Air Receiver Sizing Calculator: Time to Fall or Fill

How long a compressed-air receiver lasts between two pressures, or how long it takes to fill.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

A receiver stores air. The amount of free air (air at atmospheric pressure) it holds between two pressures is the volume times the pressure difference, divided by atmospheric pressure. Divide by the consumption to get the time the pressure takes to fall.

To fill, the compressor free air delivery (FAD) minus the consumption is what is left to raise the pressure. The calculation assumes the air stays at constant temperature (isothermal), which is a good approximation over a few minutes.

Formulas used

  • V_{fa} = \frac{V (p_1 - p_2)}{p_{atm}}
  • t_{drop} = V_{fa} / Q_c
  • t_{fill} = \frac{V(p_2 - p_1)}{(FAD - Q_c)\,p_{atm}}

Worked example

With the default numbers in the calculator above:

  1. Free air stored between the two pressures: 987.2 L
  2. Time for the pressure to fall: 1.645 min

Change the numbers in the calculator to see the full working, every step, for your own case.

Common questions

Gauge or absolute pressure?

The difference between two pressures is the same in gauge and absolute, so the calculator uses gauge pressures for the difference.

What is free air delivery?

The volume of air a compressor delivers, measured at atmospheric pressure and temperature, per minute.

How do I size a receiver for a pulsed demand?

Work out the free air used in one pulse, and choose the receiver volume so that the pressure drop during the pulse stays within what the equipment can accept.

Related calculators

Further reading: Compressed air texts and compressor manufacturers' guides.

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