Why Receiver Sizing Matters

An undersized receiver causes the compressor to cycle too frequently (short-cycling), which:

  • Overheats the motor from too many start/stop cycles
  • Causes pressure fluctuations at point-of-use tools
  • Increases compressor wear and reduces life expectancy
  • Triggers nuisance trips on thermal protection relays

An oversized receiver wastes capital cost and floor space but is rarely a problem technically. When in doubt, size up.

The Sizing Formula

The standard formula for minimum receiver volume is:

V = (Q × t × Pa) / (Pmax − Pmin)

V = minimum receiver volume (litres)
Q = peak demand flow not met by compressor (L/min free air)
t = acceptable time to deplete reserve (minutes) — typically 0.5–1 min
Pa = atmospheric pressure (1.013 bar absolute)
Pmax = system maximum pressure (bar absolute)
Pmin = minimum acceptable supply pressure (bar absolute)

Worked example

A plant has a peak demand of 500 L/min FAD. The compressor delivers 420 L/min FAD, so it cannot meet peak demand alone. The pressure band is 7.5–6.5 bar gauge (8.51–7.51 bar absolute). We want pressure to hold for 1 minute during peak demand.

Q = 500 − 420 = 80 L/min (shortfall during peak)
V = (80 × 1 × 1.013) / (8.51 − 7.51) = 81 / 1.0 = 81 litres minimum

In practice, add a safety factor of 2× and round up to a standard vessel size, giving a 200 L receiver.

Preventing Short-Cycling

For a fixed-speed compressor with on/off control, the receiver must provide enough buffer that the compressor runs for a minimum "on time" before unloading. Most manufacturers specify a minimum of 6 starts per hour, meaning the combined on+off time ≥ 10 minutes per cycle.

Required receiver volume to limit starts to N per hour:

V = (Qc × Pa × 60) / (4 × N × (Pmax − Pmin))

Qc = compressor FAD (L/min)
N = maximum starts per hour (typically 6–12)

Primary vs Secondary Receivers

Modern systems use two receivers:

  • Primary (wet) receiver — immediately after the compressor, before the dryer. Receives hot, wet air. Provides buffer volume and allows moisture to condense.
  • Secondary (dry) receiver — after the dryer, close to point-of-use. Stores dry air for peak demands. Prevents the dryer from being overwhelmed by sudden demand spikes.

The primary receiver is typically 6–10× FAD/min; the secondary is sized for the highest peak demand zone.

Pressure Vessel Compliance

Air receivers are pressure vessels and must comply with local regulations. In the UK/EU: PED 2014/68/EU and PSSR 2000 apply. A written scheme of examination (WSE) is required, and vessels must be inspected by a competent person at intervals specified in the WSE — typically every 26 months (internal) and every 4 years (hydraulic pressure test or NDT equivalent).

Never modify, weld, or remove the safety valve from an air receiver. All receivers must have a pressure relief valve set at or below the maximum allowable working pressure (MAWP).