Working pressure, point-of-use pressure and system pressure budget

Why compressor pressure and point-of-use pressure differ

A compressor controller can show a healthy 7.5 bar (108.8 psig) while a press or packing machine at the far end of the plant receives 5.5 bar (79.8 psig) and stops. Between those two points, every component consumes some pressure: aftercooler, receiver connections, dryer, filters, main header, branch lines, isolation valves, drops, regulators, point-of-use filters, hoses and quick couplings. At peak flow these losses rise sharply, because pressure drop increases roughly with the square of flow.

A clear pressure budget makes these losses visible. It lets you supply every user properly without running the whole plant at a pressure far higher than anyone needs.

Name the pressure and its location

“The plant needs 7 bar” is not a specification. Every pressure requirement should state where and under what condition it applies.

• Location: tool or machine inlet, machine manifold, branch, main header, treatment outlet, receiver or compressor discharge.
• Type: required minimum pressure for the process, normal operating pressure, and maximum allowable pressure (a protection limit, never a target).
• Condition: static (no flow) or dynamic (while the user is drawing air). Dynamic pressure is what matters; a gauge reading taken with the tool idle can hide a large drop.
• Basis: gauge (bar g, psig) or absolute (bar a). Plant gauges read gauge pressure; gas calculations often need absolute pressure. Never mix them silently.

How to build a pressure budget, step by step

1. Find the critical user. Identify the user that needs the highest pressure at its inlet, or the one furthest from the compressors, under peak flow.
2. Confirm its real requirement. Check the machine manual or measure the minimum inlet pressure at which it works properly, rather than accepting a habitual figure.
3. Add point-of-use losses. Regulator, local filter, hose and couplings at the flow the user draws.
4. Add distribution losses. Drop, branch and the section of main header between the user and the compressor house.
5. Add treatment losses. Dryer and filters at their actual flow and condition, allowing for filter loading before element change.
6. Add the control band. The difference between load and unload pressure, or the VSD control tolerance.
7. Set the compressor pressure. The unload or target pressure becomes the sum, checked against the compressor’s maximum rated pressure.
8. Verify by measurement. Log pressure at the compressor and the critical user during peak production and adjust the budget to reality.

Typical pressure losses in a well-designed system

The figures below are typical design targets used in good practice for a general industrial plant at around 7 bar. Actual losses depend on flow, pipe size, length and condition, and should be measured.

In many plants the largest single loss is at the very end: the hose, coupling and regulator at the machine. Our pressure management and point-of-use page looks at those components in detail.

A worked example

A packaging machine needs at least 6.0 bar (87 psig) at its inlet while running. The budget, working back to the compressor, might look like this:

A compressor loading at 6.9 bar and unloading at 7.5 bar (about 100–109 psig) supplies the machine properly. If the plant had instead been running at 8.5 bar to overcome a clogged filter and a narrow coupling, the extra bar would cost roughly 7% more compressor power, plus more leakage, for no benefit. Our pressure budget builder lets you set out the same calculation for your own system.

The energy cost of excess pressure

Higher pressure costs energy in two ways. First, the compressor works harder: as a widely used rule of thumb, each additional bar of discharge pressure needs about 7% more power for the same flow. Second, everything that leaks or vents to atmosphere, from leaks to open blowing and unregulated users, consumes more air at higher pressure. Lowering system pressure by finding and removing restrictions is therefore one of the most effective energy measures in compressed air, and it is usually low in cost. See our system pressure optimisation page.

What if one user needs much higher pressure?

Sometimes one machine needs 9 or 10 bar (about 130–145 psig) while the rest of the plant needs 6 bar. Raising the whole plant to suit that one user wastes energy on everything else. Better options include:

• a small dedicated compressor for the high-pressure user;
• a booster compressor taking air from the main system;
• local storage near the user if its demand is short and intermittent;
• checking whether the machine really needs that pressure, or whether its own internal losses can be reduced.

The same thinking applies in reverse to low-pressure blowing duties, which are often better served by a blower than by regulated compressed air.

