The pros and cons of variable speed drive compressors
April 2021
Variable speed drive (VSD) compressors have long since been revered for their energy saving abilities where demand for compressed air fluctuates. This is achieved by precisely matching output to actual compressed air demand. But, whether it be due to shift or production patterns - most compressed air applications actually have a fluctuating demand for compressed air. Is a VSD always the most energy efficient option? In this blog post we consider the pros and cons of VSD compressors and discuss where they may be optimally suited.
From changes in shift patterns to peaks and troughs in production and workloads, where compressed air demand fluctuates, opting for a compressor with a variable speed drive (VSD) has long since been a popular choice.
By utilising a variable speed drive (also known as a frequency controlled drive), such compressors are able to vary the speed of the airend to directly match the free air delivery (FAD) - or flow - required by the application. And, by efficiently maintaining steady discharge pressure in widely varying loads, they will be more efficient than compressors which idle a lot or use modulation control. The end result - energy savings. And, energy savings of course means reduced operating costs as well as having a positive impact on a company’s carbon footprint.
If correctly applied, another benefit of operating a VSD compressor will be an extended motor life thanks to soft motor starts, which prevent an inrush current that can overheat windings. In addition, as VSD compressors don’t operate on pressure bands, some can maintain discharge pressure within +/- 0.1 bar(g) of the set pressure. Such consistent discharge pressure also has the benefit of being kinder on air treatment and associated components downstream of the compressor.
But, the crucial point you need to remember is that variable speed drive compressors will only be the most energy efficient option under specific circumstances. It is not a one-size-fits-all option.
Let’s explain...
Why VSD compressors are not a one-size-fits-all solution
Firstly, VSD compressors are only most efficient when running from typically 40% load up to 80% loading (see diagram 1 below). This is where their performance is most economical. Outside of this the airend is running beyond its optimum efficiency ranges and not being efficient or cost effective. When used for the wrong task, they can quickly make compressed air production more expensive than it needs to be.
Diagram 1: Speed-controlled machines should be used only in their optimised loading ranges.
And, here’s something else to bear in mind when it comes to VSD compressors. They are very sensitive to heat and moisture. Their dependability in extreme ambient conditions should be considered. High moisture and fine dust can cause issues for these devices. This means they are simply not suited to every application.
So, what’s the solution?
System splitting may be a better option. In most cases, compressors can be classified by function as baseload, medium load, peak load and standby compressors. Baseload means that the basic amount of compressed air needed by an operation to function is always 100% covered. These are complemented by peak load compressors, which deliver additional volumes needed during peak periods, which vary due to the different consumers. To round-out the station, stand-by systems are integrated to ensure a reliable supply of compressed air in the unlikely event of a failure in the baseload systems.
Whether the best solution is provided by a single VSD compressor or by smaller, fixed-switching peak-load compressors can be determined through an audit of the actual compressed air demand followed by a simulation.
For optimal coverage of the various load functions, the compressors should be equipped with integrated control systems that if in the event of multiple compressors being used on site can easily communicate with a master PC based system controller - such as the Sigma Air Manager 4.0 (SAM 4.0). In the case of the SAM 4.0, this central mastermind controls the entire compressed air supply system. By constantly and in real-time analysing process data, the SAM 4.0 can identify and switch to the most efficient single compressor - or combination of compressors - to precisely meet the air demand at any one time. This not only optimises the overall efficiency of the compressed air system but it ensures that each compressor operates at its best efficiency point.
A key prerequisite for efficient – i.e. cost-effective – control is to ensure seamless graduation of the compressors. Consequently, the sum of the peak loads should be greater than that of the next baseload system to be activated. When using VSD peak load compressors, it is therefore advisable to select a control range exceeding the output of the next compressor to be activated.
Conclusion
Hopefully what the above has demonstrated, is that there is not a one-size-fits-all approach to deciding whether a VSD compressor is right for your business. Matching the exact compressor to the site demand profile is crucial, as is having the correct control measures in place to ensure that any VSD compressors you have are run only within this range.
Keep in mind that both VSD and fixed speed compressors have pros and cons. It is simply not the case that one technology is better than the other, it's more that they are better in different circumstances.
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