Airflow alone does not choose a fan
It is tempting to select a fan by airflow, as if every fan rated for the same volume were interchangeable. In practice the same fan delivers a different airflow depending on what it is connected to. The key to selection is understanding three linked quantities: airflow, pressure and system resistance. This article explains how they interact and how they point to a fan type. For a stepwise selection checklist that includes environment and noise, see the how-to article on choosing a fan.
The three quantities
Airflow is the volume of air moved per unit of time. It comes from the job: the ventilation rate for a space, the capture velocity at a hood, the air a process needs, the heat load to carry away. This is the starting point for fan airflow requirements.
Pressure is how hard the fan pushes. Fan data uses static pressure (the pressure that overcomes resistance), velocity pressure (the energy in moving air) and total pressure (their sum). For ducted systems, static pressure is usually the figure that matters for selection, but the exact convention a manufacturer uses should be checked against the documents.
System resistance is how much the ducts, fittings, filters, coils, dampers and grilles oppose the flow. It is not a fixed number.
Why resistance changes with airflow
In most systems the resistance rises rapidly as the airflow rises. As a rough rule, pressure loss in ducts and fittings increases with the square of airflow. So if you double the airflow through a system, the resistance is roughly four times higher. That relationship is the system curve.
A hypothetical example: suppose a duct run needs 200 units of pressure to move a certain flow. If you wanted 1.5 times that flow, the pressure needed would be about 200 x 1.5 x 1.5 = 450 units. These numbers are made up to show how quickly resistance grows. They are not real measurements.
The fan curve and the operating point
Every fan has a performance curve supplied by the manufacturer. At low airflow it can build high pressure; at high airflow, it can build less. The system curve rises with airflow, while the fan curve generally falls. Where they cross is the operating point, the airflow the fan will really deliver in that system.
This explains common problems.
- If the real system resistance is higher than assumed, the crossing point moves left, and the fan delivers less air than expected.
- If the system is less resistive than assumed, the fan delivers more air, possibly overloading the motor.
- If filters load with dust, resistance rises over time, and airflow slowly falls.
- Opening a branch or removing a damper changes resistance and therefore flow.
A good selection considers a range of conditions, including clean and dirty filters, not just a single design point.
How the numbers point to a fan type
Once you know the design airflow and the resistance at that airflow, the combination suggests a family.
- High airflow, low pressure. Open ventilation, wall fans, cooling of large coils, with little ductwork. Axial fans are typically a natural fit.
- Moderate airflow, moderate pressure. Ducted ventilation, mild filtration. Vane-axial, mixed-flow or centrifugal designs may all be candidates.
- Lower to moderate airflow, high pressure. Long duct runs, dust collectors, fine filters, burners and process systems. Centrifugal designs are typically preferred.
- Very high pressure. Beyond ordinary fans, multistage fans or blowers designed for higher pressure may be needed.
The characteristics of those families are described in industrial fans. Dusty or abrasive air may push the choice toward robust radial-blade wheels even if a more efficient design would work on clean air.
Efficiency and stability
The fan curve also shows where the fan runs most efficiently, usually near the middle of its range. Selecting a fan whose operating point is far to either side of this best-efficiency region wastes power and can cause noise or instability. Some fans, especially certain axial and forward-curved types, have regions on the curve where flow is unstable (sometimes called stall). The manufacturer’s documentation shows the recommended working range, and the operating point should lie well inside it, including at the extremes of expected resistance.
A fan that is too big for the job is not safe in any useful sense. It runs off its best point, often needs throttling, and costs more to run, which is discussed in fan energy use.
Speed and density
Two other factors adjust the numbers.
Speed. Changing a fan’s speed shifts its curve in a predictable way: airflow in proportion to speed, pressure with the square, power with the cube. This is why variable speed drives are such a handy way to match a fan to the system after installation.
Air density. Fan ratings are normally given for standard air conditions. Hot air, or air at high altitude, is less dense, and a fan moves a given volume at lower pressure and power. For hot process air, selection needs to account for the real density. Correction methods are in manufacturers’ literature.
Information to gather
Selection works best with:
- Required airflow at the design condition
- Estimated system resistance, with and without loaded filters
- Air temperature, moisture and contaminants
- Altitude or pressure, where relevant
- Space and mounting limits
- Noise limits and the plan for controls
Missing resistance data is the usual weak point. Calculations by a designer, or measurements on an existing system with a manometer, give far more reliable answers than a guess.
After installation
Verify performance: measure airflow and pressure where practical, check motor current against the rating, and check vibration. Compare with the design assumptions. If the fan is delivering less air than expected, review the resistance of the system before blaming the fan. A blocked filter, a closed damper or a fan running backward are all easy to overlook.
Final selection and any safety-critical decisions should involve the fan manufacturer or a qualified engineer. This article is general information, not engineering advice.