Air Receiver Sizing: Formula and Worked Example

Air Receiver Sizing: Formula and Worked Example

Ultramax Systems on 16th Oct 2026

Short answer: Size an air receiver for a demand event with V = t × (C − S) × Pa ÷ (P1 − P2). That is the event time, multiplied by the flow the compressor cannot supply, multiplied by atmospheric pressure, divided by the pressure drop you can tolerate. For a workshop whose 35 L/s compressor faces a 50 L/s demand for 60 seconds, with 100 kPa (1.0 bar) of allowable drop, the answer is about 912 litres, so a 1,000 litre receiver suits. Then check that the same receiver keeps a load/unload compressor from cycling too often.

Why receiver size matters

An air receiver stores compressed air so the system can meet short peaks without a pressure collapse. It also slows the rate at which pressure changes, which helps the compressor controls work properly. A large receiver can even let a smaller compressor serve intermittent peaks instead of buying capacity that sits idle most of the day (US Department of Energy 2016). The Australian Government's energy advice makes a related point: air compressors are most efficient near full load, and storage receivers can reduce peak loads (Australian Government n.d.).

Too little storage shows up as pressure sags at the tools and rapid load/unload cycling. The NSW Office of Environment and Heritage (2017) notes that the shorter a compressor's load/unload cycles are, the higher its average unloaded power.

Assumptions

The formulas below rest on a few simplifying assumptions that suit normal workshop and plant sizing.

  • The air in the receiver stays at a constant temperature during the event (isothermal behaviour).
  • Flows are free air delivery (FAD) at the compressor's inlet conditions, so Pa is the local atmospheric pressure, taken here as 101.3 kPa absolute (1.013 bar).
  • The compressor delivers its full rated FAD throughout the event.
  • The volume of the pipework is ignored, which adds a small safety margin.
  • P1 and P2 are measured at the receiver. Any pressure drop between the receiver and the tools must be allowed for when you choose P2.

The receiver formula for a demand event

A receiver of volume V at absolute pressure P holds V × P ÷ Pa litres of free air. When its pressure falls from P1 to P2, it releases V × (P1 − P2) ÷ Pa litres of free air. During an event that lasts t seconds, the receiver has to supply the shortfall between demand and compressor output, which is (C − S) × t. Setting the two equal and solving for V gives the standard formula:

V = t × (C − S) × Pa ÷ (P1 − P2)

No single sizing method is universally agreed, but this balance is the one commonly used (Engineering ToolBox n.d.). Marshall (2015), writing as a Compressed Air Challenge instructor, uses the same structure, defining C as the intermittent demand and S as the compressor capacity left over after normal demand.

Diagram of a compressor supplying S litres per second into a receiver of volume V, with a gauge showing P1 and P2, feeding a demand event that draws C litres per second for t seconds. The shortfall C minus S comes from the receiver. The formula V equals t times C minus S times Pa divided by P1 minus P2 is shown below.
Where each variable in the receiver formula sits in the system.
SymbolMeaningSI unit
VReceiver volume requiredlitres (L)
tDuration of the demand eventseconds (s)
CTotal air demand during the event, as free airL/s
SCompressor output available during the event, as FADL/s
PaAtmospheric pressurekPa absolute
P1Receiver pressure when the event startskPa
P2Lowest acceptable receiver pressurekPa

Because the formula uses the difference P1 − P2, you can enter both pressures in kPa gauge or both in kPa absolute. Pa must be absolute. If C and S are in L/s and t is in seconds, V comes out in litres.

Worked example: an Australian workshop

Consider a Perth fabrication and automotive workshop with one fixed-speed rotary screw compressor on load/unload control. The figures are illustrative, so substitute your own measurements.

