Can a curve measured on one fan size be carried to a larger wheel? It can provide a starting estimate only when the geometry and operating conditions support similarity. Reynolds number is one of the checks. A larger diameter changes the balance between inertial and viscous effects, but diameter alone does not tell you whether efficiency will improve, by how much, or in which part of the curve.
For an OEM comparing sizes, the useful decision is to separate a scaled prediction from a curve for the exact fan offered. Ask which geometry, test condition and correction produced the published point, then confirm the selected size at the installed airflow and pressure. The video introduces the scale question; the two diagrams below show the calculation boundary and the verification path.
What Reynolds number can and cannot tell you
Reynolds number compares inertial effects with viscous effects in a defined flow. In a common form it is characteristic velocity times characteristic length divided by kinematic viscosity: Re = U L / ν. The result is dimensionless. You must state what U and L represent before comparing two cases; choosing a different blade passage or reference velocity changes the calculated number. Fluid temperature and composition also matter because they change viscosity.
For a deliberately simple illustration, let characteristic velocity follow rotational speed times impeller diameter, let characteristic length be diameter, and hold speed and fluid constant. Reynolds number then follows speed times diameter squared divided by kinematic viscosity. Doubling diameter gives four times the illustrative Reynolds number. It does not give four times the airflow, pressure, efficiency or service life. Real fans can also change blade section, inlet geometry, clearances, casing and operating point as their size changes.
Similarity needs more than a matching outline
Two wheels may look alike while their flow passages are not geometrically similar. An inlet ring, housing, blade thickness or tip clearance that does not scale with diameter can alter the comparison. ANSI/AMCA Standard 99 describes geometric similarity for essential impeller and casing passages and treats Reynolds and Mach effects as conditions for useful fan-law conversion. The requirement is to check whether differences in those effects are small enough for the intended estimate; it is not a universal threshold that can be borrowed from another fan.
When the gas, rotational speed or diameter changes, note the reference density and viscosity. For a heated or otherwise unusual gas, use properties at the stated operating condition. At higher tip speeds, compressibility can become another limit. A supplier should be able to explain which effects were included in a converted curve and which still require a test. The ANSI/AMCA Standard 99 fan-law discussion is a useful technical reference for those assumptions.
Use the fan laws as an estimate with stated conditions
Fan laws relate flow, pressure and power at corresponding points for a similar fan under specified conditions. They are useful for screening a speed or size change, especially when the new condition remains close to a verified curve. They do not automatically preserve efficiency, sound or the complete installed duty. The system resistance curve may intersect the fan curve at a different point, and motor or drive losses belong to the selected assembly rather than to Reynolds number alone.
The old fixed efficiency-by-diameter table on this page had no controlled model curves or comparable test basis, so it has been removed. Likewise, a claim that every larger fan is quieter or that every scale conversion is conservative would overstate the evidence. To compare an array of smaller fans with one larger fan, look at each option at the same required total airflow, pressure and installation boundary. Our fan-array comparison guide is a route into that separate system decision, not a substitute for the model curves.
Move from a model curve to a size-specific decision
Begin with a source curve that identifies the exact fan, speed, density, test setup and pressure definition. Lock the impeller, inlet and casing geometry before assuming that another size belongs to the same aerodynamic family. Then calculate the intended conversion at the required operating point and review Reynolds and Mach effects, tolerances and control limits. Finally, request a curve or controlled test for the proposed production size. Compare that result with the complete system resistance, not with a maximum free-air figure.
On the LONGWELL site, the RLM plenum-fan family provides a size range and model-level material to start such a review. It demonstrates that different candidates and curves exist. The family listing does not establish geometric similarity between every model, a Reynolds correction, or a fixed efficiency advantage for a larger unit. Use the fan-curve reading guide to locate the actual duty point, then request the current drawing and performance record for the exact model and revision under consideration.
Questions to put on a supplier comparison sheet
Ask for the proposed model and revision, its impeller and inlet arrangement, the origin of each performance curve, the speed and gas state used for rating, and the distinction between measured and converted points. Specify whether the comparison uses static or total pressure and whether guards, coils, ducts or other installation effects are included. Ask what validation will be performed after the fan is mounted in the actual equipment. If a supplier provides a scaled estimate, request its assumptions and the expected uncertainty instead of treating a smooth plot as a measured result.
Reynolds number is valuable because it tells you when to question a scale conversion. It does not rank products on its own. A defensible selection joins the right flow convention, the exact curve and the installed duty.
For a model and curve review, send your required airflow, pressure, gas condition and installation constraints to sales@longwellfans.com.
