How to avoid the Control Gap

Four questions – three easy and one difficult – about your compressed air system with examples of how you can avoid a control gap. 

Bo Kuraa, Technical Director, Enersize

• Does your company have a compressed air system?
• Do you have more than one compressor?
• Is one or more of your compressors of the Variable Speed Drive (VSD) type?
• Do you have a ”Control Gap”?

Only a few can answer the last question but throughout my professional career, I have seen this problem appear over and over again. That it is a relatively unknown problem, even though it often occurs, is due to the fact that only to a lesser extent does it affect the compressed air supply and thus the production. But then, why should you worry about this? Because it increases your energy consumption, your maintenance costs and creates pressure fluctuations in the compressed air supply.

But what is “Control Gap”? In short, it is a VSD compressor that “fights” with a fixed speed compressor to be the trim compressor! It looks like this in a compressor control:

The figure shows a compressor configuration, where the VSD compressor (to the left) is smaller compared to the five other fixed speed compressors. It is also seen that the VSD compressor is 100% loaded and that compressor no. six runs in unloaded mode (which costs about 25% of the energy consumption at full load).

Approx. 2 minutes later the picture looks like this, and another 2 minutes later we are back in the situation seen in the first picture.

You are thus in a situation where the VSD compressor runs up and down in speed (including also the pressure) and with a fixed speed compressor which spends a large part of its operating hours in unloaded mode. That was probably not exactly what you wanted when you bought an expensive VSD compressor!?

But what is ”Control Gap”… a little more technical?

Over the past 15-20 years the development and sales of VSD compressors have increased significantly which means we now avoid (or should avoid) fixed speed compressors with lots of hours in unloaded mode.

In other words this has helped reduce electricity consumption which just produces heat and which in turn must be cooled, requiring yet more energy

But why not just equip your compressed air station with 3-5 VSD compressors next time you need to replace the compressors? Because a VSD compressor of the same size as a fixed speed compressor costs 30-40% more to purchase. Furthermore, a VSD compressor is typically less energy efficient when 100% loaded compared to a fixed speed compressor of the same size.

But then for example we could just purchase four compressors of the same size (three in operation and an extra in case of breakdown, or when service is required), one of which is VSD? Correct? No – definitely not !

The explanation is that a VSD compressor is not controllable in the whole range from 0-100%, but typically only >30-100%. When the demand is less than 30% it runs in start/stop mode almost like a fixed speed compressor.  For this reason there are also three pressures settings that must be established in a VSD compressor: target, load and unload pressure.

For example, if you imagine four compressors (three fixed speeds and one VSD), each of which can deliver 10 [m3/min], you will have a “Control Gap”, when the demand for compressed air is as follows:

  • 10-13 [m3/min]
  • 20-23 [m3/min]
  • … and if all compressors are suddenly in operation (perhaps because you have not had the time and the resources to implement leakage management), also at 30-33 [m3/min].

If the VSD compressor is purchased smaller than the fixed speed compressors, the problem will just get bigger, as in the first example.

A compressor control cannot prevent the problem.  Therefore, it is important to choose the right compressor configuration from the start.

The question is, what is the right size then?

There are various answers to this question:

One of them is to choose a VSD compressor, which in capacity is equal to the capacity of the fixed speed compressor plus the capacity, where the VSD compressor cannot be regulated; in this case >(10 + 4.8 + safety) ≈ 16 [m3/min]; probably the best solution!

Please see the graph – a “Control Gap” tool developed by Enersize.

Another solution could be to buy the fixed speed compressors in a smaller size, for example: 6 [m3/min], but two more compressors to get the same capacity.

A third solution could be to buy a compressor with a capacity of 6 [m3/min] and put it ”between” the VSD compressor and the other compressors. It will solve the problem and can also be the solution if a wrong purchase has already been made.

Please see the graph, where all blue and grey lines are now below the orange line.

When choosing compressors there are several other factors that should also be considered, including redundant capacity and the flow profile of the company in general. But I strongly recommend that you avoid “Control Gap”!

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