  1. Compressor output. The compressor delivers S = 35 L/s FAD. It loads at 700 kPa (7.0 bar) and unloads at 800 kPa (8.0 bar).
  2. Demand event. When the blast cabinet runs alongside the tyre bay and assembly tools, total demand reaches C = 50 L/s for t = 60 s. The shortfall is C − S = 15 L/s.
  3. Pressure limits. The worst case is an event that starts just as the compressor reaches its load pressure, so P1 = 700 kPa (7.0 bar). The tools need about 600 kPa (6.0 bar) at the receiver after allowing for pipe and hose losses, so P2 = 600 kPa. The allowable drop is 100 kPa (1.0 bar).
  4. Calculate. V = 60 s × 15 L/s × 101.3 kPa ÷ 100 kPa = 911.7 L.
  5. Select. Round up to the next common size, a 1,000 L receiver. With that receiver, pressure falls at about 1.5 kPa (0.015 bar) per second during the event and bottoms out near 609 kPa (6.09 bar), just above P2.
Chart of receiver pressure against time over 300 seconds. Pressure falls from 800 to 700 kPa while the compressor is unloaded, then drops from 700 to about 609 kPa during a 60 second demand event, stays above the 600 kPa minimum, and recovers to 800 kPa once the event ends.
Calculated drawdown for the worked example with a 1,000 L receiver, 35 L/s compressor, 15 L/s base demand and a 50 L/s event.

The load/unload control form

The second job of a receiver is to limit compressor cycling. On load/unload control, a compressor fills the receiver from the load pressure to the unload pressure, then runs unloaded while demand draws the pressure back down. With a pressure band Δp = Pu − Pl, a receiver of volume V stores V × Δp ÷ Pa litres of free air per cycle. If Q is the compressor FAD and D is the demand, the loaded time is V × Δp ÷ (Pa × (Q − D)) and the unloaded time is V × Δp ÷ (Pa × D).

Adding those two times shows that the cycle is shortest when demand is exactly half the compressor's output. At that point, the cycle time is 4 × V × Δp ÷ (Pa × Q). Kaeser (n.d.) treats the same 50% load as the critical case in its receiver calculator. Rearranging for the receiver that keeps cycling at or below Z cycles per hour gives:

V = 900 × Q × Pa ÷ (Z × Δp)

Here V is in litres, Q in L/s, Δp and Pa in kPa, and Z in cycles per hour (the 900 comes from 3,600 seconds per hour divided by 4). Kaeser's calculator gives guideline maximums of 120 cycles per hour for compressors up to 18.5 kW, 60 up to 75 kW and 30 above that (Kaeser n.d.). Your compressor manufacturer's limit takes precedence.

For the workshop example, Q = 35 L/s and Δp = 100 kPa (1.0 bar). Keeping to 120 cycles per hour needs only about 266 L, and 60 cycles per hour needs about 532 L. The 1,000 L receiver chosen for the demand event gives a minimum cycle time of about 113 seconds, or roughly 32 cycles per hour, so the event calculation governs in this case.

Chart of compressor cycles per hour against demand as a percentage of compressor output for 266 L, 532 L and 1,000 L receivers. Each curve peaks at 50% demand, at 120, 60 and about 32 cycles per hour respectively.
Calculated load/unload cycling for a 35 L/s compressor with a 100 kPa (1.0 bar) band. The worst case is always at 50% load.

Sensitivity: which inputs matter most

The table changes one input at a time from the worked example and recalculates the required volume. All values are calculated with the formula above.

Input changedNew valueRequired volume (L)
None (base case)t = 60 s, C = 50 L/s, S = 35 L/s, P2 = 600 kPa912
Event duration t30 s456
Event duration t120 s1,823
Demand C60 L/s1,520
Compressor output S40 L/s608
Minimum pressure P2550 kPa (5.5 bar), 150 kPa drop608
Minimum pressure P2650 kPa (6.5 bar), 50 kPa drop1,823

The required volume scales directly with event duration and with the shortfall, so a small rise in demand above compressor output has a large effect. The allowable pressure drop sits in the denominator, so halving it from 100 to 50 kPa (1.0 to 0.5 bar) doubles the receiver. A narrow band and a high minimum tool pressure are expensive to support with storage alone.

Rules of thumb and their limits

Rules of thumb are usually quoted in imperial units (US gallons per cfm of compressor capacity). The conversions below use about 8.0 litres per L/s (1 US gal/cfm).

