Load/unload cycle energy calculator

What does the load/unload calculator work out?

Most fixed-speed rotary screw compressors control output by switching between two states. When loaded, the inlet valve is open and the airend delivers its full free air delivery (FAD). When unloaded, the inlet closes, the separator tank blows down to a lower pressure and the motor keeps turning, ready to reload. The machine still draws a meaningful share of its loaded power while unloaded, yet it produces no useful air.

The calculator turns measured cycle timing into five practical results:

• Loaded and unloaded fractions – the share of each cycle spent in each state.
• Average package kW – the time-weighted power over a full cycle.
• Annual kWh – average kW multiplied by the hours that cycle pattern actually occurs.
• Unload energy – the kWh consumed while unloaded, the part that buys no air.
• Air-normalised specific power – average kW divided by the air actually delivered, in kW per m³/min.

It never assumes unloaded power is zero. You enter measured or verified loaded and unloaded package kW, and every result stays traceable to those inputs.

The core cycle equations

The method is a simple time-weighted average. For a two-state cycle:

1. Cycle time = t_load + t_unload
2. Loaded fraction = t_load ÷ cycle time
3. Unloaded fraction = t_unload ÷ cycle time
4. Average kW = (P_load × t_load + P_unload × t_unload) ÷ cycle time

Where more states are measured – blowdown, idle, stopped after a run-on timer, or a variable-speed trim machine running at part speed – the same idea extends to any number of mutually exclusive states: average kW = Σ(P_i × t_i) ÷ Σt_i. The only rule is that the states must not overlap and their times must add up to the observation period.

Annual energy follows directly: annual kWh = average kW × annual operating hours. Those hours must be the hours in which this cycle pattern really happens. A plant with a busy day shift and a quiet night shift has two patterns, so calculate each separately and add them, rather than averaging the cycle times first.

Why unloaded running matters

Unload energy is the most useful diagnostic the calculator produces. It is simply P_unload × unloaded fraction × hours. On a lightly loaded fixed-speed machine this figure can be a large slice of the annual total, and it is energy that delivers nothing to production.

Three things drive it:

• Unloaded power itself. It depends on the inlet valve design, how far the separator tank blows down, oil system losses and motor losses. It varies between makes and with condition, which is why it should be measured rather than taken from a brochure.
• Blowdown time. After unloading, the sump pressure falls over a period of seconds. Power drops gradually, not instantly, so short unload periods often never reach the minimum unloaded power.
• Cycle frequency. Rapid cycling wastes energy in each blowdown and loads the inlet valve, motor starter and minimum-pressure valve harder. Small effective storage and a narrow pressure band are the usual causes.

When the machine is unloaded for long periods, a run-on timer can stop the motor. The calculator treats “stopped” as its own state with near-zero power, so you can see what an auto-stop setting would change, within the motor’s permitted starts per hour.

How to measure the inputs properly

The arithmetic is easy; the inputs decide whether the answer is believable. A good measurement routine looks like this:

1. Define the boundary. Measure package input – main motor plus cooling fan and any controls wired inside the package – at the incoming supply. Record whether the dryer is included. See our motor input and electrical measurement worksheet for boundary choices.
2. Use a true power meter. Log kW, not just amps. Unloaded current can look high while real power is much lower, because power factor falls sharply when the motor is lightly loaded. Amps × volts without the correct power factor overstates unloaded kW.
3. Log for long enough. Capture complete cycles across each shift pattern. A few minutes at a quiet moment will not represent a full working day.
4. Take load and unload times from the controller or a pressure log. Many controllers record loaded hours and running hours, which give a long-term loaded fraction to cross-check the logged cycles.
5. Note discharge pressure and ambient. Loaded kW rises with discharge pressure – roughly 7% more power per extra bar on a typical screw compressor – and with poor cooling in hot summer months.
6. Record the condition. Note if the separator element, air filter or inlet valve is due for service; a sticking inlet valve or a worn minimum-pressure valve changes both loaded and unloaded behaviour.

Worked example

A fixed-speed oil-injected screw compressor runs on load/unload control at a plant working 6,000 hours a year. The figures below are illustrative inputs, not data for any specific make or model.

About a third of the annual energy goes into unloaded running. Now normalise it to air. If the machine delivers 12.5 m³/min when loaded, its loaded specific power is 82 ÷ 12.5 = 6.56 kW per m³/min. Over the whole cycle it delivers on average 0.4 × 12.5 = 5.0 m³/min for 49.6 kW, so the effective specific power is 9.92 kW per m³/min. That gap is the price of running a large fixed-speed machine at low load.

