What this register is for
Our engineering pages refer to standards, manufacturer manuals and established practice. This register lists the main references we rely on, explains in plain English what each one covers, and sets out how we use them. It helps plant engineers, buyers and consultants understand the basis of a statement on our site and judge whether it applies to their equipment.
Three points apply throughout. We summarise standards in our own words and do not reproduce their text; buy the current edition from the publisher if you need clause-level detail. A standard named on a page does not mean any equipment, site or report is certified to it. And the manufacturer’s manual for your exact model and serial number takes priority over general guidance.
Core compressed-air standards
| Reference | What it covers | How we use it |
|---|---|---|
| ISO 8573-1 | Purity classes for particles, water and total oil in compressed air | To specify air quality at the point of use and select dryers and filters |
| ISO 8573-2 to -9 | Test methods for each contaminant, such as oil aerosol, humidity and particles | To plan air-quality and dew-point measurements |
| ISO 1217 | Acceptance tests for displacement compressors, including free air delivery and specific energy | To compare stated delivery and power on a like-for-like basis |
| ISO 5389 | Performance tests for turbocompressors | For centrifugal compressor performance discussions |
| ISO 11011 | Method for assessing the energy efficiency of compressed-air systems | As the method framework for our air audits |
| ISO 12500 series | Test methods for compressed-air filters | To compare filter performance claims |
| ISO 7183 | Specifications and testing of compressed-air dryers | To compare dryer ratings and stated conditions |
| ISO 2151 | Noise test code for compressors and vacuum pumps | To read sound levels with their measurement basis |
| ISO 50001 | Energy management systems | Context for plants running formal energy programmes |
We audit by the ISO 11011 method but do not issue conformity certificates. Where a client needs certified air-quality results, testing must be carried out to the relevant part of ISO 8573 by a suitable laboratory or test body.
ISO 8573-1 water and oil classes at a glance
The water and oil classes are the ones buyers ask about most. The values below are the commonly quoted limits from ISO 8573-1; check the current edition for the full table, including particle classes.
| Class | Water: pressure dew point | Total oil: mg/m³ | Typical way to achieve it |
|---|---|---|---|
| 1 | −70 °C or lower | 0.01 or lower | Special desiccant dryer; oil-free compressor or high-grade oil removal |
| 2 | −40 °C or lower | 0.1 or lower | Desiccant dryer; coalescing and activated-carbon filters |
| 3 | −20 °C or lower | 1 or lower | Desiccant dryer; coalescing filtration |
| 4 | +3 °C or lower | 5 or lower | Refrigerated dryer; general-purpose filtration |
The class you need depends on the most demanding use on that line, its ambient conditions and whether the air touches product. Pipework after the dryer can pick up rust and condensate, so the class must hold at the point of use, not only at the dryer outlet.
Pressure equipment and statutory rules
Air receivers, dryer vessels, separator tanks and pipework hold stored energy, and their design and use are governed by codes and by law. Vessels supplied in India are commonly built to IS 2825 or to ASME Section VIII, and imported equipment may carry marks under other regimes. Inspection and testing of pressure plant in factories falls under the applicable state factory rules, and vessels storing compressed gases are covered by separate statutory rules administered by the relevant authority.
These requirements depend on the vessel, the gas, the pressure, the location and the date. We do not give a general “compliant” statement. For any receiver, we look for the maker’s data plate, design code, design pressure and temperature, hydrotest record and relief-valve setting, and we advise you to confirm statutory duties with your inspecting authority.
Manufacturer documents
For a particular compressor, the maker’s operating and service manual, parts book and service bulletins are the primary reference for intervals, settings, lubricants and torque values. Because manufacturers update these, the revision matters. A lubricant or interval from an old manual may have been replaced. When we advise on a specific machine, we work from its model and serial number and say if a figure needs confirming against the current manual.
Brochure figures need care. Free air delivery, power and sound levels are stated at defined conditions, such as inlet temperature, pressure and humidity, and at a defined working pressure. Two brochures can give different numbers for similar machines simply because their test conditions differ.
Established engineering practice
Some figures on our site are not taken from one standard but are well-established practice that experienced engineers use every day. We state them as typical values, not guarantees:
- Generating each extra bar of pressure takes about 7% more compressor power.
- A typical refrigerated dryer gives a pressure dew point of about +3 °C; desiccant dryers typically give −20 °C or −40 °C, and lower where specified.
