Quick answer: An inline fan is a fan fitted inside a duct run, so it moves air through the duct without needing to sit at a wall or roof opening. The three common types are centrifugal, mixed flow and axial. To size one, you need two numbers: the airflow you need (in m³/h or CFM) and the static pressure of the duct system (in Pa). Plot both on the manufacturer’s fan curve as your duty point, include any airflow allowance required by the project design, and pick a fan that can operate at that point within its recommended stable operating range. In the UAE, also check the motor’s heat rating and dust protection, because summer conditions are tougher than most catalogue data assumes.
The rest of this guide explains how inline fans work, which type to choose, how to size one step by step, and the mistakes that cause weak airflow or noisy installations on site.
What Is an Inline Fan and How Does It Work?
An inline fan is a duct fan whose motor and impeller sit in line with the duct work. Air enters one end and leaves from the other in the same direction. The casing is usually a round or rectangular duct section, so the fan can be hidden in a ceiling void, a plant room or a service shaft.
The motor turns an impeller, which pulls air in and pushes it forward. The fan has to overcome resistance from straight duct, bends, dampers, grilles, filters and silencers. That total resistance is called static pressure. If a fan cannot beat it, it will move far less air than its label says.
Contractors use inline fans to supply fresh air, extract stale or hot air, or lift airflow in a long duct run. Because the fan sits in the duct, it can be placed away from the occupied space, which helps keep noise down.
What Are the Main Types of Inline Fans?
There are three common types, and each suits a different job.
| Type | How it moves air | Best for | Watch out for |
| Centrifugal inline fan | Impeller throws air outward, casing turns it forward | Longer duct runs, filters, higher static pressure | Larger and heavier |
| Mixed flow inline fan | A mix of axial and centrifugal movement | Good balance of airflow and pressure in a compact body | Check the fan curve closely |
| Axial (tube axial) inline fan | Air moves straight through the impeller | High airflow at low pressure, short open ducts | Airflow drops quickly when resistance rises |
If you only need to push a little extra air through one weak branch, a duct booster fan may be a better fit than a larger inline unit.
Motor choice matters too. AC motors are simple and cost less upfront. EC motors give smooth speed control and usually use less power at part load. If the fan runs many hours a day, the energy savings can justify the higher price. Our guide to choosing fan motors for HVAC and refrigeration covers the basics.
Where Are Inline Fans Used in the UAE?
Inline fans show up across most commercial and industrial buildings:
- Plant and electrical rooms: to remove heat from equipment.
- Refrigeration machinery rooms: where mechanical ventilation is required according to the refrigerant system project design and applicable standards such as ASHRAE 15 and EN 378.
- Cold storage and logistics buildings: for fresh air and ventilation in dock areas and anterooms.
- Commercial kitchens: for make-up air or general ventilation. Standard inline fans should not be used for grease-laden cooking exhaust unless specifically designed and certified for that application.
- Toilets, basements and car parks: for general extract.
- Offices, malls and hotels: for fresh air supply and zone ventilation.
How Do You Size an Inline Fan?
Step 1: Find the airflow you need. The simplest method uses air changes per hour (ACH):
Airflow (m³/h) = Room volume (m³) × Air changes per hour
These are preliminary rule-of-thumb ranges only and are not UAE authority design requirements. Always confirm the applicable local code, consultant specification and project requirements.
| Space | Typical ACH |
| Toilets | 10 to 15 |
| Kitchens (general) | 15 to 30 |
| Plant and machinery rooms | 10 to 20 |
| Workshops | 6 to 10 |
| Warehouses | 4 to 6 |
For plant and electrical rooms, the heat-load method is more accurate:
Airflow (m³/s) = Heat load (kW) ÷ (1.2 × 1.005 × allowed temperature rise in °C)
For preliminary calculations, 1.2 kg/m³ may be used for air density, but final calculations should consider the actual design air density, especially at high ambient temperatures.
For example, a 10 kW heat load with a 5 °C allowed rise needs about 1.66 m³/s, which is roughly 5,970 m³/h.
Step 2: Work out the static pressure. Add up the resistance of every part of the system: straight duct, bends, dampers, grilles, filters and silencers. Include the expected dirty-filter pressure drop. The total, in pascals (Pa), is your static pressure.
Step 3: Add any required allowance. Include any airflow allowance required by the project design. Also check duct joints for leakage, because air leakage can significantly reduce delivered airflow.
Step 4: Check the fan curve. Every fan has a curve showing how much air it moves at each pressure. Plot the required airflow and static pressure as the duty point on the manufacturer’s fan curve, and select a fan that can operate at that duty point within its recommended stable operating range.
Worked example: A plant room measures 8 m × 5 m × 3.5 m.
- Volume: 8 × 5 × 3.5 = 140 m³
- Design rate: 15 ACH, so airflow = 140 × 15 = 2,100 m³/h
- With an assumed 10% allowance for this example: 2,310 m³/h, which is about 1,360 CFM
- Duct and fittings 120 Pa, grille 60 Pa, silencer 30 Pa, so total static pressure = 210 Pa
You need an inline fan that delivers at least 2,310 m³/h at 210 Pa. Choosing by airflow alone would give the wrong fan.
A useful conversion: 1 CFM equals 1.699 m³/h, and 1 L/s equals 3.6 m³/h.
What Happens When You Change Fan Speed?
Fan laws explain it. For the same fan under similar operating conditions, airflow changes approximately in direct proportion to speed, pressure with the square of speed, and power with the cube of speed. So a small drop in speed can cut power use a lot. That is why a variable speed drive or an EC motor pays off on fans that do not need full airflow all day.
