A double-inlet fan draws air into both sides of the wheel, while a single-inlet fan takes it from one side only. The differences are airflow, bearing loading, motor position, and how you get at the impeller for cleaning.
A double inlet wheel of the same diameter delivers roughly twice the airflow of a single inlet wheel, at the same pressure. It also balances axial thrust and needs air access from both sides. Single inlet suits ducted connections and dirty air, since the motor stays out of the airstream and the wheel is easier to reach.
The physical difference
A single inlet centrifugal fan’s impeller has a backplate on one side and an open impeller on the other, so air enters from one direction.
A double inlet centrifugal fan’s impeller is effectively two impeller sections arranged back to back, sharing a central plate. Air enters from both sides, providing a larger effective inlet area.
Consequently, the fan double inlet centrifugal fan is wider for the same diameter, and it needs clear space on both sides.
Comparing the two
| Single inlet | Double inlet | |
|---|---|---|
| Airflow for same diameter | Baseline | Roughly double |
| Axial thrust on bearings | Present | Balanced |
| Motor position | Usually outside the airstream | Often in the airstream |
| Inlet access needed | One side | Both sides |
| Cleaning access | Good | More complex |
| Typical use | Ducted extract, dirty air | AHUs, fan coils, furnaces |
Notice the motor position. It is the reason why single-inlet fans dominate greasy and dusty applications.
Where does a double-inlet centrifugal fan win?
- Air handling units, where the fan sits in a plenum with space on either side.
- Furnace and air handler blowers, which need airflow from a compact body.
- Fan coil units, where depth is limited and airflow matters.
- Applications where axial thrust would otherwise shorten bearing life.
- Situations where diameter is constrained, but airflow is not.
- Finally, clean filtered air, since the motor sits in the stream.
Most furnace blowers are double inlet for exactly this reason. Our squirrel cage blower guide covers that application.
Where does a single-inlet centrifugal fan win?
The single inlet arrangement has advantages that matter in industrial work.
It connects directly to a duct on the inlet side, which suits extraction systems. The motor of a single inlet centrifugal fan is outside the airstream, so grease, dust, and heat do not reach it. Meanwhile, the impeller is far easier to remove for cleaning, since only one side is enclosed.
For grease-laden or dusty air, single inlet is nearly always the right choice. Keeping the motor out of the airstream removes the most common cause of failure in those applications.
Installation clearances
- Allow at least half a impeller diameter of clear space at each inlet.
- On double inlet centrifugal fans, check both sides equally, since one blocked side unbalances the flow.
- Avoid mounting a double inlet fan close to a cabinet wall.
- Check inlet cone clearance on both sides during maintenance.
- Allow room to withdraw the wheel for cleaning.
- Finally, confirm the cooling airflow is not obstructed.
Uneven inlet access is a common fault on double inlet centrifugal fan installations, and it produces both performance loss and vibration.
Selection in practice
The choice is usually made by the equipment rather than by the engineer.
Air handling units and packaged equipment use double inlet fans because the geometry suits them. Ducted industrial extraction uses single inlet because the connection and the airstream demand it. Meanwhile plug fans sidestep the question entirely, since they are single inlet by nature.
Our centrifugal fan range covers both arrangements, and AMCA documents how each one is tested.
One check is worth adding to any survey. Look at what sits within half a diameter of each inlet. On a double inlet fan, a wall or a coil close to one side quietly costs airflow, and it shows up as vibration long before anybody measures the shortfall.
FAQ
Does a double inlet centrifugal fan provide twice the airflow?
Close to it, under the same diameter and speed conditions. The impeller is wider, so the fan has a larger physical footprint even though the impeller diameter remains the same.
Can I convert one to the other?
No. The impeller, housing, and bearing arrangement all differ, so it is a different fan rather than a modification.
Which is easier to clean?
The single inlet one. Only one side is enclosed, so the impeller comes out more easily, and the motor is not in the airflow path.
Do double inlet centrifugal fans need bearings on both sides?
Usually, yes. The impeller is typically supported by bearings on both sides, which helps balance the centrifugal forces generated by the rotating assembly.
A double inlet centrifugal fan draws air from both sides through a wider impeller design and requires inlet clearance on both sides. A single inlet fan provides easier motor protection and impeller access, which are often preferred in industrial applications.
