The industrial compressed air glossary

The air-end is the mechanical assembly that performs the compression: two helical rotors in a screw compressor, a vaned rotor in a vane compressor, pistons in a reciprocating compressor. It is the most expensive part to replace and the most exposed to manufacturing or operating defects (lubrication, overheating). CEZIUM covers it in full for the whole machine warranty: 3 years, extended to 5 years for as long as the installation stays supervised by SEENTINEL.

An oil-free compressor compresses with no oil in the chamber: dry screw (non-contacting rotors), scroll (interleaved spirals), rotary tooth, or water injection in place of oil. Oil may remain in the drive gearbox, separated from the chamber. ISO 8573-1 defines class 0 as stricter than class 1, at a level set by the user or the supplier; in the catalogue, the rotary tooth machines carry it with a TÜV test. Expected for processes where the air touches the final product: GMP pharmaceutical manufacturing, food and beverage, electronics, high-quality painting.

A permanent magnet motor replaces the rotor winding of conventional asynchronous motors with neodymium-iron-boron (NdFeB) or samarium-cobalt magnets. Advantages: no induced current in the rotor (hence no rotor Joule losses), maximum torque at start-up, compactness. It can reach the IE4 and IE5 classes defined by IEC TS 60034-30-2; on CEZIUM screws it is IE4, IE5 on request. The gain over an IE3 motor is counted in efficiency points: for a 4-pole motor, Regulation (EU) 2019/1781 requires 95.0% for IE3 and 96.0% for IE4 at 75 kW (93.0% and 94.5% at 22 kW). Found on almost every CEZIUM VSD compressor.

PID (Proportional-Integral-Derivative) control is the standard closed-loop control algorithm. On a variable speed compressor, the PID reads the outlet pressure (a 7.5 bar set point, for example) and adjusts motor speed in real time to hold it constant: proportional term (immediate reaction to the deviation), integral term (correction of slow drift), derivative term (anticipation of variations). Well tuned, a PID holds pressure within ±0.1 bar of the set point, which makes it possible to lower the set point to the bare minimum: each 0.1 bar gained represents 0.7% energy saved.

The PSA process exploits the difference in the adsorption of gases on a molecular sieve under pressure (carbon for N₂, zeolite for O₂). Two adsorber columns alternate in a short cycle (15-60 seconds) between an adsorption phase (high pressure, the product gas passes through) and a regeneration phase (low pressure, the sieve desorbs). It produces nitrogen at 95-99.999% purity or oxygen at 93-95%, on site, from the existing compressed air. No more dependence on cylinders or bulk tanks.

A rotary tooth compressor uses two claw-profiled rotors that turn in opposite directions at very high speed, with no contact between them or with the stator. Air is trapped between the rotors and the stator, then discharged. It is an oil-free technology certified Class 0 by TÜV, particularly suited to demanding industrial uses (pharma, food, electronics) at powers of 15-55 kW. Available in fixed-speed and VSD versions.

A scroll compressor compresses air between two spirals: one fixed, the other mobile and orbiting (without rotating on its own axis). The air trapped between the spirals is progressively compressed towards the centre, then discharged. Advantages: very few moving parts, no valves, quiet operation (55 to 70 dB(A) in the catalogue), inherently oil-free. Limits: small flows (0.3 to 5 m³/min in the catalogue) and a pressure of 8 to 10 bar. Ideal for laboratory, cleanroom and medical applications.

In a two-stage compressor, air is compressed a first time (typically from 1 to 3 bar), cooled in an intercooler, then compressed a second time (from 3 to 8-10 bar). This intercooling reduces the compression work in the second stage and cuts energy consumption by around 10% compared with an equivalent standard compressor (6 to 11% according to the US Department of Energy's compressed air Sourcebook). It saves most on sites that run long hours: the more the machine runs, the larger the annual gain.

A VSD (variable speed drive) is an electronic frequency inverter that modulates motor speed in real time so that compressed air output matches consumption exactly. Unlike a fixed-speed compressor, which alternates between full load and off-load, a VSD modulates continuously. With a permanent magnet motor (IE4 class on CEZIUM screws), the machine no longer runs unloaded when demand drops: an unloaded fixed-speed screw still draws 15 to 35% of its power while producing nothing, according to the US Department of Energy Sourcebook.

