Methods and systems for milk cooling

Effective milk cooling is essential to ensure the quality of the product. Surveys show that milk cooling accounts for 30 to 50% of the total electricity consumption of operating a dairy [1], so designing and operating an efficient milk cooling system can significantly reduce energy demand and shed operating costs.

A basic understanding of milk cooling systems can help farm managers ask good questions and make informed decisions about their milk cooling components. When it comes to cooling milk there is no one solution for all farms.

Each operation must consider:

• How much milk am I cooling and storing?
• What will the temperature of the milk be when it reaches the vat?
• How cold must the milk be stored at and how quickly must this happen?
• What is my power supply like?
• How cold is my available water for pre-cooling?
• What might my future production increases be?

Milk can be entirely cooled in the vat or chilled before it hits the vat or be cooled using a combination of pre-cooling and vat cooling.

The most widespread cooling systems use compressor-based refrigeration equipment . The use of absorption refrigeration systems may be a cost-effective alternative if waste heat is available [3].

The principle of operation is:

• Refrigerant vapour is compressed to a high pressure and temperature level by an electrically driven compressor.
• The refrigerant vapour dissipates heat in the condenser to ambient air or hot water in the waste heat regeneration system. The vapour condenses and leaves the condenser in liquefied state. Condensation occurs at a temperature that is above the highest coolant temperature (air or water).
• The liquefied refrigerant passes through the expansion valve (which reduces pressure and controls the flow of the refrigerant)
• The liquefied and depressurized refrigerant evaporates in the evaporator and cools down rapidly while evaporating. The heat energy which is necessary to evaporate the refrigerant (evaporation energy) is taken out of the milk which is cooled down to e.g. 4°C. Evaporation occurs at a temperature that is below the lowest milk temperature.

Pre-cooling is achieved by a plate chiller. It requires cold water which absorbs heat from the milk and can be used as preheated water for watering points. The significant economic effect is the reduction of electricity consumption.

This graph shows the temperature change over time (day) of the milk held in the storage tank. The red graph is without precooling, the blue is with precooling.

Widespread system configurations are

• Milk chilling by electrically powered vapour compression systems
  • without waste heat recovery or
  • with waste heat recovery for water-heating up to 60°C

  The system can be designed with direct cooling or with an ice bank. Ice banks benefit from the use of off-peak electricity to generate ice / iced water during night time and from lower power ratings than direct cooling [4]. As a drawback, ice bank systems consume more electricity.

• Two staged milk chilling by
  • pre-cooling: a plate chiller cooled by water
    (appreciated side effect: preheating water for the watering points).
  • main cooling: electrically powered refrigeration systems
    • without waste heat recovery
    • with waste heat recovery (water-heating)

1. Total electricity consumption is about 5 kWh per 100 litres of milk (average in Germany), milk cooling uses up to 2,5 kWh per 100 litres of milk.
2. vapour compression technology
3. Waste heat energy and temperature must be sufficient during the time of milk cooling. The necessary conditions may exist, if a biogas- or diesel-fueled generator is running near the milk storage.
4. Direct cooling requires a high power rating for chilling milk in as short as 2 hours per each milking. Ice banks get by on lower power ratings as they can use the whole off peak period plus a part of the peak period to produce a sufficient storage of iced water and ice.