By Steve Back
New York Blower Company
Willowbrook, Illinois
Performance curves are among the most crucial and frequently requested documents from fan manufacturers, second only to dimensional prints. While many request these documents, not everyone fully understands their significance or maximizes their potential.
Fan performance curves are generated through laboratory and/or field testing. Fans are tested in accordance to international standards such as AMCA 210 and ISO 5801 for laboratory testing along with AMCA 203 and ISO 5802 for in-situ (field) testing. Data points are collected at a set speed while the flow is modulated from full closed to full open, creating a fan performance curve representing the fan’s capabilities. Additional testing is usually performed at different speeds to get a comprehensive picture of the fan's aerodynamic and noise performance.
Performance curves serve multiple functions. They graphically depict basic performance data such as Fan Volume Airflow Rate (Q), Mass Flow Rate (ṁ), Rotational Speed (N), Fan Pressure (P), Sound Pressure Levels (Lp), Fan Efficiency (η), and Fan Power Input (Hi). Beyond these raw metrics, the curves illustrate crucial performance characteristics of various fan types, including areas of instability and the rate of change between airflow and pressure.
While capacity tables published in literature offer a convenient way to catalog this data, they are essentially simplified versions of the test results. Working with the source document — the performance curve itself — provides additional valuable information, particularly when determining capacities at varying speeds or conditions beyond published capacity tables. The data collected during testing also serves as the foundation for developing computer selection programs and published capacity tables that system designers and end users rely upon.
With a solid understanding of performance curves, engineers and technicians can make informed decisions about fan selection, system modifications, and the suitability of a fan for specific applications. These curves become invaluable tools for determining the feasibility of fan or system alterations and calculating resultant speed and power requirements.
Types of Fan Performance Curves
There are many ways to present fan performance curves. This article is going to concentrate on the three most popular ways to present fan performance curves as presented in AMCA and ISO standards. Figure 1 represents a performance curve at one speed. Figure 2 represents performance curves for an adjustable-duty fan at one speed that can alter their performance by changing blade position (variable pitch axial fans) or inlet guide vanes (axial or centrifugal fans). Figure 3 represents the performance curve for a fan operating at multiple speeds.
One item in common with all fan performance curves is that fan pressure (P) is the y-axis (vertical) and fan volume airflow (Q) or mass flow (ṁ) is the x-axis (horizontal). There may be additional y-axis’s on the right side of the graph for plotting additional fan performance variables such as fan power input (Hi), fan efficiency (η), and sound pressure levels (Lp).
Pressures
A fan produces total pressure (Pt) which is the sum of the static pressure (Ps) and velocity (dynamic) pressure (Pv). Static pressure is the force available to push air through a system against resistance (system resistance), while velocity pressure is the kinetic energy from the moving gas.
The fan performance curve’s pressure axis may be created with total or static pressure as shown in Figure 2 and 3 or both as shown in Figure 1. This is usually based on the customer’s preference, but many users prefer static pressure because it is easier to apply and understand when finding an operating point of the fan based on the system resistance. Refer to the article in the January 2026 edition of Currents, “Fan Laws and System Resistance Curves,” for a detailed explanation. There is a difference between fan static pressure (Ps) and fan static pressure rise ( ΔPs). Static pressure rise is the increase in static pressure between the fan inlet and fan outlet. Fan static pressure is the difference between static pressure at the fan outlet and total pressure at the fan inlet. (Remember that Pt = Ps + Pv).
It is important to recognize which type of pressure is being shown on the fan performance curve in order to design the system correctly or/and interpolate what point of operation the fan is operating in-situ.
Volume Airflow Rate
The fan flow rate can be displayed on the performance curve as fan volume airflow rate (Q) and/or mass flow rate (ṁ). The mass flow rate depends on the fan volume airflow rate and the air density (ρ), ṁ = Q * ρ. The industrial fan market uses fan performance curves in fan volume airflow almost entirely. A performance curve in mass flow is convenient to use if the process needing the airflow is sensitive to amount of matter (weight of air) like reactions or combustion. This article will concentrate on fan volume airflow throughout the article. Airflow rates when calculated by AMCA and ISO testing standards are based on the conditions at the fan inlet and that is how it is usually presented on performance curves.
