Plenum Fan vs Centrifugal Fan: Differences, Applications, and How to Choose

A plenum fan isn't necessarily a centrifugal fan: the impeller can also be one derived from an axial design, and when it is, the underlying physics differs too, not just the housing. There's no generic answer here: which configuration makes sense depends entirely on the application. Plenum configurations answer specific requirements, typically compact, low-resistance installations where ducting would only add pressure loss the installation doesn't need, while housed centrifugal fans take over as soon as directional control or higher static pressure comes into play.

This article clarifies the distinction between the two configurations, compares their performance characteristics, and helps identify the right choice for installation.

What is a plenum fan: design, how it works, advantages

A plenum fan is an impeller mounted without a scroll housing, engineered to operate inside a pressurised plenum chamber where the discharged air disperses freely in multiple directions rather than following a single ducted path.

Removing the scroll changes the mechanics of the airflow, not the physical principle behind it: the impeller still generates pressure through centrifugal force, but the chamber itself takes over the job the housing would otherwise perform, distributing the air and equalising pressure before it reaches the ductwork downstream.

The motor is typically direct-driven and mounted on the same frame as the impeller, keeping the assembly compact. Transmission-driven configurations exist too: they're used mostly in kilns and dryers, where temperatures run high enough that a transmission is chosen specifically to keep the motor out of the hot airflow and protect it.

This configuration brings a specific set of advantages:

• Compact installation: no scroll housing means a smaller footprint inside the air handling unit;
• Quieter operation and easier maintenance access, since the impeller sits exposed rather than enclosed, which also simplifies inspection and cleaning without dismantling a housing;
• Well suited to multi-zone AHU configurations or space-constrained installations, where several smaller plenum fans arranged in an array can replace one large housed unit, adding redundancy since the failure of a single fan does not take down the whole airflow.

What is a housed centrifugal fan

The housing is what gives the housed centrifugal fan its directional control and pressure capability. 

How the housing works

The scroll channels the air discharged by the impeller into a single, defined outlet, converting rotational energy into static pressure with far greater efficiency than an open chamber allows. This is the configuration behind ducted installations, filtration lines, fume extraction, and any high-resistance environment where air needs to travel through a defined path rather than dispersing into open space.

Blade geometry matches the fluid

Blade geometry is what differentiates one housed centrifugal fan from another, and CBI's range covers the full spectrum of that variable.

• Backward flat-blade impellers prioritise structural robustness for demanding duty cycles;
• Backward-curved impellers trade some robustness for higher aerodynamic efficiency.
  

Certain configurations are engineered specifically for conveying dusty gases, granular materials, and long fibres, using an optimised scroll housing with multiple impeller options to match the fluid handled

Pressure range and special executions

Static pressure capability spans from a few thousand pascal up to the tens of thousands, depending on the configuration selected - a range no plenum fan can approach, precisely because it depends on the housing that plenum fans don't have.

Where explosive atmospheres are a factor, ATEX-compliant execution is available on request.

Plenum fan vs centrifugal fan: key differences

Which to choose and for which application

Plenum configuration suits multi-zone AHUs, space-constrained installations, and applications where moderate pressure and quiet operation are the priority - think comfort ventilation in commercial buildings, where fan arrays inside a central plenum need to serve several zones without the bulk of individually ducted units.

Housed centrifugal fans covers a wide operating envelope: dust and fume extraction where multiple impeller configurations handle abrasive or fibrous particulate, high-temperature duty in drying plants, boilers, and furnaces, and any process requiring materials beyond standard steel, since stainless steel AISI and ATEX executions are available on request. CBI engineers and manufactures both - with CFD/FEM engineering support available for non-standard configurations.