How to filter compressed air

What contaminates compressed air? Compressed air quality according to ISO 8573-1 How to get rid of impurities in compressed air? Choosing the right filter Beware of pressure losses Where to install compressed air filters Measuring impurities in compressed air

What contaminates compressed air?

Atmospheric air drawn into the compressor contains three types of impurities:

• Solid particles: Up to 140 to 150 million polluting particles in every cubic meter of air, and even several times more in industrial operations. Most of these impurities (80%) are smaller than 2 μm, making them too small to be captured by the compressor's intake filter (which usually removes particles of 25 μm size).
• Oil enters the compressed air from both piston and screw compressors that are oil-lubricated.
• Water – water vapor in the air condenses after cooling in the piping. You can read about where moisture comes from in the air and how to get rid of it in the article Why and How to Dry Air.

Contaminated compressed air has an unpleasant odor and, furthermore, it "takes a toll" on the condition of your equipment:

• All seals wear out faster and must be replaced frequently.
• Valves in control circuits also suffer.
• Impurities from compressed air penetrate into silencers and impair their function.
• Orifices and nozzles gradually become clogged.
• Finished products and production equipment are damaged – solid particles can scratch surfaces, etc.

Compressed air quality according to ISO 8573-1

The guideline for compressed air quality is the ISO 8573-1 standard. Compressed air quality is described in the standard by seven classes, 0 to 6 (Table 1): Each class has a prescribed maximum content of solid particles, water, and oil. Different purity classes are required for various applications and flow rates.

How clean must compressed air be for different applications?

For various use cases, the ISO 8573-1 standard establishes quality classes; for example, for workshop air, a quality of 4-4-5 is mandated by the standard. This three-digit number indicates the three prescribed quality classes in this order:

• quality class for solid particles,
• quality class for water (dew point temperature),
• quality class for oil.

Table No. 2 lists the compressed air quality classes for various industrial applications.

How to get rid of impurities in compressed air?

Solid particles are captured by a suitable filter or two or more filters in series. The first barrier against the ingress of solid particles is the compressor's intake filter, but this only removes particles of 25 μm size. Additional filters are installed before or after the dryers.

Oil – can be in the air in the form of aerosol or vapor. A high-quality standard filter captures aerosol. Activated carbon filters capture oil vapor. The efficiency and lifespan of activated carbon are affected by the temperature of the compressed air and its humidity.

Water is removed from the compressed air using a suitable dryer placed after the compressor. Details and practical tips for air drying can be found in the article Why and How to Dry Air.

Before you go out to buy filters and dryers, answer these questions:

• How clean must the compressed air be for the purpose I am using it for?
  Requirements for compressed air used in a workshop for cleaning parts or in construction are entirely different from those for measurement or pneumatic transport of food. You can find the quality classes for your application in the ISO 8573-1 standard or in Table 2.
• How clean must the compressed air be so as not to damage cylinders, valves, and other elements in the distribution system?
  Contaminated compressed air used for the pneumatic drive of manipulators, pistons, and conveyors must not cause seizure of pneumatic cylinders, clogging of nozzles, or other damage to the equipment.
• How clean is the compressed air supplied by my compressor?
  When the air from the compressor does not meet the quality required by the application, it is necessary to decide on suitable filters or compressed air treatment units.

Choosing the right filter

You can choose suitable filters for your circuit from a wide range
here

When selecting a filter, base your decision on three parameters:

1. Required air quality
   The air quality you must have for your workshop or operation is specified in ISO 8573-1.
   Some application examples can be found in Table 2. Based on the quality, you choose the type of filter (microfilter, activated carbon filter...).
2. Air pressure at the filter inlet
   Depending on the pressure in the compressed air circuit, you choose a suitable filter model.
3. Performance or volumetric flow rate
   Filters are dimensioned according to the amount of air flowing through them. This amount (volumetric flow) is a basic parameter of the compressor or dryer and is referred to as output, displacement, or flow.
   It is given in liters per minute (l/min), but sometimes also in m3/hr.
   You can easily convert this: 1 m3/hr = 16.67 × 1 l/min. Many other units are used for volumetric flow, such as liters per second, etc. To avoid mistakes during conversion, you can also use an online calculator.

Quality classes in filter offers When choosing, it is necessary to ensure that the air after filtration has the content of solid particles and oil required by our application. In Table 2, find your application and then look for the corresponding filter in the filter range. Suppliers list the classes in the filter descriptions – Figure 1 shows an overview of filters from Kompresory vzduchotechnika.

Beware of pressure losses

Every filter (just like any other device, such as a dryer or fitting) causes a pressure drop, which costs money. Therefore, do not filter more than necessary.

