One of the most common solutions for moving air in ventilation ducts is the duct-type (in-line) axial fan. These fans consist of an impeller placed in a circular housing and an electric motor driving it directly; they are mounted straight into the duct line and push the air along the axis. Duct-type axial fans are preferred in many applications such as car parks, kitchen exhausts, industrial plant ventilation, and warehouse and workshop air conditioning. The heart of this system is the motor: the fan delivering the desired flow and pressure depends on the motor being selected at the right power and speed. A wrongly selected motor either moves the air insufficiently, overdraws and overheats, or fails to do its job in a hot or smoky environment. In this guide we cover step by step the selection of a duct-type axial fan motor in terms of the flow-pressure curve, motor power and speed, compact body, temperature and smoke (fire F300/F400) variant, IP protection class and flow adjustment with a VFD. The aim is to select on the first attempt the motor that provides the right flow with the right power and long life in your ventilation duct.

Duct-type (in-line) axial fan and drive motor mounted in a ventilation duct

How the Axial Fan and Motor Work

An axial fan moves air in a direction parallel to the impeller axis; this is ideal for moving high flow at low and medium pressure. In a duct-type fan the motor either runs directly coupled to the impeller (direct drive) or sits within a compact housing together with the impeller. The motor speed directly determines the flow and pressure characteristic of the fan. The choice between a centrifugal and an axial fan is made according to the pressure need; we covered this subject in detail in our centrifugal and axial fan motor selection article. General HVAC fan motor supply can be found in our fan motor supply in HVAC projects content.

Flow and Pressure Curve

The performance of a fan is defined by a curve plotted on flow (m³/h) and static pressure (Pa) axes. The point where the duct line resistance (pressure loss) intersects the fan curve is the system operating point. When selecting the motor, the power need at this operating point is taken as the basis. If the duct is long and resistive, higher pressure and therefore more power is needed. We detailed the power calculation in pumps, fans and conveyors in our motor power calculation: required kW article and load ratio and correct sizing in our motor load ratio and correct sizing content.

Speed and Pole Selection

The fan motor speed directly affects the flow and the noise level. High-speed (2-pole, 3000 rpm) motors are compact and high-flow but noisier; low-speed (4- or 6-pole) motors run quieter and more balanced. In ventilation, 1500 and 1000 rpm motors are usually preferred. We covered pole selection in our 2, 4, 6 poles which for which job article and IE4 2-pole pump-fan power selection in our IE4 2-pole 3000 rpm pump and fan content. Low-noise motor selection can be found in our noise and vibration: low-noise motor selection article.

System Resistance and Operating Point

In a duct system the real flow of the fan depends not only on the fan capacity but on the total resistance of the duct line. Elbows, narrowing cross-sections, dampers, filters and a long duct line create pressure loss; as this loss increases the fan moves less air at the same speed. For this reason, before selecting the motor, the pressure loss of the duct system must be calculated correctly and the flow the fan delivers at this pressure must be checked. The gradual clogging of filters also shifts the operating point. We covered a similar holistic view in pump systems in our real efficiency in a pump system article and motor selection in in-line circulation applications in our in-line and circulation pump motor selection content.

Motor Power and Correct Sizing

The fan motor power must be selected to match the shaft power at the operating point. A motor chosen too small heats up and burns out; a motor chosen too large runs inefficiently at low load and brings unnecessary cost. A point to watch in fan applications is that the fan draws more air (and therefore more power) when the duct is open or the filter is clean; for this reason the motor must be selected according to the worst-case power need. We covered the effect of efficiency class on long-term cost in our total cost of ownership (TCO) article and the IE4 threshold in pumps and fans in our IE4 threshold in pumps, fans and compressors content.

Efficiency and Energy Saving

Ventilation fans often run long hours, even continuously. For this reason the motor efficiency makes a noticeable difference in the annual energy cost. High-efficiency IE3 and IE4 motors pay back the investment quickly in continuously running fans. Moreover, since regulation also makes a high efficiency class mandatory at certain powers, the correct efficiency class selection is important for both legal compliance and cost; we covered the efficiency mandate in our IE3 and IE4 efficiency mandate article. We covered efficient motor supply in HVAC and air handling unit fans in our IE4 motor HVAC air handling unit and AHU fan supply article and ventilation motors in our aspirator and dust collection fan motor selection content.

High-temperature rated (F300/F400) duct-type axial fan motor for smoke exhaust

Temperature and Smoke Exhaust (F300/F400) Variant

A special application of the duct-type axial fan is smoke exhaust. In car parks, tunnels, malls and enclosed spaces, the fans that expel smoke during a fire must withstand high temperature for a certain time. These fans are defined by a temperature-time class: for example F300 (at 300°C for a certain time) or F400 (at 400°C for a certain time). In this application the motor must be of a special design that can protect its winding and bearing at high temperature. We detailed smoke exhaust fan motor supply in our smoke exhaust fan motor: mall and car park projects article and tunnel and metro jet fan motors in our tunnel, metro and motorway jet fan motors (F300/F400) content.

Power Derating in Hot Environments

At high ambient temperature or high altitude the power the motor can deliver decreases; this is called power derating. Fan motors carrying hot air or working in a hot environment must be selected taking this effect into account. We covered insulation class and cast iron body selection at high temperature in our insulation class in hot and dusty environments article and the derating calculation in our derating at high altitude and hot environment content. Power derating in facilities where the ambient temperature exceeds 40°C can be found in our derating at high ambient temperature article.

