There is an invoice in the plant for electricity, gas, and water. Not for compressed air. It is available at every wall, used as a matter of course, and does not appear as a separate item in any cost centre. In many companies, compressed air is therefore regarded as a practically free medium – and is simultaneously monitored less effectively than any other energy source.

In fact, compressed air is not a raw material, but a converted utility. Electrical energy is turned into pressure, and a large proportion of the energy used leaves the compressor again as heat. In Germany, compressed air generation accounts for around 7% of industrial electricity consumption. For a well-utilised industrial compressor, energy costs make up by far the larger part of the life cycle costs; figures of 65 to 85% are cited in technical literature.

This article shows how compressed air costs per m³ can be calculated, which four key variables determine them, and which three audits a plant can perform using existing operating data.

Why compressed air is underestimated in operation

Three factors come together. Firstly, there is no meter: the electricity consumption of the compressors is hidden in the plant’s total invoice and is rarely assigned to a specific line or process. Secondly, the purchase price is visible, but the operation is not. The system is paid for once, the air every day. Thirdly, an increase in consumption is hardly noticeable as long as the network pressure remains stable: the compressor automatically compensates for additional losses as long as it has reserve capacity.

This shifts perception. A system that runs reliably is considered to be in order – even if it generates significantly more air than production actually requires.

What a cubic metre of compressed air costs to generate

The price per cubic metre is not found in any table. It results from your own system. The decisive variable for the basic calculation is the compressed air performance indicator of generation: the electricity consumption of the compressed air generators including controllers, frequency converters, fans, and other ancillary units, divided by the volume of compressed air generated in the same period. It is specified in kWh/m³. The specific energy requirement of dryers and other active components of compressed air treatment is not included. It must be recorded separately, allocated to the volume of compressed air generated, and additionally taken into account in the full operating cost calculation. Pressure losses through dryers, filters, and piping must also be included in a full system assessment.

For an operating pressure of 7 bar (g), a range of 0.10 to 0.12 kWh/m³ is considered a good to acceptable value. If the determined indicator is significantly higher, it is worth searching for the cause. The compressed air performance indicator of generation describes the generation system and indicates how much electricity the compressed air generators, including the aforementioned ancillary units, require per cubic metre generated.

In contrast, the specific power requirement describes a single machine and relates its total electrical power input to its free air delivery (FAD), as specified in the data sheet according to ISO 1217. Machines can only be reliably compared if the delivery volume, operating pressure, and reference conditions match. The nominal motor power in kW is unsuitable for both considerations.

Electricity costs of compressed air generation per m³ = compressed air performance indicator of generation (kWh/m³) × unit price (€/kWh)

Additional electricity costs of treatment per m³ = energy consumption of treatment (kWh) ÷ volume of compressed air generated (m³) × unit price (€/kWh)

For the full ongoing energy cost calculation, both values must be added together. The following example calculation shows only the first part, i.e. the electricity costs of compressed air generation.

Example calculation: Electricity costs of compressed air generation

Note: Example calculation with disclosed assumptions. The values are not transferable to other systems and do not replace a measurement.

• Delivery volume: 5 m³/min
• Operating hours: 4,000 h per year
• Compressed air performance indicator: 0.110 kWh/m³ (assumption, 7 bar)
• Unit price: 0.22 €/kWh (assumption)

Annual volume: 5 m³/min × 60 × 4,000 h = 1,200,000 m³. Electricity consumption: 1,200,000 m³ × 0.110 kWh/m³ = 132,000 kWh. Electricity costs: 132,000 kWh × 0.22 €/kWh = 29,040 €. This corresponds to around 2.4 cents per cubic metre for compressed air generation, excluding the energy requirement for treatment.

Two variables determine the result: how much air is generated and how much energy each cubic metre costs. How the actually required volume can be determined is shown in the guide for calculating compressed air requirements.

Generation costs are not the full total costs

The calculations above answer a clearly defined question: What does ongoing operation cost per cubic metre generated? They are therefore an operating or energy cost calculation, not a full consideration of the total cost of ownership.

For a full consideration over the life cycle, the following are also included: purchase price of the compressors and treatment, installation, piping, electrical connection and, if applicable, foundation, maintenance, wear and spare parts, depreciation and financing, modifications in the event of changes in demand, as well as residual or disposal value at the end of the service life. Only this sum results in the total costs. How the individual blocks are distributed over the service life is covered in the article on the total costs of an industrial compressor.

The cost lever model: four central key variables

In a simplified model, four central key variables can be distinguished. The first two have a multiplicative effect: anyone who improves both by ten per cent reduces the electricity costs of generation by around a fifth.