Although we can easily compensate for the pressure loss by increasing the pressure at the compressor outlet, it is not free. Energy consumption will rise (by 6 to 10% for a 1 bar increase in pressure) and, unfortunately, the temperature of the compressed air will also rise with the pressure, thereby increasing its humidity.

• Pre-filters and dust filters with a coarser structure of filter elements reduce pressure the least.
• Fine filters for capturing oil and moisture have a higher pressure loss.
• Regular replacement of filter elements also helps in the fight against pressure loss. A clogged filter reduces pressure more than a clean one.

A differential pressure gauge will tell you...
You can check the pressure loss on the differential pressure gauge, which is part of some filters.

Where to install compressed air filters

• An intake filter is installed before the compressor intake to filter out larger solid particles so they do not damage the compressor.
• It is suitable to place a coarse filter after the aftercooler and air receiver to protect the dryer mainly from solid particles.
• A fine filter can be placed after a refrigerant dryer to remove remaining condensate.
• A fine filter must be placed before an adsorption dryer to protect it from oil contamination. A coarse filter is placed after the dryer.

Microporous filters with regular pores (pre-filter, coarse filter)

This type of filter captures solid particles from the compressed air before the dryer. It can handle solid particles from 3 μm, but only minimally with oil, water, and fine particles. The filters contain replaceable filter elements – hollow cylinders made of porous sintered bronze, ceramics, polyethylene, or polypropylene.

Microfilters with disordered microfibers (coarse filter) If the compressed air passing through the refrigerant dryer is not sufficiently clean, we place a filter made of disordered microfibers after the dryer, which can reduce the oil content in the air down to 0.01 mg/m3 and captures solid particles larger than 0.01 μm (Figure 2). Oil particles collide with the glass microfibers and form larger droplets, which are carried by the air stream to the outer edge of the element into a foam jacket. In it, the oil droplets fall to the bottom of the filter by gravity.

Activated carbon filters (fine filter)

After the compressed air has passed through the dryer and filters, oil vapors still remain in it. Therefore, an activated carbon filter is placed after the microfilter, which reduces the oil content in the air down to 0.005 mg/m3. The porous activated carbon filter works on the principle of adsorption. Compressed air flows through the activated carbon filling, and the hydrocarbons contained in it bind to it.

Filter design versions

For connection, filters are equipped with a thread or a flange. Filters also have these elements:

• differential pressure gauge to measure the air pressure drop during passage
• condensate drain - manual or automatic.

Filters are color-coded according to their parameters. The operator can see at a glance what type of filters are installed in the system and what filter elements will be needed.

When designing a filter system, remember that filters are often built into dryers, cyclone separators, or air receivers and other equipment.

Compressed air treatment units

In production facilities, we encounter very specific requirements for compressed air quality. In these cases, treatment units come in handy. These devices usually contain a pressure regulator, filters, and lubricators in a single unit, and it is possible to choose a suitable model exactly according to the properties the compressed air should have in your operation.

Measuring impurities in compressed air

How to check if the compressed air does not contain too many impurities? To verify the purity of compressed air, use:

• oil detectors
• solid particle detectors
• portable measuring stations

Oil detectors

Photoionization detectors, PID, are used to determine the oil content in compressed air.

Measurement principle A photoionization detector uses a UV lamp to ionize* the molecules of the measured air. UV radiation is not capable of ionizing molecules of normal components in the air (oxygen, nitrogen, CO2, argon, water, etc.). However, it can reliably ionize hydrocarbons, which are components of oils. The strength of the ion flow (and thus the electrical signal) is directly proportional to the concentration of ionized molecules. The electrical signal is amplified and displayed on the screen as the total oil content. These detectors only measure air without aggressive, corrosive, toxic, caustic, flammable, and ignitable substances*). Hint: During ionization, an electrically neutral molecule becomes a molecule with an electric charge, i.e., an ion. An electrical signal is created by the flow of ions.

Oil detector installation

Oil detectors are installed on a vertical supply pipe with upward flow. Two installation options are shown in the diagrams (Figure 4):

• direct installation
• installation with by-pass piping

Solid impurity detectors Solid impurities in compressed air (and other technical gases) are measured by transillumination using a laser diode placed perpendicular to the pipe. Instruments measure the number of particles in a so-called "normal cubic meter" – Nm3 (volume at normal pressure and temperature). The device can also distinguish particle sizes – it measures how many particles of size 0.1 to 0.5 mm, 0.5 to 1 mm, and 1 to 5 mm the gas contains.

Portable measuring stations Portable measuring stations are also used to monitor compressed air quality, which include: oil content detector solid impurity detector dew point sensor The station is simply brought to the compressor to measure the compressed air quality.