Pressure optimisation and artificial demand in compressed-air systems

What is artificial demand and why does it matter?

Artificial demand is the portion of air consumption that exists only because the system pressure is higher than the process needs. Any use that is not held at a fixed pressure by a regulator consumes more air as supply pressure rises. Leaks, open blow-off nozzles, air-driven cooling, drain valves stuck open, unregulated cylinders and many vortex or venturi devices all behave like fixed orifices: the higher the upstream absolute pressure, the more free air passes through them.

That gives over-pressure a double cost. The compressor works harder to reach the higher discharge pressure, and the plant then consumes more air at that pressure. As a working rule, each extra bar on an oil-injected screw compressor adds about 7% to power at the same delivery. On a plant where leaks and unregulated uses are a large share of demand, the flow penalty can be as important as the pressure penalty.

Artificial demand is easy to miss because nothing looks broken. The plant runs, the machines load and unload, and the gauges read high. The waste only shows when pressure, flow and power are logged together and compared with what the process actually requires.

How much pressure does the process really need?

The starting point is not the compressor setpoint. It is the minimum dynamic pressure each critical user needs at its own inlet during its peak demand event. A pneumatic press, a packaging machine or an instrument-air panel may have a stated minimum, but that figure means something only when it is measured while the machine is drawing air, not at idle.

Many plants carry a historical setpoint inherited from an older compressor, a since-removed restriction or a single complaint years ago. A historical setpoint is evidence of past operation, not proof of present need. To separate the two:

• List the critical users and their documented minimum pressures from the machine builders or process owners.
• Measure dynamic pressure at the inlet of each critical user during its real operating cycle.
• Identify the user that sets the plant minimum, and check whether it is a genuine high-pressure need or a symptom of a local restriction.
• Where one user truly needs much higher pressure than the rest, consider a local booster or a dedicated supply instead of raising the whole plant.

Our page on pressure management at the point of use covers how to set and protect those user minimums in detail.

Building a pressure-loss ledger

Once the critical minimum is known, every pressure drop between the compressor discharge and that user must be accounted for. We call this a pressure-loss ledger: a simple table that assigns the loss to each stage under the relevant flow case.

An illustrative example: at the critical event, treatment loses 0.2 bar, main and branch piping 0.2 bar and the final local assembly 0.3 bar, giving 0.7 bar downstream of the compressor. If the process needs 5.8 bar(g) at the user, the compressor-room boundary needs roughly 6.5 bar(g) before any justified control margin. If the plant is actually running at 7.5 bar(g), about 1 bar is available to recover, but only after the ledger proves it.

The ledger often shows that the local connection, not the header, is the largest loss. A generous main header can still feed a machine through a small hose, a restrictive coupling and an undersized regulator. Fixing that assembly is usually cheaper than buying a larger compressor.

Static pressure is not the optimisation boundary

A gauge reading at idle tells you very little. A machine can show a comfortable static pressure and still starve during its peak draw because the losses rise sharply with flow. Pressure drop in piping rises roughly with the square of flow, so a network that works on the night shift can be restrictive at production peak.

For this reason pressure and flow must be logged on the same time base. A pressure dip without the matching flow or process state can lead to the wrong conclusion. It may be a demand spike, a compressor that has not yet loaded, a dryer switching, or a downstream restriction. Our pressure profiling service records multiple points at once so these events can be separated.

A common pattern is that operators raise compressor pressure to cure one machine’s low-pressure complaint. That restores the one machine but raises pressure throughout the plant, increasing power and artificial demand everywhere else. The right fix is to find the restriction feeding that machine.

Control margin and pressure band

Some margin above the calculated minimum is legitimate, but it should be justified rather than added “for safety”. Reasonable reasons for margin include sensor accuracy, compressor control response time, receiver and system storage dynamics, and documented process variation.

The control band also matters. A fixed-speed compressor running load/unload between, say, 6.5 and 7.5 bar(g) spends much of its time above the lower limit, so the average pressure the plant sees is higher than the minimum. Narrowing the band, adding storage, using a variable-speed trim machine or a central sequencer can bring the average pressure down without lowering the minimum.

Our compressor sequencer page explains how multi-machine pressure bands are coordinated.

How to run a controlled pressure-reduction trial

Pressure reduction should be a controlled trial with the production team, not a quiet change to a setpoint. A typical method is:

1. Log baseline pressure at the compressor room and at critical users, total flow and compressor power for at least one representative production period.
2. Correct the obvious restrictions found in the ledger: change loaded filter elements, open partly closed valves, replace restrictive hoses and couplings.
3. Agree the critical users to watch, who will report problems and how to restore the old setting quickly.
4. Reduce the setpoint in small steps, commonly 0.1–0.2 bar at a time, holding each step through full production cycles.
5. At each step, check dynamic pressure at the critical users, product quality and any machine alarms.
6. Stop at the step where a critical user approaches its minimum, then return to the last stable step plus the justified margin.
7. Retune the compressor bands or sequencer to the new target and confirm the machines no longer overlap or short-cycle.
8. Log the same period again and compare power, flow and pressure with the baseline.

