The volute, the spiral housing around a centrifugal wheel, collects and directs the flow, and its shape directly sets how much pressure the fan develops and how efficiently it does so.
What the Volute Does
Air leaves the spinning wheel in all radial directions at high speed. The volute gathers that flow and converts part of the velocity into static pressure as the cross-section grows toward the discharge. If the growth rate matches the flow, the conversion is smooth and losses are low; if the volute is too tight or too loose for the duty, the wheel works against recirculation and efficiency drops by several points while noise rises. The volute is where velocity energy becomes the usable static pressure the duct system actually needs, so its shape is as important as the wheel itself.
The cut-off, the tongue near the wheel tip, is especially sensitive: too close and it scrapes the flow, too far and it lets back-flow. Its clearance is a key tuning parameter that the designer sets against the duty's noise and loss target, and small changes there move both efficiency and sound level.
Volute proportions also fix where a fan is stable. A scroll sized for a high-flow duty pushes the fan into steep, unstable territory when the system turns out more restrictive than calculated, and on a forward-curved wheel that appears as a flat curve region where flow wanders without a pressure change to correct it. Wheel outlet width and scroll width have to correspond as well: a wide wheel discharging into a narrow scroll throws flow against the side plates, and the loss leaves as heat and noise rather than pressure. This is why re-using a housing for a different duty point rarely works even when the wheel diameter is unchanged.
Volute Configurations
Volutes are sized by the specific speed of the wheel. Forward-curved wheels suit a compact, high-volume volute; backward-inclined wheels need a larger, slower volute for efficiency; radial wheels use a robust volute for material-laden air where scrubbing resistance matters more than peak efficiency. The spiral expansion rate is typically set so the area grows roughly with the cumulative flow angle, which keeps the static pressure recovery even along the scroll.
| Item | Specification | Note |
|---|---|---|
| Expansion | Spiral growth | Matches flow angle |
| Cut-off clearance | 6โ12% wheel dia | Tunes loss and noise |
| Wheel type | FC / BI / radial | Sets volute form |
| Static gain | 20โ40% | Velocity to static |
| Discharge | Flanged outlet | To duct connection |
| Scroll width | Matched to wheel outlet | Mismatch shows as noise and heat |
| Blade passing frequency | rpm × blades / 60 | Dominant tone at the cut-off |
On a tested fan, volute quality shows up as the gap between total and static efficiency: a good volute recovers more velocity into usable static pressure, which is what a ducted system actually needs, while a poor one leaves that energy in a turbulent jet at the outlet that the duct then has to absorb as loss.
Noise from a centrifugal fan is largely made at the cut-off, and it is predictable. The dominant tone sits at blade passing frequency, the product of running speed and blade count, so a 12-blade wheel at 1,450 rpm produces a tone near 290 Hz that no amount of general lagging will remove; the cure is clearance and cut-off profile at design stage, or an attenuator tuned to that band. Structure matters too. Volute side plates are large flat panels that drum, so stiffeners, heavier gauge, or double-skin construction lower radiated noise measurably. In abrasive service a replaceable wear liner at the cut-off and along the outer scroll protects the very geometry the efficiency depends on.
Design Checklist
- Match the wheel: Pair volute form to wheel type, not to whatever housing is on the shelf.
- Tune the cut-off: Set clearance for the duty's noise and loss target, not a generic value.
- Confirm recovery: Compare static to total efficiency on the test report to see the recovery.
- Flange the outlet: A square discharge eases duct connection and reduces leak loss at the joint.
- Check the scroll: Avoid a volute sized for a different duty point or the wheel runs off its efficient zone.
- Match scroll to wheel width: Do not re-use a housing for a different duty even with the same wheel diameter.
- Predict the tone: Calculate blade passing frequency early so attenuation is tuned to the right band.
Why Volute Shape Counts
Two fans with the same wheel but different volutes can differ by several efficiency points, which over years is a large power bill on a continuously running fan. Keruite develops volute geometry per wheel series so the housing recovers pressure instead of wasting it, and the quoted efficiency reflects the built volute, not an ideal one.
Housing development is invisible in a quotation and obvious in an electricity bill, which is why a volute is worth engineering per wheel series rather than per order. Fans built in an 80,000 m² plant across more than 30 years of production are measured as complete assemblies in an in-house performance laboratory, so the recovery a scroll actually achieves appears in the published curve instead of being assumed. ISO 9001:2015 controls the panel forming and cut-off clearance the curve depends on, CCC certification covers the electrical build, and with units running in more than 50 countries the same geometry is documented for both 50 Hz and 60 Hz speeds.
Our engineers respond to selection enquiries within 24 hours, including volute and wheel matching for centrifugal fan efficiency and pressure duty.