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Variable Speed Drive Compressors – not always the most efficient solution !

Over the years we have had lots of customers request variable speed drive compressors regardless of the air demand profile of the site and when we ask why they are asking for a variable speed drive they often say one of two things:
“We just want one; we’re putting everything on VSD’s” or “It’s the most efficient type of compressor.”

However, we do not quote a variable speed drive (VSD) compressor just because the customer wants one as it would be irresponsible of us to do so. VSD compressors are more complicated and the VSD controllers have internal electronic losses of 3% which need to be considered. VSD compressors are also a great deal more expensive than fixed speed compressors to purchase.

In an ideal situation the air usage would have a static air demand which was perfectly matched to the size of a fixed speed compressor.

However, if a customer has a varying air demand between 50 and 100% of a compressor output then a VSD compressor may be the most efficient way to generate the compressed air and would warrant the additional capital expenditure.

As already eluded to – a VSD compressor is a very expensive investment and surprisingly in less than 50% of situations are VSD compressors the correct solution.

In an ideal world the proper use of a VSD compressor is the trim compressor in a multiple compressor installation and this is where we get the greatest energy savings.

Some of the important points to consider when buying a VSD compressor are:

1.  A variable speed drive compressor loses about 3% of energy right from the beginning due to the controller losses.
2.  A load/unload compressor (or compressors) with a properly sized tank will be more efficient in nearly every situation.
3.  VSD compressors are a great deal more expensive to work on when they fail.
4.  VSD compressors are more sensitive to the ambient conditions – they’re more fragile than regular compressors.
5.  If you’re getting a VSD due to having a low demand shift and a high demand shift, usually a multiple compressor solution of load/unload compressors are more efficient.
6.  If you want to expand with another compressor, you may have to worry about a control gap.
7.  VSD compressors have a higher initial cost than load/unload fixed speed compressors of the same size.
8.  VSD compressors, if sized correctly, will hardly ever turn off.  That increases your maintenance costs as they will have more run hours on the compressor.

Buying a VSD when you don’t need one would be like buying a supercar to do the daily city commute in.  Sure it will work, but you’ll pay more up front, waste a lot of fuel, the servicing costs will be greater and as the car is more complicated it will have a greater chance of failure. However, had you purchased a city car designed for running at lower speeds then the capital costs are less, running costs are less and repair costs are less. Furthermore as the car is simpler by design it will also be more reliable.

Here at Glaston we will always perform detailed analysis to determine the correct solution for any compressed air system. In many cases retrofitting a VSD controller to an existing compressor will offer all the VSD advantages at a fraction of the cost.

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