Three reasons to size a receiver
- Cover a demand peak that is larger than the compressor can supply, for a known time.
- Limit compressor cycling on load/unload or start/stop control.
- Bridge a gap, such as the time a standby compressor needs to start after the running machine trips.
Work out the volume for each reason that applies, and use the largest.
1. Covering a peak
The storage needed to supply extra air for a set time, while pressure falls from an upper to a lower limit, is:
V = t × Q × pa ÷ (p1 − p2)
- V = receiver volume, m³ (× 1,000 for litres)
- t = duration of the peak, minutes
- Q = air needed above what the compressors supply, m³/min of free air (1 m³/min ≈ 35.3 CFM)
- pa = atmospheric pressure, about 1.013 bar absolute
- p1 − p2 = allowable pressure drop during the peak, bar
Example: a blow-off uses 50 CFM more than the compressor can supply for 1 minute, and users can accept a drop from 7.0 to 6.0 bar. Q = 50 ÷ 35.3 = 1.42 m³/min. V = 1 × 1.42 × 1.013 ÷ 1.0 = 1.44 m³, about 1,450 litres. Allowing only 0.5 bar of drop doubles the volume to about 2,900 litres. The allowable pressure drop has as much influence on size as the peak itself.
2. Limiting compressor cycling
A load/unload compressor needs enough storage to keep its cycles within the manufacturer's limit. A widely used formula is:
V = 0.25 × q × pa ÷ (f × Δp)
- V = receiver volume, litres
- q = compressor free air delivery, litres per second (1 CFM ≈ 0.472 l/s)
- f = maximum load cycles per second the machine allows (for example 1 cycle per 30 seconds = 0.033)
- Δp = difference between load and unload pressure, bar
The worst case occurs when demand is half the compressor's output, which is where the 0.25 factor comes from.
Example: a 212 CFM (100 l/s) compressor, 1 cycle per 30 seconds allowed, 0.5 bar band: V = 0.25 × 100 × 1.013 ÷ (0.033 × 0.5) ≈ 1,535 litres. Widening the band to 1 bar halves this, but raises average pressure and energy use.
Variable-speed compressors need less storage for cycling, but still benefit from a receiver near the machine for stable control and condensate removal.
3. Bridging a standby start
If the running compressor trips, how long does the standby need to start and load, and how far can pressure fall before production stops? Use the peak formula with Q equal to the full demand and t equal to the start-up time.
Rough guide
For a single load/unload screw compressor in an ordinary industrial system, a total receiver volume of very roughly 6 to 12 litres per CFM of compressor capacity is common. Piston compressors on start/stop control often need more, to keep motor starts within limits. Treat these as a starting point only; use the formulas above where peaks, sensitive users or several compressors are involved.
| Compressor capacity | Common starting range of total receiver volume |
|---|---|
| 50 CFM | 300 to 600 litres |
| 100 CFM | 600 to 1,200 litres |
| 200 CFM | 1,200 to 2,400 litres |
| 400 CFM | 2,400 to 5,000 litres |
| 800 CFM | 5,000 to 10,000 litres |
Where to put the volume
Storage can be split between a primary receiver near the compressors (for control and water removal) and secondary receivers near large intermittent users. Remember that pipework also holds air, and that a pressure regulator downstream of a receiver stops it releasing stored air below the regulator setting.
Try the receiver storage calculator, or send us your compressor details and demand pattern and we will size it for you: +91 99590 60718, reddycompressors@gmail.com.