What are plug fans and why are they useful

Technical guide: Air treatment for compact industrial applications

You have limited technical space (e.g., an air handling unit, a compact ventilation unit, an industrial oven) and can’t install a traditional centrifugal fan with its bulky casing. Or you’re designing an air handling unit and want to maximize aerodynamic efficiency while reducing mechanical space. In both cases, a plug fan is the right technical solution. In this guide, we explain what a plug fan is, how it works, which applications it’s best used for, and how to size it correctly.

What is a plug fan

A plug fan is a centrifugal fan with a free-flowing impeller , meaning it lacks the casing that characterizes traditional centrifugal fans . The impeller operates directly in a pressurized plenum, without a casing to direct the outflow.

This makes it fundamentally different from a standard centrifugal fan not only in appearance but also in its operating principle: air is drawn in axially, accelerated by the impeller, and diffused radially within the plenum, where the pressure is evenly distributed before being conveyed to the subsequent sections of the system. The result is a compact, highly aerodynamically efficient component, specifically designed for integration into systems where space is limited and pressure drops must be kept to a minimum.

The structural difference from the traditional centrifugal

In a conventional centrifugal fan, the impeller is housed in a casing that collects the airflow and directs it to a tangential outlet. The casing takes up space, adds weight, and imposes specific installation constraints such as orientation, flange connections, and minimum compartment dimensions.

In the plug fan, the impeller operates without a casing, is mounted directly on the motor shaft, and is sized to operate at available static pressure. The system’s efficiency does not depend on the casing’s geometry, but on the impeller’s characteristic curve and the pressure of the plenum in which it is installed. The most common plug fan impellers have backward-curved blades or airfoil blades , which ensure:

• high aerodynamic efficiency (up to 75-80% in models with airfoil blades)
• no overload at maximum flow rate, the power curve decreases towards the right of the characteristic curve
• quiet operation compared to forward-bladed impellers

The motor is mounted directly on the impeller (direct coupling) or on supports external to the flow (indirect coupling), with the possibility of combining it with an inverter for speed regulation.

When to use and main applications

A plug fan isn’t a generic replacement for a centrifugal fan and has a specific application. It’s the right choice when one or more of the following conditions apply:

• Air handling units (AHUs) and ventilation units This is the primary use case. In modern AHUs, the plug fan has largely replaced the traditional centrifugal fan with a scroll, as it takes up less space in the delivery section, allows for a smaller unit size, and can be easily combined with inverters for flow modulation.
• High-efficiency industrial HVAC systems. Industrial buildings, logistics centers, hospitals, and data centers require continuous flow regulation and monitor energy consumption. A plug fan + IE3/IE4 motor + inverter is currently one of the most efficient combinations for high-efficiency systems compliant with EU Delegated Regulation 2024/1834.
• Industrial furnaces and heat treatment tunnels In forced air circulation furnaces (metal heat treatment, ceramic drying, paint polymerization), plug fans are used for internal hot air recirculation. In this context, the impeller is typically made of stainless steel or high-temperature alloys, and the motor is external to the heat flow.
• Paint booths and compact filtration systems. Where the fan must be housed in limited technical spaces, often requiring downstream filtration. The absence of a casing significantly reduces the overall size of the fan section.
• Cooling systems in electrical cabinets and racks Low pressure applications with extremely limited spaces, where the compact plug fan replaces traditional ducted axial fans.

Concrete technical advantages

The first noticeable advantage of installing a plug fan is the space it saves. Without a scroll, the volume of the fan assembly is reduced by 30–50% compared to a traditional centrifugal fan of the same capacity, which in AHUs translates into lower sections, more compact units, and greater freedom in the design phase.

Aerodynamically, the impellers with airfoil blades achieve total static efficiencies exceeding 75%, in line with the best centrifugal fans with scrolls. The non-overloading power curve adds a significant safety margin: even an imperfect motor selection rarely leads to operational problems.

The plug fan is also designed to work with an inverter. Speed ​​modulation is stable across the entire operating range, without the risk of instability or pumping, and the cube law does the rest: reducing the speed by 20% leads to energy savings of approximately 49%.

Maintenance is quicker than with a traditional centrifugal fan because the impeller is accessible from the front, without removing casings or scrolls. In AHUs with downstream filters, this translates into significantly reduced inspection and cleaning times.

In terms of acoustics, backward-curved and aerofoil impellers emit 5–10 dB less than forward-curved impellers of the same flow rate. In environments with regulated acoustic constraints, such as hospitals or office buildings, this is often the deciding factor.

Plug fan vs traditional centrifugal fan

Characteristic	Plug fan	Centrifugal juicer with snail
Snail	Absent	Present
Encumbrance	Reduced	Greater
Installation	In pressurized plenum	On flanged connection
Noise	Low–medium	Average
Main application	Air handling units	Localized extraction, ATEX
Dust resistance	Low-medium	High

When NOT to use a plug fan Suction of dust, shavings or suspended materials (risk of clogging of the open impeller), systems with high pressure drops in the ducts, ATEX environments (in this case use the special MZ Aspiratori certified series).

Key parameters for sizing a plug fan

Sizing a plug fan requires at least four starting data:

1. Required air flow rate (m³/h) Calculated based on the volume of the plenum or room, the required air changes/hour, and the pressure drops of the downstream system (filters, coils, dampers).
   Basic formula: Q (m³/h) = Volume (m³) × Changes/hour
2. Available static pressure (Pa) This is the pressure the plug fan must overcome to move the required flow rate through the system. It includes the resistances of the filters (clean and saturated), heat exchange coils, dampers, and ducts. Common mistake: sizing based on the pressure with a clean filter without considering the increase in resistance with a dirty filter.
3. Fluid temperature (°C) affects air density and therefore absorbed power. For applications with hot air (ovens, recirculation), the selection curve must be corrected with the density factor.
4. Installation conditions Plenum size, motor position (internal or external to the flow), noise constraints, and the need for an inverter. A plenum that is too small for the impeller reduces efficiency and increases noise.

In our many years of technical support, the most common mistake we encounter is underestimating the available static pressure. Those who size systems based on the nameplate data of a clean filter often find themselves with insufficient flow rates after just a few weeks of operation, when the filters begin to saturate. If you don’t have all the parameters, our technical team can help you calculate them. All you need is the system size and the type of application.