Milk must be cooled from 98 degrees F. (37 degrees C.) to storage temperature, typically about 38 degrees F., to preserve its quality. The cooling process involves removing 56 BTUs of energy from each pound of milk (27 kilojoule per kg). Typically, a refrigeration system does this by using a special refrigerant fluid to remove heat from the milk and “reject” the heat (usually) into the outside air.
The basic refrigeration system is made up of a refrigerated bulk tank, a refrigeration compressor unit and an air-cooled condenser unit. There are several technologies that can be added to the milk cooling systems on dairy farms to reduce the refrigeration requirements or to capture waste heat for pre-heating water:
- Refrigeration heat recovery (RHR) units will make a refrigeration system more efficient by collecting heat that would normally be wasted to the air and using it for water heating. An RHR unit captures heat from the system refrigerant and transfers it to water, preheating it before it enters a water heater.
- Scroll compressors are 15 to 20% more efficient than traditional reciprocation compressors yet have fewer moving parts and are only slightly more expensive than reciprocating compressors. Scroll compressors have been used in the dairy industry with good results for over 15 years. If you are purchasing a new bulk tank or replacing a failed reciprocating compressor, you should specify that the compressors be a scroll type. The additional investment is a modest cost for the improvement in efficiency.
- Well Water Precoolers are heat exchangers that use well water to cool the milk before it reaches the bulk tank. Properly sized, they can reduce milk cooling costs by up to 60%, assuming 55°F well water. Undersized water lines and water system capacity are the two largest reasons that precoolers do not perform up to their potential. Caution: If an RHR unit is being used on the dairy, an energy audit should be done, as precoolers and RHR units are competing technologies. It is usually more cost effective to maximize water heating with an RHR than precooler.
It is also important to keep a farm’s refrigeration system clean and well maintained. Dirty coils or low refrigerant pressures will reduce efficiency and increase operating costs. Many systems have a “watch glass” that can be used to determine if the refrigerant needs to be recharged – if the fluid in the watch glass is bubbly instead of clear, it is time to call your refrigeration technician and schedule a service.
- Energy Conservation in Agriculture. Scott Sanford, University of Wisconsin Extension bulletins. A link to Energy Conservation bulletins in Agriculture from the University of Wisconsin Extension.
- Energy Conservation in Agriculture: Refrigeration Systems. Scott Sanford, University of Wisconsin Extension bulletin A3784-4. Details refrigeration heat recovery units and their use on dairy farms, how they work, sizing a unit, flow diagrams, and discussion of precooler and RHR unit interactions.
- Energy Conservation in Agriculture: Well Water Precoolers. University of Wisconsin Extension bulletin A3784-3, 2003.
Contributors to this Article
- Scott Sanford, Senior Outreach Specialist, University of Wisconsin - Madison. This article is excerpted from Refrigeration Systems, University of Wisconsin Extension, bulletin A3784-4, 2003.