- Author
- Byron Raal, CAS Founder-Editor About the author
- Checked against
- AS 1210AS/NZS 3788AS 4343
- Date last checked
- 16 August 2026
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The short answer
Size an air receiver from the demand event you need it to survive, not the compressor nameplate. Work out how much free air the plant pulls, how long the event lasts, and how far header pressure can fall before production notices. Then run the drawdown equation: V = (Q × t × Pa) / (P1 − P2), where V is receiver volume in litres, Q is the free air flow it must supply in litres per second, t is the event duration in seconds, Pa is atmospheric pressure, and P1 and P2 are the receiver pressures at the start and end of the event in the same units. A 30 second burst pulling 50 L/s on a 1 bar swing needs about 1,520 L. Rules of thumb are a sanity check only, and once you convert the published ones to the same units they run from 3 to 40 litres per litre per second, so they disagree by more than a factor of ten. In Australia your receiver is also a pressure vessel: specify it to AS 1210, classify it under AS 4343, and register it or not depending on your state.
What the receiver is actually doing
Plenty of sites size the receiver off the nameplate and then wonder why the machine still short-cycles. It has four jobs, and only one of them relates to the compressor’s rating. If the compressor rating itself is still open, settle it first with the air compressor sizing calculator.
It buffers demand events. When a blow-off station or a bank of cylinders fires at once, the plant pulls more air in three seconds than the compressor makes in three. The receiver covers the gap. This is what the drawdown equation sizes for, and it’s the job that gets skipped when someone sizes off the nameplate.
It cuts load cycles, because an unloaded fixed-speed compressor still draws power to do nothing. It drops moisture out, so less water reaches your dryer and filters.
It steadies pressure, which lets you run a lower set-point, and that one is worth money. Per our demand profiling guidance, dropping the set-point by 1 bar typically saves 6 to 7 per cent of compressor energy. Take a 45 kW machine at 6,000 hours a year, run at full load, on an indicative $0.30 per kWh: that’s an $81,000 electricity bill, so 6 to 7 per cent is roughly $4,900 to $5,700 a year. Use your own tariff and your own load profile, because both move the answer.
The rules of thumb, and why they don’t agree
There is no single industry rule of thumb here. At least four are in circulation, the published numbers span 3 to 40 litres per litre per second once they are all converted to the same units, and several don’t survive that conversion.
| Published rule | Source | In litres per litre per second |
|---|---|---|
| 10 to 15 L per L/s, fixed-speed load/unload | Atlas Copco Australia | 10 to 15 |
| 3 to 5 L per L/s, VSD | CAS receiver hub | 3 to 5 |
| “At least 6 to 10 times FAD in L/s”, attributed to BCAS | Anglian Compressors, a branch of Atlas Copco Compressors (UK) | 6 to 10 |
| “Three to five gallons per CFM” (US gallons) | Compressed Air Best Practices, Derrick Taylor of PneuTech USA | 24 to 40 |
A word on the 6 to 10 figure, because you’ll see it quoted as gospel. It’s a real published rule and it’s traceable: Anglian Compressors, a branch of Atlas Copco Compressors in the UK, attributes it to British Compressed Air Society guidance, in those words. What we could not do is open a BCAS publication and read it there. BCAS puts some fact sheets out in the open, including one on air receiver conformity, but that sheet is about compliance rather than sizing, and the best practice guides sit behind registration.
So here’s exactly where we stand. We can tell you who prints the rule and who they say it came from. We can’t tell you we’ve read it at source, so we won’t dress a UK distributor’s attribution up as an Australian engineering standard. We’re not calling it wrong either. We just can’t reach the document that would settle it, and you should know which of those two things we’re saying.
The conversions are worse than the spread suggests. Atlas Copco offers “3 to 4 gallons of tank volume for every cubic foot per minute (CFM)” as being “equivalent to 10 to 15 litres for every litre per second”. It isn’t. One US gallon per CFM is 3.785411784 litres divided by 0.4719474 litres per second, which is 8.02 litres per litre per second, so 3 to 4 gallons per CFM is really 24.1 to 32.1. Their two numbers are meant to be the same rule expressed twice, and they sit a factor of two to two and a half apart (24.1 against 10 at the bottom, 32.1 against 15 at the top). When the shortcut can’t convert its own units, you calculate.