The comparison usually shows two effects: lower specific power at the compressor and lower total flow because leaks and unregulated users now take less air. Both should be reported separately, along with any changes in production rate.

Finding and reducing artificial demand

Lowering pressure reduces artificial demand automatically, but the best results come when the pressure work is combined with direct action on the uses that react to pressure.

• Leaks: tag, repair and re-survey. Our leak detection page explains ultrasonic surveys and how to rank leaks by size.
• Open blowing: replace open pipe ends with engineered nozzles and fit regulators so blowing runs at the pressure it needs.
• Unregulated cylinders and actuators: fit regulators on the return stroke or on lightly loaded actions.
• Idle production lines: isolate them with a valve or solenoid so they do not leak when not in use.
• Inappropriate uses: review air-driven cooling, mixing, vacuum generation and cabinet cooling for more suitable alternatives.

A useful test is a shutdown or weekend flow measurement with production stopped. Whatever the compressors deliver with nothing running is mostly leakage, and it is the first number to bring down.

Indian plant conditions that affect pressure optimisation

In Telangana and Andhra Pradesh, summer compressor-room temperatures can be high, and dust loads on intake and cooler surfaces are significant. Hot intake air and fouled coolers can make a compressor deliver less free air, which operators often try to compensate for by raising pressure. Dryer performance also suffers in heat, and a struggling dryer adds differential pressure. Check room ventilation and cooler cleanliness before blaming the setpoint.

Voltage variation and frequent power interruptions in some industrial areas can make operators nervous about low pressure, so they keep a large margin. A better answer is adequate storage near critical users and, where justified, a standby machine, rather than running the whole plant high all the time.

How Reddy Compressors helps

From our office at Tarnaka, Secunderabad, our team works on compressed-air systems of all major brands, including ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and others, as an independent multi-brand specialist rather than an authorised dealer. Our service engineers and technicians cover the whole of Telangana and Andhra Pradesh.

• Pressure and flow logging at the compressor room and critical users, with a written pressure-loss ledger.
• Leak surveys and repair support, combined with pressure trials so the result is measured, not assumed.
• Retuning of compressor pressure bands and sequencing on existing machines.
• Filter, dryer and piping recommendations where treatment or distribution losses are high.
• Supply of new or used compressors, variable-speed trim machines and receivers where the trial shows a real capacity or control gap.
• Service and AMC support so the improved settings and clean filters are maintained.

For a full baseline of your system, see our compressed-air audit and measurement services.

What to send us

• Make, model and rated pressure of each compressor, and whether fixed or variable speed.
• Current load/unload or setpoint values and any sequencer settings.
• Dryer and filter types with their location in the system.
• A simple layout or sketch of the main header and the critical users.
• The users that complain of low pressure, and when they do so.
• Any recent power bills or running-hour readings, if available.

Frequently asked questions

How much energy do I save by reducing compressor pressure by 1 bar?

As a typical rule for oil-injected screw compressors, each bar of discharge pressure adds about 7% to power at the same delivery. Reducing pressure also lowers leakage and unregulated consumption, so total savings can be larger than the pressure effect alone. The actual result depends on your compressor control, how much demand reacts to pressure, and whether the machine is loaded or unloading, so it should be measured before and after.

What is artificial demand in compressed air?

Artificial demand is extra air consumed only because system pressure is higher than needed. Leaks, open blowing, stuck drains and unregulated users act like fixed orifices, so they pass more air as pressure rises. The compressor must supply that extra flow at the higher pressure. Bringing pressure down to the true process minimum, and regulating pressure-sensitive users, removes this waste without affecting production.

Why does one machine get low pressure when the compressor gauge reads high?

Usually there is a restriction between the compressor and that machine: a loaded filter, an undersized drop, a partly closed valve, a small hose or a restrictive coupling. These losses rise sharply with flow, so the machine starves during its peak draw even though static pressure looks fine. Measure pressure progressively towards the machine while it is running to locate the loss, then correct the restriction rather than raising the plant setpoint.

Is it safe to lower my compressed-air pressure?

It is safe when done as a controlled trial. Measure the dynamic pressure each critical user needs, correct restrictions first, then reduce the setpoint in small steps through full production cycles while the production team watches quality and alarms. Keep a quick way to restore the old setting. Stop at the step where a critical user nears its minimum and return to the last stable value plus a justified margin.

What pressure should a factory compressed-air system run at?

There is no single right figure. Many general manufacturing plants run between about 6 and 7 bar(g), roughly 87–101.5 psig, but the right value is the critical users’ minimum dynamic pressure plus the measured losses through treatment, piping and local connections, plus a small justified margin. If one user needs far more than the rest, a local booster or dedicated supply is usually better than raising the whole plant.

Does a variable-speed compressor help reduce pressure?

A variable-speed trim compressor holds pressure close to a setpoint instead of swinging between wide load and unload limits, so the average plant pressure can come down. It works best when it is sized to stay within its useful speed range and is coordinated with the fixed-speed machines. It does not remove restrictions or leaks, so those still need attention for the full benefit.

Related pages

• Working pressure and system pressure budget
• System pressure optimisation
• Leak detection and air-loss management
• Pressure-drop audit
• Compressed-air distribution and piping
• Compressed-air energy and efficiency