FAD (free air delivery) is the standardised measure of a compressor's usable flow: the volume of compressed air delivered at the outlet, referred to ambient inlet conditions (temperature, atmospheric pressure, humidity). It is measured to ISO 1217, whose Annex C covers electrically driven packaged compressors. This is the value to compare between manufacturers, not the theoretical flow calculated from the swept volume.

A compressor's flow can be expressed in m³/min (cubic metres per minute, at actual inlet conditions: site altitude, temperature, humidity) or in Nm³/h (normal cubic metres per hour, referred to 1,013 mbar and 0 °C). A compressor data plate generally gives the FAD (free air delivery): the actual flow measured at the outlet, referred to inlet conditions. Always compare flows on the same basis: 6 m³/min FAD ≠ 6 Nm³/h. The approximate conversion: 1 m³/min ≈ 56 Nm³/h for air drawn in at 20 °C.

Predictive maintenance instruments the compressor with sensors (bearing vibration, oil/air temperature, filter and separator differential pressure) whose data feed continuous monitoring. Intervention is triggered when a threshold is exceeded or a drift is detected, not by a fixed calendar. Average benefits reported by the US Department of Energy's O&M best practices guide (2010): 70 to 75% of breakdowns eliminated, 35 to 45% less downtime, 25 to 30% lower maintenance costs; only what needs changing is changed, and components last longer. It is justified first where a stoppage is expensive: a compressor that is critical to production, with no standby machine.

Specific power is the ratio of power input to delivered flow, in kW/(m³/min). It is the reference metric for comparing the energy efficiency of two compressors: a 30 kW machine delivering 5 m³/min consumes 6 kW/(m³/min), a 30 kW machine delivering 5.5 m³/min consumes 5.45, i.e. 9% better. For the same 5 m³/min, at constant specific power, the second one draws only about 27.3 kW instead of 30: 2.7 kW less for every running hour, to multiply by your annual hours and your price per kWh. Often left out of sales literature: always recalculate it yourself.

ISO 8573-1 gives class 0 no figure: it defines it as stricter than class 1 (0.01 mg/m³ of total oil), at the level the user or the supplier specifies. A class 0 is therefore stated with its threshold and its measurement method. In the trade, and at CEZIUM, it means a machine with no oil in the compression chamber: scroll, rotary tooth, dry screw, water-injected screw. A lubricated screw followed by filtration can approach that level at the point of use, but the machine itself is not class 0. It is the usual requirement in pharmaceuticals, food and electronics.

The pressure dew point (PDP) is the temperature at which, at the pressure considered, the water vapour in the air starts to condense into liquid droplets. It is the indicator of residual moisture after drying. A standard refrigerant dryer reaches +3 °C PDP (ISO 8573-1 Class 4), an adsorption dryer reaches −40 °C PDP (Class 2) or −70 °C PDP (Class 1). The lower the PDP, the drier the air and the more costly the drying energy: choosing the right class is a trade-off between energy and process.

ATEX stands for the French "ATmosphères EXplosibles" (explosive atmospheres). There are two European directives: 2014/34/EU for equipment manufacturers (Ex marking, certification) and 1999/92/EC for employers (risk assessment, zoning of the site). Zones are classified as 0 (explosive atmosphere present permanently), 1 (occasionally), 2 (rarely). A compressor installed in an ATEX zone must be ATEX-compatible (equipment, fittings, electrics). It concerns chemicals, petrochemicals, painting, food processing (grain dust), etc.

GMP (Good Manufacturing Practices) are the manufacturing practices enforceable in the pharmaceutical industry. In Europe, the reference framework is EudraLex Volume 4, whose Annex 1 (2022 revision) deals specifically with the manufacture of sterile medicinal products. For compressed air in contact with the product, GMP asks for a defined quality, a qualified installation (IQ/OQ/PQ) and monitoring over time; the ISO 8573-1 class is set by the operator in its file, often with oil in class 1 or 0.

HACCP is a systematic method for identifying, evaluating and controlling hazards (microbiological, chemical, physical) that are significant for food safety. It is mandatory for all food business operators under European Regulation 852/2004. For compressed air in contact with foodstuffs (bottle blowing, pneumatic conveying of ingredients, packaging), the method sets no ISO 8573-1 class: the operator sets it in its risk analysis and documents it, most often aiming for oil in class 1 or better for air that touches foodstuffs.

