Skip to main content

A VSD Compressor Doesn’t Save You Money

VSD (Variable Speed / Frequency Drive) Compressors

The energy savings were huge… of course. The pay-back time would be a few months, or days, or less.. for sure!

Stop here!

Yes, a VSD compressor can save huge amounts of energy (and thus money), but only if a propoer homework is one, make some calculations and use your common sense.

[For those in the dark: a VSD or variable-speed compressor uses a frequency drive to match the compressor speed (and thus capacity) to the amount of compressed air needed. This as opposed to 'load-unload' machines that runs at a fixed speed but 'loads' and 'unloads' between a minimum and maximum pressure).

And IF a VSD compressor is a good idea in your situation, buy the right size of VSD compressor.

Here's when a VSD compressor will save you money: when it runs at less than 80% speed for most of the time and when it runs continuously (daily).

In these cases, the cost-savings can be huge. Tens of thousands of dollars per year in energy costs can be saved.

But: A VSD compressor that runs at 100%, fully loaded, uses 5 to 10% more energy than a fixed-speed compressor! This is because of the 'overhead' of the electronic frequency drive.

The optimum frequency range for a VSD is 50 – 80% speed for maximum savings.

Don't just believe the salesman with his fancy brochures and his pitch about all the wonderful things a VSD compressor does to your budget.

I hate to say this, but does he know what he's talking about? As an engineer I have had many talks with salespeople and their pitches and brochures and claims… and 90% is either hyped up, not applicate to your situation, or simply not true.

The sad truth is that in my years as a compressor mechanic, I have seen many, many VSD compressors that were just not needed, were only costing money, or were simply slowly dying.

Here are a few examples:

1) A VSD compressor that was way too big. It ran for only 5 to 10 minutes every hour, at maximum 30% speed.

This machine was slowly dying from water and rust because it never really heated up. A smaller fixed-speed machine, matched to the actual needed output would be way more logical. (this was a 90 kW machine!)

2) A VSD compressor that ran between 30 and 50% all the time, every day.

Sure, it was saving money over a load-unload machine of the same size. But a VSD compressor that was half the size and ran at 80% would be more cost-effective (both in investment and running costs).

3) A VSD compressor that was needed for one specific machine and was only used intermittently

This compressor was like one unit with the machine it supplied air to, and was way too big. The air demand was basically stable, and was only needed for 10 or 20 minutes at a time, a few times per day.

A basic compressor of correct size with a large air receiver would be far more cost-effective in this case.

4) A VSD compressor that ran at 100% each and every day, all day.

That customer invested extra money for a variable speed compressor, and ended up paying 5 to 10% MORE in energy costs each and every year! Yeah, great!

Again, a VSD compressor CAN be a great choice, and can save you a lot of money. And this is true in many cases.

But, beware of the salesman with a hyped-up story about energy savings, who doesn't know when NOT to try to sell the a VSD compressor.

Or you end up paying MORE in energy costs, MORE in maintenance costs and MORE in up-front capital investment.

And the compressor mechanics can try and clean up the mess afterwards ;)


Popular posts from this blog

Decoding the Duty Cycle Rating of Piston Compressors

Selecting the correct air compressor for your application can be a complicated process. Before making a purchase, operators need to specify the quality and amount of air their application requires. Each compressor technology (rotary, piston, centrifugal, etc.) has an optimal flow output, so users should choose compressor technology based on the compressed airflow requirement of their application. Some compressors, like rotary screw and centrifugal, are designed to run continuously at full speed while maintaining peak airflow (defined in cubic feet per minute, or CFM). The motors and cooling systems of these compressors are engineered to run 100 percent of the time without overheating. However, this isn’t true for all compressors. Piston compressors do not have the cooling capabilities to run continuously for extended periods of time. Therefore, each piston compressor must have a specified flow output and pressure, as well as the perc…

Did you buy the wrong type of compressed air dryer?

Do you have a desiccant compressed air dryer in your plant?
Do you know the annual running cost of that dryer?
It's plugged into a power socked, so it probably uses a bit of electricity… and you need to replace the desiccant every so many years. Right?
But for many types of desiccant air dryers, there's a hidden cost… and it is substantial!
In fact, the yearly cost will probably SHOCK you!
The culprit?
Purge air!
Purge air is air that is used to regenerate the desiccant. (the desiccant needs to be regenerated every few hours, to remove the water that it previously absorbed).
There are 2 main types of desiccant dryers:
Heated blower regenerated
This type uses an external blower and heater to regenerate the desiccant.

It requires only electricity (around 1 kW for every m3/min of compressed air).
Purge regenerated
This type doesn't use electricity to regenerate the desiccant...
Great, right! 
It uses precious compressed air!!!
This compressed air flow is between 10 and 20% of na…
Splash and Pressure Lubrication in Piston Compressors
Piston compressors have been around for centuries. They can be either oil-injected or oil-free, depending on the application and end use. In oil-injected models, the oil typically serves three crucial purposes: cooling, sealing and lubricating. But not all oil-injected piston compressors lubricate components the same way. There are two common methods of lubricating the pump in piston compressors: splash and pressure lubrication.

Splash lubrication In splash lubrication, oil is applied to the cylinders and pistons by rotating dippers on the connecting-rod bearing caps. Each time they rotate, the dippers pass through an oil-filled trough. After running through the oil trough, the dippers splash oil onto the cylinders and pistons to lubricate them. While splash lubrication is effective for smaller engines and pumps, it’s not a precise process. Parts of the pump may be insufficiently oiled or oiled too much. The amount of oil in the trou…