The drawdown method
A receiver at absolute pressure P and volume V, as an ideal gas at constant temperature, holds as much gas as P × V / Pa litres of free air at atmospheric pressure Pa. Falling from P1 to P2 releases V × (P1 − P2) / Pa litres; a plant drawing Q litres per second for t seconds needs Q × t litres. Set them equal and rearrange:
Air receiver drawdown method
Size the tank from the event, not the nameplate
One equation, four inputs, and a worked burst that shows why the compressor’s contribution matters.
V = ( Q × t × Pa ) / ( P1 − P2 )
V = receiver volume (litres)
Q = free air flow it must supply (L/s)
t = duration of the demand event (seconds)
Pa = atmospheric pressure (absolute)
P1, P2 = pressure at start and end, same units
Q = 50 L/s, t = 30 s, Pa = 101.3 kPa, P1 − P2 = 1 bar = 100 kPa
Compressor output during the event assumed zero
V = (50 × 30 × 101.3) / 100 = 1,519.5 litres, call it 1,520 L
Net the compressor back in and the answer moves.
If the compressor is already putting 30 L/s into the header, the receiver only has to make up 20 L/s:
V = (20 × 30 × 101.3) / 100 = 608 litres. Two and a half times smaller, and the only thing that changed was one assumption.
Source: CAS receiver drawdown method, isothermal ideal gas, Pa 101.3 kPa absolute.
V = ( Q × t × Pa ) / ( P1 − P2 )
where:
V = receiver volume (litres)
Q = free air flow the receiver must supply (L/s)
t = duration of the demand event (seconds)
Pa = atmospheric pressure (absolute)
P1 = receiver pressure at the start of the event
P2 = receiver pressure at the end of the eventThe units cancel: Q × t is litres of free air, and Pa over (P1 − P2) is dimensionless provided both are in the same units.
One trap: P1 and P2 can be gauge or absolute so long as they match, but Pa in the numerator is always absolute. That’s where spreadsheet versions go wrong.
Worked example: a 30 second, 50 L/s burst on a 1 bar swing
Stated assumptions. Isothermal expansion. Ideal gas. Atmospheric pressure 101.3 kPa absolute. The 1 bar swing is a difference, so gauge or absolute doesn’t change the answer. Compressor output during the event assumed zero: the receiver carries the burst alone. Pipework volume ignored, which is conservative.
Inputs. Q = 50 L/s of free air. t = 30 s. Pa = 101.3 kPa. P1 − P2 = 1 bar = 100 kPa.
Step 1. Free air the event demands: Q × t = 50 × 30 = 1,500 litres.
Step 2. Multiply by Pa, divide by the swing.
V = (1,500 × 101.3) / 100 = 151,950 / 100 = 1,519.5 litres. Call it 1,520 L, or 1.52 m3.Sanity check one. Run it backwards. A 1,519.5 L vessel dropping 100 kPa releases 1,519.5 × (100 / 101.3) = 1,500 litres of free air, exactly the demand. Spec the rounded 1,520 L and you release 1,500.5 L, so the rounding buys you half a litre.
Sanity check two. Receiver and free air volumes come out nearly identical because Pa and the swing are both close to 100 kPa. On a 1 bar swing you need roughly one litre of receiver per litre of free air consumed.
Yes, 1,520 L looks big, and we’re not rounding it down. Put the same 50 L/s machine through the three metric rules in that table and you land between 150 L and 750 L, well under our answer. Put it through the US gallons-per-CFM rule and you land at 1,200 to 2,000 L, straddling it. The published rules bracket the calculation rather than agree with it. That’s the point.
The 150 to 750 L gap isn’t an arithmetic error either. Those rules assume the compressor keeps contributing through the event; this example assumes it contributes nothing. Net it back in and the answer moves. If the compressor is already putting 30 L/s into the header, the receiver only has to make up 20 L/s:
V = (20 × 30 × 101.3) / 100 = 608 litres
Two and a half times smaller, and the only thing that changed was one assumption. That’s why we ask for a measured demand profile before quoting a size.
As a cross-check, run the cycle-frequency variant from our receiver hub, V = (C × T × Pa) / (4 × (P1 − P2)). On the same 50 L/s machine and the same 1 bar control band, the 10 to 15 L per L/s range works out at 500 to 750 L, which buys a full load/unload cycle of about 39 seconds at the bottom of the range and about 59 seconds at the top: roughly 90 down to 60 cycles an hour. Both numbers move with the band you set, so if anyone quotes you a cycle time, make them quote the band with it.
