Inline Duct Fan

Overview

An inline duct fan is a fan built into a short length of tube so it can be inserted anywhere in a run of round ductwork, in a ceiling void, a loft, or a riser, rather than at a wall or roof termination. The format dominates residential and light-commercial extract ventilation (bathrooms, kitchens, offices), duct boosting, and applications like grow-room and server-closet ventilation where airflow against several hundred pascals of duct resistance is needed from a compact, quiet package. Units are made in the standard European duct diameters from 100 to 315 mm, moving from about 100 to 2,500 m³/h.

The product combines an Mixed-Flow Impeller and EC Motor cartridge inside a tubular Fan Housing, with a Speed Controller, an external Terminal Box, and Mounting Hardware hardware.

Mixed-flow aerodynamics

The defining component is the mixed-flow Impeller Wheel: a one-piece glass-filled polymer impeller whose conical hub and backward-swept blades accelerate air partly axially (along the duct) and partly radially (outward). The air leaves the blade row at an angle, is turned and de-swirled by fixed Outlet Guide Vanes, and exits along the duct axis. This hybrid geometry develops two to three times the pressure of a pure axial fan in the same tube diameter, without the bulky scroll housing a centrifugal fan needs, which is exactly the trade an in-duct package requires. An Inlet Contraction Ring contracts the incoming flow smoothly into the blades, and small Balance Clip weights added at the factory bring the rotor within balance grade G6.3, the main determinant of transmitted vibration. The wheel locks onto the Motor Shaft through its Impeller Hub.

EC motor

Nearly all current inline fans use an electronically commutated motor: a permanent-magnet rotor carrying Neodymium Magnet poles, a wound Stator Assembly, and Integrated Drive Electronics built into the motor itself. The onboard inverter rectifies 230 V mains and switches the windings through six Power MOSFET devices, with Hall Sensor feedback for commutation. The result is better than 70% motor efficiency where the shaded-pole AC motors of older fans managed about 25%, plus an intrinsic capability the AC motor never had: smooth, stepless speed control from an external 0–10 V or PWM signal, with constant-torque behaviour that holds airflow as filters load up. The rotor spins in two sealed-for-life Ball Bearing units rated beyond 40,000 hours, which in continuous extract service is the real service life of the product.

Housing and installation

The Casing Shell is molded ABS on smaller sizes and rolled galvanized steel on larger ones. Each end carries a Duct Spigot sized to EN 1506 nominal diameters with a rubber lip seal, so the fan push-fits between duct sections and clamps with standard duct clips. The motor hangs in the center of the airway on a Motor Support Spider spider; better designs split the casing in half so the whole motor-impeller cartridge can be swapped without disturbing the duct. Installation is orientation-free: a Mounting Bracket saddle clamps the casing to wall, ceiling, or threaded rod through rubber Anti-Vibration Mount bobbins, and a short Flexible Duct Connector sleeve at each spigot breaks the rigid acoustic path so motor and flow noise do not telegraph down the sheet-metal duct. Field wiring lands in the IP44 Terminal Box through compression Cable Gland entries onto the Terminal Block.

Control

Control ranges from trivial to closed-loop. The simplest installations switch the fan from the room light and rely on the built-in Run-On Timer, adjustable from about 2 to 30 minutes, to keep extracting moisture after the bathroom empties. A Speed Potentiometer on the controller or motor sets maximum speed at commissioning. More capable versions pair the Controller Board with a humidity sensor or a duct Pressure Sensor: in constant-pressure mode the fan serves several intermittently used extract points, speeding up automatically as valves open. Because the EC motor accepts a standard 0–10 V command, the same fan drops into building-management systems without any added drive hardware.

Ratings

Aerodynamic performance is measured to ISO 5801 and published as a pressure-flow curve; sound data distinguishes duct-radiated from casing-radiated levels, typically 25–45 dB(A) at 3 m for the casing path. Within the EU, the ErP regulation 327/2011 sets minimum fan efficiency grades that effectively eliminated AC shaded-pole designs from the market, and household-installation safety falls under IEC 60335-2-80.