How to Size an Axial Fan: A Step-by-Step Method With a Worked Example

Sizing an axial fan means finding the airflow the process needs, adding up the resistance the fan must overcome, correcting for the real air density, and then checking the operating point against a fan curve. The most common errors are skipping the density correction, ignoring velocity pressure, and adding a large safety margin that pushes the fan toward stall. This guide walks through the method with a worked example whose numbers you can reuse.

Step 1: Establish the Required Airflow

Airflow comes from the purpose of the fan.

Heat removal. Airflow = sensible heat load ÷ (air density × specific heat × allowed temperature rise). A hall with 400 kW of sensible heat and a 6 K allowed rise, using 1.15 kg/m³ and 1.006 kJ/kg·K, needs 57.6 m³/s.

Air changes. Codes and good practice set air changes per hour for many spaces. Convert to m³/s by multiplying room volume by air changes and dividing by 3,600.

Contaminant dilution. For a steady contaminant source, required airflow is the generation rate divided by the allowed concentration difference. Local exhaust at the source is more effective than dilution for hazardous contaminants.

Process needs. Cooling towers, coolers, and drying processes state their own airflow requirement.

Write the airflow in both m³/s and m³/h, and state whether it refers to actual conditions or standard air.

Step 2: Add Up the Resistance

List every element the air passes through and estimate its pressure loss at the design flow: inlet louvres or guards, bellmouth, ducts and bends, dampers, silencers, filters, and outlet cowls or weather hoods. Resistance rises with the square of flow, so a system curve can be drawn from one calculated point.

Add an allowance for fouling. Filters load, louvres collect dust, and screens block. Calculate the system at clean and at fouled resistance and check the fan against both.

Step 3: Choose a Trial Fan Diameter and Find Velocity Pressure

Velocity pressure is the energy in the moving air leaving the fan. It is calculated from air density and velocity:

Velocity pressure = ½ × density × velocity²

Velocity depends on flow and the fan’s outlet area, so diameter matters. For 30 m³/s at a site air density of 0.921 kg/m³:

Fan diameter	Outlet area	Outlet velocity	Velocity pressure
1,250 mm	1.227 m²	24.4 m/s	275 Pa
1,400 mm	1.539 m²	19.5 m/s	175 Pa
1,600 mm	2.011 m²	14.9 m/s	102 Pa

A larger diameter reduces velocity and velocity pressure, which cuts energy and noise, but costs more and takes more space. Our fans reach 1,600 mm diameter.

Step 4: Compute the Required Total Pressure

Fan total pressure combines the static resistance and the velocity pressure at the outlet when the outlet velocity is lost to the atmosphere.

Worked example. Take 30 m³/s (108,000 m³/h) through a 1,400 mm fan at a site 1,800 m above sea level with air at 35 °C, giving a density of 0.921 kg/m³. The system static resistance, at site density, is 300 Pa.

• Velocity pressure = 175 Pa
• Required total pressure = 300 + 175 = 475 Pa

Step 5: Correct for Air Density

Fan curves are normally published at standard density, about 1.204 kg/m³. To read a standard-air curve, convert the site requirement:

Equivalent standard-air pressure = required pressure × (1.204 ÷ site density)

In the example: 475 × (1.204 ÷ 0.921) = 621 Pa. So you select a fan that delivers 30 m³/s at about 621 Pa on the standard-air curve. Ignoring this step would under-size the fan by about a quarter. See air density and the fan laws.

Step 6: Select From the Fan Curve

Plot the system curve and read the operating point on the candidate fan’s curve. Check:

– flow and pressure meet the duty at both clean and fouled conditions, – the operating point is clear of the stall region, with margin, – efficiency is near the best-efficiency region, – tip speed and predicted noise are acceptable.

Our guide to reading an axial fan curve explains these checks.

Step 7: Size the Motor

Air power = flow × total pressure. For the example: 30 × 475 = 14.25 kW. At an assumed 75% total efficiency, shaft power is about 19 kW. Motors are sized above the calculated shaft power to allow for tolerances, density variation, and start-up. A margin of around 10% to 15% is common practice, but confirm it against the applicable standard and the worst-case power point, since power is highest at the densest, coldest air and may rise at lower flow. Do not oversize the motor without reason, because a large margin on the fan itself invites operation nearer stall.

Step 8: Decide the Control Method

If the load varies, plan for control from the start: a variable-speed drive, adjustable-pitch blades, or staging of parallel fans. Sizing for the maximum duty and controlling down is usually better than throttling. Our guide to adjustable pitch versus VFD control compares the options.

Step 9: Split the Duty When One Fan Is Not Enough

Our axial fans reach 185,000 m³/h, about 51 m³/s. If the duty is larger, or if you want redundancy, split it across two or more fans in parallel. The heat-removal example above needs 207,000 m³/h, so two fans of about 103,700 m³/h each is a natural arrangement. Parallel fans give partial operation if one fails, allow staged control, and make maintenance easier. Check the combined curve and the isolation of an idle fan, since air can recirculate backward through a stopped fan unless a damper or backdraught shutter prevents it.

Common Sizing Mistakes

1. Comparing static pressure on one fan with total pressure on another.
2. Forgetting the velocity pressure that is lost at a free discharge.
3. Skipping the air density correction at altitude or high temperature.
4. Using only clean-filter resistance.
5. Adding a large safety factor to the flow and pressure, which moves the operating point toward stall.
6. Sizing the motor for the design point only.

Have Us Size Your Fan

Send your airflow, system resistance, and site conditions, and we’ll size the fan, check the operating point, and confirm the motor. Contact us for a free quote.

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