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Best practice for compressed air treatment

11 July 2019

Compressed air is not clean!  In fact, in a typical system there are 10 main contaminants that require treatment if the system is to operate safely.

The concentration of airborne contaminants present during the compression process means that the compressed air will invariably need some form of air treatment before the point of use.

As technology for compressed air treatment continues to evolve, there is now a wide range of equipment available which can satisfy the most demanding of compressed air treatment needs. The selection of the right equipment can be a complex task, affecting everything from maintenance schedules to the ongoing costs associated with achieving the required standards, while avoiding generating excessive purge air and preventing significant pressure drops.

Following the British Compressed Air Society’s (BCAS) publication of its ‘Filtration and Drying of Compressed Air’ best practice guide earlier this year, Roy Brooks, Technical Development Officer at the British Compressed Air Society (BCAS) outlines some of the key areas to consider when selecting the appropriate air treatment equipment for the application.

Which contaminant?

Commonly, compressed air contaminants are combined into three distinct categories; particles (including viable and non-viable microbiological organisms), water and oil. ISO 8573-1, the international standard for compressed air purity (quality), refers to the main contaminants in this format.

When selecting purification equipment, it must be remembered that contaminants will be in one of three different phases (states of matter). For example, water and oil in a compressed air system will be found in liquid form, as an aerosol (fine mist) and in a vapour (gaseous) phase and a different purification technology will be required depending upon the phase of the contaminant.

Which standard?

Depending on the application, there are number of different compressed air standards and best practice guidelines which can assist the end user. Prior to the purchase of new compressed air treatment equipment, the user should assess carefully the air purity (quality) requirements of the system or application. The air purity (quality) required should be specified to all suppliers to assist product selection.

ISO8573 series is the most commonly used standard for compressed air (excluding breathing air or medical air). It is made up of nine separate parts. Part 1 refers to air purity (quality), while parts two to nine provide details on the equipment and methodology to be used to measure for different contaminants in a compressed air system (and meet the air purity (quality) classifications shown in part one).

ISO8573-1 – International Standard Relating to Compressed Air Purity (Quality) ISO8573-1 provides the user a way of specifying an air purity (quality) required for the entire compressed air system and/or for individual usage points, based upon application requirements. It also allows equipment manufacturers to show product performance easily and specify purification equipment to meet the end users air purity (quality) specification.

In addition, there are also specific requirement for compressed air that comes in to direct or indirect contact with food or beverage products as well as the HTM02-1 purity recommendation for medical and surgical air and HTM2022 for dental air. Finally, EN 12021:2014 is the specified standard for breathable air, indicating maximum permitted contaminant levels both in the UK and the EU.

Specifying correctly

Whether designing new systems, or reviewing existing systems, the first step should be to define the precise compressed air purity (quality) requirements, ideally using the ISO8573-1 standard. To achieve the degree of air purity (quality) specified by ISO8573-1, a careful approach to system design, commissioning and operation must be adopted.

It is recommended that compressed air is treated prior to entry in to the distribution system and at critical usage points and application, to ensure that contamination already in the distribution system is removed.

Purification equipment should ideally be installed where the air is at the lowest possible temperature, i.e. downstream of air receivers, but also protected from freezing. Point-of-use purification equipment should be installed as close as possible to the application.

To allow correct sizing and selection of purification equipment, the following primary operating parameters must be obtained from the user’s site:

  • The maximum compressed air flow rate into the filters/dryer
  • The maximum operating temperature into the filters/dryer
  • The maximum ambient air temperature where the equipment is to be installed (required for some dryer technologies)
  • The required dewpoint (dryers)

Individually, each of the primary operating parameters can influence product sizing, however collectively they can have a major impact on product sizing and performance.

Many manufacturing plants only need a proportion of the compressed air to be treated to a very high purity (quality). In these cases, excellent savings are achievable by treating all the generated air to the minimum acceptable level and improving the purity (quality) to the desired level at the usage point.

If most of the compressed air is needed at a high purity (quality), it can make sense to treat all the compressed air to the level required by the highest purity (quality) application.

The specification of the most appropriate air treatment can be a complex task and this brief overview only illustrates the basic principles to consider.  BCAS recommends that users consult their compressed air supplier for expert advice.

In addition, detailed guidance can be found in the ‘Filtration and Drying of Compressed Air - Best Practice Guide 104.

The guide provides a comprehensive overview of the entire specification process, covering all stages of system design; from an overview of the sources of contamination, to the relevant standards for air purity.  It also examines the extensive range of air purification technologies available and how these operate, including after-cooling, dryers, air receivers, filtration grades and condensate management.

With compressed air energy cost a major contributing factor to a site’s overall electricity consumption, the guide also focuses on the varying energy efficiencies of the different treatment technologies available as well as guidance on the ongoing maintenance of the system.

The guide is aimed at typical industrial compressed air applications operating at low pressure, with a range between 7 bar g to 20 bar g.  

To download a free copy of the guide, please visit www.bcas.org.uk/airtreatment.  Printed copies of the guide are available from BCAS for £4.00 each plus postage by emailing enquiries@bcas.org.uk