Required receiver volume
-Free air stored p1 to p2
-Refill time
-Hold-up time, existing tank
-All results
| Net flow from the receiver (demand minus compressor) | - |
| Free air needed during the peak | - |
| Required volume with no compressor help | - |
| Pressure band p1 − p2 | - |
| Atmospheric pressure used | - |
| Required volume with 20 % allowance (non-isothermal discharge) | - |
| Existing tank: free air stored p1 to p2 | - |
Method and formulas
Isothermal gas law (Boyle) applied to the usable pressure band, so every figure can be checked by hand.
| Quantity | Formula |
|---|---|
| Net flow from the tank | Qnet = Qd − Qc |
| Required volume | V = Qnet · t · pa / (p1 − p2) |
| Free air stored in the band | Vfree = V (p1 − p2) / pa |
| Refill time | tfill = V (p1 − p2) / (Qc pa) |
| Hold-up time of a given tank | t = V (p1 − p2) / (pa Qnet) |
Pressures p1 and p2 are gauge pressures; only their difference enters the formulas. Atmospheric pressure pa is 101.325 kPa (14.696 psi). Free air is air at atmospheric conditions (scfm or L/min). The result assumes constant temperature: fast discharge cools the air, so the 20 % allowance row is a rough margin, not a calculation.
Worked example: 600 L/min peak for 30 s, 8 to 6 bar(g), 200 L/min compressor
Qnet = 600 − 200 = 400 L/min = 6.667 L/s, so the free air needed is 6.667 × 30 = 200 L. V = 200 × 1.01325 / (8 − 6) = 101.3 L. That volume holds 200 L of free air between 8 and 6 bar and takes 1.00 min to refill at 200 L/min. Without the compressor the demand is 300 L and V would be 152.0 L. An existing 150 L tank would hold the 400 L/min net demand for 44.4 s.
Questions
How do you size an air receiver?
For a short burst of demand, V = Q t pa / (p1 - p2): Q is the free-air flow the compressor cannot supply, t the duration, pa atmospheric pressure, p1 the pressure at which the compressor cuts out and p2 the lowest pressure the equipment can accept (all absolute, or as gauge differences). The receiver is the buffer that gives up free air as its pressure falls from p1 to p2.
How much compressed air does a receiver store?
Between two pressures a receiver of volume V holds V (p1 - p2) / pa of usable free air, with pressures in the same units as pa. For example 150 L between 8 and 6 bar gauge gives 150 x 2 / 1.01325 = about 296 L of free air. Most of the gas in a tank is unusable because the equipment needs a minimum pressure.
How long does it take to refill a receiver?
The compressor refills it at about t = V (p1 - p2) / (Qc pa), where Qc is its free-air delivery. In practice the compressor also has to serve the live load, so use only the surplus Qc minus the average demand, and allow for heating of the air during compression.
Is the calculation isothermal or adiabatic?
Isothermal: the air is assumed to stay at the ambient temperature while the tank discharges, which is reasonable for slow discharge and a tank with good heat exchange with its surroundings. Fast bursts cool the air in the tank and deliver somewhat less than predicted, so many designers add 10 to 25 percent.
What cut-in and cut-out pressures should I use?
p1 is the compressor cut-out pressure and p2 is the lowest pressure that keeps the machine working, not the cut-in pressure unless the compressor is off during the demand. A wider band needs a smaller tank, but a high cut-out costs more energy to compress, and regulators and line losses need extra margin. Check your compressor data sheet.
Where should the receiver go and what do the codes say?
Wet receivers near the compressor smooth pulsations and drop out water, and dry receivers at the point of use cover local peaks. Receivers are pressure vessels: they need a safety valve, drain, pressure gauge and a rating plate, and are subject to national pressure-equipment rules (for example the EU Pressure Equipment Directive, ASME Section VIII and OSHA or local inspection rules). This page does not replace them.
What does this calculator not cover?
Compressor duty cycles and unloading control, pipework volume and pressure losses, temperature effects at high flow, dryers and moisture, gas other than air and altitude correction other than the atmospheric pressure shown. It is a first-pass sizing aid.
Engineering aid for guidance. It uses an isothermal model and ignores compressor control, pipework and pressure losses. Pressure vessels must be designed, tested and inspected to the applicable code: verify critical designs.