The scroll is the part everyone ignores, and the volute cutoff inside it is ignored twice over. Together they decide roughly half the performance.
Two features do most of that work. One is the widening spiral. The other is the volute cutoff, where the scroll passes closest to the wheel.
A scroll housing collects fast air leaving the impeller and slows it in a widening spiral, so velocity turns into static pressure. The volute cutoff is the point where the scroll passes closest to the wheel, and its clearance controls both pressure recovery and blade passing noise. Typical clearance is 8 to 12 percent of wheel diameter.
What the spiral actually does
Air leaves an impeller fast, often 40 to 90 m/s on a mid-size wheel. That speed is energy, and it is useless unless something converts it.
The scroll widens progressively around the wheel, so the accumulated flow always has room. As the passage grows, average velocity falls, and part of the kinetic energy reappears as static pressure.
Consequently the scroll is a diffuser wrapped around a circle. Get its expansion rate wrong and the recovery suffers.
The volute cutoff is the critical dimension
The volute cutoff, sometimes called the tongue, separates air leaving through the outlet from air carrying on around the scroll.
| Clearance | Effect |
|---|---|
| Very tight, under 5% | Higher pressure, much more tonal noise |
| Typical, 8–12% | Balanced pressure and noise |
| Loose, over 15% | Quieter, noticeable recirculation losses |
| Sharp tongue edge | Stronger blade passing tone |
| Rounded or angled tongue | Lower tone, slight pressure loss |
Every blade passing the cut-off creates a pressure pulse, and blade count times speed gives the frequency. So tightening the clearance raises both pressure and tone together.
Why wheels and housings are matched
This explains one of the most common field mistakes.
The scroll width, expansion rate, outlet area and cut-off clearance are all designed around a specific wheel. Fitting a different wheel of the same diameter changes every one of those relationships, even though it physically fits.
A mismatched wheel typically delivers less pressure, uses more power and makes considerably more noise. Meanwhile the assembly balance is unknown, so vibration usually follows.
Angled volute cutoff designs and noise control
Manufacturers often angle or step the volute cutoff rather than running it straight across the wheel width.
- An angled tongue spreads the pulse over time rather than concentrating it.
- That converts a strong tone into weaker broadband noise.
- The pressure penalty is small, usually one or two percent.
- Rounded edges reduce vortex shedding at the tongue.
- Some designs use a serrated edge for the same reason.
- Finally, clearance can be increased slightly where noise matters more than pressure.
Our fan noise guide covers why tonal noise is more objectionable than broadband noise at the same level.
Housed against plug fans
A plug fan dispenses with the volute housing entirely and blows into a plenum instead.
That loses the pressure recovery, typically 5 to 10 percent, and removes the cut-off tone at the same time. Meanwhile it saves considerable cabinet length and allows fan arrays.
So the volute is not always worth having. Our plug against housed comparison works through the trade-off.
Outlet velocity and the system beyond
Whatever the scroll achieves can be undone immediately downstream.
A damper right on the outlet, a sharp elbow or an abrupt expansion all destroy the recovery the volute just produced. Consequently the first metre of duct matters almost as much as the housing design.
For the pressure losses involved, AMCA Publication 201 documents system effect factors, and our system effect guide covers the practical cases.
Volute design FAQ
Can I fit a different wheel in my volute housing?
Physically often yes, aerodynamically almost never. Cut-off clearance, scroll width and outlet area are matched to the original wheel, so performance and noise both change unpredictably.
Why is my fan making a musical tone?
That is usually blade passing noise from the cut-off. Check clearance, look for a damaged tongue, and verify the inlet is not distorting the flow before adding attenuation.
Does a bigger outlet give more airflow?
Not by itself. The outlet area is part of the diffusion design, so enlarging it changes velocity and recovery rather than simply releasing more air.
Do plug fans really lose much pressure?
Typically 5 to 10 percent of what a housed fan recovers. In an air handling unit the freed space usually returns more than that through lower system resistance.
The scroll turns speed into pressure, and the volute cutoff decides what that costs in noise. Neither is an accessory, and neither tolerates a mismatched wheel.
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