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Open resource1. Purpose and Engineering Decision
Compressed-air receivers are often discussed as if more storage automatically fixes low pressure. It does not. A receiver can support a short transient only when three quantities are known: the net air deficit during the event, the usable pressure band, and the event duration.
The calculator supports two primary modes: size a receiver for a defined transient, or enter an existing receiver and estimate how long it can support a defined net deficit before pressure falls to the minimum allowable value.
Decision mode | Primary output |
|---|---|
Size receiver | Required geometric receiver volume |
Check existing receiver | Estimated transient hold-up time |
Check usable stored air | Free-air-equivalent release across P1 to P2 |
Compare local vs central storage | Same storage equation, different assumed supply response and local restrictions |
2. Core Storage Relationship
For a short isothermal event with the receiver gas at approximately constant temperature and with flow stated at a defined reference pressure, the usable free-air-equivalent storage is proportional to receiver volume and the absolute pressure difference across the usable band.
V_receiver = Q_net,ref x t x P_ref,abs / (P1,abs - P2,abs)
Q_net,ref is the net reference flow deficit: transient demand minus compressor or upstream supply that continues to reach the protected zone during the event. P1 and P2 must both be absolute pressures.
Variable | Meaning |
|---|---|
V_receiver | Required geometric receiver volume |
Q_net,ref | Net deficit at the selected flow reference condition |
t | Transient duration |
P_ref,abs | Absolute pressure of the flow reference basis |
P1,abs | Receiver pressure at start of usable discharge |
P2,abs | Minimum acceptable receiver pressure |
3. Generalized Mass-Balance Form
The website default should remain the transparent isothermal form above, while the advanced method is available only when the additional inputs are known.
V = [Q_ref x t x P_ref/(Z_ref x T_ref)] / [P1/(Z1 x T1) - P2/(Z2 x T2)]
If T1 = T2 = T_ref and Z1 = Z2 = Z_ref, the generalized equation reduces to the simpler pressure-band formula.
4. Net Transient Demand
Q_net = max(Q_event - Q_supply_during_event, 0)
Receiver sizing must use the air deficit that storage actually has to cover, not automatically the full event demand. If the compressor, header or upstream receiver continues to supply part of the event flow, that contribution reduces the required local storage. If supply response is uncertain, the calculator should allow a conservative zero-contribution scenario and a measured/verified contribution scenario side by side.
Event behaviour | Recommended input |
|---|---|
Short actuator/pulse event | Peak event flow minus steady supply reaching the zone |
Compressor unloaded then reloads slowly | Use the flow delivered during the actual response interval, not rated capacity |
Protected instrument-air hold-up | Often use conservative supply loss or reduced-supply scenario |
Remote branch with restrictive piping | Use measured/estimated branch supply during the event |
Sustained demand increase | Do not rely on receiver sizing alone; investigate supply capacity |
5. Input Model
Input group | Field | Validation / note |
|---|---|---|
Mode | Size receiver / Hold-up time / Usable storage | Explicit selection |
Demand | Event flow | Must use a declared flow basis |
Demand | Supply during event | 0 to event flow for storage-deficit mode |
Time | Event duration | Required for sizing mode |
Receiver | Existing volume | Required for hold-up mode |
Pressure | Start pressure P1 | Gauge or absolute, converted internally |
Pressure | Minimum pressure P2 | Must be lower than P1 |
Reference | Flow reference pressure | Absolute |
Reference | Flow reference temperature | For advanced temperature correction |
Advanced | Receiver temperature / Z factors | Optional; default isothermal Z=1 approximation |
Output | Volume unit | L, m3, US gal where implemented |
6. Pressure Band - The Most Important Design Choice
The receiver is useful only across the pressure band that can be released without violating the process minimum pressure. A receiver charged to 8 bar(g) but protecting a machine that must remain above 7.5 bar(g) has far less usable storage than the same receiver protecting a 6.0 bar(g) user.
The tool must not encourage raising plant pressure merely to create storage; pressure elevation has energy and equipment consequences that belong to the wider system decision.
7. Reference Test Case A - Local Receiver for a 20-Second Peak
A machine cluster requires 18 m3/min FAD for 20 seconds. The branch can continuously supply 10 m3/min during the event. Receiver pressure may fall from 8.0 bar(g) to 6.5 bar(g). Atmospheric/reference pressure is 1.01325 bar(a).
Input | Value |
|---|---|
Event demand | 18 m3/min FAD |
Supply during event | 10 m3/min FAD |
Net deficit | 8 m3/min FAD |
Duration | 20 s |
P1 | 8.0 bar(g) = 9.01325 bar(a) |
P2 | 6.5 bar(g) = 7.51325 bar(a) |
V = 8 x (20/60) x 1.01325 / (9.01325 - 7.51325) = 1.801 m3
Reference result: approximately 1801 litres geometric receiver volume before design margin, piping volume, control response and site-specific constraints are considered.
