CALCULATORS / PUMPS, FANS AND VENTILATION

Duct pressure drop calculator

Darcy-Weisbach duct loss with supplied hydraulic diameter and factor.

pumps-duct-pressure-drop.napkin / editable

SCENARIO COMPARISON

Compare the numbers.

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

Scenario	lossPa
	7.5
	6

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.

duct loss budget7.50 Pa✓ passes

BASELINE INPUTS

length5 mdiameter0.2 mvelocity5 m/sdensity1.2 kg/m^3factor0.02

CALCULATED

loss7.5 Pa

Edits change the baseline source.

THE SOURCE

Edits below update the same model.

# Duct pressure drop
# Scope: Darcy-Weisbach duct loss with supplied hydraulic diameter and factor.
# Status: illustrative worked example; engineering review pending.
# Inputs are example values. Checks are editable example requirements, not standards.

length = 5 m
diameter = 0.2 m
velocity = 5 m/s
density = 1.2 kg/m^3
factor = 0.02

loss := factor*length/diameter*density*velocity^2/2 [Pa]

check duct loss budget: loss <= 20 Pa

scenario alternative: length = 4 m

Calculated from your source · not saved

Baseline · Baseline

duct loss budget7.50 Pa ✓ passes

alternative · alternative

duct loss budget6.00 Pa ✓ 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.

loss: loss = factor*length/diameter*density*velocity^2/2 [Pa]

Original worked example

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

Given

length: 5 m
diameter: 0.2 m
velocity: 5 m/s
density: 1.2 kg/m^3
factor: 0.02

Calculated

loss: 7.5 Pa

Assumptions and limits

Darcy-Weisbach duct loss with supplied hydraulic diameter and factor.

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: Duct pressure drop. Inputs: duct geometry; airflow; roughness; fittings. Outputs: pressure loss; velocity. 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: duct geometry, airflow, roughness, fittings
Outputs: pressure loss, velocity
Modes: calculate, compare, sweep
Drawing: System and equipment curves

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