The one piece of fan physics every plant accountant should know.
Reviewed by Jitamitra application engineering
Share on LinkedInMost industrial fans are selected for the worst case and then run somewhere below it — with a damper eating the difference. The physics of that choice has been settled for a century: fan shaft power varies with the cube of speed. Cut the speed by 20%, and the power demand falls by about half.
| Fan speed | Flow (approx.) | Shaft power (cube law) |
|---|---|---|
| 100% | 100% | 100% |
| 90% | 90% | ~73% |
| 80% | 80% | ~51% |
| 70% | 70% | ~34% |
The three common ways to reduce a fan's flow do not cost the same. As flow drops, an outlet damper consumes the most power, inlet guide vanes less, and speed control (a VFD) the least — a ranking the US Department of Energy documented in its industrial-fan sourcebook. That gap, hour after hour, is the energy bill. Brake power is read off the fan curve at the operating point, and that is the quantity the cube law scales when the speed changes.
The cube law is real, but it sets the ceiling — the engineering sets the number. A credible retrofit respects four caveats: (1) a minimum stable speed; (2) resonance skip bands set at commissioning; (3) shaft and bearing protection against VFD bearing currents; and (4) the actual saving depends on your system curve and your motor and drive efficiencies. We size and commission all four into every retrofit — so the number you're promised is the number you get.
Jitamitra Electro Engineering · Fan-engineering notes, written for the engineer.
About half. Fan shaft power varies with the cube of speed: at 90% speed the shaft power is about 73%, at 80% about 51%, and at 70% about 34%, with flow tracking speed. Brake power is read off the fan curve at the operating point, and that is what the cube law scales when the speed changes.
It very likely is. Most industrial fans are selected for the worst case and then run somewhere below it, with a damper eating the difference. The physics has been settled for a century — shaft power varies with the cube of speed — so that difference, hour after hour, shows up as the energy bill.
The three common ways to reduce a fan's flow do not cost the same. As flow drops, an outlet damper consumes the most power, inlet guide vanes less, and speed control the least — a ranking the US Department of Energy documented in its industrial-fan sourcebook. That gap, hour after hour, is the energy bill.
Three profiles carry the money. Fans that run continuously but rarely at full duty — the classic oversized selection. Processes with genuine demand variation: seasonal loads, batch operations, multi-shift profiles. And dampered fans running permanently part-closed, where the damper position is the saving, and it is visible today.
Not as it stands. The cube law is real, but it sets the ceiling — the engineering sets the number. The actual saving depends on your system curve and on your motor and drive efficiencies, so the figure has to be worked out for your installation rather than read off the speed ratio.
Four things, engineered in rather than assumed. A minimum stable speed. Resonance skip bands, set at commissioning. Shaft and bearing protection against VFD bearing currents. And the saving checked against your system curve and your motor and drive efficiencies. We size and commission all four into every retrofit — so the number you're promised is the number you get.
Flow, static, gas temperature, application — or attach a spec, GA drawing or a multi-fan schedule. Engineer to engineer.
ISO 9001:2015 quality system · performance-tested to IS 4894 / ISO 5801 / AMCA 210 method · witnessed FAT on request.
*For our standard range. ATEX and special projects need 5 to 7 working days.
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