- Normal oil-injected screw discharge temperature is about 75–95 °C.
- Specific power is expressed in kW per m³/min of delivered air at stated pressure.
- 1 bar is 14.5 psi, and 1 m³/min is about 35.3 cfm.
Where a page gives a figure from our own work, such as servicing hire machines every 300 running hours, it says so.
How standards apply in practice
Standards are often misapplied. These examples show the questions we ask.
- “500 cfm” on a brochure. Is that free air delivery to ISO 1217, at what working pressure and inlet conditions? Site delivery in a hot, dusty plant room can differ, so we compare machines on the same basis.
- “Class 1.4.1 air”. An ISO 8573-1 class has three figures, for particles, water and oil. Is the class needed at the point of use, and where and how will it be checked? A filter’s catalogue class is not a measured result in your plant.
- “Oil-free, Class 0”. Class 0 is a level stated by the manufacturer and tested to defined conditions. It describes the compressor’s output, not what reaches your process after pipework and treatment.
- Energy savings claims. A percentage from one case study does not transfer to another plant. Savings must be measured before and after, with the same production and boundaries.
- Sound levels. A dB(A) figure means little without the distance and operating state. Do not average two figures measured differently.
How we keep references accurate
When we write or revise a page, our engineers check each technical statement against its basis: a standard, a manufacturer document, a measurement or established practice. We note the edition or revision where it matters and narrow statements that only apply to certain equipment or conditions. Standards are revised over time, so we review pages when a key reference changes.
If you find a statement that looks wrong, out of date or applied too broadly, tell us. Send the page address, the statement and the reference you believe applies. We will check it and correct the page where needed.
Limits of this register
This register is general guidance. It is not legal advice, a design code or a substitute for the current edition of a standard. It does not certify any product, installation or report. For safety-critical duties such as breathing air, medical air, oxygen service and hazardous areas, the specific code for that duty and the competent authority govern, and specialist design is required.
How Reddy Compressors helps
Reddy Compressors, Tarnaka, Secunderabad, has served industry since 2002 as an independent multi-brand specialist in compressor sales, rentals, service, audits and spare parts. We are not an authorised distributor of any brand. We have tie-ups with all major and reputed brands. Our engineers help clients turn standards into practical decisions: specifying air quality by ISO 8573-1 class, comparing compressor offers on a like-for-like ISO 1217 basis, planning dew-point and air-quality checks, and carrying out energy audits by the ISO 11011 method. Service teams cover Telangana and Andhra Pradesh; equipment and spares are supplied across India.
What to send us
- The standard or specification clause you need to meet
- Equipment nameplates and the manufacturer’s data sheets
- Where the air is used and the purity class required
- Any existing test reports, with dates and methods
- The page and statement if you are reporting an error
Frequently asked questions
What is ISO 8573-1 and what does a class like 1.4.1 mean?
ISO 8573-1 sets purity classes for compressed air. The three numbers give the class for solid particles, water and total oil, in that order. Class 1.4.1 means class 1 for particles, class 4 for water (a pressure dew point of about +3 °C or better) and class 1 for oil. Lower numbers mean cleaner air, and class 0 is a stricter level agreed by specification.
What does free air delivery mean on a compressor brochure?
Free air delivery is the volume of air the compressor actually delivers, converted back to its inlet conditions, measured to a test standard such as ISO 1217 at a stated working pressure. It is lower than swept volume or displacement. Always compare machines by free air delivery at the same working pressure, and allow for higher inlet temperatures at site.
Do air receivers need inspection in India?
Generally yes. Air receivers are pressure vessels, and the applicable state factory rules typically require periodic inspection and testing by a competent person. Requirements depend on the vessel and location, so confirm them with your inspecting authority. Keep the maker’s data plate, design certificate, hydrotest record and relief-valve details available for the inspector.
Is ISO 11011 a certification for compressed-air systems?
No. ISO 11011 sets out a method for assessing the energy efficiency of a compressed-air system, from demand through distribution to supply. It describes how an assessment should be planned, carried out and reported. We carry out audits by this method, but we do not issue conformity certificates, and a plant does not become certified by having an assessment.
Which matters more, the standard or the manufacturer’s manual?
For a specific machine, follow the manufacturer’s current manual for intervals, settings, lubricants and safety limits. Standards set general methods and classes, while the manual applies them to that design. Where law or a site code sets a stricter rule, that applies. If the manual is old, check whether service bulletins have changed it.
Related pages
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.