Why does an air compressor need staged compression?

Many people know that air compressors have one-stage compression and two-stage compression. Two-stage compression is suitable for higher efficiency and producing high pressure. But sometimes, atmospheric pressure also requires more than two compressions. So why is staged compression necessary, and what are its benefits?

When the required working pressure of the compressed air is high, using single-stage compression is not only uneconomical but sometimes even impossible to achieve. Therefore, two-stage or multi-stage compression must be employed. Multi-stage compression involves passing the air through several compression chambers, undergoing multiple compression and pressure increases from the point of suction, until it reaches the required working pressure.

1. Save power consumption

By adopting two-stage or multi-stage compression, intermediate coolers can be set up between stages. This allows the compressed air to undergo isobaric cooling after being compressed in one stage, thereby reducing its temperature before entering the next compression chamber. As the temperature drops and the density increases, it becomes easier to compress further, significantly saving power consumption compared to single-stage compression. Therefore, under the same pressure, the work area of multi-stage compression is much smaller than that of single-stage compression, and the more stages there are, the more power consumption is saved, approaching isothermal compression.

Note: The oil-injected screw air compressor is already very close to an isothermal process. If it continues to compress and cool after reaching saturation, condensed water will precipitate out. If this condensed water enters the oil-gas separator (oil tank) together with the compressed air, it will emulsify the cooling oil, affecting the lubrication effect. As the amount of condensed water continues to increase, the oil level will also rise continuously. Eventually, the cooling lubricating oil will enter the system along with the compressed air, contaminating the compressed air and causing serious consequences to the system.

Therefore, to prevent the generation of condensate water, the temperature inside the compression chamber must not be too low and must be higher than the condensation temperature. For an air compressor with an exhaust pressure of 11 bar (A) and a condensation temperature of 68°C, condensate water will precipitate when the temperature inside the compression chamber is lower than 68°C. Therefore, the exhaust temperature of an oil-injected screw air compressor cannot be too low, which means that the application of isothermal compression in oil-injected screw compressors is limited due to the issue of condensate water.

2. Improve volume utilization rate

Due to reasons related to manufacturing, installation, and operation, the residual volume within the compressed air chamber is always unavoidable. This residual volume not only directly reduces the effective volume of the air chamber, but also requires the residual high-pressure gas to expand to the suction pressure before the air chamber can start inhaling fresh gas, further reducing its effective volume.

It is not difficult to understand that as the compression ratio increases, the residual gas in the clearance volume expands more violently, resulting in a smaller effective volume of the gas chamber. In extreme cases, the gas in the clearance volume may fully expand within the gas chamber, yet the pressure remains no lower than the suction pressure. At this point, suction and exhaust cannot continue, and the effective volume of the gas chamber becomes zero. If multi-stage compression is employed, the compression ratio of each stage is small, and the residual gas in the clearance volume only needs to expand slightly to reach the suction pressure. This naturally increases the effective volume of the gas chamber, thereby enhancing the utilization rate of the gas chamber volume.

3. Reduce exhaust temperature

The exhaust temperature of a compressor rises with the increase of compression ratio. The higher the compression ratio, the higher the exhaust temperature, but an excessively high exhaust temperature is often not permissible. This is because in oil-lubricated compressors, a high temperature of lubricating oil can reduce its viscosity, exacerbating wear and tear. When the temperature rises too high, it is easy to form carbon deposits in the gas chamber and on the valves, which can further aggravate wear and tear, and sometimes even lead to explosions. Due to various reasons, the exhaust temperature is greatly limited, so multi-stage compression must be adopted to reduce the exhaust temperature.

Note: Graded compression can reduce the exhaust temperature of a screw air compressor and also make the thermodynamic process of the air compressor approach isothermal compression as much as possible, achieving energy-saving effects, but this is not absolute. Especially for oil-injected screw air compressors with an exhaust pressure below 13 bar, since low-temperature cooling oil is injected during the compression process, the compression process already approaches an isothermal process, making it unnecessary to perform secondary compression. If graded compression is further implemented, it will complicate the structure, increase manufacturing costs, and also increase gas flow resistance and additional power consumption, which is somewhat counterproductive. Furthermore, if the temperature is too low and condensed water forms during the compression process, it will deteriorate the system state, leading to serious consequences.

JOY MACHINERY always adheres to the corporate vision of "building a leading brand of oil-free air compressors in the world", and hopes to provide high-quality, clean and safe air for food, beverage and other production enterprises on the road of oil-free air compressor research and development, production and sales. Source protection, and also contribute a little bit to people's health and safety.

JOY MACHINERY oil-free air compressor series is widely used in the food and beverage processing industry due to its clean air source and continuous and stable gas transmission. If you have any needs, please contact us! JOY Machinery (Shanghai) Co., Ltd. has a professional design team with rich experience and welcomes customization. Diversified products can meet most applications and meet your one-stop shopping needs. The air compressor series includes power frequency screw air compressors, variable frequency screw air compressors, permanent magnet variable frequency screw air compressors, two-stage compression screw air compressors, integrated screw air compressors, laser cutting air compressors, medium and high pressure air compressors. Air compressors, oil-free water-lubricated screw air compressors (special oil-free screw air compressors for the medical industry), diesel-moving screw air compressors, electric-moving screw air compressors and other products.

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