Compressed Air Quality Standards and Class

When air is collected to be compressed, other particles are collected too - this air is contaminated. When air is compressed, the concentration of contaminants increases exponentially. In addition, further contaminants could be added during the process of compressing air. 

Air treatment is necessary to remove contaminants from the compressed air system. The types of contaminants found in a compressed air system include:

1. Solid particles / Dust 

2. Water (in liquid or vapour form)

3. Oil content (in vapour or aerosol form)

When air is treated properly, it is considered clean and safe. But the quality of compressed air is not only defined by how clean it is but also by how dry it is. To determine how clean and dry the compressed air is, we must count the number of particles of a specific size present in one cubic metre of air, the dew point, and the amount of oil aerosols and vapour.

The International Organisation for Standardisation (ISO) has developed the international standard to test compressed air quality, known as ISO 8573-1. The ISO air quality standard measures three types of contaminants present in compressed air: water, oil content and solid particles. It does not consider microorganisms and gases. 

A specific compressed air class is assigned depending on the amount of contaminants found. The air quality class is set according to ISO 8573-1. This standardised system defines parameters from the least to most contaminated sources of compressed air.

In the context of compressed air specifications, air compressors are graded according to the purity class after compression. So you can start to determine which type of compressor you need by looking at what air purity class your application requires.

Compressed air is used in many industries, such as mining, manufacturing, textile production and food processing. The quality of the air used in industrial applications has a direct influence on the work process, installed machines and product quality. It is, therefore, vital for the compressed air to be clean and free from contaminants. 

The cleaner the air is, the lower the risk of contamination, breakdowns and product rejection. This is critical in industries like food and beverage, and pharmaceuticals. There is a chance that the air will come into direct contact with the product or indirectly with the packaging. 

High air quality is important in many industries, but the most sensitive application is in medical services. In terms of the medical air supply for hospital patients, air purity must be 100% guaranteed. That is where oil-free compressors, which produce the cleanest air, are indispensable.

It is recommended that only compressed air classified as Class 0 be used in critical processes to eliminate the risk of air contamination. This level of classification does not mean zero contamination. Class 0 refers to the highest air quality possible with minimum contamination present in the air and has to be lower in contamination than Class 1. 

A combination of compressed air equipment can be installed to produce clean air. This may include various air filters and dryers. Identifying which contaminants need to be removed will help you to determine which equipment you need.

A filter is used to separate air particles from contaminants. However, each filter is only effective to a certain degree, as no filter can separate all particles. Particles between 0.1μm and 0.2μm are the most difficult to filter. 

Oil and water in aerosol form behave similarly to other particles and can be separated using a coalescing filter. In the filter, these liquid droplets coalesce and become heavier, so they sink to the bottom of the filter. 

The filter can separate oil in aerosol as well as in liquid form. However, if the oil is in liquid form, it will result in a high-pressure drop and oil carry-over. If the oil is in vapour form, it is more difficult to separate and requires a filter that contains adsorption material, usually activated carbon.

All filtering inevitably results in a pressure drop which the compressed air system to lose energy. Finer filters with a tighter structure separate more contaminants, but, they also cause a higher pressure drop and are more likely to clog faster. This leads to more frequent filter replacement and, consequently, higher maintenance costs.

Additionally, filters must be dimensioned to handle the nominal flow properly and have a larger capacity threshold. This makes it easier to manage some pressure drops due to a certain amount of blockage.

Refrigerant or adsorption dryers are used to remove moisture from compressed air. Refrigerant dryers are used when the maximum air quality required is Class 4, meaning the dew point is 3°C or below. If compressed air with less moisture (a lower pressure dew point) is required, an adsorption dryer must be installed.