Wet, dry, and where the vessel goes
A wet receiver sits between the compressor and the dryer, letting hot saturated air cool and drop condensate. A dry receiver sits after the dryer and filters and holds treated air ready to use. Neither position is universally the one to buy first. The wet receiver upstream helps the compressor and the dryer: cooler air, condensate dropped before the dryer sees it, longer filter life and a steadier pressure signal to the controller. The dry receiver downstream helps the process: it rides out a short demand peak on already-treated air, so the surge never gets pulled through the dryer above its rated flow and lifts the dew point exactly when you need it. Plenty of sites run storage on both sides. If the budget only covers one, pick its position from your dryer’s rated capacity, your measured demand profile, the pressure dew point you have to hold and what you want the controller to do, not from a rule of thumb.
Where the demand event sits a long way from the plant room, split storage beats one big tank: central storage handles the compressor’s control problem, and a local receiver at the blow-off station handles the three second event the plant room can’t reach through 80 metres of pipe. Our system design guide covers distribution.
The compliance step, and why it belongs in the sizing decision
In Australia a receiver is a pressure vessel, and the number you choose changes your regulatory position. AS 1210 is the design and construction standard for unfired pressure vessels; buy certified to it, with the plate fixed and legible.
Hazard level. AS 4343 classifies pressure equipment by hazard level, and it isn’t merely guidance: Schedule 5 of the model WHS Regulations adopts the AS 4343 hazard-level scheme, and the harmonised states and territories enact that schedule. The formula is H = P × V × Fc × Ff × Fs; for clean compressed air at standard service those factors take known values (Fc = 10, Ff = 1.0, Fs = 1), so it collapses to H = 10 × P (MPa) × V (litres) in MPa.L. Run your numbers through the AS 4343 hazard level calculator.
This is where sizing bites. Our 1,520 L answer at 800 kPa design pressure gives H = 10 × 0.8 × 1,520 = 12,160 MPa.L. Level C runs above 1,000 up to 10,000 MPa.L and Level B above 10,000, so at 800 kPa design pressure the crossover sits just above 1,250 L, and our vessel is well past it. The equation just moved the receiver up a hazard level, which is worth knowing before the purchase order goes out, not after. An H value landing exactly on a boundary takes the lower level under Clause 2.2.8; ours is nowhere near one, and if yours is, have a competent person call it.
Registration. Hazard level sets the duty, but the trigger sits with each state and territory regulator, so there’s no single national threshold anyone can quote you. In the harmonised jurisdictions, design registration generally applies at Levels A to D and item registration to pressure vessels at Levels A to C, subject to exceptions. Victoria isn’t one of them: it runs its own regime under the Occupational Health and Safety Regulations 2017 and hasn’t required item registration since 1 July 2014, so if a supplier quotes you a Victorian duty out of a harmonised checklist, they’ve got the wrong instrument. Check your own state in our pressure vessel registration guide.
Inspection. AS/NZS 3788 is the technical in-service inspection reference standard and it guides how the duty is met; it isn’t itself the legal source. Cadence turns on the pV product, not on hazard level: above pV 150 MPa.L the standard sets external inspection at 2 years and internal at 4, with a 12-year extended interval available on its own conditions. At or below 150 MPa.L the intervals sit at the competent person’s discretion. Our vessel’s pV is 0.8 × 1,520 = 1,216 MPa.L, so it lands in the 2 and 4 year band. Detail in our air receiver inspection requirements guide.
Indicative sizing reference
| Compressor output (FAD) | Load/unload (10 to 15 L per L/s) | VSD (3 to 5 L per L/s) |
|---|---|---|
| 20 L/s reference point (7.5 kW class) | 200 to 300 L | 60 to 100 L |
| 20 to 50 L/s (about 11 to 22 kW) | 200 to 750 L | 60 to 250 L |
| 50 to 90 L/s (about 30 to 37 kW) | 500 to 1,350 L | 150 to 450 L |
| 90 to 130 L/s (about 45 kW) | 900 to 1,950 L | 270 to 650 L |
This table answers the compressor’s control problem, not your demand events. The worked example above shows the size of that gap: the load/unload band puts a 50 L/s machine at 500 to 750 L, and the drawdown calculation on an unassisted 30 second event puts it at 1,520 L, two to three times more. Compressor sizing sits in our sizing guide.