Density
The fan performance curve will be based on a given density of the gas being moved and rotational speed. A fan is a constant volume device which means that given a constant speed, it will move the same volume of gas even if the density changes. However, it does not create the same amount of pressure and require the same power input. These will change with a density change. When using a fan performance curve, make sure the density of the gas and speed listed on the curve match the process requirements or actual conditions in-situ. It is important to remember that the density of the gas is dependent on the properties of the gas itself plus elevation (barometric pressure), pressure inside the ducts, humidity, dust, products of the process, e.g., combustion in the gas stream.
Standard, Normal and Actual Conditions
Many performance curves are created based on Standard or Normal air conditions. There are several definitions for these terms by different entities in the world. Two of the common referenced entities in the industrial fan market are AMCA and ISO. AMCA states that Standard air is 70 deg F (21 deg C) at atmospheric pressure of 29.92 in Hg (101.3 kPa), 0% humidity which has a density of 0.075 lbm/ ft3 (1.20 kg/m3) and ISO states that Standard air is 20 deg C (68 deg F) at atmospheric pressure of 101.3 kPa (29.92 in Hg), 0.4% humidity which has a density of 1.20 kg/m3 (0.075 lbm/ft3). ISO also defines Normal air at 15 deg C (59 deg F), 101.3 kPa (29.92 in Hg), 0% humidity which has a density of 1.225 kg/m3 (0.076 lbm/ft3). If the flow on the performance curve has an “S” or “N” in front of the units, e.g., scfm (standard cubic feet per minute - sft3/m) ncms (normal cubic meters per second - nm3/s), this means it is based on “Standard” or “Normal” conditions. A third designation in front of the units may be an “A”, e.g., acfm (actual cubic feet per minute - aft3/m) which means that the flow is based on the actual density of the gas at the operating temperature, elevation and humidity.
Compressible Airflow, Reynolds and Mach Numbers
Fan performance curves may or may not have been corrected for compressibility, Reynold* and Mach** Numbers from laboratory or field tests. For this article, the assumptions have been made that the fan performance curve from the manufacturer has been corrected and the system is moving standard dry air at a pressure so that the change in density, compressibility, Reynolds or Mach number is small.
* Reynolds Number (Re) is a dimensionless quantity in fluid dynamics used to help determine flow patterns.
** Mach Number (M) is a dimensionless quantity in fluid dynamics used to calculate the ratio of the flow velocity to the speed of sound.
Performance Data
The pressure curve on a fan performance curve serves as the foundation for all fan performance calculations. The pressure curve represents the fan’s performance capability at one specific speed and can be used to determine the fan’s pressure capability at any volume. To locate a fan’s point of operation on the performance curve, follow these steps:
- Locate the required pressure on the pressure scale (y-axis) at the left of the curve
- Draw a horizontal line to the right until it intersects with the pressure curve
- From this intersection (point of operation), draw a vertical line down to the airflow scale (x-axis)
This process reveals the fan’s volume airflow capability for that pressure at the given speed and density. For example, as illustrated in the performance curve (Figure 4), this particular fan provides 8750 acfm (4.1 acms) at 12 in. wg (2.98 kPa) static pressure (Ps) when operating at 1750 rpm with dry air at a density of 0.075 lb/ft3 (1.20 kg/m3). Pressure can be expressed in terms of the height of a water column. Common pressure units for industrial fans is "in. wg" which is inches of water column gage and "mm wg" which is millimeters of water gage.
Once the flow and pressure have been determined, the required power input for the fan can be determined. An accurate power requirement is crucial for two purposes: properly sizing the motor, coupling or V-Belt drive, variable speed drives and the required electrical system. Performance curves include a fan power input (Hi) vs fan airflow curve that allows for rating determination at specific capacities. To find the required power input at any specific operating point, draw a horizontal line rightward from where the vertical flow (Q) line intersects with the power input curve.
It is important to note that it is common that power input required shown on the performance curve does not include power losses from couplings, V-belt drives and bearings. Therefore, the motor for the fan must be larger to accommodate for those losses.
