Efficient use of vacuum pumps

How to cut vacuum pump running costs through better system control and the use of variable speed drives.

The vacuum pump accounts for about 20-25% of energy use for a normal dairy farm. It’s clearly a necessary part of the milking system so its use cannot be avoided. However by using better control and more efficient equipment it’s possible to cut running costs considerably.

The key to lower costs is avoiding the generation of excessive vacuum. With conventional milking systems, the vacuum level is managed by the controlled admission of air at atmospheric pressure from the vacuum regulator. When the vacuum level increases, air is admitted to reduce the level. When the vacuum level drops, air admission ceases and the vacuum recovers to the pre-set level. These valves should be well maintained and adjusted to ensure that they do not leak and that they give the appropriate vacuum.

Nevertheless, even a pressure valve operating effectively is an inefficient solution, as it is only set to give one vacuum level and so in the face of varying demand can only work by effectively removing the vacuum produced by the pump and therefore wasting the energy that has been used to produce it.

A solution to the above is to vary the vacuum being created by the pump such that it matches the demand on it. This is usually achieved by installing a variable speed drive to the motor of the vacuum pump which alters its speed in relation to vacuum demand, to create more or less as needed.

A sensor (pressure transducer) is fitted in the vacuum line to allow the variable speed drive to follow the vacuum demand. This varies quite dramatically between standby, milking and cleaning loads, where cleaning cycles are often the periods of greatest demand and least time.

Variable speed drives work best with a lobe pump (also called a Roots blower) rather than the more common oil ring or vane pumps. Whilst it is possible to retrofit a variable speed drive to a vane pump the practical limit on their turndown is such that less energy savings will be made.

Surveys have shown energy savings of between 40% and 70% using variable speed control. Capital costs for retrofit can be highly variable and paybacks can be anywhere between 4 and 8 years. Also, in most cases you need to consider the availability of 3 phase electricity as variable speed drives invariably require this type of supply (although you may be able to include a phase converter to enable installation on single phase).