Water heating: Heat recovery units

A look at the two main ways to recover heat from the milk refrigeration system and the typical savings that can be achieved by efficient heat recovery.

Water heating can be 30% of the energy use in a dairy parlour and is mainly carried out using electric immersion heaters.  Since the refrigeration system emits to ambient the heat it extracts from the milk, it seems logical to try and recover this to heat the water.  In some cases water heating costs can be halved by using this technique, where there is no plate cooler and a poorly maintained refrigeration system a heat recovery unit will generate most hot water. However it’s always advisable to improve milk cooling performance first.

There are two main ways to recover heat from the milk refrigeration system:

• Water storage heat recovery – here the hot gases from the refrigeration system are routed through coils/heat exchangers which heat a static volume of stored water.
• Continuous flow heat recovery – the hot gases are passed through one circuit of a plate heat exchanger, with the water to be heated routed through the other circuit. The heated water can either be recirculated through the heat exchanger to build up water temperature or put through as a single pass.
• There are usually practical considerations which will make one or other of the techniques more suitable. The former is simple to engineer but may be comparatively expensive. The latter is cheaper to install but needs careful tuning to get the most from it.

As heat recovery units do not heat the water to full circulation cleaning temperature, the water temperature has to be topped up with an immersion heater. Usually the heat recovery system can achieve about 60⁰C temperature if working well. 

One spin-off from using this technique it that it can improve the performance of the milk cooling system, especially in the summer time. Refrigeration systems work more efficiently if the hot gases which go through the condenser coil are cooled more effectively. Heat recovery supplements the work of the condenser and therefore helps to achieve this although it is always advisable to improve milk cooling efficiency first.

For a parlour heating 400 litres per day to a temperature of 90°C heat recovery will save about £600 per annum (based on 10p/kWh night rate electricity).