A receiver tank smooths the compressor’s pulses and builds a reserve, but it is also the largest store of energy in an air system: a 100 L tank at 8 bar gauge holds 80,000 to 200,000 J depending on how the release is counted, yet running down from 8 to 6 bar gauge yields only 200 L of usable free air. Every receiver needs a relief valve, a gauge and a drain at its lowest point, and the drain matters because condensate corrodes the tank from the inside where nobody looks.
Part of Pneumatic Systems
Storing compressed air safely means a vessel that can hold the pressure, devices that limit it, and a way to remove the water that compression squeezes out.
A Receiver Buys Time, Not Much Air
The usable free air between two pressures is tank volume x (high absolute pressure - low absolute pressure) / atmospheric pressure. For a 100 L (0.1 m³) tank running from 8 bar gauge (9 bar absolute) down to 6 bar gauge (7 bar absolute), the lowest pressure a tool will tolerate in this example:
- Free air = 0.1 m³ x (9 - 7) bar / 1 bar = 0.2 m³ = 200 L
A tool that eats 68 L/min or more empties that in under three minutes (68 L/min is the least a 300 W vane motor can use; see Air Drill and Grinder). The receiver evens out the compressor’s pulses, lets a small compressor serve a tool that uses air in bursts, and gives the air time to cool and drop its water. It does not replace compressor capacity.
A Receiver Is a Bomb With a Gauge on It
Stored energy rises with pressure and volume. For the same 100 L at 8 bar gauge the energy released if it burst, counted three ways (the working is in Receiver Tank Construction), is 80 kJ or 80,000 J (pressure x volume, the convention the tire article uses), about 106 kJ (adiabatic) and about 200 kJ or 200,000 J (isothermal). Compare 4,000 J, the figure the corpus gives for a tire, which it already treats as a hazard. Keep stored energy down by using the smallest vessel and the lowest pressure that does the job.
Three Fittings Every Receiver Needs
- A relief valve on the vessel itself, with no valve of any kind between it and the tank, set at or below what the vessel is rated for. Details in Safety Devices.
- A gauge you can read from where you stand, checked against a known reference.
- A drain valve at the lowest point. Open it daily.
Condensate Corrodes From Inside
Compression squeezes water out of the air and it collects at the bottom of the tank, in the pipes and in any cooler. The water sits on bare steel and rusts the wall thinner from the inside, in the one place nobody can see without taking the tank apart. A tank that passed its test when new gets weaker every month it is not drained. Draining the water, cooling the air before it enters, and keeping the tank sloped to its drain are what keep the wall thick enough; see Moisture Removal.
Common Mistakes
- Treating a rusty tank as strong because it held pressure last year. Corrosion thins the wall in places a gauge cannot reveal.
- Fitting a shut-off valve between the tank and its relief valve. The valve can be closed by mistake, and then the relief is useless.
- Using a tank whose rating you do not know. The rating must come from the vessel’s own marking or from a real engineering reference; this corpus cannot supply it for an unmarked tank.
- Skipping the drain because it “never has much water”. Even a few hundred millilitres a day is enough to rust through a tank over months.
Safety
A burst receiver is an explosion
A compressed-air vessel that fails at a weld, a corroded patch or an over-pressured wall releases tens or hundreds of kilojoules in an instant and throws fragments. Never run a receiver without a relief valve, never raise a relief setting to stop it lifting, never weld or drill a tank that has been in service, and never trust a tank with no legible rating. Stand clear of the tank while it fills the first time, and test it with water first, as in Receiver Tank Construction.
What’s Next
- Learn next: Receiver Tank Construction — building and testing the vessel
- Combine with: Moisture Removal — getting the water out