Some misconceptions about oil-free compressed air – does anyone know them?

Thread Content

As the second most important power source after electricity, compressed air is used in various industries. Different industries have varying quality requirements regarding various pollutants in compressed air. Some industries have high demands for the quality of compressed air, and there is also a need for oil-free compressed air. However, many people hold the following misconceptions about oil-free compressed air: while most users know that oil-free compressed air is necessary, they are unclear about the specific numerical values related to its quality standards. The oil removal equipment available on the market for compressed air typically relies on conventional filtering systems, and such technology is unable to achieve sustained, thorough oil removal. Moreover, few companies are equipped with devices for detecting oil content; as a result, when the quality of compressed air fails to meet standards, it is often simply assumed to be due to oil contamination. At the same time, users generally believe that the air produced by oil-free air compressors is oil-free compressed air. In light of the above, we discuss oil-free compressed air here, analyzing it primarily from the following aspects. 1 What is oil in compressed air? It is clearly defined in the standard ISO 8573-1-2010 \"Compressed air – Part 1: Contaminants and purity classes\" and the standard GB/T 13277.2-2015 \"Compressed air – Part 2: Methods for measuring the content of suspended oils\": Oil refers to hydrocarbon mixtures composed of 6 or more carbon atoms (C6+), which are collectively referred to as oil. 2 What is oil-free compressed air? Before understanding oil-free compressed air, it is necessary to first know what components oil contains in compressed air. Referencing the standard GB/T 13277.1-2008 \"Compressed air – Part 1: Grades of cleanliness\" (eqv.ISO 8573-1), it is specified that oil in compressed air consists of three components: liquid oil, suspended oil, and oil vapor ； We consider suspended oil to be another form of liquid oil; it exists in particle form and remains suspended in the gas through Brownian motion, whereas oil vapor exists uniformly in the gas in a single-molecule state. The total of their contents represents the oil content in compressed air. As can be seen from the table above, among the 5 grades defined for the oil content in compressed air, Grade 0 corresponds to a standard set by the equipment user or manufacturer, requiring a total oil content of ≤0.01 mg/m3; oil-free compressed air represents the oil content level equivalent to Grade 0, and this can be quantified with a specific numerical value. 3 Main sources of oil in compressed air: Earlier, we defined the composition of the total oil content based on standards: liquid oil, suspended oil, and oil vapor. The sources of oil in compressed air are mainly two: one is the trace amounts of oil already present in the atmosphere, which exist primarily in the form of oil vapor, and the oil content varies depending on the degree of air pollution. Secondly, there is the grease introduced into the compression chamber during the compression process through oil lubrication; screw compressors using oil lubrication are the ones most affected by this issue, as the air in the compression chamber mixes thoroughly with the lubricating oil. The oil content in the compressor’s exhaust air is controlled by the oil-gas separator built into the compressor. The principle of oil-gas separation involves removing oil from the compressed air through the collision and aggregation of liquid droplets, as well as through filtering materials with different pore sizes, all at a certain flow rate. When operating properly, the suspended oil content is 1–3 mg/m3, while the oil vapor content depends on the temperature of the compressed air. 4. Oil content in the atmosphere: Oil in compressed air is defined as hydrocarbon compounds of C6+. In areas where people live, such as urban environments, industrial zones, and roads, any human activity inevitably leads to the release of volatile organic compounds, many of which are C6+ hydrocarbon compounds; therefore, ambient air necessarily contains oil. As can be seen from the data in the table above, the levels of hydrocarbon compounds in the air inside buildings, on roads, and in industrial areas are relatively high. This is mainly due to the production activities of nearby factories and enterprises or vehicle exhaust emissions; inside buildings, volatile organic compounds tend to accumulate in enclosed environments, resulting in higher oil content levels ； In light of this, a set of data was also collected regarding the air conditions in the factory’s air compressor room, with the average oil content reaching 0.297 mg/m3. 5 The acquisition of oil-free compressed air: It is commonly assumed that the compressed air produced by an oil-free air compressor is oil-free, but this is actually an inaccurate understanding. In oil-free lubrication compressors, no oil is involved in the lubrication process, which essentially ensures that there is no oil contamination during compression. However, the oil present in the ambient air cannot be ignored; therefore, in most cases, especially in areas with harsh conditions, the use of oil-free compressors does not guarantee completely oil-free compressed air. It is difficult to produce oil-free compressed air relying solely on oil-free air compressors; however, it is more reliable to use reliable post-treatment oil removal and purification equipment at the later stage. With the support of reliable oil removal and purification equipment, the air compressors used at the front end are not limited to oil-free compressors; oil-lubricated compressors can also be used to produce oil-free compressed air, and there are numerous examples of this in actual production. Therefore, by dispelling misconceptions and gaining a new understanding, it becomes possible to determine the right approach to obtaining high-quality oil-free compressed air.