1. Volume generated. Every cubic metre that is not needed still costs money. This includes leakages, continuous consumers, consumers that remain on the network outside of shifts, and applications for which compressed air is not the appropriate medium.
2. Compressed air performance indicator. This summarises how much electricity generation requires per cubic metre. This is affected by the pressure setpoint, compression technology, control strategy, utilisation, maintenance status, and all pressure losses between the compressor and the point of application.
3. Unit price and heat recovery (HR). The price per kWh goes directly into every cubic metre. The largest part of the electrical drive energy is converted into heat. Part of this heat can be used via heat recovery (HR). Where a suitable heat consumer is available and heat generation and heat demand coincide in time, HR reduces the total energy costs of the operation because it replaces heat that would otherwise have to be purchased.
4. Fixed ancillary costs. Maintenance, filter and oil changes, wear parts, maintenance of the dryers, and condensate treatment continue regardless of the volume consumed.

The order is chosen deliberately: volume and compressed air performance indicator can usually be influenced more quickly and cheaply in existing systems than the unit price or a new investment.

Where costs arise in everyday operation

Leakages

Leakages are permanent consumers. They draw air even at night, at weekends, and during breaks – exactly when no cubic metre of benefit is created. As a guide: in smaller networks, the leakage rate should not exceed about 5%, in larger industrial networks around 10%, and in very large networks about 13 to 15%. However, studies in industrial plants show that the actually measured proportions are often significantly higher.

Typical locations are quick-release couplings, hoses, screw connections, maintenance units, valves, and cylinder seals. The compressed air station itself is also a possibility, for example via stuck condensate drains or defective relief valves. It is important to separate two questions: the mathematically derived leakage rate indicates how much is lost. Where the leakages are located is only shown by detection, usually with ultrasound during ongoing operation.

Pressure setpoint and pressure losses

The required pressure level starts at the consumer, not at the compressor. The pressure setpoint of the station results from the process pressure at the most sensitive consumer, the permissible tolerance, the pressure losses across treatment and the network, and a justified safety margin. Every bar of higher compression increases the energy requirement by about 6 to 10%; the exact value depends on the system, control, and load profile. Which pressure stage suits which network is covered in the article on the correct operating pressure.

As a guideline, the pressure drop from the compressed air receiver to the consumer should not exceed one bar, with the majority of this usually occurring in the connection accessories – in hoses, couplings, filters, and maintenance units – and only a small part in the pipe network. The most common mistake is increasing the pressure as a quick fix. Before the pressure setpoint is increased, dirty filters, dryer condition, couplings, hose cross-sections, pipe network routing, load peaks, and individual critical consumers should be put to the test.

Idling and load changes

A compressor with load-idle control also consumes energy when it is not delivering air. The power requirement during idling is usually 20 to 30% of load operation. Short load changes, an undersized compressed air receiver, or several machines without a higher-level controller extend these unproductive phases. Variable speed control adapts the delivery volume to the demand and reduces idling and switching cycles. Whether it pays off is decided by the measured load profile and not the brochure: speed-controlled machines work most efficiently in a medium utilisation range. At full load, the losses of the frequency converter are added. Only with permanent magnet synchronous motors can the higher motor efficiency partially compensate for these converter losses. This cannot be transferred to speed-controlled compressors with asynchronous motors.

Compression technology also contributes to this lever. In the STRONG 2S series, two-stage compression with intercooling reduces energy consumption; according to the catalogue, over 10% energy can be saved compared to single-stage compression. Why two machines with the same nominal motor power deliver different amounts of air per kilowatt hour is shown in the article on the two-stage screw compressor.

Treatment and condensate

Dryers, filters, and condensate technology cost twice: via their own energy requirement and via the pressure drop that the station must compensate for. An increasing filter differential pressure due to loaded elements drives the pressure setpoint upwards; as a guide, the pressure drop across a line filter should not exceed 0.1 bar. Adsorption dryers also require purge air, which the compressor must first generate. Which design is necessary when is clarified by the comparison of refrigerant dryers and adsorption dryers.

A frequently overlooked item is condensate drains. A defective or permanently open drain continuously loses compressed air; poorly set time-controlled drains release more air than necessary during each cycle. How the chain of separator, dryer, filter stages, and condensate technology works together is described in the guide to compressed air treatment. Basically: a higher air quality than the process requires increases investment and operating costs without benefit.

The first three audits for your plant

The following audits can be performed with existing operating data. They provide an initial assessment and show whether a reliable measurement is necessary. They do not replace a reliable measurement.

1. Evaluate load and idle hours. Read the operating hours and load hours counters. The difference gives the idle hours. If the idle proportion is below about 10% of the total operating hours, this is unremarkable; above that, it is worth checking the run-on time, receiver size, machine sizes, and higher-level controller. In future, note the meter readings monthly – an inexplicable increase is often the first indication of a new problem.
2. Check consumption during production downtime – under the appropriate conditions. If a compressor regularly runs under load while production is at a standstill, it is working against leakages and continuous consumers. The leakage rate can be roughly estimated from the ratio of load running time to total time during the standstill.