ISO 1217 "Displacement compressors - Acceptance tests" defines the protocol for measuring the performance of positive displacement compressors (screw, piston, vane). Annex C (the most widely used commercially) specifies how to measure air flow, pressure and power input under standardised conditions. Always ask for ISO 1217 performance data: it is the only reliable basis for comparison between brands.

ISO 8573-1 "Compressed air - Contaminants and purity classes" is the worldwide reference for specifying and verifying compressed air quality. It defines 3 class columns: solid particles (size and concentration), water (dew point), oil (total concentration in mg/m³). The class is written [particles:water:oil], for example [1:4:1]; the operator sets the class for each use, according to what the air touches.

The CEE scheme (Certificats d'Économies d'Énergie, France's energy savings certificates) obliges energy sellers to generate energy savings among consumers, whether or not they are their customers. In practice, for an air compressor, variable speed falls under standardised sheet IND-UT-102 "Système de variation électronique de vitesse sur un moteur asynchrone" (11,900 kWh cumac per kW, 15 years) or, for a permanent magnet motor, under sheet IND-UT-114 "Moto-variateur synchrone à aimants permanents ou à réluctance" (9,200 kWh cumac per kW, 20 years). The incentive covers part of the extra cost of VSD over fixed speed, depending on the negotiated price per MWh cumac. The calculation is made in kWh cumac (kWh cumulated and discounted over the conventional lifetime). The CEE funder's commitment must be made no later than the acceptance of the quotation, otherwise the operation becomes ineligible.

Almost all of the electricity drawn by a compressor ends up as heat. According to the US Department of Energy, the available heat amounts to 80 to 93% of the electricity drawn, and a properly designed recovery unit recovers 50 to 90% of it, i.e. 40 to 84% of the electricity, at a usable temperature: oil at 70-90 °C (the main source), discharge air at 30-50 °C, ambient air at +10-20 °C. Three technologies: air-to-air (returned to the workshop), air-to-water (heat exchanger on the oil for domestic hot water or space heating), water-to-water (industrial water circuit). It makes sense on a site that needs heat for a good part of the year. Covered by CEE (France's energy savings certificates) sheet IND-UT-103 "Système de récupération de chaleur sur un compresseur d'air" (heat recovery system on an air compressor).

The IE scale has five classes: IE1 (standard), IE2, IE3 (Premium), IE4 (Super Premium) and IE5 (Ultra Premium). IEC TS 60034-30-2 (2016) defines them for variable speed motors, including permanent magnet synchronous motors fed by a variable speed drive; IEC 60034-30-1 covers motors connected directly to the grid. In Europe, Regulation (EU) 2019/1781 has required at least IE3 from 0.75 to 1,000 kW since 1 July 2021, and IE4 from 75 to 200 kW (2, 4 or 6 poles) since 1 July 2023. Permanent magnet motors can reach IE5; on CEZIUM screws they are IE4, IE5 on request.

The kWh cumac (cumulated and discounted) is the unit of the CEE market. It represents the total energy saved by an energy efficiency operation over the conventional service life of the equipment (15 years under sheet IND-UT-102, 20 years under IND-UT-114), discounted at 4% per year from the second year. It is valued at the price negotiated with the intermediary. For a 75 kW compressor, the sheets give 892,500 kWh cumac (IND-UT-102) or 690,000 kWh cumac (IND-UT-114).

Load factor (average load) is the ratio between a site's actual average air consumption and the maximum capacity of the installed compressor. It is measured over a week with a data logger. A continuous steel plant runs close to full load, a mechanical workshop well below. The drive gains most when the average load is well below capacity and varies; at a constant full load it brings little, because the motor already runs close to its rated speed.

Pressure drop (or ΔP) is the fall in pressure between the compressor outlet and the point of use, caused by friction in the pipework, bends, valves, filters and the dryer. A well-designed network loses much less than 10% of the discharge pressure (US Department of Energy). Beyond that, the operator compensates by raising the compressor set point, and each extra bar costs about 7% more energy. Reducing pressure drop (replacing clogged filters, re-sizing the network) is often the first energy efficiency measure to take.