Atlantic Compressors performs compressed air system audits across Central and Southwest Virginia and Southern West Virginia, and one of the most common inefficiencies our auditors find is a control gap between a facility’s fixed-speed and variable-speed drive (VFD) compressors. A control gap happens when a facility pairs a variable speed compressor with an existing fixed speed compressor and sizes them incorrectly. If the fixed-speed compressor’s output capacity matches or exceeds the VFD compressor’s control range, the system will develop a control gap, a pressure band where neither machine can respond efficiently.
What Is a Control Gap, and Why Does It Happen?
A control gap is the range of compressed air demand that falls outside what either compressor can serve efficiently on its own. It typically shows up during equipment upgrades, when a facility adds a VFD compressor to an existing fixed-speed unit without first mapping out how the two machines’ capacities overlap.
Here’s a real example from one of our audits:
- Kaeser DS 171 (125 hp, fixed speed): produces 625 cfm at 125 psig
- Sullair TS200TS-150L/A (150 hp, VFD): produces 200 to 800 cfm at 100 psig, a 600-cfm control range
On paper, that looks like plenty of coverage. In practice, this pairing creates two control gaps: below 200 cfm, and between 800 and 825 cfm. The facility sized the VFD compressor larger than the fixed-speed compressor, and a 25-cfm gap was enough to put the system into a fight it never won.
The 2015 Audit: Two Compressors Battling for Hours
During a 2015 audit, our team recorded system pressure alongside the load on each compressor. As demand pushed system pressure down, the fixed-speed Kaeser loaded up to compensate. At the very same moment, the VFD Sullair unloaded, working against the Kaeser instead of alongside it. That pattern repeated for hours, with the two compressors essentially arguing over the same narrow band of demand while production upstream never noticed a thing.
That last point is what makes control gap issues easy to miss. Production output stayed steady, so from the plant floor everything looked fine. Back in the compressor room, though, the two units were consuming extra energy fighting each other instead of sharing the load efficiently. Most facility teams we work with already know the term “control gap,” but far fewer have a clear picture of what it actually costs them in wasted electricity over a year of continuous operation.
Why “Production Didn’t Suffer” Isn’t the Same as “All Is Good”
It’s tempting to treat a control gap as a non-issue once you confirm output wasn’t affected. But an inefficient compressor room quietly inflates a facility’s energy bill month after month, even when nothing downstream ever slows down. A properly sized VFD compressor should absorb the swings in demand while the fixed-speed unit handles a steady base load, not compete with it. When sizing goes wrong, the facility pays for that mismatch indefinitely, whether or not anyone notices it.
How Atlantic Compressors Prevents Control Gap Issues
Atlantic Compressors designs compressed air systems around the full picture, not just individual equipment specs. Our consultative auditing service maps supply-side and demand-side data together, so a new VFD compressor’s control range is sized correctly against whatever fixed-speed equipment is already running, before a control gap has the chance to form. If you’re not sure whether your compressor room has a control gap of its own, we offer a no-obligation, thorough walkthrough of your compressed air system.
Frequently Asked Questions
What causes a control gap in a compressed air system? A control gap forms when a facility’s fixed-speed compressor capacity equals or exceeds the control range of its variable speed drive compressor, leaving a band of demand that neither machine can serve efficiently.
Can a small control gap, like 25 cfm, really waste energy? Yes. Even a narrow control gap can cause two compressors to load and unload against each other for hours at a time, since the system searches for stability across a demand range neither unit is sized to handle alone.
Does a control gap always affect production? Not necessarily. Production can run normally while the compressor room absorbs the inefficiency, which is why control gaps often go undiagnosed until an energy audit uncovers them.