New Air Compressor Designs Explode Myth of VFD Savings

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The narrow range of VFD effectiveness is often eclipsed by 100%-efficient, direct-drive air compressors.

The myth behind VFDs is that they are misapplied

While much popular folklore, whether true or not, refuses to die, plant managers and facility engineers are now waking up to the fact that variable frequency drives (VFDs) don't always save money. For many commercial and industrial users, the up-front costs of integrating VFDs into their air compressor systems often fail to reap the expected utility savings because of the misapplication of these drives -- which only offer efficiencies within a limited range (30-70%) of operation.

In contrast, direct-drive rotary compressors reduce electricity usage over a full range of loads from 1-100%, accounting for their increasing presence in manufacturing and process plants that require air compressor systems.

"The most efficient range of operation for a variable speed drive is limited to 30-70%, but some users run above or below that level," notes Steve Van Loan, President of Sullivan-Palatek. "For example, if you buy a 100 hp compressor with a VFD and mainly run it at a 25% duty cycle, then the compressor actually draws more current trying to turn the big air end at that slow a speed. You might as well have saved some money and bought a 25 hp machine in the first place. At the other end of the spectrum, if you run the compressor at 100%,, then that doesn't save any electricity either, since no drive is 100% efficient."

Sullivan-Palatek of Michigan City, Indiana, manufacturers a line of direct-drive rotary-screw, industrial air compressor systems that allow users to profit from greater energy savings in their pressurized-air operations. The increased efficiency (ranging from 3-8%) of this design stems from the rotary screw that eliminates unnecessary moving parts such as belts and gears.

Under a 100% duty-cycle application that called for 100 hp, a direct-drive, rotary-screw air compressor would only requires 100 hp to supply a given air flow rate at a given pressure, whereas a chain or belt-drive compressor might require 112 hp to provide the same amount of air while making up for the parasitic losses of the extra moving parts. At 100% usage, even a VFD could not offset this loss. Using the industry standard average of $.08 cents per kWh, the 12 hp saved by the direct-drive compressor would reduce electricity costs by approximately $4200 per year.

"The myth behind VFDs is that they are misapplied," stresses Van Loan. "In order to save the same eight cents per kW with a non-direct drive compressor with a VFD as compared to a non-direct drive, it's going to take a user a long time to get a payback on the 25-30% cost premium for adding the frequency drive -- maybe 7-8 years. Most companies can't wait that long."

However, properly applied Variable Frequency Drive machines (ie. Utilization between 30 and 75%) can show a pay back in 2 to 4 years.

Established in 1984, Sullivan-Palatek manufactures industrial equipment such as electric and diesel driven high performance rotary screw air compressor systems, along with a complete line of accessory items that include air dryers, filters, remediation systems and construction air tools.


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Steve Van Loan
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