Chilled Water Supply System in a Dairy Plant

As already discussed, the cooling of milk by using chilled water is an indirect method of cooling. In this method first the supply water is stored in a large tank where it is cooled/ chilled by a separate refrigerating unit. Then this chilled water is supplied through pipes to the various milk cooling equipment in the plant. This method is very commonly used in most of the dairy plants due to its various advantages over direct cooling system. In a dairy plant, large quantity of milk is handled/processed. The milk is collected from the milk supply tankers at a particular time. All this milk cannot be processed at a time and needs to be kept in chilled condition to preserve its quality. Moreover, the cooling of milk/milk-products may be required in some other processes also. Thus, the cooling requirement is not continuous or steady. And any time any amount of cooling may be required. For this type of requirement, it is generally not feasible to install refrigeration unit ateach cooling point. On the other hand, the indirect cooling suits very much due to its various advantages as given below:

1. As the volume of chilled water being less than the volume of refrigerant vapours,the piping cost is very less in case of indirect cooling.

2. Simply controlling the flow of chilled water can easily control the amount of cooling.

3. A small size refrigeration plant is needed for indirect cooling. This small unit may run continuously and keep the water in chilled condition in the insulated tank. On continuous running, there may be partially freezing of water in the tank resulting in the formation of ice. In the peak hour of cooling load and also in case of power failure, the required amount of chilled water can be made
available from the ice/chilled water tank to meet the cooling load.

4. In case of any leakage in cooling pipes, only chilled water will come in contact with milk and the toxic refrigerant if any will remain away from milk/milk- products. Thus it is also safe to use indirect cooling.

i. Chilled Water Tank/ Ice-Bank Unit
The chilled water tank is also called ice-bank unit because when there is no supply of chilled water to dairy plant and refrigeration unit is continuously cooling the water, ice-formation will take place. This ice works as a bank of chilled water,which can be supplied during the peak of cooling requirement. It is a metallic tank whose outside surfaces are insulated with glass wool and brick wall. The tank contains a network of cooling coils through which the refrigerant flows. The cooling coil/evaporator coil is of flooded type and remain filled with refrigerant liquid generally ammonia. The liquid ammonia from the receiver of refrigeration plant enters the coil through a low side float valve. The low side float valve reduces the pressure and temperature of liquid ammonia and maintains a constant level of it in the cooling coil. The liquid ammonia absorbs heat and cools the water outside the pipe in the tank. On absorbing heat the liquid ammonia evaporates and its vapours lift up and accumulates in the accumulator above the coils. From the accumulator, these vapours are sucked by compressor through an insulated pipe and again compressed and sent to condenser. A float valve keeps the water level constant in the tank. An agitator installed on one end of tank keeps the water flowing over cooling coil for better heat transfer. Outside the tank on one side, the cooling water pumps are installed which take water from the bottom of tank through their suction pipes and supply it to the dairy plant through the network of chilled water pipes. After fulfilling the cooling requirement, the chilled water becomes normal water and is again supplied to the chilled water tank from the top near the agitator.