Milk pre-cooling: Plate heat exchangers

The role of plate heat exchangers in milk cooling, how to operate them at maximum efficiency in practice and an indication of how much energy and money you might save.

Milk has to be reduced in temperature from 37°C to 4°C with the bulk tank refrigeration equipment usually doing most of the work. Many farmers reduce the load on the bulk tank by using a plate heat exchanger with mains or borehole water to pre-cool the milk.

Plate heat exchangers can reduce milk cooling costs by as much as 50%. Savings in energy aren’t free where there is a capital cost of installation and the cost of water to consider.

Where water is taken from the mains it is worth seeing if this can be substituted by borehole water and to maintain water efficiency it is always recommended that water is re-used, such as in drinking systems or for high volume washdown.

Using water to cool milk involves the use of a plate heat exchanger, a device which passes water and milk either side of heat conducting plates to allow the water to partially cool the milk. Theoretically it is possible to achieve a milk temperature exiting the plate cooler to be equal to the water temperature entering it (around 10°C) however this would require a very large plate cooler and a considerable water flow.

The exiting milk target temperature should be around 20°C in real world conditions. To achieve this there must be sufficient plates in the plate cooler and the right volume of water available for cooling. Over the course of a milking you should expect to use up to twice as much water as your milk volume. So, for instance, 4,000 litres of milk per day would need between 4,000 to 8,000 litres of cooling water per day.

Water usage can be controlled by the addition of a solenoid valve connected to the milk pump control. When the pump starts, the solenoid allows the water to flow.  When the milk pump stops, the water should continue to flow for around 20 seconds to ensure that the plate cooler is ready for the next flow of milk.

To enhance the performance of the plate cooler milk flow should be minimised and water flow maximised. Some farms have a partially closed valve fitted in the milk delivery line to slow the throughput of milk (where one is fitted it’s important to ensure it is fully opened for washing periods) although a much better solution to this is a variable speed milk pump.

Water flow can be maximised by ensuring that the water delivery and drain pipes are well sized and the water delivery pressure is high.  A separate pump set can be used to boost pressure if natural water pressure is low.

To check performance, measure the temperature of the milk entering the tank following passage through the cooler. A milk exit temperature of 22°C and ideally close to 17°C should take place. Milk exit temperature should be measured as close to the plate cooler as possible, you should not rely on the milk tank temperature gauges. The best time to check performance is on a warm day as this is when you want the plate cooler to be at its most effective.

For a 200 cow herd producing 1.6M litres of milk per year and having an ice bank cooling system, a plate cooler reducing milk temperature to 20°C will save approximately 20,666 kWh of electricity, worth approximately £2,040 per year.