Milk Cooling

Illustration 2: Well water Precooler (heat exchanger). Source: Scott Sanford

Illustration 2: Well water Precooler (heat exchanger). Source: Scott Sanford

There are three technologies that cut electrical energy use when cooling milk: scroll-type refrigeration compressors, well-water cooled precoolers (heat exchangers) and refrigeration heat recovery units.

• Scroll compressors are newer units and are 15 to 20 percent more efficient, have fewer moving parts, and are only slightly more expensive than traditional reciprocation compressors. Scroll compressors have been used in dairy refrigeration systems since the mid- 1990s with good results. If you are purchasing a new bulk tank, you can specify the compressor be a scroll type. If your existing reciprocating compressor fails replacing it with a scroll compressor adds about $300-500 more than the cost of a new reciprocating compressor. The extra cost is for re-wiring and new mounting holes. This is a modest cost for the improvement in efficiency [3].

• Well-water precoolers are heat exchangers that use well water to cool the milk before it reaches the bulk tank. If sized properly and have an ample water supply, they can reduce milk cooling costs by up to 60 percent. This is assuming 55°F well water to reduce the milk temperature to within 3 or 4°F of the well water temperature. Undersized water lines and water system capacity are the two largest reasons that precoolers do not perform up to their potential. It requires 1 to 3 gallons of water to cool 1 gallon of milk depending in the design of the heat exchanger (2 gallons water per gallon milk common). The precooler water should be re-used for watering cows or general clean-up. The water can be stored on a plastic holding tank until needed. If a refrigeration heat recovery (RHR) unit (see below) is being used, an energy audit should be done to assure adding a precooler will not increase energy requirements, because precoolers and refrigeration heat recovery units are competing technologies. If the milk is cooled with a precooler, then there is less heat available to pre-heat water in the RHR unit. If an electric water heater is being used, it is usually more cost-effective to maximize water heating with an RHR unit rather than to precool milk to save energy [4].

• A refrigeration heat recovery (RHR) unit is typically a water tank with a heat exchanger jacket around it through which the refrigerant from the milk cooling system flows before it goes to the air heat exchanger (condenser). Some of the heat from the refrigerant is transferred through the jacket and tank wall into the water stored in the RHR tank. An added benefit is a small increase in the efficiency of the refrigeration system due to a larger heat transfer area [5].

Variable Speed Milk Pumping
Variable speed drives (VSDs) can be used on milk pumps to control the flow of milk through a precooler or heat exchanger to increase its cooling efficiency. A constant speed milk pump can move milk at a rate of 30 gallons per minute (gpm) or more. To get maximum milk cooling, the water to milk flow rate ratio should be a minimum of 1:1, so a minimum of 30 gpm of water is needed. However, many precoolers require a water to milk ratio of 2:1 for maximum heat exchange rate thus 60 gpm of water is needed for a system with a constant speed pump. Most farms do not have this size of water system capacity. A VSD on the milk pump can lower the average milk flow through the precoolerwhich reduces the water demand (gpm) needed to achieve maximum efficiency from the precooler [6].