The inlet box is the first thing air meets before the impeller, and a poorly shaped one distorts the flow and steals efficiency before the wheel does any work.
Why the Inlet Matters
A centrifugal fan makes pressure by accelerating air through the wheel, but it performs best when that air arrives evenly and axially. The inlet box sets the approach: if it turns the flow sharply, creates a swirl, or presents a non-uniform velocity profile, the wheel sees a distorted inlet, surges locally, and loses 3โ8% efficiency while running noisier. Because the loss is upstream of the useful work, it is pure waste that no impeller improvement can recover once the air is already disturbed.
Good inlet design also reduces the risk of low-flow instability, where an uneven approach pushes part of the wheel toward stall. A fan that stalls at part load is hard to control, so the inlet quality protects the operating range as well as the efficiency, which matters on systems with widely varying demand.
Distance from an obstruction matters as much as the shape of the box. A wall, floor, or filter face closer than about half an inlet diameter to the fan eye restricts the approach and behaves like a partial blanking plate, so a fan pushed hard against a plenum wall loses pressure for no visible reason. Double-inlet units are doubly sensitive, because both eyes have to be fed equally; an off-centre plenum starves one side and the rotor then carries an unbalanced load. On single-inlet fans an inlet cone alone is enough where air already arrives axially, and a full box with vanes is only needed where the approach has to turn.
Design Guidelines
The inlet should provide a generous bell-mouth or rounded entry, a straight settling length, and any required turning done with well-rounded vanes rather than a hard elbow. The cross-section should transition smoothly to the wheel eye without sudden contraction or offset. Where the fan must draw from a side or bottom, an inlet box with turning vanes restores uniformity that a bare elbow would destroy, and the extra length pays for itself in recovered pressure.
| Item | Specification | Note |
|---|---|---|
| Entry | Bell-mouth radius | Rounds the approach |
| Settling length | 1โ1.5ร inlet dia | Straightens flow |
| Turn vanes | Well-rounded | If a turn is needed |
| Loss target | < 5% of dynamic | Entry pressure loss |
| Uniformity | ยฑ 10% | Velocity at wheel eye |
| Obstruction clearance | ≥ 0.5× inlet diameter | From wall, floor, or filter face |
| Double-inlet balance | Equal feed to both eyes | Off-centre plenum unbalances the rotor |
The test for a good inlet is the velocity profile at the wheel eye: within about ยฑ 10% across the annulus and free of swirl. AMCA 210 testing captures the effect as a higher measured efficiency versus a fan tested with a poor inlet, which is why the inlet is part of the rated assembly, not an afterthought bolted on by the installer.
An inlet box is also where several other components want to live, and each has a flow consequence. Inlet guide vanes fitted inside the box can regulate volume with less power penalty than a discharge damper, but they introduce a designed swirl the fan curve must already account for. An isolation damper placed immediately at the eye becomes a distortion source when partly closed, so it belongs at the far end of the settling length rather than against the wheel. On cold or humid duty the box is the coldest large surface in the assembly, so external insulation and a drain point stop condensate collecting in the bottom of an otherwise sealed volume.
Design Checklist
- Give straight length: Resist cramming the fan against a bend; allow settling space for uniform flow.
- Use turning vanes: If a turn is unavoidable, round it with vanes, not a sharp elbow that sheds vortices.
- Round the entry: A bell-mouth is cheap insurance against separation at the lip.
- Verify on test: Compare inlet-boxed efficiency to the bare-wheel curve to see the gain.
- Mind the offset: Avoid side entry that lands off the wheel eye centre, a common distortion source.
- Keep clearance: Allow at least half an inlet diameter to any wall, floor, or filter face.
- Place dampers away from the eye: Fit isolation dampers at the far end of the settling length, not hard against the wheel.
Why Inlet Design Counts
A small investment in inlet shaping returns as lower power draw and quieter running across the fan's whole life, and the saving recurs every operating hour. Keruite designs inlet boxes as part of the centrifugal fan characteristic, so the efficiency quoted includes the real approach, not an ideal one that the installer would have to recreate on site.
Inlet quality is the cheapest efficiency available on a fan installation, because it costs sheet metal rather than motor power. Centrifugal fans built in an 80,000 m² plant by a manufacturer with more than 30 years of experience are rated with their inlet arrangement in place, and an in-house performance laboratory measures the boxed assembly rather than the bare wheel, so the quoted efficiency is the one the site will see. ISO 9001:2015 controls the panel and vane fabrication that determines uniformity, CCC certification covers the electrical build, and with fans in more than 50 countries the arrangements suit both plenum-draw and ducted approaches.
Our engineers respond to selection enquiries within 24 hours, including inlet box and approach duct guidance for centrifugal fan installations.