Air Compressor Works
Post

Variable Speed Drive – not always the most efficient

January 12, 2018

Lately we have seen a lot of customers ask us for quotes on variable speed drive (VSD) compressors.  When we ask why they are asking for a variable speed drive, they sometimes say one of two things:

“We just want one; we’re putting everything on VSD’s”
or
“It’s the most energy efficient.”

Coincidentally we’ve also been doing a lot of repairs of other brands’ VSD compressors and getting a lot of trade-ins on VSD compressors that were improperly sized, or where it was just the wrong situation for a VSD.

We’re not going to quote a variable speed drive compressor, just because the customer wants one.  It would be irresponsible of us to do so.  However, our competitors seem to be doing a lot of it and making a killing off it, while taking advantage of the customer.  The compressor company makes a ton of money by selling one of the most expensive compressors they have for that size, while the customer is stuck with lots of problems, lots of failures, and eventually having to buy another compressor.

We just replaced a 60hp VSD of a different brand with two 20hp Kaeser Compressors.  There was nothing wrong with their 60hp VSD compressor, other than it was the completely wrong compressor for the job.  The two Kaeser 20hp compressors were less money up front, less money in maintenance, and used less energy than the 60hp VSD.

A variable speed drive compressor is a very expensive investment.  You should only buy one in special situations.  In about 90% of cases it’s not right for a single compressor installation.

The proper use of a VSD compressor is as a trim compressor in a multiple compressor installation.  If that’s what your compressor system is like, then a VSD will probably be the best fit and save you money on your electrical bills.  If that’s not what you have here is why you should not buy a VSD compressor:

1.  A variable speed drive compressor loses about 3% of energy right from the beginning.
2.  A load/unload compressor (or compressors) with a properly sized tank will be more efficient in nearly every situation.
3.  VSD compressors are 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 will be more efficient.
6.  When a VSD runs in low turndown, it has major problems, usually excessive moisture in the oil and excessive oil-carryover.
7.  If you want to expand with another compressor, you may have to worry about control gap.
8.  VSD compressors have a higher initial cost than load/unload compressors of the same size.
9.  VSD compressors, if sized correctly, will hardly every turn off.  That increases your maintenance costs, because you have more run hours on the compressor.

Buying a VSD when you don’t need one is like buying a machine gun to hunt ducks.  Sure it will work, but you’ll pay more up front, waste a lot of bullets, and it’ll jam more often.  You should have just bought that shotgun that was designed for duck hunting.  If the company that sold it to you didn’t warn you about that, then find a new company to buy from.

If you really think you need a VSD, get an Air Demand Analysis.  That is the correct way to know if you need one or not.

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