Ask the Expert: Will my shop benefit from a variable frequency drive compressor?

Q: Will I benefit from a variable frequency drive compressor?

A: Variable frequency drive (VFD), or variable speed drive, compressors have gotten a lot of publicity for being the energy-saving "silver bullet" for compressed air systems. To be fair, VFD compressors can provide exceptional energy savings in large systems and where electricity is expensive. They must, however, be properly applied. You can buy the most efficient piece of equipment but if it is not properly applied you will not save much on your electric bill.

VFD compressors are great for applications where there is widely varying, but not sporadic demand between 40 and 80 percent of the compressor’s full load. Think of it this way. A shop with multiple bays, where at any given time there is always some air being used, may benefit from a VFD.  But a smaller shop with only one or two bays that does not have a constant demand for air, would not get the benefit.  

VFD compressors have a higher initial purchase price, and can be finicky about their power supply. If you are debating between a fixed and variable speed compressor, you should first get at least a rough idea of your annual compressed air costs using this formula:  

Compressor power (hp X .746 kW/hp) X annual operating hours X $/kWh (electricity rate)  

With this information you can judge if the estimated savings from a VFD compressor will justify the cost. While the allure of energy savings may be tempting, you may be better off going with a fixed speed compressor and a larger receiver tank for some extra air storage.

Information provided by: Kaeser Compressors, Inc.