Pressure stability and storage

A pressure budget assumes the system can hold pressure during peaks. Short, large demands such as cylinder banks, blow-off cycles or filter pulse cleaning can pull pressure down faster than compressors can respond. Correctly sized and placed receivers, a stable control band and sensible sequencing keep pressure within the budget. Dynamic pressure logging at the compressor and the critical user, taken at the same time, shows whether dips come from distribution losses or from supply response. Our pressure profiling page explains how this is measured.

Common pressure-budget mistakes

• Setting pressure from a habit or an old complaint rather than the critical user’s measured need.
• Measuring user pressure only when the user is idle.
• Ignoring filter loading, so the budget works on day one and fails a few months later.
• Small-bore quick couplings and long thin hoses on high-flow tools.
• Locating the compressor pressure sensor before the dryer and filters, so the controller does not see their loss.
• Raising pressure to cover a distant user instead of fixing the pipe that feeds it.

How Reddy Compressors helps

Reddy Compressors, at Tarnaka in Secunderabad, has supplied, rented and serviced compressed-air equipment since 2002. As an independent multi-brand specialist, not an authorised dealer, we work on systems built around ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Sullair, Doosan, Kirloskar and other makes. We have tie-ups with all major and reputed brands.

• Pressure surveys and logging from compressor to point of use, for plants across Telangana and Andhra Pradesh and on projects elsewhere in India.
• Pressure-drop mapping across dryers, filters and piping, with clear recommendations.
• Correction work: filter and drain service, regulator and coupling changes, piping modifications and setpoint resetting.
• Booster, dedicated or additional compressors, new, used or on rental from our own fleet, where the budget shows they are needed.
• Ongoing service and AMC support from over 20 service engineers and technicians.

What to send us

• Compressor make, model and nameplate photo, with current load and unload settings.
• Dryer and filter types and their position in the system.
• A simple piping sketch showing the compressor room, main header and far users.
• The machines that complain of low pressure and their required inlet pressure.
• Pressure readings at the compressor and at those users during peak production, if available.

Frequently asked questions

What working pressure should my air compressor be set to?

Set it to the lowest pressure that supplies your most demanding user at peak flow. Start from that user’s required inlet pressure, add the losses through hoses, regulators, filters, piping and dryer, then add the control band. For many general plants that works out at around 7 bar (101.5 psig), but the right figure comes from your own pressure budget and measurements, not a standard value.

How much pressure drop is acceptable in a compressed air system?

Good practice aims to keep total loss from compressor discharge to the point of use as low as practical, often around 1 bar or less including treatment. Main headers are typically designed for a very small drop, while dryers and filters each take a few tenths of a bar. Large losses usually appear at the point of use, in hoses, quick couplings and regulators, so measure there while the machine runs.

How much energy does reducing compressor pressure save?

As a widely used rule of thumb, each extra bar of discharge pressure needs about 7% more compressor power for the same flow. Reducing pressure also reduces air lost through leaks and unregulated uses. The actual effect depends on compressor type and control, so it should be verified by logging power and pressure before and after the change on the same basis.

Why is pressure at my machine lower than at the compressor?

Air loses pressure as it flows through dryers, filters, pipes, valves, regulators, hoses and couplings, and the loss rises steeply with flow. Clogged filter elements, undersized branch pipes, long dead-end runs and small quick couplings are the usual culprits. Measure pressure at several points at the same time during peak demand to find where the biggest drop occurs.

What is the difference between psig and bar?

Both are units of pressure. One bar equals about 14.5 psi, so 7 bar is 101.5 psi. The “g” in psig or bar g means gauge pressure, measured above local atmospheric pressure, which is what plant gauges show. Absolute pressure adds atmospheric pressure, about 1 bar, and is used in gas calculations. Always state which basis a figure uses.

Should I use a booster for one high-pressure machine?

Often yes. If one user needs much higher pressure than the rest of the plant, raising the whole system to suit it wastes energy everywhere. A booster or small dedicated compressor serves that machine while the main system runs at the lower pressure most users need. Whether it pays off depends on that user’s flow, running hours and pressure, so measure them first.

Related pages

• Pressure management and point of use
• Pressure optimisation and artificial demand
• Compressed-air distribution and piping
• Pressure-drop audit
• Pressure budget builder
• System pressure optimisation