A fan datasheet is useful only when its numbers describe the same build and the same operating condition. A maximum airflow entry and a maximum pressure entry usually sit at different ends of a curve. Combining them into a single promised duty point is a selection error. Begin with the airflow and pressure your equipment actually needs, then ask which exact fan curve reaches that point.
This guide is for an engineer or buyer comparing candidate fans for a defined system. It explains what to mark on a curve, what to read in the small print, and what to request when the page is incomplete. The accompanying video walks through the same review.
Start with one required operating point
Write the required airflow and the pressure the fan must overcome, with units and a clear static or total pressure definition. Plot that requirement on the candidate curve at the speed and air density stated for the test. The working airflow is set where the fan curve and the system resistance curve meet. If the expected system curve is uncertain, show the filter, coil, duct, inlet and outlet assumptions used to estimate it. A changing filter resistance or different duct layout can move the operating point.
Once that point is identified, read power, efficiency and any sound information at the same airflow and speed. Do not transfer a value from a neighboring speed line or another fan suffix. The fan-curve walkthrough expands the axis and intersection method.
Lock the exact document and product identity
Record the full model suffix, document revision and date before copying any value. One LONGWELL controlled example, the LWBE3G310-138NT-05-00 specification at revision A0 dated 20 January 2026, separates nominal data, control information, a dimensional drawing and an airflow-pressure chart across its pages. Those pages describe that named configuration only. The revision does not make a result transferable to every LWBE3G310 fan, or establish that it fits a particular cabinet or air-handling unit. Compare the offered drawing and curve with the purchase configuration rather than relying on a family photograph.
If the seller supplies a revised impeller, control setting, guard, inlet cone or electrical option, request the corresponding controlled curve and drawing. A family page such as the LONGWELL EC backward-curved fan range is useful for locating candidates; it is not an exact-model selection or an installation approval.
Read the test boundary before comparing figures
Check whether the pressure axis is fan static pressure or fan total pressure, which inlet and outlet test arrangement was used, the reference air density, the rotational speed and the included accessories. An airflow curve without these conditions is difficult to compare fairly with another supplier’s curve. Ask whether the published points were measured for this configuration or derived from another size or speed, and request the applicable method and tolerance statement. Avoid treating a standard name printed on a document as proof of a particular certificate or test result.
Compare electrical power at one boundary
A shaft-power curve, a motor input figure and complete electrical input are different boundaries. State which one a table reports. Ask for input power and current at the selected duty point and supply condition, especially when comparing an integrated EC fan with an AC fan and separate drive. Do not calculate an energy saving from two headline values taken at different speeds or pressures. The selection needs the same air duty and declared electrical boundary on both sides.
Likewise, an efficiency peak is not the efficiency at every operating point. Use the efficiency definition supplied for the relevant curve and read it at the chosen airflow. If a value is absent, request a measured or properly documented result for the quoted configuration rather than inserting a generic percentage.
Put sound data in its measurement context
A single dB(A) number cannot describe every installation. Check whether the sheet reports sound power or sound pressure, the measurement distance or arrangement where relevant, and the operating point. A project acoustic review may need octave-band information and the equipment enclosure or duct path. Do not promise an installed noise level from a free-field fan number alone. If the required acoustic target is strict, include it in the inquiry so the manufacturer can state the test and calculation boundary.
Check the drawing and the installed system
The curve is only one part of the purchase decision. Compare the dimensioned drawing with the available envelope, mounting pattern, inlet and outlet clearances, cable route and service access. Verify voltage, control signal, environmental range and the exact protection scope on the offered revision. The A0 example documents some of these items for its named suffix, but it is not a blanket statement about the entire product family. Installation details may alter the real airflow. AMCA Publication 201 describes the system-effect problem and why field conditions need their own allowance.
Send a selection brief that can be answered
Send the required airflow and static or total pressure, air density or temperature, system layout, electrical supply, control method, acoustic target, environmental conditions, mounting envelope and applicable market requirements. Include the candidate model and revision if already chosen. Ask the supplier to return the matching curve, drawing, input data at the duty point, test basis and any exceptions. If the system resistance is still an estimate, identify the uncertain part so the response can show what must be verified during commissioning.
A defensible purchase decision is a named fan configuration operating at a stated point under stated conditions. Headline maxima can help sort a catalogue, but they cannot replace that short evidence package.
