant circuit (see figure). The main components of this circuit are:
Scheme refrigerant circuit
In a milk cooling tank an evaporator is located mostly at the bottom, where at a low pressure a liquid refrigerant is injected. This refrigerant is heated by the warm milk, whereby it evaporates and is converted to gas. In this way the milk is cooled.
The gas is sucked from the compressor to the evaporator and is compacted (compressed). The pressure increases and at the same time, the temperature of the gas also increases.
The heat discharged from the condenser is useless to the environment. Instead, you can cool the hot gases with water. Thus, the heated water can be used in a storage / boiler and for use in the barn or house. The longer the cooling, the warmer the water. In practice, the water is heated to 55° C. Higher temperatures are technically possible, but strains the compressor too much, this will adversely affect the life of the compressor. The amount of water depends on the cooling time and milk volume. A simple rule of thumb is that with 1.5 litre of milk about 1 litre of water can be heated to 50° C . With a heat recovery system, it is also possible to warm up small amounts of water quickly. The longer the cooling, the more hot water is produced.
The hot gas is compressed by the compressor to the condenser and cools down herein. The capacitor has plates, whereby a large cooling surface is formed. A fan which is mounted on the capacitor, accelerates the cooling of the gas. The heat of the gas is released into the environment. The fins of the condenser should be cleaned regularly, so that dirt does not influence the cooling effect of the capacitor and thus the cooling effect of the whole milk cooling unit. During the passage of hot gas through the condenser, it further cools down the gas and liquefies it. At the exit of the condenser, the gas is completely liquid.