For example, (see Figure 5) when a fan operates at 8750 acfm (4.1 acms) @ 12 in. wg (2.98 kPa) static pressure, it requires 30 bhp (22.3 bkW). The “b” in front of the unit designates “brake” which means the power required at the fan shaft not including drive losses. The fan performance curve may not always use this designation, but in the industrial fan market, it is best to assume required fan input power is based on “brake” power. The required power input for the fan depends on what efficiency (η) the fan operates. A fan has a static efficiency (ηs) and a total efficiency (ηt ).
The required power input is calculated from the formula Hi = Q * P / η. The formula can use either total pressure or static pressure and the pressure used determines what efficiency value to use. If using static pressure, use static efficiency and if using total pressure, use total efficiency.
Using Performance Curves
Figure 6 provides a visual representation of the operating point for a variable pitch axial fan selected to deliver 4.2 kPa (16.9 in wg) at 160 am3/s (339,000 acfm) of dry air at sea level at 1480 rpm. This figure serves as a valuable tool for troubleshooting potential issues with the system.
If the system fails to deliver the required volume airflow, taking measurements and comparing them to the performance curve in Figure 6 can help pinpoint the source of the problem. For example, if a tachometer reading reveals the fan is running at 1200 RPM instead of the intended 1480 RPM, the volume airflow would be 130 am3/s.
In such a scenario, the system was initially calculated correctly, and the solution lies in adjusting the fan speed back to the original 1480 RPM. However, if the tachometer reading indicates the correct speed but the airflow is still lower than expected, this suggests several other possibilities that need to be investigated.
One or more of the following factors may contribute to the fan not performing;
- The in-situ system resistance has changed since the design stage. Refer to Figure 7. The system has greater resistance requiring the fan to deliver more pressure. The intersection of the in-situ system resistance curve vs the as designed curve shows the fan will be delivering 137 am3/s (290,286 acfm) at 5.6 kPa (22.5 in wg). The power requirement will also be higher at 1000 bkW (1341 bhp)
- The fan is adjustable duty and the fan blades (variable pitch axial fan) or inlet guide vanes are at a different setting as shown in Figure 8. The fan performance curve intersects with the system resistance curve at a new point. In this case, the in-situ operating point will cause the fan to deliver 135 am3/s (286,048 acfm) at 3.0 kPa (12 in wg). In this case, the power requirement will be lower at 580 bkW (778 bhp).
- The density of the in-situ air-stream differs from the design value because of temperature, water saturation (humidity), elevation, system process variables affecting the air-stream such as dust and products of combustion.
- Disturbance in the air-stream before or after the fan which affects the airflow pattern. These are called system effects. Duct turns, blocked ducts, incorrect type of damper, expanding or contracting ducts can cause system effects and reduce the pressure and airflow output of the fan.
Fan Performance and Stability
Each fan type exhibits distinct airflow characteristics, resulting in unique performance curves.
Fan types can be based on how the flow moves through the fan (axial, mixed flow, centrifugal aka radial), the blade shape (airfoil, backward inclined, radial, radial tip, forward curve, bi-directional axial blade, curved backward inclined, etc.), the arrangement (layout of the fan) and ability to be adjustable (variable pitch axial, inlet vane control, inlet damper control, etc.). The fan performance curve will have different characteristics for each of these variables.
The “best” type of fan for the application depends on the situation and design constraints. But one of the most important considerations is to ensure the fan will perform in a stable condition for all current and future operating points on the system resistance curve. Some fan types will operate in a stable condition across all points of the curve but some will not. Generally speaking, when a curve peaks and drops into a valley, this is an unstable region. It is also known as the stall region. The fan will constantly search for a stable operating point and deliver an inconsistent pulsating pressure and flow. Refer to Figure 9 for some examples. Best practice is to make sure all operating points will be to the right of the peak on the curve. But, there are always exceptions to the rule and there are applications and fan types that will perform to the left of the unstable region.
By understanding these factors, engineers can optimize fan selection and ensure efficient system operation.
Conclusion
A good working knowledge of performance curves is necessary to understand the performance characteristics and capabilities of different fan equipment. Use of performance curves in the selection of fan types and sizing will assure stable and efficient operation as well as future flexibility.
This document is New York Blower’s Engineering Letter EL-03 revised and updated by Steven F. Back, PE for nomenclature, clarity and consistency with AMCA/ANSI Standard 99, SI units and ease of use. 09/16/2025
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