In this exclusive article, the Head of NBC (Nordic Baltics Countries) Torbjörn Lundberg from SMC shares insights on the ever-evolving world of automation and its profound impact on various industries.
Water is a necessity for life and is not only found in lakes and streams. An adult consists of approximately 60% water and children up to 80%, but the air we breathe also contains large amounts of water.
Humidity causes problems
When talking about humidity, it is almost always the relative humidity that is meant. It is expressed as a percentage (%) and is dependent on the temperature of the air. The warmer – the more water/moisture the air can absorb. If the relative humidity is below 100%, the water is not noticeable, but is in the form of water vapor. If you then compress the air, there is less room for water and then the water vapor condenses into liquid water, something that can disturb and be harmful in compressed air systems.
A potential problem with water in connection with compressed air systems is that the water can carry debris from the inside of the pipes, give rise to rust if steel pipes are used, etc. Examples of problems are spray painting where the air quality is crucial for a good result. Water in the compressed air is also a possible problem in the food industry. Here the reason may not be as obvious, but the compressed air is used here to blow clean bottles before they are filled, cool pastries, etc.
Water in the air also affects the life of the compressed air components that are exposed to the water. Most components such as cylinders, valves, rotary actuators etc. are permanently lubricated. The risk is that if water condenses in the compressed air system, this will affect the lifespan of the grease that is there to lubricate the component. If there is enough water, the fat can even be "flushed" away. The consequence in both cases is a significantly shortened lifespan for the compressed air component.
What do you do to avoid problems with water in compressed air systems?
Most compressors today have aftercoolers with water separators. When the air is compressed in the compressor, the temperature rises. What the aftercooler, which is simply a heat exchanger, does is lower the temperature of the compressed air and condense the water vapor that is in the compressed air. It is then separated using the water separator. However, the compressed air still has an elevated temperature and a fairly large water content. To further reduce the water content in the compressed air, a refrigeration dryer is often used. The principle is the same as for the aftercooler, lower the temperature to condense out liquid water. Here, however, refrigerants are used to achieve this, the same method as in a refrigerator. With a refrigeration dryer, you can get down to dew points of a few plus degrees Celsius. The dew point is the temperature when the relative humidity is 100% and when the water vapor in the air begins to condense into liquid water. Usually, a dew point of a few +oC is not enough, but you must lower the dew point further.
To get even drier air and less water, additional drying is required. So-called adsorption dryers provide the lowest dew point down to -70 oC. However, they consume energy in the form of both regeneration of the adsorbent and a pressure drop. Another way to achieve really dry air is the membrane dryer, where the water vapor is separated through a semi-permeable membrane – somewhat reminiscent of osmosis. Membrane dryers can reach dew points as low as adsorption dryers but require no electricity. They are also small in relation to capacity and the investment is usually considerably lower. They are also very energy efficient and the only extra energy used is so-called purge/flush air, which is part of the principle of membrane dryers, but it is negligible in the grand scheme of things.
Does the entire compressed air system have to be dried?
The methods of drying the air described are all intended to be placed directly after the compressor, but you may not need "instrument air" everywhere in your facility. Some of the applications may require ultra-dry air and then you can dry the air extra, just for them. It may not be possible to deploy an adsorption dryer for every application, and perhaps not a membrane dryer either. For entire sections in a compressed air system, it can work and then the membrane dryer is likely to be a more cost-effective alternative.
Especially small actuators/actuators (cylinders) are sensitive to water in the air. This is because the ratio between the volume in the hose leading up to the cylinder and the volume in the cylinder risks becoming too large. It is the ratio that governs whether the water vapor should condense into liquid water and can be calculated using the General Gas Law. For ideal gases, the Pressure times the Volume through the Temperature is constant. The water is formed when the temperature drops rapidly during deaeration and the relative humidity increases until the temperature becomes low enough and reaches down to the dew point.
IDK is a solution
But there are solutions where you can locally, and for each individual compressed air component, dry the air so that condensation is avoided. SMC Automation manufactures hoses made of fluoropolymer plastic that function as small membrane dryers where water can penetrate through the hose wall while the compressed air remains. The hoses are linear, 100 mm, 200 mm or spiral and are easily connected, with standard couplings, between the cylinder and existing hose. The series of fluoropolymer hoses is called IDK and is available in several different diameters. This solution is most suitable for smaller components and especially suitable for high frequency applications, where the risk of condensation is greatest. The IDK hose works best for cylinders up to approx. 20 mm in diameter. SMC Automation has a calculation program where you can easily check if additional drying is necessary. Just like the membrane dryer, no extra energy is required in the form of e.g. electricity without the IDK hose is completely complete as is.
