Pressure lost before it reaches the plant

Ask what pressure a plant runs at and you will usually hear the compressor setpoint. But the machines don't use the pressure at the compressors. They use what is left after the air has passed through the dryers, the filters, the headers, the drops and the hoses. The difference is pressure the compressors make only to lose it.

What a psi costs

Each 2 psi of added discharge pressure costs about 1% more compressor power. So a system that loses 10 psi between the compressors and the plant pays about 5% more for every cubic foot, all year.

Take a system averaging 150 kW of compressor power, running around the clock at $0.12 per kWh. Ten psi of loss is about 7.5 kW, or about $7,900 a year.

And that is only the compressors. To get enough pressure to the far end, the whole system runs higher. Every leak and every use without a regulator then draws more air than it would at the lower pressure, and that air is paid for too.

Where the pressure goes

Dryers

Every dryer has a rated pressure drop at its rated flow. Fouled heat exchangers, worn valves and blocked strainers raise it over time. Dryers that run beyond their rated flow lose more.

Filters

A clean filter loses little. A loaded one can lose many times as much, and keeps getting worse until the element is changed. Elements changed on the calendar, not on the pressure across them, are often changed too late, or too early.

Piping

Headers too small for today's demand, long runs, sharp bends, restrictive fittings, quick-connects and long hoses all take their share. A line sized when the plant was half the size is a common cause.

Valves and other equipment

A valve left part-closed after maintenance, a check valve that sticks, or a separator or aftercooler that has fouled can take several psi on its own, and is easy to miss.

Losses that grow with the flow

Pressure lost through a dryer, filter or pipe rises faster than the flow does. Double the flow and the loss is roughly four times as large. That is why a system can look fine on a quiet shift and starve the far end at peak demand. A single reading at one time of day can hide the problem.

Lost, or held back on purpose?

Not every drop is waste. A flow controller or a regulator at a machine holds the pressure down on purpose, so that the plant uses less air. That drop is doing a job. The question is whether the rest of the loss is what the installed equipment should lose, or more. When the loss is well beyond what the equipment is rated for, something has changed: a loaded filter, a fouled dryer, a valve, or a pipe too small for the flow.

The Sample Plant on our Examples page shows the pressure each compressor loses on the way, against what the installed equipment should lose.

How to find it

• Measure at several points at the same time. At the compressor discharge, after the dryers and filters, at the main header, and at the far end of the plant.
• Log through peak demand. A week of readings, logged together, shows how the losses change with the flow. Spot readings rarely catch the worst of it.
• Compare against the ratings. Each dryer and filter has a rated drop. Losses well above it point to what needs attention.
• Fit gauges across the filters. Change the elements on the pressure across them, not the date.

Fixing it, and getting the savings

• Clean or change what has fouled. Filters, dryer heat exchangers, strainers and drains.
• Fix the bottlenecks. Open part-closed valves, replace restrictive fittings and quick-connects, and enlarge or loop the headers that are too small for today's flow.
• Size new dryers and filters for low pressure drop at the flow they will really see, not just to fit the budget.
• Then lower the setpoint. This is the step that is often missed. Fixing the losses saves nothing until the compressors are set to make less pressure. Lower it in steps, and watch the far end as you do.

Pressure is the quiet cost in compressed air. It doesn't hiss like a leak, and no one sees it. But every psi made only to be lost is energy that delivers nothing.

Source: U.S. Department of Energy, Improving Compressed Air System Performance: A Sourcebook for Industry, pressure drop and controlling system pressure. The example assumes $0.12 per kWh, as in our calculators.