Control can also be based on temperature. Fit a compatible speed controller where variable speed is required. A thermostat can be used for on/off control, or with a compatible controller, depending on the motor and control system. You can browse thermostats and temperature controllers to match the control to the fan.
Why Does UAE Climate Change Inline Fan Selection?
- Heat: Summer outdoor temperatures often go above 45 °C. Check the motor’s maximum ambient rating, especially for roof and plant-room installations.
- Dust and sand: Choose sealed motors, protect intakes with filters and plan regular cleaning. Select an IP rating suitable for the actual installation environment and project requirements.
- Humidity and coastal air: Along the coast in Dubai, Abu Dhabi and Sharjah, salt and moisture speed up corrosion, so look for coated or corrosion-resistant casings.
- Fire safety: A standard inline fan is not a smoke extract fan. Smoke control needs fans certified for high temperature, such as units tested under EN 12101-3 (for example F300, which means 300 °C for 60 minutes). Always follow the project’s fire and life safety requirements.
For the wider ventilation rules that sizing decisions come from, the ASHRAE Standard 62.1 fact sheet explains minimum ventilation rates for indoor air quality.
What Mistakes Lead to Weak Airflow or Noisy Inline Fans?
- Sizing by airflow only. Ignoring static pressure is the most common reason a fan under performs.
- Installing right next to a bend. This causes turbulence and noise. Leave straight duct before and after the fan where possible.
- Choosing a fan that is too big. It wastes energy and adds noise.
- Skipping flexible connectors or vibration isolation. Where the installation and the manufacturer require them, leaving them out lets vibration travel through the duct work.
- Forgetting service access. A fan that cannot be reached does not get cleaned or checked.
- No speed control for variable loads. The fan runs flat out when it does not need to.
- Using a standard fan for smoke extract. It will not survive the temperatures that smoke control requires.
Which Inline Fans Does Haste Offer?
Haste currently lists the Klemax SW series of inline fans, all 220V duct fans:
These suit small to medium duct runs. Confirm the actual connection diameter from the manufacturer’s data-sheet or the verified product specification for each model, rather than relying on the model name.
Here is a quick match: a small store room measures 3 m × 3 m × 2.8 m, which is about 25 m³. At 10 air changes per hour you need about 252 m³/h. Add a 10% allowance and the required airflow is about 280 m³/h. That is below the SW-150’s listed maximum airflow, but final suitability still depends on the airflow available at the actual system static pressure.
One caution: the listed maximum airflow may be measured at or near free-air conditions. Once you add bends, grilles and filters, real airflow drops, so always check the manufacturer’s fan curve at your required static pressure. A load like the plant room example above (2,310 m³/h) is beyond a single SW-250, so talk to the team about the options.
Inline Fan Selection Checklist
Before you order, confirm:
- Required airflow (m³/h or CFM)
- Total static pressure (Pa)
- Duct size and shape
- Fan type (centrifugal, mixed flow or axial)
- Motor type (AC or EC) and power supply
- Ambient temperature and IP rating
- Noise limit in dB(A) and whether a silencer is needed
- Speed control needs
- Performance data tested to a recognized method, such as AMCA 210
- Access for service
Frequently Asked Questions
What is an inline fan used for? An inline fan moves air through a duct for supply, extract or boosting. It is common in plant rooms, kitchens, toilets, car parks and cold storage buildings.
How do I calculate the CFM I need for an inline fan? Multiply the room volume by the air changes per hour to get m³/h, then divide by 1.699 to get CFM. Add any allowance required by the project design.
What is the difference between an inline fan and a duct booster fan? An inline fan is the general type of fan that sits in the duct. A duct booster fan is a smaller unit used to lift airflow in a weak branch.
Which is better, an EC motor or an AC motor? EC motors save more energy and give smooth speed control. AC motors cost less to buy. For long running hours, EC is usually the better choice.
Can I use a standard inline fan for smoke extraction? No. Smoke extract needs fans certified for high temperatures, such as those tested to EN 12101-3.
Can inline fans be used outdoors in the UAE? Yes, if the fan has the right IP rating, weather protection and a motor rated for local ambient temperatures.
How often should an inline fan be serviced? Follow the manufacturer’s maintenance schedule and inspect more frequently in dusty or high-use installations. Look at the impeller, bearings, vibration, filters and electrical connections.
Do I need a variable speed drive? If airflow demand changes, a variable-speed solution may save energy, but first confirm that the fan motor is compatible with a VFD, or use an EC fan with suitable speed control.
Summary
- An inline fan sits inside the duct and moves air through it. The main types are centrifugal, mixed flow and axial.
- Size it with two numbers: airflow (m³/h or CFM) and static pressure (Pa).
- Airflow = room volume × air changes per hour, plus any allowance required by the project design.
- Always check the manufacturer’s fan curve, not just the maximum airflow on the label.
- UAE heat, dust and coastal air affect motor rating, IP protection and casing choice.
- A standard inline fan is not a smoke extract fan.
- EC motors and compatible speed control save energy on fans that run long hours.
Getting it right comes down to knowing your airflow and pressure, choosing the right type, and checking the fan curve before you order.
Haste Trading has supplied HVAC/R products in the UAE since 1983 and delivers across Dubai, Abu Dhabi and Sharjah. If you are unsure which fan fits, send us your airflow, static pressure and duct size and get in touch with our team.
This guide draws on general HVAC ventilation practice and published standards such as ASHRAE 62.1, ASHRAE 15, EN 378 and EN 12101-3. ACH values are typical rules of thumb. Always confirm the exact requirement against the project specification, the consultant’s design and local authority rules.