KEVOS® MAKEOS · Hydraulics
Pump and Fan Affinity Laws
What flow, head and power will this centrifugal pump or fan give at a new speed or impeller size?
Flow, head (pressure) and power of a centrifugal pump or fan at a new speed or trimmed impeller diameter by the affinity laws, with the power saving.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Present flow | m³/h | Flow at the present operating point, from the pump curve or a flow meter. |
| Present head | m | Head (or fan pressure) at that point. |
| Present shaft power | kW | Shaft power at that point, from the curve or measured input × motor efficiency. |
| Present speed | rpm | Present speed. |
| New speed | rpm | New speed (e.g. with a VSD or new pulleys). |
| Present impeller diameter | mm | Present impeller diameter. |
| New impeller diameter | mm | New (trimmed) impeller diameter; equal to the present one for speed changes only. |
Method and formulas
- Q₂ = Q₁ (n₂/n₁)(D₂/D₁).
- H₂ = H₁ (n₂/n₁)²(D₂/D₁)².
- P₂ = P₁ (n₂/n₁)³(D₂/D₁)³.
- Example: slowing from 2900 to 2400 rpm cuts power from 7 to 3.9677 kW.
Limitations
- Applies along a system curve with no static head; with static head, the operating point and savings are smaller.
- Impeller trimming beyond about 10–20% departs from the laws and lowers efficiency.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Estimate savings from a variable speed drive
- Predict the effect of trimming an impeller
- Re-rate a fan at a new pulley ratio
Who it is for
- Hydraulic and process engineers
- Maintenance engineers
- Design engineers
What you get
- New flow, head and power
- Power saving
Interpreting the result
- Plot the new pump curve against the system curve to find the real operating point.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 3024–3027. Topic reference; no source tables or recommended settings are reproduced.
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