This estimation is only meaningful under certain conditions. It requires that production is actually at a standstill and known continuous consumers are factored out beforehand. It works with machines that count load and idle times separately; for speed-controlled compressors, no air volume can be derived from the running time ratio because the delivery volume varies with the speed. The delivery volume used as a basis comes from manufacturer specifications and can deviate due to wear or internal compressor leaks, which is why a good maintenance status is a prerequisite. In plants with continuous production, this procedure is not applicable; here, measurement alternatives are required. And where compressed air supplies safety-relevant functions, the supply must not be interrupted for the audit.

• Compare the pressure setpoint against actual demand. Determine the required pressure at the most sensitive consumer and compare it with the pressure setpoint at the station. The difference is your surcharge for pressure losses and safety. In doing so, read the differential pressure displays on pre- and after-filters and check the condensate drains for permanently blowing or excessively long-opening valves.

If these audits reveal abnormalities, the next step is worthwhile. For the precise quantification of delivery volume, base load, peak load, and idle proportion, measured values over a representative operating week under real operating utilisation are required; a compressed air audit provides this data basis.

Common mistakes in cost assessment

• Comparing compressors solely by nominal motor power in kW instead of by specific power requirement related to the delivery volume.
• Comparing delivery volumes without common reference conditions.
• Increasing the pressure setpoint before the cause of the pressure drop has been located.
• Determining the leakage rate once and not following up on it afterwards.
• Excluding treatment, condensate technology, and the pipe network from the cost calculation.
• Adopting a flat-rate cent value per cubic metre instead of calculating with your own operating data.

Frequently Asked Questions

Why is compressed air more expensive than the electricity that generates it?

Because during compression, the vast majority of the electrical energy is generated as heat and only a fraction reaches the tool as usable compressed air. In addition, there are losses in the network, in treatment, and through leakages. If you compare the energy requirement of a compressed-air-powered application with a suitable direct electric drive for the same task, the compressed air solution is usually significantly more energy-intensive. However, how large the difference is depends heavily on the application, the pressure level, the efficiency, and the load profile.

For which applications is compressed air not worthwhile?

Wherever a simpler form of energy performs the same task. For blowing off, an industrial vacuum cleaner is usually the better choice; for cooling and conveying, a blower or a conveyor belt; for drives, an electric motor; and for linear movements, potentially a linear drive or hydraulics. Where compressed air remains the technically correct choice, optimised nozzles and demand-dependent control noticeably reduce consumption.

How much does the intake temperature affect costs?

Warm intake air has a lower density, so the compressor must suck in a larger volume for the same mass of air. If the intake temperature rises from 20 °C to 35 °C, the specific power requirement increases by about 5%. Supply air, exhaust air, and heat dissipation are therefore part of the cost consideration; the planning of the compressor room directly affects the price per cubic metre.

How does treatment factor in relatively?

In addition to the dryer, compressed air treatment also includes line filters and compressed air receivers. In a full system assessment, the piping and its installation method are also added. The energy and cost share of drying depends heavily on the drying process, the required pressure dew point, the utilisation, the pressure loss, and the control. Adsorption dryers can require significantly more energy than refrigerant dryers due to the regeneration of the desiccant and the purge air requirement. They are therefore only sensible where the process actually requires a negative pressure dew point. The difference arises primarily from the regeneration of the desiccant and the purge air requirement. An adsorption dryer is therefore only sensible where the process actually requires a negative pressure dew point.

Is heat recovery worthwhile for the cost calculation?

Mathematically almost always, practically only under certain conditions. The largest part of the electrical drive energy is converted into heat, and part of this heat can fundamentally be used. Decisive factors are the temperature level, a heat sink nearby, the temporal overlap of heat generation and heat demand, and a sufficient number of full-load hours – idle hours provide very little heat. The STRONG 2S series is optionally available with an integrated heat exchanger that makes the heat of compression usable as hot water.

Evaluate compressed air costs with your own operating data

A reliable cost assessment begins with your operating data. Helpful information includes the number and delivery volume of the compressors, control strategy and year of manufacture, the set pressure setpoint and the pressure actually required at the point of application, operating and load hours, shift model, the unit price per kWh, the existing treatment, and known problems with pressure drop, leakages, or condensate.

On this basis, SCC air compressors categorises the situation technically and identifies the most effective levers in your network. The scope of services includes auditing the system for faults and leakages, analysing the compressed air demand with a data logger, and simulating the compressor settings; this results in economic efficiency and savings calculations for your specific system. The product overview provides an overview of the suitable series. Describe your initial situation via the technical advice from SCC – the more accurate the operating data provided, the more specific the assessment will be.