The problem is not full load
Almost every compressor is reasonably efficient at full load. The difference between machines shows up when demand falls below capacity, which is most of the time in most plants. A fixed speed screw compressor running at full load draws its rated power. When the plant only needs part of that air, the machine has two options: load and unload, or run against a throttled inlet. Both keep the motor turning and consuming a large share of full-load power while delivering little or no useful air.
What variable speed changes
A variable speed drive matches motor speed to demand. At 60 percent demand, the machine runs near 60 percent speed and draws close to 60 percent power. The saving is not a small percentage of the bill; it is the difference between paying for air and paying for rotation.
| Operating point | Fixed speed | Variable speed |
|---|---|---|
| Full load | Efficient | Efficient, plus small drive losses |
| 70 percent demand | Load / unload cycling, power stays high | Speed falls, power follows demand |
| 30 percent demand | Poor - frequent cycling, more wear | Good, but below the drive's efficient floor a fixed machine may be better |
| Zero demand | Unloaded running or stop / start | Stops or idles at minimum speed |
When fixed speed is still the right answer
If a plant genuinely runs at full capacity for long uninterrupted blocks, a fixed speed machine at its design point can beat a variable speed unit, because there is no drive loss to pay for. The same applies when demand is steady and a small receiver plus load / unload control already runs efficiently. The decision depends on the load profile, not on the technology being newer.
What the load profile has to show
- Demand over a shift - how often and how far demand drops below capacity.
- Cycling frequency - a fixed machine that loads and unloads many times an hour wears its inlet valve and costs more than the power figure suggests.
- Storage volume - more receiver volume lets a fixed machine run in longer, more efficient blocks.
- Future growth - a variable speed unit sized at the current peak leaves little headroom; sizing at the future peak leaves it running at part load, which is where it belongs.
The practical rule
Size the machine for the highest demand you expect, then choose control that keeps it efficient at the average demand. Most plants find the average is well under the peak, which is exactly the case variable speed is built for.
Send us your working pressure, required flow and site conditions and we will propose a configuration. Contact OPPAIR.