Compressors: litres per minute, pressure and tank size

The air flow is given in the data sheets as l/min, m³/h or cfm. Gast often gives two of them, for example 1.3–11 cfm or 37–311 l/min at 60 Hz for the piston compressors.

The total of everything running at the same time

Add up the consumption of everything that uses air at the same time. Equipment used one after the other is only counted once. Examples from the catalogue:

• Rotabroach Raven, air-powered magnetic drilling machine — 13 l/s (28 CFM) at 5.5–6.9 bar
• Gast air motor 4AM — max. 78 CFM (132.52 m³/h)
• Gast gear motor 1UP-GR11 — max. 21 CFM (35.68 m³/h)

Compare with what the compressor delivers

Gast's oil-free piston compressors deliver up to 9 cfm at 50 Hz (11 cfm at 60 Hz), and the rocking piston compressors up to 10 cfm at 50 Hz (12 cfm at 60 Hz). A Raven with a stated consumption of 28 CFM therefore uses more air than any of them is rated to deliver.

In other words, air-powered tools require a system sized for the consumption, not just the smallest compressor that can reach the right pressure.

How many bar does the equipment need?

The compressor must be able to deliver the pressure the equipment needs, with some margin for losses in hoses and couplings. Raven and Eagle need 5.5–6.9 bar, and Gast's oil-lubricated air motors are rated at a max. operating pressure of 7 bar, except for the 1UP at 5.6 bar.

Check the pressure limit for the model, not for the series. As a series, Gast's piston compressors go up to 125 psi (8.62 bar), but many models stop lower: the max. pressure for individual models ranges from 3.45 to 8.62 bar. The rocking piston series goes up to 12 bar, but some models stop at 1.03 bar.

Higher pressure than necessary does no extra work; it only adds wear and energy use. Turn the pressure down to what the equipment needs.

How big a tank does the compressor need, and how long can it run?

The duty cycle tells you how much of the time the compressor can be running. Jun-Air's oil-lubricated compressors are designed for 50%, and the user manual sets a limit of 15 minutes in each cycle at 8 bar. The Jun-Air systems with Gast's 86R and 87R are rated at 100% and can run continuously, but the user manual for the 87R-4P says 50% or less. If consumption is steady and prolonged, the compressor should be able to handle continuous operation or have a good margin.

The tank stores air between starts and stops. A larger tank means fewer starts and handles short peaks, but if consumption over time is higher than the compressor delivers, the tank will empty anyway, just later. Gast's tank-mounted piston compressors come in a simplex version with tanks from 7.6 to 113.6 litres, and Jun-Air's systems are available in several tank sizes.

Look for the 50 Hz figure

Gast often gives the air flow for both 50 and 60 Hz, and the motor runs faster at 60 Hz. On the Norwegian power grid, which is 50 Hz, it is the lower figure that applies. For example, the belt-driven rotary vane compressor 1550 is rated at 11.5 cfm at 50 Hz and 14.5 cfm at 60 Hz.

Also watch out for summary figures. For the motor-mounted rotary vane compressors, the air flow of 0.6–13 cfm is given at 60 Hz.

Clean, dry air

If the air is to be used in laboratories or for medical and dental applications, it is not enough for the flow to be right. In that case, the requirements for the equipment determine whether the air must be oil-free and dry. Jun-Air has both oil-free and oil-lubricated compressors for such applications, and supplies oil-free compressors in a cabinet with a dryer where the air needs to be clean and dry.

When you contact us

With the following details, we can work out the right compressor:

• The equipment that will use air, with consumption and pressure from the data sheet
• How much of it runs at the same time, and for how many hours per day
• Whether the air must be oil-free or dry
• Where the compressor will be located, and whether people are present there
• The voltage and phase available