Calculation sequence
Step-by-step method
- Divide new wheel diameter by baseline wheel diameter.
- Apply the third-power diameter relation to flow.
- Apply the second-power relation to pressure and fifth-power relation to shaft power.
MechanicalAdvancedFans
Fan Wheel-Diameter Change Calculator
Estimate flow, pressure, and power changes for geometrically similar fans at the same rotational speed.
Why it matters: Diameter relationships use different exponents from speed-only affinity laws and require geometrically similar equipment.
Calculated result
Engineering evaluation
Predicted airflow for similar fan (Q2)7,703 cfm
- Predicted fan pressure (P2)
- 2.101 in. w.g.
- Predicted shaft power (W2)
- 4.854 hp
- Wheel-diameter ratio (rD)
- 0.917 ratio
Engineering interpretation
For geometrically similar fans at the same speed and density, the entered diameter change predicts 7703 cfm, 2.101 in. w.g., and 4.854 hp.
Important limitations
- Do not apply geometrically similar diameter laws to a trimmed wheel in an unchanged housing without manufacturer data and approval.
Motor-driven equipment
Estimate electrical demand and energy use
Use nameplate information and an operating schedule to screen motor input power, current, annual energy, and operating cost. Results update automatically.
Choose the motor nameplate output-power convention.
Rated shaft output, not electrical input.
Used to estimate current; it does not change the hp-based energy result.
Use efficiency at the evaluated load point when available.
Use measured or defensible operating load rather than motor nameplate size alone.
Used only for the estimated current calculation.
Assumes all selected motors operate concurrently at the entered load.
Replace the profile with the actual occupied, process, or control schedule.
$/kWh
Use the applicable marginal utility energy charge; demand charges are not included.Estimated result
Total motor input power (Pin,total)6.21 kW
- Current per motor (I)
- 9.18 A
- Total coincident current (Itotal)
- 9.18 A
- Daily energy (Ed)
- 50 kWh/day
- Annual operating hours (Hy)
- 2,080 h/year
- Annual energy (Ey)
- 12,925 kWh/year
- Annual energy cost (Cy)
- $1,551/year
Equations and assumptions
Nameplate-based screening method
Horsepower is converted using 1 hp = 0.7457 kW. This estimate assumes steady average load and the entered operating-point efficiency. Voltage and power factor affect estimated current, but hp, load, efficiency, quantity, and schedule establish the energy estimate. VFD speed profiles, cycling, standby power, demand charges, and process variation require separate evaluation.
Technical basis: U.S. Department of Energy — Determining Electric Motor Load and EfficiencyEngineering knowledge center
Understand the calculation—not just the answer
Governing relationships
rD = D2/D1
Q2 = Q1rD³
P2 = P1rD²
W2 = W1rD⁵
Current substitution
rD = 22/24 = 0.9167; Q2 = 10000×0.9167³ = 7703; P2 = 2.101; W2 = 4.854.
Equation legend
Inputs and calculated quantities
- Q1
- Baseline airflow — cfm or m³/s
- P1
- Baseline fan pressure — in. w.g. or Pa
- W1
- Baseline shaft power — hp or kW
- D1
- Baseline wheel diameter — in. or mm
- D2
- New wheel diameter — in. or mm
- Q2
- Predicted airflow for similar fan — cfm
- P2
- Predicted fan pressure — in. w.g.
- W2
- Predicted shaft power — hp
- rD
- Wheel-diameter ratio — ratio
Applications
How this calculation is used
Building design
Screen scale effects between similar fan sizes before curve selection.
Construction
Evaluate a proposed wheel-size substitution before manufacturer approval.
Commissioning
Check whether a documented diameter change could explain a measured performance shift.
Quality controls
Assumptions and limitations
Assumptions
- Fans are geometrically similar and operate at the same rotational speed and air density.
- Efficiency and operating region remain sufficiently similar for preliminary scaling.
Limitations
- Wheel trimming within one fan housing is manufacturer-specific and may not follow geometrically similar diameter exponents.
- Use manufacturer curves and approved modification limits for actual equipment decisions.
Field use
Verification procedure
- Confirm the approved reference condition, instrument range, and calculation boundary.
- Stabilize the system and record calibrated field measurements.
- Compare the result with current manufacturer data and project requirements before disposition.
Common engineering mistakes
- Mixing measurements from different operating conditions or equipment boundaries.
- Treating a calculated screening status as automatic acceptance.
Technical basis
References and source standards
- U.S. Department of Energy—Improving Fan System PerformanceFederal sourcebook covering fan-system relationships, fan laws, power, efficiency, system curves, and field performance considerations.
- ASHRAE Handbook—FundamentalsCurrent handbook chapters covering fluid flow, pressure relationships, system curves, and mechanical-system fundamentals; the licensed edition must be verified during professional review.
Engineering calculation disclaimer
These engineering tools are provided for educational, preliminary evaluation, field verification, and commissioning support. Results depend on the accuracy of user-entered information and the assumptions stated for each calculation. Every colored status and comparison is a screening indicator only; project criteria, contract requirements, applicable codes, manufacturer instructions, measurement uncertainty, and authorized engineering judgment govern. The tools do not replace project-specific engineering analysis or the judgment of a licensed professional engineer.
Read the full use and limitation statement