A receiver-mounted automatic air compressor starts when tank pressure reaches a lower cut-in setting and stops at a higher cut-out setting. This animation follows that pressure-switch cycle. Enter receiver volume, the two pressures and free-air delivery to estimate the air stored between the settings and the ideal fill time with no downstream air use.
Automatic Air Compressor Interactive Calculator
Vary tank size, pressure switch settings, and pump delivery to see pump-up time and stored free air between cut-in and cut-out.
Equation Used
This calculator estimates how long the compressor must run to raise the receiver from cut-in to cut-out. The pressure band is converted into equivalent free air using 14.7 PSI atmospheric pressure, and gallons are converted to cubic feet before dividing by pump CFM.
- Tank volume uses US gallons; pump flow is free-air cubic feet per minute.
- Gauge pressures are converted using 14.7 PSI atmospheric pressure.
- Pump free-air delivery is treated as constant across the pressure band.
- Equal-temperature storage estimate with no downstream demand or leakage.
The start-stop pressure cycle
The motor drives the reciprocating pump, which draws in air and delivers it through a check valve to the receiver. As pressure rises to cut-out, the pressure switch stops the drive. When downstream use lowers pressure to cut-in, the drive starts again.
On systems fitted with a head unloader, the trapped discharge volume between pump and receiver check valve can be relieved at shutdown. The check valve separates that volume from stored receiver air. Exact unloader and motor-control arrangements depend on the equipment.
Parts and functions
- Pump and drive: supply compressed air.
- Receiver: stores air between the operating pressures.
- Pressure switch: initiates start and stop at the setpoints.
- Check valve: limits reverse flow from the receiver.
- Unloader, where fitted: relieves the appropriate pump-side volume.
- Pressure regulator: controls downstream pressure within its operating limits; it is separate from the tank pressure switch.
The animation’s falling-pressure interval represents air use but has no entered demand rate. Its duration is illustrative. Only the no-demand pump-up interval is calculated. Equal or reversed setpoints give a stationary preview and zero pressure-band outputs.
Receiver storage and changing demand
A receiver can supply short bursts of air and slow the pressure change when demand varies. The US Department of Energy describes storage as part of the control strategy for managing demand events and maintaining system pressure.
A larger receiver stores more air over the same pressure band, but does not increase the pump’s continuous delivery. Tool demand, leakage and flow losses must be considered separately when deciding whether a system can sustain a duty.
Equipment selection also requires documented pressure limits, duty rating, motor-start limits and air-quality requirements. This model does not establish breathing-air or clinical suitability, select a vessel, or specify pressure-switch adjustments. The entered pressures are calculation inputs rather than instructions to change a machine’s settings.
No-demand pump-up time
Convert tank volume V from US gallons to cubic feet: V_ft³ = V/7.48052. Pressure Band ΔP = max(0, P_out − P_in), with both gauge pressures in PSI. Stored Free Air A = V_ft³ ΔP/14.7, in cubic feet referred to the 14.7 PSI reference pressure at the assumed common temperature.
Pump-up Time = 60A/Q seconds, or A/Q minutes, where Q is constant free-air delivery in cubic feet per minute on a compatible reference basis. The gauge-pressure difference equals the absolute-pressure difference because the atmospheric offset cancels.
The calculation assumes equal-temperature ideal-gas storage, no air use and no leakage during filling. Real receiver air heats during compression, and delivery can vary with pressure. A quoted SCFM value may use different temperature, pressure and humidity conventions; compare the supplier’s reference conditions before using it.
With a separately known constant demand D less than Q, an extended steady-flow model gives fill time 60A/(Q − D), idle time 60A/D and running fraction D/Q. Demand is not an input to this calculator. Under these assumptions, increasing tank volume lengthens both intervals but does not reduce that running fraction.
Worked example: storage between two pressures
Use a hypothetical 30 US gal receiver, cut-in 100 PSI, cut-out 130 PSI and constant free-air delivery 9.4 CFM. These are arithmetic inputs, not specifications for a named compressor.
The pressure band is 30 PSI. Tank volume is 30/7.48052 = 4.010417 ft³, and Stored Free Air is 8.184523 ft³. With no air use, fill time is 8.184523/9.4 × 60 = 52.2416 seconds, or 0.8707 minutes.
Doubling the receiver to 60 US gal doubles stored air to 16.369046 ft³ and fill time to 104.4833 seconds. Doubling pump flow instead halves the original fill time without changing the stored-air quantity.
For comparison outside the calculator, suppose constant demand is 4 CFM on the same reference basis. Net fill is 5.4 CFM, giving 90.9391 seconds filling and 122.7678 seconds idle. The running fraction is 4/9.4 = 42.5532%. A larger receiver stretches both times in this model; it does not change the 42.5532% fraction or establish that any compressor can sustain it.
Storage, delivery and control are different choices
| Change | Effect at other fixed inputs |
|---|---|
| Larger receiver | More stored free air and a longer no-demand fill interval. |
| Wider pressure band | More stored free air; actual allowable settings still depend on equipment ratings. |
| Higher free-air delivery | Shorter fill time, with the same stored-air quantity. |
| Higher downstream demand | Less net air available for filling; not included in the calculator. |
Noise, air purity, service life and maintenance schedules require specific equipment data. Compressor type or horsepower alone does not supply those values.
Pressure-cycle questions
No. It is the equivalent free-air quantity added between the two entered pressures, not the total air present at cut-out.
More air must be supplied across the same pressure band. At fixed delivery, stored quantity and no-demand fill time both grow in proportion to volume.
Not in the ideal constant-demand start-stop model. Both fill and idle times grow together, while running fraction remains demand divided by pump delivery.
No. Its drain interval has no entered demand rate, so a complete timed cycle is not calculated.
The model clamps the pressure band to zero. This identifies invalid pressure ordering, not a usable operating cycle.
The calculator does not authorize or specify such an adjustment. Receiver ratings, controls, relief protection and tool requirements must be checked for the actual equipment.
No. The model has no electrical, unloader, valve or temperature diagnostic inputs. Unexpected pressure or a stalled drive requires equipment-specific assessment.
In the constant-flow extension, there is no positive net fill rate. Increasing storage may delay pressure loss but cannot supply an unlimited continuous deficit.
Compressed-air storage reference
US Department of Energy, Improving Compressed Air System Performance, Fact Sheet 6: Compressed Air Storage, explains receiver storage and its relationship to system control.
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