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In the pharmaceutical industry, clean compressed air is widely used as a process gas source in the production and manufacturing of drugs. Since it comes into direct contact with raw materials, it is an important factor affecting product quality. At present, the standards for the quality of pharmaceutical-grade clean compressed air are not well-defined both domestically and internationally; therefore, this paper explores the quality requirements for clean compressed air in pharmaceutical engineering designs, and analyzes the selection of materials for clean compressed air pipelines as well as the considerations in pipeline design. 1 Compressed air 1.1 Uses and classifications of compressed air Based on their uses, compressed air can generally be divided into two main categories: one is compressed air used in production, which comes into direct contact with materials and medicines ； Another type is compressed air used as a power source; it does not come into direct contact with drugs, but it is mostly used within clean areas. Therefore, clean compressed air must be used for pharmaceutical compressed air. Clean compressed air requires control over the amount of solid particles, moisture and oil content, as well as microbial levels, and it should be odorless. 1.2 Quality standards for compressed air The quality standards for compressed air can refer to GB/T 13277.1-2008 \"Compressed air – Part 1: Purification levels of contaminants\" (equivalent to ISO 8573-1, etc.). This standard classifies quality grades based on four control parameters: solid particle size and quantity, humidity and liquid water level, and oil content level, as shown in Table 1. Currently, regarding compressed air used in pharmaceutical manufacturing, only Appendix 1 of the \"Good Manufacturing Practice for Pharmaceutical Products\", titled \"Sterile Drugs\", requires that the compressed air used in sterile production areas be subjected to sterilizing filtration, with the integrity of such filters being checked regularly. For sterile products, the guidelines for sterile drugs require that the quality standards for compressed air be as follows: dew point of -20 ℃, and an oil content of no more than 0.1 mg/m3. The microbial limits for compressed air are determined based on the risk associated with the product; when entering sterile areas, the compressed air must undergo sterilizing filtration to ensure that its microbial content is less than 1 CFU/m3. However, for non-sterile or other situations, the new version of GMP does not provide detailed requirements. For compressed air used in non-sterile production, the design-recommended quality grade should be no lower than grade 2.4.1 (i.e., solid particle grade of 2, humidity grade of 4, and oil content grade of 1). The quality specifications for compressed air in clean areas should be no lower than the cleanliness level requirements of those areas. 2 Compressed air system equipment 2.1 Air compressors Air compressors are generally classified into piston, screw, and centrifugal types. Screw air compressors have small structural dimensions, produce no pulsating airflow, generate low noise and vibration, require minimal maintenance, and feature a high level of automation; therefore, oil-free screw air compressors are recommended for use in pharmaceutical engineering designs. 2.2 Air Storage Tank The air storage tank is an important component in an air compression system. Its main functions include buffering, cooling, water removal, and oil removal. 2.3 Drying Equipment Devices for drying compressed air, generally divided into cryogenic and adsorption types. When the pressure dew point requirement for compressed air is greater than or equal to 3 °C, a refrigerated dryer is typically used. If it is below 3 °C, an adsorption dryer or a combined freezing + adsorption drying unit is used. 2.4 Filters Filters are divided into oil-removing filters and dust-removing filters. When selecting filters, different types should be combined based on the substance to be filtered; filters that do not meet the required precision levels not only fail to achieve the intended filtering purpose but also incur high economic costs. If an oil-lubricated air compressor was used in the initial stage, it is possible to achieve a oil-free state using post-treatment oil removal devices; however, to obtain oil-free compressed air, highly efficient catalytic oxidation-based oil removal devices are necessary. When multiple filters are installed in a compressed air system, the principle of installing coarse filters first and then finer ones must be followed. 3 Compressed air pipelines 3.1 Selection of pipeline materials For compressed air pipelines, clean pipeline materials are generally used. Austenitic stainless steel sanitary-grade pipes, due to their various advantages such as smooth surface, corrosion resistance, good weldability, and stability, are widely used in pharmaceutical engineering. AISI 304 is recommended; for higher requirements, AISI 316L can be used. Stainless steel clean ball valves are recommended for valves. Pipe connections are generally made by welding, while valve connections are usually made using clamps. Stainless steel pipes should be passivated before use. The main pipes are installed in the technical interlayer, while the branch pipes that lead into the clean area must undergo surface polishing, with a surface roughness of Ra≤0.8 μm. 3.2 Considerations for pipeline design: The main compressed air pipeline should have a certain slope, with a requirement of not less than 0.002, and a drain valve should be installed at the end to facilitate the drainage of condensate water from the pipeline ； The branch pipe should lead out from above the main pipe to prevent condensate from entering the branch pipe and affecting the end-use devices. The pressure of the clean compressed air system in pharmaceutical manufacturing facilities is generally less than 1.6 MPa, and it is required that the pressure loss from the compressor outlet to the most unfavorable point in the system shall not exceed 10% of the compressor outlet pressure. Taking the pressure drop into account, the pipe diameter and gas flow velocity should be selected appropriately during design. The flow velocity inside the pipes for clean compressed air is generally kept at no more than 15 m/s to avoid excessive pressure losses. Clean compressed air pipelines can adopt a circular piping system or a branched piping system. Through the various choices and combinations of air compressors, air storage tanks, drying devices, and filters, impurities in the air are thoroughly removed to obtain clean compressed air suitable for pharmaceutical use, with technical specifications that meet the required standards. This air is then delivered to various consumption points via an appropriate piping system, thereby creating a comprehensive compressed air system within the cleanroom facility.