Rule of thumbLitres per L/sFor a 35 L/s compressor
Older rule (1 gal/cfm) (Marshall 2015)8about 280 L
Atlas Copco range (3 to 4 gal/cfm) (Atlas Copco 2020)24 to 32about 840 to 1,120 L
Load/unload base compressors (5 gal/cfm) (Mehltretter n.d.)40about 1,400 L

Marshall (2015) notes that the 8 L per L/s (1 US gal/cfm) rule dates from modulating compressors, and argues for more storage on load/unload machines where efficiency matters. The workshop's 1,000 L receiver works out at about 28.6 L per L/s (3.6 US gal/cfm), which sits inside the Atlas Copco range. Be careful with metric versions of these rules. The same Atlas Copco (2020) page converts its rule (3 to 4 US gal/cfm) to 10 to 15 litres per L/s, but the arithmetic gives 24 to 32 litres per L/s.

Bar chart of receiver volumes from three rules of thumb for a 35 L/s compressor: about 280 L, about 840 to 1,120 L and about 1,400 L. Lines mark the 912 L event calculation and the selected 1,000 L receiver, which sits inside the 24 to 32 L per L/s (3 to 4 gal/cfm) range.
Rules of thumb converted at 8.0 L per L/s (1 US gal/cfm), compared with the worked example result.

Rules of thumb ignore the demand profile, so use them as a cross-check. The event formula has limits too:

  • Fast events. In a sudden drawdown, the air in the receiver cools as it expands. With no heat transfer at all, the usable air between 700 and 600 kPa (7.0 and 6.0 bar) falls to about 73% of the isothermal value, so allow a margin for events lasting only seconds.
  • Lost pressure band. Pressure drop between the compressor and the first receiver eats into the usable band, and the NSW Office of Environment and Heritage (2017) reports that losing more than 30% of the band this way is not uncommon.
  • Compressor state. If the compressor is unloaded or stopped when the event begins, S is lower for the first part of the event.
  • Point of use. For one large intermittent user, a receiver near that user with a check valve and a metering valve can refill slowly between events without disturbing the rest of the system (US Department of Energy 2004).

Australian requirements for air receivers

An air receiver is a pressure vessel, so sizing is only part of the decision. AS 1210 sets minimum requirements for the materials, design, manufacture, testing and inspection of pressure vessels (Standards Australia 2010). Safe Work Australia (2015) advises choosing a receiver whose design pressure is at least the maximum working pressure, fitting a safety relief valve set no higher than that design pressure, and fitting a pressure gauge, an inspection opening and a drain valve at the lowest point. Pair that drain with an automatic condensate drain so it is not left to a daily manual task.

Registration depends on the vessel's hazard level, which is determined under AS 4343 (Standards Australia 2014). In Western Australia, pressure vessels in hazard level A, B or C require item registration, although serially produced vessels are excluded (WorkSafe WA 2022). Receivers also need periodic external and internal inspection, and Safe Work Australia (2015) points to AS/NZS 3788 for in-service inspection; the current edition is AS/NZS 3788:2024 (Standards Australia and Standards New Zealand 2024). Check registration and inspection obligations with your state regulator or a competent person.

Schematic of a vertical air receiver with a safety relief valve, pressure gauge, inspection opening, outlet and inlet, and a drain valve at the lowest point feeding an automatic condensate drain. A panel lists AS 1210, AS 4343 and AS/NZS 3788.
Fittings Safe Work Australia advises for an air receiver, and the Australian Standards that apply (schematic).

Key takeaways

  • For a demand event, size the receiver with V = t × (C − S) × Pa ÷ (P1 − P2), using L/s, seconds and kPa.
  • For load/unload cycling, use V = 900 × Q × Pa ÷ (Z × Δp), which covers the worst case at 50% load.
  • Size for the larger of the two results. Halving the allowable pressure drop doubles the receiver.
  • Rules of thumb of roughly 24 to 40 litres per L/s are useful cross-checks, not a substitute for the calculation.
  • Design to AS 1210, check the AS 4343 hazard level for registration, and inspect in service to AS/NZS 3788.

Frequently asked questions

How do I find the demand C for an event?

Measure it with a flow meter if you can. Otherwise add up the rated air consumption of the tools that run together.

Does a variable speed compressor need as much storage?

Usually less for cycling control, because it matches output to demand instead of loading and unloading. You still need enough storage to ride through demand events and the start-up of a standby compressor, so run the event calculation.

Related reading

References

This article is general information only. Check manufacturer documentation and applicable Australian Standards for your installation, and have pressure equipment assessed by a competent person.