Figure

HVAC

Fan laws

Use proportional, squared, and cubed relationships to compare airflow, speed, pressure, and power.

At constant fan size and air density, airflow follows speed, pressure follows the square of the speed ratio, and power follows its cube. The examples express these relationships through Figure’s proportion mode using consistent units.

These proportional estimates apply to the same fan, or aerodynamically similar fans at corresponding operating points, with comparable efficiency and unchanged air density. A different fan geometry or system condition needs its own assessment rather than a simple ratio.

Find new fan speed

Use airflow/speed on both sides. Enter numbers in the same CFM and RPM units.

Start a new ratio

  1. Press Clear entry, then Clear entry.

1. Open Ratios

If A/B/C/D are already on the keys, skip this step. Otherwise close the current mode and tap Ratios. Skip Close if no mode is open.

  1. Press Close calculation mode, then Open Ratios.

2. Current airflow (A)

  1. Current airflow (A)

    Press 1250, then Assign value A.

3. Current speed (B)

  1. Current speed (B)

    Press 1400, then Assign value B.

4. Target airflow (C)

  1. Target airflow (C)

    Press 750, then Assign value C.
  1. Example · D840

Keep in mind

D is the new speed in RPM. Enter airflow and speed as numbers, without Feet or Inch. Leave the answer empty. It fills when you assign the third value. Correct a known value to update the same answer. Close returns to the regular keys; values stay until you clear the calculation.

Find airflow after changing speed

Use airflow/speed on both sides. Leave C empty for the new airflow.

Start a new ratio

  1. Press Clear entry, then Clear entry.

1. Open Ratios

If A/B/C/D are already on the keys, skip this step. Otherwise close the current mode and tap Ratios. Skip Close if no mode is open.

  1. Press Close calculation mode, then Open Ratios.

2. Current airflow (A)

  1. Current airflow (A)

    Press 14000, then Assign value A.

3. Current speed (B)

  1. Current speed (B)

    Press 855, then Assign value B.

4. Target speed (D)

  1. Target speed (D)

    Press 1050, then Assign value D.
  1. Example · C17192.98

Keep in mind

C is the new airflow in CFM. Enter airflow and speed as numbers, without Feet or Inch. Leave the answer empty. It fills when you assign the third value. Correct a known value to update the same answer. Close returns to the regular keys; values stay until you clear the calculation.

Find new fan pressure

Use airflow squared/pressure on both sides. Square both airflow values; pressure is a number without Inch units.

Start a new ratio

  1. Press Clear entry, then Clear entry.

1. Open Ratios

If A/B/C/D are already on the keys, skip this step. Otherwise close the current mode and tap Ratios. Skip Close if no mode is open.

  1. Press Close calculation mode, then Open Ratios.

2. Current airflow (A)

  1. Current airflow (A)

    Press 1000, then Square.

  2. Press the keys

    Press Assign value A.

3. Current pressure (B)

  1. Current pressure (B)

    Press 0.5, then Assign value B.

4. Target airflow (C)

  1. Target airflow (C)

    Press 1500, then Square.

  2. Press the keys

    Press Assign value C.
  1. Example · D1.125

Keep in mind

D is pressure in inches of water. Square both airflow values. Enter pressure as a number; don’t press Inch. Leave the answer empty. It fills when you assign the third value. Correct a known value to update the same answer. Close returns to the regular keys; values stay until you clear the calculation.

Find new fan power

Use airflow cubed/power on both sides. Cube both airflow values; use consistent airflow and power units.

Start a new ratio

  1. Press Clear entry, then Clear entry.

Allow large numbers

Turn Scientific notation on in Settings to display large values.

Scientific notation
On

1. Open Ratios

If A/B/C/D are already on the keys, skip this step. Otherwise close the current mode and tap Ratios. Skip Close if no mode is open.

  1. Press Close calculation mode, then Open Ratios.

2. Current airflow (A)

  1. Current airflow (A)

    Press 1000, then Second, then Cube.

  2. Press the keys

    Press Assign value A.

3. Current power (B)

  1. Current power (B)

    Press 2, then Assign value B.

4. Target airflow (C)

  1. Target airflow (C)

    Press 1500, then Second, then Cube.

  2. Press the keys

    Press Assign value C.
  1. Example · D6.75

Keep in mind

D is power in BHP. Cube both airflow values. Use the same airflow units throughout. Leave the answer empty. It fills when you assign the third value. Correct a known value to update the same answer. Close returns to the regular keys; values stay until you clear the calculation.

Find airflow from pressure

Use airflow squared/pressure. Leave C empty, solve, then take the square root to get airflow.

Start a new ratio

  1. Press Clear entry, then Clear entry.

1. Open Ratios

If A/B/C/D are already on the keys, skip this step. Otherwise close the current mode and tap Ratios. Skip Close if no mode is open.

  1. Press Close calculation mode, then Open Ratios.

2. Current airflow (A)

  1. Current airflow (A)

    Press 1000, then Square.

  2. Press the keys

    Press Assign value A.

3. Current pressure (B)

  1. Current pressure (B)

    Press 0.5, then Assign value B.

4. Target pressure (D)

  1. Target pressure (D)

    Press 2, then Assign value D.
  1. Example · C4000000

5. Find airflow in CFM

  1. Press the keys

    Press Close calculation mode.

  2. Press the keys

    Press Square root.
  1. Example result2000

Keep in mind

C is airflow squared. Tap Close, then take its square root to get CFM. Enter pressure as a number, without Inch. Leave the answer empty. It fills when you assign the third value. Correct a known value to update the same answer. Close returns to the regular keys; values stay until you clear the calculation.

Find airflow from a power limit

Use airflow cubed/power. Leave C empty, solve, then take the cube root to get airflow.

Start a new ratio

  1. Press Clear entry, then Clear entry.

Allow large numbers

Turn Scientific notation on in Settings to display large values.

Scientific notation
On

1. Open Ratios

If A/B/C/D are already on the keys, skip this step. Otherwise close the current mode and tap Ratios. Skip Close if no mode is open.

  1. Press Close calculation mode, then Open Ratios.

2. Current airflow (A)

  1. Current airflow (A)

    Press 1000, then Second, then Cube.

  2. Press the keys

    Press Assign value A.

3. Current power (B)

  1. Current power (B)

    Press 2, then Assign value B.

4. Target power (D)

  1. Target power (D)

    Press 16, then Assign value D.
  1. Example · C8000000000

5. Find airflow in CFM

  1. Press the keys

    Press Close calculation mode.

  2. Press the keys

    Press Second, then Cube root.
  1. Example result2000

Keep in mind

C is airflow cubed. Tap Close, then take its cube root to get CFM. Use the same airflow and power units throughout. Leave the answer empty. It fills when you assign the third value. Correct a known value to update the same answer. Close returns to the regular keys; values stay until you clear the calculation.

References

Keep the guide beside your calculation.

These examples are built into Figure. Open Help while you calculate, then return to the same guide and reading position.

See Help in Figure →