Unload and idle loss reduction for air compressors

Why does an unloaded compressor still use so much power?

When a fixed-speed screw compressor reaches its upper pressure setting, the inlet valve closes and the blowdown valve vents the separator tank. The airend stops delivering air, but the main motor keeps turning the rotors, the cooling fan keeps running, oil keeps circulating and the controller stays live. Until the separator pressure has fallen, the machine is still working against back-pressure, so the drop in power is gradual rather than instant.

The result is a machine that can show a low load factor on its display while the energy meter tells a different story. A compressor that is loaded for 40% of its running hours can easily spend a large share of its total kWh in the unloaded state. That is why unload and idle loss is often the quickest saving found in a compressed-air audit, and why it is usually invisible until someone logs power and pressure together.

The aim is not zero unloaded time. Some unloaded running is needed for pressure stability, oil and motor cooling, restart limits and standby response. The aim is zero unjustified unloaded energy.

The operating states you need to separate

Before calculating any saving, define the states the machine actually spends time in. Most modern controllers record loaded and running hours; the difference between them is a first indication of unloaded time, but it does not show when the unloaded running happened or why.

These figures are typical engineering ranges, not values for any particular model. Your machine’s data sheet and a power logger are the only reliable basis for a saving estimate.

Common causes of excessive unloaded running

Long unloaded periods are symptoms. Changing the run-on timer without understanding the cause usually moves the problem somewhere else. The usual causes are:

• Too many compressors left online, so two or more machines sit unloaded at the same time while one could carry the load.
• Overlapping or poorly staggered pressure bands, so machines fight each other instead of acting as base and trim.
• A fixed-speed machine used as the trim compressor when a VSD machine is available, or a VSD machine set to a higher pressure than the fixed-speed base.
• Too little effective storage, so the pressure rises quickly when demand falls and the compressor unloads and reloads frequently.
• Night, weekend and holiday running to supply leaks and open drains rather than production.
• Long run-on timers set years ago to protect a motor that has since been replaced, or set by default at commissioning.
• Faulty pressure transducers, sticking inlet valves or failed blowdown solenoids that make the machine behave as though demand is low.

How to measure unload and idle loss

A credible estimate needs power and pressure logged at the same time, at a resolution that matches the load/unload cycle. One-minute averages hide cycles that last 20–40 seconds; a logging interval of a few seconds is better for cycling machines.

1. Log three-phase true power on each compressor, with a current-only logger used only where voltage and power factor are known and stable.
2. Log discharge or header pressure at a few seconds’ resolution, plus pressure at the critical consumer if possible.
3. Record the controller’s loaded and running hours at the start and end of the logging period.
4. Cover at least one full production week, including a night and a weekend or holiday.
5. Classify each interval as loaded, blowdown, unloaded, modulating or stopped from the power and pressure trace.
6. Calculate kWh in each state per machine and for the whole station.

Our power and flow logging method uses this approach. Where several compressors share one header, logging them together is essential, because simultaneous unloading is a station problem rather than a machine problem.

Worked example: how a low load factor hides high energy

Consider a 75 kW fixed-speed screw compressor that runs 6,000 hours a year. The controller shows 3,600 loaded hours and 2,400 unloaded hours. Assume a measured loaded power of 80 kW at the working pressure and an unloaded power of 22 kW after blowdown, which sits in the typical range above.

• Loaded energy: 3,600 h × 80 kW = 288,000 kWh.
• Unloaded energy: 2,400 h × 22 kW = 52,800 kWh.
• Unloaded share of total energy: 52,800 ÷ 340,800 ≈ 15%.

The machine is loaded 60% of the time, yet about 15% of its electricity produces no air at all. If better sequencing and auto-stop cut unloaded running by half, the station saves about 26,000 kWh a year before counting any pressure reduction. The real figure depends on measured unloaded power and how much of the unloaded time can safely be removed, so treat this as a method, not a promise.

Is auto-stop the right fix?

Auto-stop switches the main motor off after a set period of unloaded running and restarts it when pressure falls to the lower setting. It can remove most of the unloaded energy on a machine with long, low-demand periods. It is not a universal setting, because every start puts thermal stress on the motor and starter and wear on contactors and couplings.

Motor manufacturers limit starts per hour, often more strictly for larger motors and for direct-on-line or star-delta starters than for soft starters or VSDs. Before shortening the run-on time, check:

• The motor and starter manufacturer’s maximum starts per hour and minimum time between starts.
• Whether the restart pressure gives enough time for the machine to start and load before the critical consumer drops below its minimum.
• Whether oil temperature reaches a stable value during typical running periods, to avoid condensation in the oil on short cycles in humid conditions.
• Whether the panel, dryer and drains behave correctly when the compressor stops.

If auto-stop causes more starts than the motor allows, the answer is usually more effective storage, a wider pressure band or a smaller trim machine, not a shorter timer.

Storage, pressure bands and cycling

Effective storage slows the rate of pressure change, so a load/unload compressor spends longer in each state and switches less often. Useful storage depends on the volume between the compressor and the consumer and on the pressure band the controls actually use, not on the nameplate volume of the receiver alone. A receiver installed behind a large pressure drop, or isolated by a valve, contributes little.

Widening the pressure band reduces cycling but raises the average pressure, which costs about 7% more power for each extra bar at typical working pressures. The better approach is to fix the pressure losses that forced the high setting, then set the narrowest band the storage allows. See compressed air storage for energy optimisation for the sizing method and system pressure optimisation for setting the target pressure.