IP Protection Class and Mounting

A duct-type fan motor must be selected in an IP protection class suited to its operating environment. While IP54/IP55 is usually sufficient in indoor clean-air ducts, higher protection is needed in humid, dusty or outdoor-exposed ducts. In exhaust and kitchen hood lines oil and moisture threaten the motor; in outdoor ducts water and dust do. We covered IP protection class selection in our IP protection class selection: IP55, IP65, IP66 article and IP protection in cooling tower and outdoor fan motors in our cooling tower and chiller fan motors content.

Compact Body and Mounting Type

In a duct-type fan the motor must be in a compact body that fits the duct diameter. The mounting type (B3, B5, B14, B35) is chosen according to the fan structure; in fans directly coupled to the impeller, a flanged (B5/B14) connection is usually used. We covered mounting types in our B5 or B14 mounting type selection article and shaft and coupling compatibility in our shaft diameter and key dimensions content. If you are replacing an old fan motor, our direct replacement of an old-brand motor article guides equivalent selection.

Application Examples and Sector Supply

The duct-type axial fan motor is used across a very wide range of applications. Process ventilation in industrial plants, air circulation in food and textile facilities, car park exhaust and general air conditioning are among the foremost. Each sector has a different ambient condition; humid, dusty, oily or hot environments require different protection and insulation. We covered HVAC sector motor supply in our fan motor supply in HVAC projects article and vacuum and blower motors in our vacuum pump and industrial blower motors content. Motor supply for uninterrupted ventilation in continuous process can be found in our IE4 motor continuous process paper and textile line article.

Flow Adjustment with a VFD

The ventilation need changes through the day; always running at full flow is both a waste of energy and a source of unnecessary noise. With a frequency drive (VFD) the flow can be adjusted by reducing the fan speed. In variable-torque loads such as fans, by the affinity law, reducing the speed by a small ratio reduces the power consumption by a much larger ratio; this makes the VFD the most profitable saving tool in ventilation. We covered pump-fan saving with a VFD in our VFD energy savings in pumps and fans article and when a frequency drive is needed in our frequency drive (VFD) with asynchronous motor content. The efficient motor and drive combination can be found in our high-efficiency motor + frequency drive article. For our product range you can visit our homepage.

Vibration and Quiet Operation

A ventilation fan often works close to occupied spaces; for this reason the vibration and noise level matter. An unbalanced impeller or a poorly aligned motor creates both noise and early bearing failure. We covered vibration acceptance values in our vibration and balance ISO 10816/20816 acceptance values article and quiet operation in IE4 motors in our quiet and low-vibration operation in IE4 motors content.

Cooling and Bearing Life in Continuous Operation

Since ventilation fans often run throughout a shift or continuously, the motor must be suitable for continuous duty (S1) and have a long bearing life. In a continuously running fan motor the bearing lubrication, the cleanliness of the cooling fins and the bearing type determine the real life of the motor. We covered bearing type and life in our bearing type and life article and duty type selection in our duty type (S1-S6) selection content. Cooling methods can be found in our cooling methods IC411 and IC416 article.

Frequently Asked Questions

How should I size the motor for a duct-type axial fan?

First you need to determine the fan operating point: the point where the duct line pressure loss intersects the fan curve gives the required flow and pressure. A motor matching the shaft power at this point, with some safety margin, is selected. It must not be forgotten that the fan can draw more air (and power) when the filter is clean or the duct is open; the motor should be selected according to the worst-case power. The speed (pole count) is determined by the desired flow and the acceptable noise level. The most accurate result is to share your fan curve and system resistance with the supplier.

Is a smoke exhaust fan motor different from a normal fan motor?

Yes, it is markedly different. A smoke exhaust fan motor must be of a special design that can withstand high temperature (for example 300°C or 400°C) for a certain time during a fire. The winding insulation, bearing and connection materials are selected to suit this temperature-time class (such as F300/F400). A normal ventilation motor cannot do its job under these conditions. Therefore, in smoke exhaust applications a purpose-built motor of the appropriate class must always be supplied.

Does it make sense to use a VFD on a fan motor?

In continuously running ventilation fans whose flow need changes, a VFD usually makes a lot of sense. A fan is a variable-torque load; by the affinity law, reducing the speed a little reduces the power consumption much more. This saves energy, lowers noise and extends mechanical life through soft starting. In drive operation the motor should be selected as inverter duty and low-speed cooling should be considered. In applications requiring a fixed, unchanging flow, the cost of a VFD may not always be necessary.

Get a Quote

Let us determine together the duct-type axial fan motor in the right flow, pressure, speed and protection class, and in the smoke exhaust (F300/F400) variant if needed, for your ventilation duct. Share your fan curve, operating point and ambient conditions; we will quickly offer you a suitable solution. To request a quote, reach us through our contact page or call now: +90 (532) 345 49 86.

Duct-Type Fan Motor Selection Checklist

  • Has the fan operating point (flow and static pressure) been determined?
  • Have the shaft power at the operating point and the safety margin been calculated?
  • Has the worst-case (clean filter/open duct) power need been taken into account?
  • Has the speed/pole count been selected according to flow and noise level?
  • Has the efficiency class (IE3/IE4) been evaluated for continuous operation?
  • If smoke exhaust is needed, has an F300/F400 class motor been selected?
  • Has power derating been calculated for a hot environment or high altitude?
  • Has an IP protection class suited to the environment (IP54/IP55 or higher) been selected?
  • Are the compact body and mounting type (B5/B14 etc.) suitable for the fan?
  • If flow adjustment is needed, have a VFD and an inverter-duty motor been planned?
  • Are the vibration and noise level within acceptance values?