Variable Speed Drive - When to Buy

Last week I wrote about variable speed drive (VSD) compressors and when they should not be purchased.  It got a lot of attention, and it may have appeared that I'm biased against VSD compressors.

That's not true - VSD's are excellent compressors when applied correctly in appropriate situations.  They are specially equipped, higher priced compressors, designed for special situations.  So here is a follow-up on when a VSD is the right compressor.

First and foremost, if you can afford to get a VSD, then you can afford to get an Air Demand Analysis (ADA).  You should always get an Air Demand Analysis before buying a VSD compressor - it's just the right way to do it.  An ADA can keep you from making a costly mistake.

If you have a new installation or a situation where an ADA cannot be performed, here are some situations where a VSD compressor is a good fit:

1.  You have a large, multiple compressor installation.  Your flow varies within the shifts and from shift to shift.  A VSD is an excellent choice as a trim compressor in this situation, especially when the compressors are controlled by a master controller.

2.  You're low on space, and have a situation where you occasionally have a high flow application or shift that lasts for an hour or more, but most of the time your flow is lower.  You don't have the room for two compressors and a large tank.  I would argue that you should find some space, because you should always have a back-up.  If you don't have the space, then a VSD might be the correct solution.

3.  Your power company gives rebates for VSD compressors.  Don't just jump immediately into purchasing a VSD because of the rebate, but a rebate certainly can make a VSD a more viable option.

4.  Your installation has a limit on starting amps.  It's rare occurrence, but there are situations where you cannot install the correct breaker or disconnect fuse to start the compressor that you need.  Maybe you'll exceed the overall ampacity of your building.  One of the benefits of a VSD is that it's very soft-starting.  The starting amps slowly ramp up and you'll never exceed the full load amp rating of the compressor.

If you are in the situation where you want to make the jump to a VSD, here are things to watch out for:

1.  Make sure you have great voltage and solid electrical service.  This is a must.

2.  Put the compressor is in a climate controlled room.  There are work-arounds for this, so it's not a must, but it makes your life a lot easier.  VSD control cabinets can get really hot, so make sure it's a well ventilated area with good air flow.

3.  Keep it out of a dusty area.  Again the VSD cabinet can get hot, so it needs proper air flow.  Don't let dust clog up the ventilation.

4.  Make sure that most of the time you're in the bottom of the curve, or the sweet spot.  If you look at the CAGI data of a VSD compressor, you'll see a curve.  The compressor is most efficient at the bottom of the curve.  Here is the CAGI data sheet for a Kaeser SFC-110S.  Take a look at the curve:

You want your VSD compressor to be at the bottom of that curve most of the time.  That is where it's most efficient.  You want to avoid the very beginning of the curve - that is the problem area for VSD compressors.

5.  Watch out for control gap when installed with other compressors.

A variable speed drive compressor can save you energy and can be the most efficient, if the situation is appropriate and you size it correctly.  Remember that it will have a higher initial cost, so make sure you crunch the numbers or get an ADA before you buy.  Make sure you get that return on investment.