Fan selection: curves, efficiency and system effect

Fans are the heart of every air system, and they are often among the largest energy users in a commercial building. A well-selected fan runs quietly and efficiently across its operating range. A poorly selected fan wastes energy, makes noise, surges or fails to deliver its design airflow, sometimes all of these at once.

Fan selection is more than picking a model that meets the design airflow and pressure. It involves understanding fan curves, choosing the right fan type, operating near peak efficiency and accounting for how the installation affects performance. This guide covers each of these.

Reading a fan curve

A fan curve shows how a fan performs at a given speed. The main curves are:

• Pressure versus airflow: as airflow increases, the pressure the fan can develop generally decreases.
• Power versus airflow: shows the brake power required at each point.
• Efficiency versus airflow: shows where the fan converts shaft power to air power most effectively.

Multi-rating charts show these at several speeds, with lines of constant efficiency or power overlaid.

Fan curves are based on laboratory tests under ideal inlet and outlet conditions, typically performed to AMCA Standard 210 (ANSI/ASHRAE 51). Real installations rarely match these conditions, which is where system effect comes in.

The system curve and operating point

The system curve shows the pressure the ductwork, coils, filters, dampers and terminals require at each airflow. For fixed ductwork, pressure rises roughly with the square of airflow. The operating point is where the fan curve and system curve intersect.

If the actual system pressure is higher than calculated, the fan delivers less airflow than design. If lower, it delivers more. Field balancing then adjusts fan speed to reach design airflow. See fan laws and how to apply them.

Fan types

Backward-inclined and airfoil fans have a non-overloading power characteristic: power peaks near the middle of the curve and does not increase indefinitely with airflow. This protects motors if system pressure turns out lower than expected. Forward-curved fans lack this protection. See fan arrays and fan wall technology for multi-fan options.

Selecting the operating point

Stay to the right of the peak

Most fan curves have a peak in the pressure curve. To the left of the peak, operation can be unstable: the fan may surge, causing pulsating airflow, noise and vibration. Select the design point comfortably to the right of the peak, and consider where the operating point will move at low airflow in VAV systems.

Select near peak efficiency

The best selections place the design point near the fan's peak total efficiency, with some allowance for VAV turndown moving the operating point. A fan selected far from its peak efficiency wastes energy for its entire life.

Consider the whole operating range

In a VAV system, the fan spends most of its time well below design airflow. Check efficiency and stability across the expected range, not just at design. A fan that is efficient at design but surges at 40% airflow is a poor choice.

Efficiency metrics: FEG and FEI

The industry has moved from fan efficiency grade (FEG, AMCA 205) to fan energy index (FEI, AMCA 208). FEI compares a fan's power at a specific duty point with a reference fan. An FEI of 1.0 or higher indicates the fan meets the baseline efficiency at that point; higher values are better.

Because FEI is calculated at the actual duty point, it rewards correct selection, not just efficient fan models. Recent editions of ASHRAE 90.1 and some North American regulations have adopted FEI-based requirements. NECB requirements for fan power are expressed differently; check the edition adopted for the project.

System effect

System effect is the loss of fan performance caused by installation conditions that differ from the laboratory test. It is not a pressure loss in the ductwork itself but a reduction in what the fan can deliver. AMCA Publication 201 quantifies system effect factors for common conditions.

Common causes

• Elbows close to the fan inlet, which create uneven, swirling flow into the wheel.
• Inlet boxes and plenums with insufficient clearance around the inlet.
• Abrupt transitions or elbows at the fan outlet, before the velocity profile has developed.
• Obstructions such as bearings, guards, belt drives or structural members at the inlet.
• Inlet swirl in the direction opposite to wheel rotation, which increases power, or with rotation, which reduces capacity.

Accounting for system effect

1. Identify inlet and outlet conditions during design and layout.
2. Use AMCA 201 factors to estimate system effect pressure for each condition.
3. Add system effect to the calculated system static pressure when selecting the fan.
4. Better still, improve the layout: provide straight duct runs at inlet and outlet, use turning vanes where appropriate and allow proper clearances.

System effect is a common reason new fans fail to deliver design airflow. During air balancing, measured static pressure profiles often reveal significant losses at fan inlets and outlets.

Drives and motors

• Direct drive fans with variable frequency drives eliminate belt losses and maintenance. Fan arrays almost always use direct drive. See belt drives vs direct drives.
• Belt drives allow speed adjustment by sheave changes but lose efficiency through belt slip and wear.
• Motor selection should allow for the full operating range, altitude, VFD duty and service factor.
• Electronically commutated (EC) motors are common on small fans and fan arrays and are efficient at part load.

Selection checklist

1. Confirm the design airflow and calculated system static pressure, including system effect.
2. Choose a fan type suited to the pressure, airflow, space and noise requirements.
3. Select a size and speed with the design point near peak efficiency and to the right of the pressure peak.
4. Check stability and efficiency at minimum expected airflow.
5. Check FEI at the duty point and compliance with the applicable energy code.
6. Check sound power against the noise criteria for served spaces.
7. Size the motor and drive, considering altitude and VFD operation.
8. Coordinate layout to minimize inlet and outlet system effect.

Frequently asked questions

Why does a new fan not deliver design airflow?

Common causes include higher than calculated system pressure, system effect at inlets or outlets, incorrect rotation, wrong speed and closed or partially closed dampers. A static pressure profile across the unit usually identifies the problem.

Is a bigger fan more efficient?

Often a larger, slower fan operating near peak efficiency uses less power than a small fan running fast. The trade-off is cost and space. Compare selections at the actual operating point.

What is surge?

Surge is unstable operation to the left of the fan curve's pressure peak, where airflow pulsates. It causes noise, vibration and possible damage, and is avoided by proper selection and controls.

Key takeaways

• The operating point is where the fan curve meets the system curve.
• Select near peak efficiency, to the right of the pressure peak, and check the full VAV range.
• FEI rewards correct selection at the actual duty point.
• System effect from poor inlet and outlet conditions reduces fan performance and must be designed out.
• Direct drive and variable speed improve efficiency and reduce maintenance.