CALCULATORS / PUMPS, FANS AND VENTILATION

Pump affinity-law scaling calculator

Speed affinity at unchanged impeller and fluid; similarity assumed.

pumps-pump-affinity.napkin / editable

SCENARIO COMPARISON

Compare the numbers.

This source describes an analytical model. Its quantities and checks are the view.

Scenario	newFlowL/min	newHeadm	newPowerW
	12	14.4	172.8
	9.6	9.216	88.4736

All calculated quantities are listed with the inputs. Switch to Source to inspect the equations.

SCENARIOS / CALCULATED VALUESSAME SOURCE

Change an input to see the consequences.

motor power budget172.80 W✓ passes

BASELINE INPUTS

speedRatio1.2 flow10 L/minhead10 mpower100 W

CALCULATED

newFlow12 L/min

newHead14.4 m

newPower172.8 W

Edits change the baseline source.

THE SOURCE

Edits below update the same model.

# Pump affinity-law scaling
# Scope: Speed affinity at unchanged impeller and fluid; similarity assumed.
# Status: illustrative worked example; engineering review pending.
# Inputs are example values. Checks are editable example requirements, not standards.

speedRatio = 1.2
flow = 10 L/min
head = 10 m
power = 100 W

newFlow := flow*speedRatio [L/min]
newHead := head*speedRatio^2 [m]
newPower := power*speedRatio^3 [W]

check motor power budget: newPower <= 200 W

scenario alternative: speedRatio = 0.96

Calculated from your source · not saved

Baseline · Baseline

motor power budget172.80 W ✓ passes

alternative · alternative

motor power budget88.47 W ✓ passes

Illustrative first-order example. Checks are editable requirements, not design standards. The broader topic may require additional equations, data, and validation.

Formula and units

This example uses the following equations. Input names and units match the editable source above.

new flow: newFlow = flow*speedRatio [L/min]
new head: newHead = head*speedRatio^2 [m]
new power: newPower = power*speedRatio^3 [W]

Original worked example

These are the baseline values in the downloadable source. Edits above let you test your own values.

Given

speed ratio: 1.2
flow: 10 L/min
head: 10 m
power: 100 W

Calculated

new flow: 12 L/min
new head: 14.4 m
new power: 172.8 W

Assumptions and limits

Speed affinity at unchanged impeller and fluid; similarity assumed.

The requirement checks use the limits written in this example. Passing a check does not establish that the model covers every load, material behavior, or operating condition in a real design.

Download this .napkin example ↓

Take it further with your agent

Napkin keeps the inputs, equations, checks and assumptions together in one study. Your agent sets it up; you change the numbers.

Try with Claude ↗

codex 'Use Napkin (https://napkin.engineering/mcp, or the local Napkin plugin) to build a study: Pump affinity-law scaling. Inputs: baseline curve; speed or impeller change. Outputs: scaled flow, head and power; similarity limits. Model scope: Measured or manufacturer curves and declared fluid conditions. Read napkin_capabilities and the current snapshot. Ask me for the values that matter, label assumptions with units, author .napkin source, validate it and apply it with napkin_source using the current revision, then open the study. Add checks for the limits I supply. Keep unsupported analyses explicit.'

Claude needs the Napkin connector; Codex needs the Napkin plugin. Set up your agent ↗

The broader study

The topic brief below describes the broader scope. The worked example above states the particular calculation it implements.

Inputs: baseline curve, speed or impeller change
Outputs: scaled flow, head and power, similarity limits
Modes: calculate, compare, sweep
Drawing: System and equipment curves

EXECUTABLE WORKED EXAMPLE · FIRST-ORDER MODEL · CHECK THE SCOPE & ASSUMPTIONS