Scenario comparison then asks: what if a properly sized receiver lengthened the cycle, or a smaller machine or variable-speed trim compressor took the swing load? Enter the predicted cycle for each case and compare kWh. Express the benefit in kWh and kW per m³/min; convert to money only with your own tariff.

How to interpret the results

The numbers point to different actions depending on the pattern you see.

Do not chase a single number. A good result is a stable cycle at the lowest pressure that still serves the critical users, with machines running near full load and spare capacity held in reserve rather than idling.

Limits of the method

• It assumes stable states. Modulating inlet control, dual control and variable-speed drives do not follow a clean two-state pattern. Use logged kW directly, or more states, for those machines.
• Delivered air is inferred. The air-normalised result assumes loaded FAD at site conditions. Inlet temperature, altitude and wear all change it; a flow meter gives a better basis. Our FAD and reference condition conversion tool explains the corrections.
• Measurement uncertainty carries through. A small error in unloaded kW multiplies across thousands of hours. The measurement uncertainty worksheet shows how to put a confidence band on the answer.
• Operating hours must match the pattern. Weekend, holiday and seasonal demand should be handled as separate patterns.
• It is not a control recommendation on its own. Starts per hour, motor heating, process pressure needs and standby cover must all be checked before changing settings.

How Reddy Compressors helps

Since 2002 we have sold, rented and serviced compressors of many makes – ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Sullair, Doosan, Kirloskar and others – as an independent multi-brand specialist, not an authorised dealer. We have tie-ups with all major and reputed brands. Our team of over 20 service engineers and technicians works from our office at Tarnaka, Secunderabad and covers the whole of Telangana and Andhra Pradesh.

For load/unload energy questions we can:

• Log package power, pressure and load/unload state over representative shifts as part of an air audit.
• Check inlet valves, blowdown, minimum-pressure valves, separators and coolers that push unloaded or loaded power up.
• Review receiver sizing, pressure bands and sequencing, then model the alternatives with the same method shown above.
• Supply new or used/refurbished compressors, including variable-speed trim machines, where the data shows a better fit, and rent machines for trials.

We report results in kWh and kW per m³/min so you can apply your own tariff and approve changes on evidence.

What to send us

• Compressor make, model, serial number and a nameplate photo.
• Controller readings: running hours, loaded hours, set pressures.
• Any power or pressure logs you already have, with the measurement point.
• Number of compressors, receivers and dryers, and how they are sequenced.
• Shift pattern and annual operating hours.
• Your contact details: call +91 99590 60718 or +91 92468 88812, or email reddycompressors@gmail.com.

Frequently asked questions

How much power does a screw compressor use when unloaded?

It varies by make, inlet valve design, blowdown pressure and condition, so it should be measured rather than assumed. Many fixed-speed screw compressors still draw a substantial share of their loaded power when unloaded, because the motor, airend and oil system keep running. Use a true kW logger at the package supply and read the value after blowdown has settled, not immediately after unloading.

How do I calculate the average kW of a load/unload compressor?

Multiply loaded kW by the load time and unloaded kW by the unload time, add the two, then divide by the total cycle time. For example, 82 kW for 40 seconds and 28 kW for 60 seconds gives 49.6 kW on average. Multiply the average by the hours that pattern runs each year to get annual kWh.

Why does my compressor load and unload so often?

Rapid cycling usually means there is too little effective storage between the compressor and the demand, or the pressure band between load and unload set points is very narrow. Restrictive piping or a dryer between compressor and receiver can also hide storage from the controller. Frequent cycling wastes blowdown energy and wears the inlet valve and starter, so it is worth correcting.

Is a variable-speed compressor always better than load/unload?

No. A variable-speed drive saves energy where demand varies within its speed range, but a fixed-speed machine running fully loaded is efficient, and a VSD machine running at full speed all day gains little. The load/unload calculation tells you how much unloaded running you have, which is the starting point for deciding whether a VSD trim machine would pay back.

Can I use motor amps instead of kW for this calculation?

Only as a rough screen. Unloaded motors run at low power factor, so amps stay relatively high while real power falls. Multiplying volts and amps without the correct power factor overstates unloaded kW and exaggerates the unload penalty. A true power meter or logger that records kW directly gives a far more reliable result.

What is a good loaded fraction for a compressor?

There is no single target. A base-load machine should ideally run close to fully loaded, while one trim machine handles the swing in demand. If several fixed-speed machines all show low loaded fractions, the system is probably oversized for current demand or poorly sequenced, and the energy is going into unloaded running rather than useful air.

Related pages

• Unload and idle loss reduction
• Compressed air storage for energy optimisation
• Base, trim and VSD energy optimisation
• Part load fleet dispatch calculator
• Specific-power assessment
• Compressed air knowledge and calculation centre