8. Reference Test Case B - Hold-Up Time of an Existing Receiver
An existing 2,000 L receiver starts at 8.0 bar(g) and may discharge to 6.0 bar(g). A protected zone loses 6 m3/min more air than the upstream source can supply. Reference pressure is 1.01325 bar(a).
t = 2.0 x (9.01325 - 7.01325) / (6 x 1.01325) = 0.658 min
Hold-up time is approximately 39.5 seconds under the stated isothermal assumptions.
9. Reference Test Case C - Why Compressor Contribution Matters
A 12 m3/min transient lasts 10 seconds across the same 8.0 to 6.5 bar(g) band. Compare two cases: no supply contribution and 7 m3/min continuing supply.
Scenario | Net deficit | Required volume |
|---|---|---|
No supply contribution | 12 m3/min | 1.351 m3 (1351 L) |
7 m3/min continues | 5 m3/min | 0.563 m3 (563 L) |
This comparison is why the calculator asks for supply during the event rather than assuming either zero supply or full compressor nameplate flow.
10. Local vs Central Storage
Storage location | Engineering advantage | Limitation |
|---|---|---|
Local near cyclic user | Supports the transient before long/restrictive branch losses dominate | Needs local space, drainage, isolation and pressure control |
Central receiver | Provides system-wide buffering and compressor control volume | May not correct a severe downstream restriction |
Distributed storage | Can protect multiple zones and reduce local pressure collapse | Requires coordinated pressure bands and maintenance access |
Process-protected receiver | Can provide controlled hold-up for critical utility users | Must be isolated from noncritical drains on stored air |
11. Usable Storage Output
For an existing receiver, the calculator should also show usable free-air-equivalent volume across the selected pressure band.
V_free,usable = V_receiver x (P1,abs - P2,abs) / P_ref,abs
For the 2.0 m3 receiver in Test Case B, usable free-air-equivalent storage from 8.0 to 6.0 bar(g) is approximately 3.95 m3 at the stated reference pressure.
12. Output Specification
Output | Display rule |
|---|---|
Required receiver volume | Primary result in L and m3 |
Net transient deficit | Demand minus continuing supply |
Usable pressure band | P1, P2 and absolute pressure difference |
Hold-up time | Shown when existing receiver volume is entered |
Usable free-air storage | Equivalent reference-volume release |
Assumptions | Isothermal / Z / temperature / flow basis / atmospheric pressure |
Confidence | High / medium / preliminary |
Warnings | Sustained deficit, narrow pressure band, basis mismatch, P2 >= P1, uncertain supply contribution |
13. Validation and Guardrails
Condition | Required behaviour |
|---|---|
Event flow <= 0 | Block calculation |
Supply during event < 0 | Block calculation |
Supply during event > event flow | Set net storage deficit to zero and explain no receiver volume is required for that defined deficit |
P2 >= P1 | Block calculation |
Absolute P1 or P2 <= 0 | Block calculation |
Duration <= 0 | Block sizing mode |
Existing volume <= 0 | Block hold-up mode |
Flow basis missing | |
Gauge pressure without atmosphere | Require Patm or visible accepted default |
Very long event / large volume | Warn that receiver storage may be the wrong primary solution |
14. Design Margin and Non-Ideal Effects
The calculated geometric volume is a physics-based starting value, not the final vessel selection. A practical installation may require allowance for control delay, uncertainty in event flow, pressure drop between receiver and user, temperature change, inaccessible full vessel volume, future demand, simultaneous events and the actual compressor-control response.
The base result and margin-adjusted result should both remain visible.
V_selected_basis = V_calculated x (1 + margin/100)
Calculators and worked examples
These tools give indicative figures for screening, comparison and for preparing an enquiry. They run in your browser and send nothing. Prices, tariffs and efficiencies shown are example values: replace them with your own. Results are not a performance guarantee or a design certificate; final selection is confirmed by an engineer against the machine data sheet and the site.
Receiver storage sizing
Estimate the receiver volume needed to ride through a short burst of demand without the pressure falling below the limit.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Burst demand above compressor output (m³/min free air): 5
- Burst duration (s): 20
- Allowed pressure fall (bar): 0.7
Result
- Receiver volume needed: 2,412 litres
- 2.41 m³
- Practical size (with 25% margin): 3,016 litres
Method. V = Q × t × P_atm / ΔP, where Q is the shortfall in free air (m³/min), t the time (min), ΔP the allowed pressure fall (bar) and P_atm = 1.013 bar. Isothermal; for a real system add a margin.
What to take from it. A receiver buys time. It does not create capacity: once it is spent the pressure keeps falling until the compressors catch up.