Direct expansion (DX) tank cooling

Direct expansion (DX) is the simplest and most energy efficient way of cooling milk but cooling must take place quickly to prevent total bacterial content from rising. What are the key points to consider?

Direct expansion (DX) is the simplest and most energy efficient way of cooling milk. This is because the evaporator forms the outer skin or jacket of the inner tank surrounding the milk; as such, there are fewer heat transfer processes.

DX tanks tend to be the industry standard solution because of their inherent efficiency and simplicity. Often replacing older ice bank tanks, they are capable of storing greater volume of milk for a given footprint, because they don’t require space for an ice jacket. They can also provide very rapid cooling where other solutions would struggle.

The cooling of milk in a DX tank needs to take place quickly, during and shortly after milking, to prevent the total bacterial content from rising. Therefore the compressor must be large enough to cope with the peak cooling load. In some cases, the existing electricity supply may be insufficient to meet the compressor power rating, bearing in mind other concurrent uses such as vacuum pumps. On smaller traditional units, this can often be a problem.

Power required may be calculated from the volume of milk collected and the session duration, as per the following example, in which a morning 2.5-hour milking session collects 6,000 litres of milk which are pre-cooled to 15°C before reaching the mechanical cooler:

• Heat to be removed (calculated as Mass × Specific Heat Capacity × Temperature Difference and converted from kJ to kWh)= 6000litres × 1.03kg/litre × 3.93kJ/kg.C × (15-4)C ÷ 3600kJ/kWh = 74kWh
• Time in which to remove that heat = milking session duration + 30 minutes = 3h
• Cooling duty = rate of removal of heat = 74kWh ÷ 3h = 25kW
• Electrical power required by compressor = cooling duty ÷ Coefficient of Performance = 25kW ÷ 3 = 8.2kW

Another disadvantage of needing to deliver instant cooling is that use of cheap night rate electricity is not possible – you pay at the tariff in force when you milk.

DX tanks can also be used in conjunction with ice builders to perform the final cooling. Ice is built up at night, using cheaper electricity, And milk is cooled by way of a PHE, further cooling is delivered directly to the tank during milking. The cooling system of the tank itself may be smaller, as the peak demand is reduced.