Four ways sites get this wrong
Fudging the compressor’s contribution. The worked example moved from 1,520 L to 608 L on that assumption alone. If your supplier can’t tell you which assumption sits behind their number, it’s decoration.
Buying for today’s load. Storage is cheap to oversize at purchase and expensive to redo later, because redoing it means another vessel and another installation. Whether it also costs you in registration isn’t automatic: in NSW a design registration covers every item built to that design, Queensland accepts a design registered under a corresponding law in another state, and Victorian design registrations don’t expire. What can still bite is item registration, which turns on the new vessel’s hazard level, the exceptions and your own state’s scheme. Price the vessel and the install, then check the registration position rather than assuming a fresh one.
Leaving registration and inspection out of the purchase decision. A vessel crossing into a higher hazard level or pV band carries compliance cost for its whole life. Put that in the capital comparison, not a surprise invoice three years later.
Sizing storage to hide a leak load. If a chunk of your measured demand is leaks, you’re buying steel to store air you throw away. A single 3 mm leak at 7 bar discharges about 7.3 L/s of free air and costs roughly $3,500 to $6,900 a year in electricity, per our 2026 leak cost report. Find them first, then size.
Frequently asked questions
How do I calculate air receiver tank size?
Use V = (Q × t × Pa) / (P1 − P2), with V in litres, Q the free air flow the receiver must supply in litres per second, t the event duration in seconds, Pa atmospheric pressure at 101.3 kPa absolute, and P1 and P2 the receiver pressures at the start and end of the event in the same units. A 30 second event drawing 50 L/s on a 1 bar drop needs about 1,520 litres, or 608 litres if the compressor supplies 30 of it.
Is there a rule of thumb for air receiver sizing?
There are several and they don’t agree. Atlas Copco publishes 10 to 15 litres per litre per second of free air delivery for fixed-speed machines, and CAS uses 3 to 5 for variable speed. A figure of 6 to 10 circulates in the United Kingdom, attributed to British Compressed Air Society guidance, though we couldn’t read it in a BCAS primary publication. A US trade-journal rule of three to five gallons per CFM converts to 24 to 40 litres per litre per second, higher than any of the metric rules. Converted to common units the published numbers span 3 to 40, so treat any of them as a sanity check rather than the answer.
Should the receiver be sized for the compressor or the demand?
For the demand. The compressor rating drives the cycle-frequency calculation, which stops a fixed-speed machine cycling too often. The demand event drives the drawdown calculation, which stops header pressure collapsing when processes fire together. Run both and buy the larger.
Does a bigger air receiver save energy?
Indirectly. More storage means fewer load and unload cycles and a steadier header, which lets you run a lower set-point, and per our demand profiling guidance reducing the set-point by 1 bar typically saves 6 to 7 per cent of compressor energy. Storage doesn’t create air, so it won’t fix a capacity shortfall.
Does my air receiver have to be registered in Australia?
It depends on its hazard level under AS 4343 and on your state or territory. Schedule 5 of the model WHS Regulations adopts the AS 4343 hazard-level scheme, and for clean compressed air at standard service the hazard level is 10 times design pressure in megapascals times volume in litres. In the harmonised jurisdictions, design registration generally applies at Levels A to D and item registration to pressure vessels at Levels A to C, subject to exceptions. Victoria isn’t one of them: it runs its own OHS regime under the Occupational Health and Safety Regulations 2017 and hasn’t required item registration since 1 July 2014. There’s no single national threshold, so confirm the duty with your own regulator.
Want the sizing checked against your real demand profile?
For a receiver enquiry, send your compressor free air delivery, demand during the pressure-dip event, event duration, normal header pressure, minimum acceptable process pressure, ongoing compressor supply during the event, and the permitted cycling rate or relevant manufacturer control information. State units and identify estimates. These inputs help a supplier assess storage, controls and recovery together. We can review your brief and discuss a suitable Australian provider if one is available. We ask for your written permission before an introduction and explain any referral payment arrangement. There is no cost to enquire.
CAS does not sell, install or service receivers. There is no charge to enquire or for an introduction if one is arranged. Any referral payment arrangement is explained in writing before an introduction, and we ask for your written permission before sharing your details. You can also use the contact page.
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