Compressed Air Quality
From Compressor to Tool
Compressed air is an energy carrier — a gas under pressure used to transport energy from one place to another.
We input electrical energy into a compressor, which compresses the air to a higher pressure.
This pressure is then transferred through piping to a tool or a cylinder, where the stored energy is converted into mechanical work.
The process seems simple, but there are some important considerations.
Contamination
The air we breathe is far from ideal for producing compressed air. It is a fairly constant mixture of 16 different gases. Nitrogen and oxygen are the main components, but it also contains hydrogen, carbon dioxide, carbon monoxide and a small proportion of noble gases.
However, our air is also full of impurities: a single cubic metre can easily contain up to 100 million dust particles. These include airborne substances such as soot, pollen, dust, and water vapour. The presence of such contaminants varies from one location to another and depends heavily on weather conditions.
In addition, we should not forget that the compressor itself can introduce a considerable amount of oil into the air during the compression process.
Volume percentage per element
| Symbol | Name (Dutch) | Name (English) | Volume percentage (%) |
|---|---|---|---|
| N₂ | Stikstof | Nitrogen | 78.08 |
| O₂ | Zuurstof | Oxygen | 20.95 |
| Ar | Argon | Argon | 0.93 |
| CO₂ | Kooldioxide | Carbon dioxide | 0.03 |
| Ne | Neon | Neon | 0.0018 |
| He | Helium | Helium | 0.0005 |
| Kr | Krypton | Krypton | 0.00011 |
| SO₂ | Zwaveldioxide | Sulphur dioxide | 0.00001 |
| CH₄ | Methaan | Methane | 0.000002 |
| H₂ | Waterstof | Hydrogen | 0.000005 |
| O₃ | Ozon | Ozone | 0.000007 |
| Xe | Xenon | Xenon | 0.0000009 |
| NO₂ | Stikstofdioxide | Nitrogen dioxide | 0.0000002 |
| I₂ | Jodium | Iodine | 0.0000001 |
Filtering Compressed Air
In the ISO 8573-1:2010 standard, compressed air quality is defined in seven classes. The air quality is described objectively, allowing you to specify the required purity level for your compressed air.
A demolition hammer can operate with a lower air quality than the air used to aerate a delicate dessert. The higher the required air quality, the more expensive it becomes to produce compressed air at that level.
Air Quality Table According to ISO 8573-1:2010
| Quality class | 0.1–0.5 μm | 0.5–1.0 μm | 1–5 μm | Moisture content (Pressure dew point °C) | Oil content (mg/Nm³) |
|---|---|---|---|---|---|
| 0* | |||||
| 1 | ≤ 20,000 | ≤ 400 | ≤ 10 | –70 (≤ 0.003 g/Nm³) | ≤ 0.01 |
| 2 | ≤ 400,000 | ≤ 6,000 | ≤ 100 | –40 (≤ 0.11 g/Nm³) | ≤ 0.1 |
| 3 | – | ≤ 90,000 | ≤ 1,000 | –20 (≤ 0.88 g/Nm³) | ≤ 1 |
| 4 | – | – | ≤ 10,000 | +3 (≤ 6.0 g/Nm³) | ≤ 5 |
| 5 | – | – | ≤ 100,000 | +7 (≤ 7.8 g/Nm³) | – |
| 6 | – | – | – | +10 (≤ 9.4 g/Nm³) | – |
* Specific requirements are defined by the user and are stricter than Class 1.
Moisture
When air is compressed, all contaminants and moisture are compressed along with it. Moisture accelerates wear because it reduces lubrication. It also promotes corrosion and can lead to frost damage or mechanical damage caused by water hammer. In addition, moisture can cause blockages by making fine dust particles clump together.
Finally, moist air creates more resistance during transport, making the system less efficient and therefore more expensive to operate.
It is therefore essential to keep the moisture content in your compressed air system as low as possible. Further on in this article, you can read about methods to minimise moisture in your compressed air system.
The table below shows how much water vapour is present at different temperatures.
| Temperature (°C) | Maximum water vapour content (g/m³) |
|---|---|
| 40 °C | 50.67 |
| 35 °C | 39.29 |
| 30 °C | 30.08 |
| 25 °C | 22.83 |
| 20 °C | 17.15 |
| 15 °C | 12.74 |
| 10 °C | 9.36 |
| 8 °C | 8.24 |
| 7 °C | 7.73 |
| 6 °C | 7.25 |
| 5 °C | 6.79 |
| 4 °C | 6.36 |
| 3 °C | 5.95 |
| 2 °C | 5.57 |
| 1 °C | 5.21 |
| 0 °C | 4.87 |
Example Calculation
(Assuming air at 25 °C with a relative humidity of 60%) In one cubic metre of air, there is 0.6 × 22.8 = 13.7 grams of water vapour. When this air is compressed to 8 bar, it contains 123.3 grams of moisture per cubic metre (P + 1 is the multiplication factor). However, air at 25 °C can hold a maximum of only 23 grams of water vapour. This means that 100 grams of liquid water will remain — water that we do not want in our compressed air system. In addition, there is still 23 grams of water vapour that may later turn into liquid water (for example, through condensation at a lower temperature).
Practical Example
Some time ago, a customer approached us with a problem involving cracked pilot heads on their valves. After some investigation, we discovered that the technician was getting soaked every time he replaced one of these pneumatic components.The cause turned out to be quite simple: the compressed air system consisted of a low-cost compressor that blew hot air directly into the compressed air piping network, where it then cooled and condensed. As a result, large parts of the system were filled with water.When the valves were actuated, the existing column of water caused enormous pressure surges throughout the system.
You Can Dry Compressed Air in Three Ways
Refrigeration Dryers
When air is cooled, its ability to hold water vapour decreases. By cooling the air, condensation occurs, and this water can easily be separated and removed. The point at which water vapour in the air condenses is called the pressure dew point.
As the amount of water vapour dissolved in the air decreases, the dew point drops.
Refrigeration dryers do not achieve a dew point lower than +2°C.
A refrigeration dryer can be compared to a refrigerator through which compressed air flows.
Adsorption Dryers
In this process, the moist air passes through an adsorbent material that removes the water vapour from the air. Usually, this consists of two units: while one is drying, the other is being regenerated by blowing an excess of air through it (about 20% of the compressor capacity) and/or by heating. Vacuum regeneration (vapour pressure reduction) is also a common method.
With this technique, pressure dew points as low as –70°C can be achieved.
Membrane Dryers
Here, the air to be dried flows through hollow membrane fibres. The membrane’s properties allow water vapour to pass through, separating it from the air. On the other side of the membrane, this vapour condenses into water, which is then removed using compressed air (about 10–15% of the compressor capacity).
Pressure dew points of around –20°C can be achieved. However, in all cases, there is only a relative reduction in dew point. The performance of this type of dryer strongly depends on the pressure, temperature and flow of the air passing through it.
The range of dryers at Q Plus
When Is Lubrication Required for a Cylinder?
It is not normally necessary to lubricate pneumatic cylinders or valves with oil. Both components are factory-lubricated and designed to operate for many years without additional lubrication.
When cylinders are lubricated using an oil mist lubricator, the existing grease is dissolved and flushed away by the oil-containing air. If the oil mist lubricator ever runs empty, the cylinder will no longer be lubricated, which can cause heavy movement or even complete seizure — with all the resulting consequences.
You should only lubricate a cylinder when its speed exceeds 1 m/s, and ensure a continuous oil mist supply.
Once oil mist lubrication has been started, it must not be discontinued.
Use an oil mist lubricator only when strictly necessary.
