Are variable speed compressors a sensible option?

Pneumatic systems & components

Third Quarter 2004 Pneumatic systems & components Perimeter Security, Alarms & Intruder Detection

Generally it is believed that variable speed technology offers the most efficient method of producing compressed air.

In the First World where it was conceived, it may be a plausible solution when one considers the cost of their electricity and high operating standards. However, in South Africa, we probably have the cheapest electricity in the world, which casts an overwhelming doubt on the actual benefit of variable speed - energy saving, in relation to the capital cost of this type of technology.

The fact that a variable speed drive compressor can only be totally effective in a plant that has extreme fluctuations, (variable), in air demand, implies that the plant has to either be running at maximum efficiency, in other words with zero air wastage, or alternatively, the compressor has to be substantially over-sized in order to obtain the energy-saving benefit of operating at various speeds.

Rather a catch-22, I would say!

Invariably however, such plants do not exist in the real world. It is thus very dangerous to believe that variable speed technology will offer the best economical solution to your air requirement. Inherently, this type of technology is maintenance-intensive and can become costly in terms of sophistication if the plant is neglected. Particularly in SA, we have to accept that the level of skills on the workshop floor is way below First World standards, due to the fact that product knowledge is kept privy by the local suppliers and training is only available at a premium.

Besides these primary contentions, one also has to consider that the physical design and character of the rotary screw air end prohibits optimum efficiency at varying speeds and therefore the quoted percentages of energy savings, due to variable speed operation, become intangible values rather than real savings. A further consideration is the life expectancy of the unit. Generally, the VSD compressors design parameter, prescribes an optimum efficiency at mid-speed, (3000 rpm), where the unit is intended to run most frequently, which means that the chosen air end on the relative model compressor, would have the best performance at this speed. If the unit has to run at the top speed, (6000 rpm), it will not be running efficiently or comfortably and it becomes very expensive to maintain and operate.

It is thus most important to understand that if a VSD compressor is installed in an unbalanced compressed air reticulation that is not constantly maintained, the unit will eventually become a full base load compressor due to the fact that it had to be oversized during the original selection process and without consistent monitoring, which is normally the case with compressor plant, the unit will eventually run unnoticed at top speed, thereby defeating the original objective.

For more information contact Kevin Rushmer, Air Bank, +27 (0) 11 908 4169.

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Are variable speed compressors a sensible option?

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