Compressed air storage isn't 1 gallon per cfm

Ask how big an air receiver should be and the answer is usually "1 gallon per cfm", sometimes 3 to 5 gallons for a load/unload compressor. A 450 cfm compressor gets a 450 gallon tank, and the job is done.

That rule has a purpose: it gives the compressor enough of a buffer that it doesn't load and unload every few seconds. It was never meant to say how much air the plant can draw on when demand jumps, a compressor trips, or the next machine is still starting. That is a different question, with a different answer.

A tank only gives air when the pressure falls

A receiver doesn't hold air the way a bucket holds water. It holds air under pressure, and it only gives that air to the plant as the pressure falls. A tank sitting at 105 psig gives the plant nothing unless the pressure is allowed to drop.

So the useful size of a receiver isn't its gallons. It is how much air comes out for each psi the pressure falls, times the number of psi the plant can afford to lose. As a guide, a 1,000 gallon receiver gives about 9 cubic feet of free air for every psi it drops.

What 1 gallon per cfm really buys

Take that 450 cfm compressor with its 450 gallon receiver. The compressors run between 100 and 110 psig, and the machines at the far end need at least 90. Allowing for the pressure lost in the dryers, filters and piping, the receiver can fall to about 100 psig before the plant notices. That is about 10 psi of useful pressure.

Ten psi from 450 gallons is about 40 cubic feet of air. If a large user starts and the plant suddenly needs 300 cfm more, that air lasts about 8 seconds. A backup compressor that takes 30 seconds to start and load will be late. The pressure sags, and someone turns the setpoint up "to be safe". From then on, every compressor makes more pressure than the plant needs, all day, to cover a few seconds of shortfall.

Size storage from what it has to carry

Storage for the system starts from three questions:

• What is the biggest event? The largest sudden demand: a blow-off, a big cylinder, a baghouse pulse, a compressor that trips or unloads.
• How long until help arrives? The time for the next compressor to start and load, or for the event to end.
• How many psi can the plant give up? From the pressure the compressors hold, down to the lowest pressure the plant can accept at the far end, less what is lost on the way.

The air the event needs over that time, spread across the psi the plant can give up, sets the storage. In the example, 300 cfm for 30 seconds is 150 cubic feet. Spread across 10 psi, that is about 1,650 gallons, almost four times what the rule gives.

Where the storage goes matters too

• Storage near the compressors (often after the dryer) steadies the compressors and their controls.
• Storage near a large intermittent user feeds that user from close by, so its demand doesn't pull the whole header down.
• Storage behind a flow controller can be held at a higher pressure than the plant needs, so it has more psi to give up, without raising the pressure the plant runs at.

Large headers count as storage too, and are often left out when someone adds it up.

Signs a system is short of storage

• The pressure dips sharply every time a particular machine starts.
• Compressors load and unload rapidly, or a backup comes on for a few minutes and goes off again.
• The setpoint has been raised over the years to cover dips, and no one is sure why.
• Pressure complaints at the far end at the same times of day.

Logged pressure and compressor power, side by side over a week, show which of these is happening and how often. They also show how fast the pressure falls in each event, and so how much air the system was short.

More storage isn't always the answer. If the pressure is lost in piping that is too small, a bigger tank in the compressor room won't help the far end. But a system sized only to "1 gallon per cfm" is often paying for higher pressure every hour to cover a few seconds a day.

Source: U.S. Department of Energy, Improving Compressed Air System Performance: A Sourcebook for Industry, compressed air storage. Figures are rounded and use free air at sea level.

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