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Knowledge of compressed air system

Compressed air system, in a narrow sense, is composed of air source equipment, air source purification equipment and related pipelines. In a broad sense, pneumatic auxiliary components, pneumatic actuators, pneumatic control components, vacuum components, etc. all belong to the category of compressed air system. Usually, the equipment of an air compressor station is a compressed air system in a narrow sense. The following figure shows a typical compressed air system flow chart:

The air source equipment (air compressor) sucks in the atmosphere, compresses the air in the natural state into compressed air with higher pressure, and removes moisture, oil and other impurities in the compressed air through purification equipment.

The air in nature is composed of a mixture of various gases (O₂, N₂, CO₂…etc.), and water vapor is one of them. Air that contains a certain amount of water vapor is called humid air, and air that does not contain water vapor is called dry air. The air around us is moist air, so the working medium of the air compressor is naturally moist air.
Although the water vapor content of humid air is relatively small, its content has a great influence on the physical properties of humid air. In the compressed air purification system, the drying of compressed air is one of the main contents.

Under certain temperature and pressure conditions, the content of water vapor in humid air (that is, water vapor density) is limited. At a certain temperature, when the amount of water vapor contained reaches the maximum possible content, the humid air at this time is called saturated air. The moist air without the maximum possible content of water vapor is called unsaturated air.


At the moment when unsaturated air becomes saturated air, liquid water droplets will condense in the humid air, which is called “condensation”. Condensation is common. For example, the air humidity is high in summer, and it is easy to form water droplets on the surface of the water pipe. In the winter morning, water droplets will appear on the glass windows of the residents. These are all formed by the cooling of humid air under constant pressure. Lu results.

As mentioned above, the temperature at which the unsaturated air reaches saturation is called the dew point when the partial pressure of water vapor is kept constant (that is, the absolute water content is kept constant). When the temperature drops to the dew point temperature, there will be “condensation”.

The dew point of humid air is not only related to temperature, but also related to the amount of moisture in the humid air. The dew point is high with high water content, and the dew point is low with low water content.

The dew point temperature has an important use in compressor engineering. For example, when the outlet temperature of the air compressor is too low, the oil-gas mixture will condense due to the low temperature in the oil-gas barrel, which will make the lubricating oil contain water and affect the lubrication effect. therefore. The outlet temperature of the air compressor must be designed not to be lower than the dew point temperature under the corresponding partial pressure.

Atmospheric dew point is the dew point temperature under atmospheric pressure. Similarly, pressure dew point refers to the dew point temperature of pressure air.

The corresponding relationship between the pressure dew point and the normal pressure dew point is related to the compression ratio. Under the same pressure dew point, the larger the compression ratio, the lower the corresponding normal pressure dew point.

The compressed air coming out of the air compressor is dirty. The main pollutants are: water (liquid water droplets, water mist and gaseous water vapor), residual lubricating oil mist (mist oil droplets and oil vapor), solid impurities (rust mud, metal powder, rubber fines, tar particles and filter materials, fine powder of sealing materials, etc.), harmful chemical impurities and other impurities.

Deteriorated lubricating oil will deteriorate rubber, plastic, and sealing materials, causing malfunction of valves and polluting products. Moisture and dust will cause metal parts and pipes to rust and corrode, causing moving parts to be stuck or worn out, causing pneumatic components to malfunction or leak air. Moisture and dust will also block throttling holes or filter screens. After the ice causes the pipeline to freeze or crack.

Due to poor air quality, the reliability and service life of the pneumatic system are greatly reduced, and the resulting losses often greatly exceed the cost and maintenance costs of the air source treatment device, so it is absolutely necessary to correctly select the air source treatment system.
What are the main sources of moisture in compressed air?

The main source of moisture in compressed air is the water vapor sucked by the air compressor along with the air. After the humid air enters the air compressor, a large amount of water vapor is squeezed into liquid water during the compression process, which will greatly reduce the relative humidity of the compressed air at the outlet of the air compressor.

For example, when the system pressure is 0.7MPa and the relative humidity of the inhaled air is 80%, although the compressed air output from the air compressor is saturated under pressure, if converted to the atmospheric pressure state before compression, its relative humidity is only 6~ 10%. That is to say, the moisture content of the compressed air has been greatly reduced. However, as the temperature gradually drops in the gas pipeline and gas equipment, a large amount of liquid water will continue to condense in the compressed air.
How is oil contamination in compressed air caused?

The lubricating oil of the air compressor, the oil vapor and suspended oil droplets in the ambient air and the lubricating oil of the pneumatic components in the system are the main sources of oil pollution in the compressed air.

Except for centrifugal and diaphragm air compressors, almost all air compressors currently in use (including various oil-free lubricated air compressors) will have more or less dirty oil (oil droplets, oil mist, oil vapor and carbon fission) into the gas pipeline.

The high temperature of the compression chamber of the air compressor will cause about 5%~6% of the oil to vaporize, crack and oxidize, and deposit in the inner wall of the air compressor pipe in the form of carbon and varnish film, and the light fraction will be suspended in the form of steam and micro The form of matter is brought into the system by compressed air.

In short, for systems that do not require lubricating materials during operation, all oils and lubricating materials mixed in the compressed air used can be regarded as oil-contaminated materials. For systems that need to add lubricating materials during work, all anti-rust paint and compressor oil contained in the compressed air are considered as oil pollution impurities.

How do solid impurities enter compressed air?

The main sources of solid impurities in compressed air are:

①The surrounding atmosphere is mixed with various impurities of different particle sizes. Even if the air compressor suction port is equipped with an air filter, usually “aerosol” impurities below 5 μm can still enter the air compressor with the inhaled air , mixed with oil and water into the exhaust pipe during the compression process.

②When the air compressor is working, the friction and collision between the various parts, the aging and falling off of the seals, and the carbonization and fission of the lubricating oil at high temperature will cause solid particles such as metal particles, rubber dust and carbonaceous fission to be brought into the gas pipeline.

Post time: Apr-18-2023