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Air compressors are general-purpose mechanical devices used in industrial production, and they are widely applied in sectors such as petroleum, chemicals, power and metallurgy, mining, construction, light industry, pharmaceuticals, civil use, and defense, playing an important role in the national economy. Therefore, it is crucial to choose the appropriate air compressor model that meets the requirements of the production process; this can significantly reduce the operating costs of air compressors, and it is of great importance for saving energy, increasing output, reducing the burden on enterprises, and boosting their revenue. Compressed air produced by ordinary air compressors usually contains a certain amount of lubricating oil as well as water. In some applications, the presence of oil and water is not allowed; in such cases, it is necessary to pay attention to the selection of the air compressor, and in industries such as oxygen and nitrogen production as well as the food industry, post-treatment equipment must be added. If an oil-free air compressor is used, it can ensure that the air delivered contains no oil, but not that it is free of water. In either case, an additional stage or two of purification devices or dryers should be installed on the air compressor, so as to reduce the oil content and moisture level in the compressed air to levels below a certain threshold, thereby meeting the process requirements. The degree of drying and purification of compressed air also varies depending on customer requirements, leading to different choices. Typically, our standard configuration is as follows: air compressor + air storage tank + main pipeline filter of grade C (pre-filter) + refrigeration dryer + oil removal filter of grade T + precision filter of grade A. I. Understanding the principles for selecting an air compressor: 1. Air properties: local altitude, atmospheric pressure, local relative humidity, etc. 2. The diameter and concentration of solid dust particles in the air. 3. Air temperature: (°C) 4. Operating voltage and frequency (high voltage for domestic mines; voltage standards abroad) 5. Required flow rate (Nm3/min or Nm3/h) 6. Flow rate, which is often referred to as displacement by domestic users. Flow rate refers to the amount of gas discharged by the air compressor per unit of time at the desired exhaust pressure, expressed in terms of the intake condition. 7. Domestic: The flow rate is measured in m3/min (cubic meters per minute) or L/min (liters per minute); 1 m3 (cubic meter) = 1000 L (liters) ； Abroad: The unit for flow rate is CFM (cubic feet per minute); 1 CFM = 0.02832 m3/min. Generally, the commonly used unit for flow rate is m3/min (cubic meters per minute) ； 9. Flow rate is also known as displacement or nameplate flow rate. 10. Pressure: design pressure, maximum operating pressure, operating pressure. (There are many ways to express pressure units; here, the common units used for pressure measurement in centrifugal air compressors are introduced.) 11. Operating pressure, which is often referred to as exhaust pressure by domestic users. Working pressure refers to the highest pressure of the gas discharged by the air compressor ； 12. The common units for expressing working pressure are bar, MPa, and kg/cm2, as well as psi (Pounds per square inch). 1 bar = 0.1 MPa = 1 kg/cm2; 1 psi = 0.006895 MPa. Therefore, 100 psi is approximately equal to 100 × 0.006895 ≈ 0.69 MPa. 13. Generally, users refer to pressure in terms of kg/㎡, with 1 bar being equivalent to 1 kg/㎡. II. How is the operating pressure (exhaust pressure) determined? 1. When a user is ready to purchase a compressor, the first step is to determine the operating pressure required at the point where air is used, and then add a margin of 1–2 bar. The pressure of the air compressor should be selected based on this value; this margin takes into account the pressure losses that occur over the distance from the location where the compressor is installed to the actual point where the air is used. The appropriate margin of pressure is determined depending on the distance, ranging from 1 to 2 bar. Of course, the size of the pipeline diameter and the number of bends are also factors that affect pressure loss; the larger the pipeline diameter and the fewer the bends, the lower the pressure loss ； Conversely, the pressure loss will be greater. Therefore, when the distance between the compressor and the pipelines at the various air-consuming ends is too large, the diameter of the main pipeline should be appropriately increased. If the environmental conditions meet the installation requirements of the air compressor and the operating conditions permit, it can be installed near the point where air is used. 2. The higher the altitude, the thinner the air and the lower its density; at the same rotational speed, the volume of air compressed by the compressor decreases. 3. For new projects, the selection can be made based on the flow rate values provided by the design institute ； 4. Consult the gas equipment suppliers regarding the flow rate parameters of such equipment to select the appropriate model ； 5. For the renovation of the air compressor station, the original parameter values can be used as a reference, along with the actual gas consumption requirements, to select the appropriate model ；