Inline Fans

Inline fans — also called in-line duct fans — install directly into the run of circular ducting, hidden away in a ceiling void, loft or service riser. Because the fan sits in the duct rather than on the wall or ceiling, you get powerful, quiet extraction with nothing visible in the room but a grille. At Alpha Air we supply inline duct fans from 100mm to 400mm diameter for everything from a single bathroom to full commercial kitchen and office ventilation systems.

Which inline fan do I need?

For domestic and light commercial extract, the TD inline mixed flow fans (100–400mm, with single and three phase options on the largest sizes) are our most popular choice, and the TD-SILENT ultra-quiet versions (100–315mm) suit noise-sensitive installs such as bedrooms, hotels and studies. The Turbo In-line range (100–315mm) and the compact Vents TT fans from Blauberg UK (100–150mm) give further options at domestic sizes, while the Versaline VL metal tube fan (100–315mm) offers a metal-cased alternative.

Where the duct run is long or resistance is high, choose a centrifugal or high-pressure design: the JETLINE inline centrifugal fans (100–315mm), the all-metal Elta Versaline VL (100–315mm) for low-airflow, medium-pressure systems, the EC-motor Elta Jetflow SJMF (100–355mm), or the Elta Multiflow SEL mixed flow fans (315–400mm available) with static pressures up to 1290 Pa across the range.

Delivery and accessories

Alpha Air is a Leicester-based UK ventilation supplier. Stocked items ordered by 3pm are dispatched for next-day delivery, and mainland UK delivery is free on orders over £400 ex VAT. Complete the job with fast clamps and flexible fan connectors, and add a speed controller to AC models for adjustable airflow.

What Is an Inline Fan?

An inline fan (also called a duct fan or tube fan) is a powered ventilation device designed to mount within the ductwork, rather than at the wall or ceiling extract point. The fan motor and impeller are housed in a cylindrical or box-section casing with spigot connections that join directly to the spiral or flexible duct on each side. Because the fan sits within the duct run — often in a ceiling void, riser, or plant room — it can be positioned away from occupied spaces, significantly reducing noise at the grille.

Types of Inline Fan

Mixed-Flow Inline Fans

Mixed-flow impellers combine axial and centrifugal airflow principles, delivering high airflow with moderate to high static pressure in a compact casing. They are the most popular choice for commercial ventilation, offering an excellent balance of performance, noise, and energy efficiency. Examples include the S&P TCBB/TCBT series and the Blauberg Turbo range.

Axial Inline Fans

Axial fans move air in a straight line parallel to the fan shaft. They deliver high airflow but lower static pressure than mixed-flow designs. Suitable for short, simple duct systems with minimal bends and fittings. Compact and cost-effective.

Centrifugal Inline Fans

Feature a centrifugal (scroll) impeller inside an inline casing. They generate higher static pressure than axial or mixed-flow fans, making them suitable for longer duct runs, systems with multiple bends, and applications with filters or other high-resistance components.

Selecting the Right Inline Fan

Key selection criteria include:

  • Required airflow (m³/h or l/s) — calculated from the room volume and the required air change rate
  • System static pressure (Pa) — the total resistance of the ductwork, bends, filters, dampers, and grilles that the fan must overcome
  • Noise level (dB(A)) — critical in occupied spaces; check both inlet and outlet sound power
  • Duct diameter — the fan connections must match your ductwork size
  • Temperature rating — standard fans operate up to 40°C; kitchen extraction requires 300°C or 400°C rated models

Plot the required airflow and pressure on the manufacturer’s fan curve to verify the fan operates in its efficient zone. Our technical team can perform this analysis for you — call 0116 246 0031.

Installation

Mount inline fans on anti-vibration pads or mounts to prevent vibration transmitting to the building structure. Use flexible duct connectors on both sides of the fan to isolate duct-borne vibration and noise. Ensure adequate straight duct length (minimum two diameters) upstream and downstream of the fan for optimal performance. Provide an accessible location for future maintenance and motor replacement.

Why Choose Alpha Air?

With many years supplying ventilation fans to the UK trade, Alpha Air offers expert fan selection advice backed by deep technical knowledge. We stock inline fans from S&P, Blauberg, and other leading brands for same-day dispatch. Free delivery on orders over £400.

Frequently Asked Questions

How do I choose between a mixed-flow and an axial inline fan?

If your system has a long duct run, multiple bends, filters, or dampers, choose a mixed-flow fan for its higher pressure capability. For short, straight duct runs with minimal resistance, an axial fan may be sufficient and will cost less. Check the fan curve against your system resistance to confirm.

Can inline fans be speed controlled?

Yes. Most inline fans can be connected to an external speed controller (transformer-type for AC motors, or an inverter for EC motors) to adjust airflow. Speed control reduces noise and energy consumption when full-speed operation is not needed.

Do I need flexible connections on an inline fan?

Flexible connections on both the inlet and outlet of an inline fan are strongly recommended. They absorb vibration from the motor and impeller, preventing it from transmitting through the rigid ductwork and into the building structure, which would cause noise and potential fatigue damage to joints.

What is a kitchen-rated inline fan?

A kitchen-rated inline fan is certified for continuous operation at elevated temperatures, typically 300 degrees C for 1 hour or 400 degrees C for 2 hours. UK regulations require this rating for commercial kitchen extraction to ensure the fan continues operating during a fire. We stock fire-rated models from S&P.

Related Categories

Flexible DuctingCased Axial FansPlate Fans