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.
  

 

Variable Speed Drive - not always the most efficient

Lately we have seen a lot of customers ask us for quotes on variable speed drive 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 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.

Leasing - a great option

We talked last week about how compressed air is expensive.  Getting a good compressor is a significant investment, and it's important that you avoid the "lowest cost wins the bid" attitude.

Sometimes you just don't have the budget or cash flow for the initial cost of the compressor.  In that case, leasing can be the answer to your problem.

Here are the benefits of leasing:
• Requires less money up front
• Permits soft cost financing
• Preserves bank credit lines
• Offers flexible payment options
• Allows upgrades and add-ons quickly and easily
• Protects against technological obsolescence
• Minimizes balance sheet liabilities

So when you see that Kaeser Compressor you really wanted, but don't have the immediate funds to purchase, consider leasing.

At Air Compressor Works, we work hand-in-hand with Marlin Leasing, and they've done an excellent job for our customers.  Please contact us or them for more information.