Multi-compressor stations and simultaneous unloading

When two or more fixed-speed machines are unloaded at the same moment, the station is carrying spare capacity that is costing energy. This is common when each compressor has its own local pressure settings and no sequencer, or when an old sequencer has been bypassed.

A correctly configured station has one trim machine handling demand changes, base machines fully loaded or stopped, and standby machines off. A VSD machine should normally be the trim machine, operating in a band below the base machines so that it never runs unloaded while a fixed-speed machine is also unloaded. A VSD machine that is too large for the minimum demand can still run unloaded or start and stop frequently, so its size matters as much as its control. Our page on base, trim and VSD energy optimisation covers dispatch in more detail.

Non-production hours need their own mode

Many plants in Telangana and Andhra Pradesh run one or two shifts but keep compressors on through the night to supply a few small users, a packing line or simply leaks. In that period the compressor may load briefly every few minutes only to replace leakage. That is pure loss.

1. List what genuinely needs air outside production: instruments, fire-system or valve actuators, a single process line.
2. Isolate idle production zones with valves so leaks in those zones are not fed overnight.
3. Supply the remaining night load from the smallest suitable compressor, or a dedicated small machine.
4. Switch off dryers and treatment that serve isolated zones, following the manufacturer’s procedure.
5. Use a timer or master controller schedule, with a manual override that resets automatically.

Leakage is often the real load at night, so a leak detection survey is usually the companion project.

Common bad fixes

• Shortening run-on timers below the motor’s start limits, which shifts cost from electricity to motor and contactor failures.
• Switching off all compressors at night without checking instrument air, valve actuators or safety duties.
• Raising the pressure band to stop cycling, which trades unload loss for permanent pressure loss.
• Adding a receiver in the wrong place, behind a restriction, where it cannot deliver air quickly.
• Ignoring maintenance faults: a leaking blowdown valve or sticking inlet valve can cause false unloaded running that no setting will fix.
• Making changes once, then letting manual overrides return the station to its old state within a few months.

How Reddy Compressors helps

Reddy Compressors has worked with industrial compressed air since 2002 from our office in Tarnaka, Secunderabad. Our team of more than 20 service engineers and technicians works on ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other brands as an independent multi-brand specialist, not an authorised dealer. We have tie-ups with all major and reputed brands. For unload and idle loss we:

• Log power and pressure on each compressor and on the header over a representative week.
• Read controller hours, settings, start counts and alarm history.
• Check inlet valves, blowdown valves, minimum pressure valves and transducers for faults that cause false unloading.
• Recommend sequencing, pressure-band, auto-stop and storage changes within the motor and machine limits.
• Carry out the agreed changes and verify the result with a second logging period.
• Supply new, used or rental compressors where a smaller trim machine or a VSD unit is the right answer.

Our service teams cover the whole of Telangana and Andhra Pradesh, and we supply equipment, rentals and spares across India. Ongoing settings and maintenance can be kept under an annual maintenance contract.

What to send us

• Make, model, motor rating and serial number of each compressor, ideally with a nameplate photo.
• Controller screenshots showing running hours, loaded hours and pressure settings.
• Shift pattern, including nights, weekends and holidays.
• A simple sketch of the compressor room, receivers, dryers and main header.
• Any existing power bills, logging data or audit reports.

Frequently asked questions

How much power does a screw compressor use when unloaded?

A typical oil-injected screw compressor draws roughly a quarter to a third of its full-load power once the separator has blown down, and more during the blowdown period itself. The exact figure depends on the design, the inlet valve, the fan arrangement and the condition of the machine. The only reliable number for your plant comes from logging true power on the compressor while it runs in the unloaded state.

Is it bad to run an air compressor unloaded for long periods?

Running unloaded is not harmful to the machine in itself, and it can protect the motor from too many starts. The problem is cost: an unloaded compressor uses electricity without producing air. If a machine regularly runs unloaded for long periods, it is usually a sign that it is not needed online, that the sequencing is wrong or that auto-stop could safely be used.

What run-on time should I set before auto-stop?

There is no universal value. The run-on time must keep the motor within the manufacturer’s permitted starts per hour, which depend on motor size and starting method. Start with the motor and starter limits, check how often the pressure falls to the restart point, and then set the shortest run-on time that respects those limits. Storage and pressure bands should be reviewed at the same time.

Will a VSD compressor eliminate unload losses?

A correctly sized VSD compressor removes most unloaded running because it matches speed to demand. However, every drive has a minimum speed. When demand falls below that, the machine will stop, unload or cycle. A VSD that is oversized for the plant’s low-demand periods, or set at a higher pressure than the fixed-speed machines, can still waste energy. Sizing and settings matter as much as the drive.

How do I know if two compressors are unloading at the same time?

Log the power of each compressor on the same clock, at a few seconds’ resolution, for at least a week. If the traces show two or more machines at their unloaded power simultaneously, the station is carrying unnecessary spare capacity. A sequencer or master controller, staggered pressure bands, or simply switching one machine to standby will usually correct it.

Why does my compressor load and unload every few seconds?

Rapid cycling usually means too little effective storage, a very narrow pressure band, a pressure transducer in the wrong place or a fault such as a sticking inlet valve. It wastes energy in repeated blowdown and wears the inlet valve and solenoids. Check receiver volume and connection, the pressure settings and the control components before changing the timer or raising the pressure.

Related pages

• Compressed-air energy optimisation hub
• Base, trim and VSD energy optimisation
• Compressed air storage for energy optimisation
• Compressor sequencers
• Power and flow logging
• Leak detection