Knowledge of compressed air systems

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In a narrow sense, a compressed air system consists of air source equipment, air source purification equipment, and related piping; in a broader sense, pneumatic auxiliary components, pneumatic actuating elements, pneumatic control elements, vacuum components, and so on all fall under the category of compressed air systems. Typically, the equipment in an air compressor station constitutes what is meant by a compressed air system in a narrow sense. The following diagram shows a typical flow chart of a compressed air system: The air source device (air compressor) draws in atmospheric air and compresses it from its natural state into compressed air at higher pressure; subsequent purification devices are used to remove moisture, oil, and other contaminants from the compressed air. The air in nature is composed of a mixture of various gases (O₂, N₂, CO₂, etc.), and water vapor is one of them. Air containing a certain amount of water vapor is called moist air, while air without water vapor is called dry air. The air around us is humid, so the working medium of air compressors is naturally also humid air. Although the water vapor content in wet air is relatively low, it has a significant impact on the physical properties of such air. In compressed air purification systems, drying the compressed air is one of the key tasks. Under certain temperature and pressure conditions, the amount of water vapor in moist air (i.e., the water vapor density) has a certain limit. At a certain temperature, when the amount of water vapor contained reaches its maximum possible value, the moist air at that point is called saturated air. Wet air that has not reached its maximum possible water vapor content is called unsaturated air. At the moment when unsaturated air becomes saturated air, liquid water droplets form in the humid air; this phenomenon is known as \"dew formation\". Dew formation is a common phenomenon; for example, in summer when the air humidity is high, water droplets tend to form on the surface of tap water pipes, and on winter mornings, water droplets appear on residents’ window panes. All these are the result of moist air cooling down at constant pressure and forming dew. As mentioned above, the temperature at which unsaturated air is cooled, while keeping the water vapor partial pressure constant (i.e., maintaining the absolute moisture content constant), until it reaches a saturated state is called the dew point. When the temperature drops to the dew point, \"dew formation\" occurs. The dew point of moist air is related not only to temperature but also to the amount of moisture present in it. A higher water content results in a higher dew point, while a lower water content leads to a lower dew point. The dew point temperature has important applications in compressor engineering. For instance, when the outlet temperature of an air compressor is too low, the oil-gas mixture inside the receiver tank may condense due to the low temperature, causing the lubricating oil to become water-contaminated and thus affecting its lubrication performance. Therefore. The outlet temperature of the air compressor must be designed to be no lower than the dew point temperature at the corresponding partial pressure. The atmospheric dew point is the dew point temperature at standard atmospheric pressure; similarly, the pressure dew point refers to the dew point temperature of air under a certain pressure. The relationship between the pressure dew point and the atmospheric dew point is related to the compression ratio; at the same pressure dew point, the higher the compression ratio, the lower the corresponding atmospheric dew point. The compressed air coming out of the air compressor is very dirty. The main pollutants include: water (liquid water droplets, water mist, and water vapor in gaseous form), residual lubricating oil mist (oil droplets in mist form and oil vapor), solid impurities (rusty sludge, metal powder, rubber particles, tar particles, as well as fine particles from filtering materials and sealing materials), harmful chemical impurities, and other types of impurities. Spoiled lubricant can degrade rubber, plastics, and sealing materials, causing valve malfunction and contaminating the products. Moisture and dust can cause metal components and pipes to rust and corrode, leading to jamming or wear of moving parts. This, in turn, can result in malfunction or air leakage in pneumatic components. Moisture and dirt can also clog restrictor orifices or filters. In cold regions, frozen moisture can cause pipes to freeze or even crack. Due to poor air quality, the reliability and service life of pneumatic systems are reduced; the losses resulting from this often exceed the cost of air source treatment devices as well as their maintenance expenses. Therefore, it is absolutely necessary to choose an appropriate air source treatment system. What is the main source of moisture in compressed air? The main source of moisture in compressed air is the water vapor drawn in along with the air by the air compressor. When wet air enters the air compressor, a large amount of water vapor is compressed into liquid water during the compression process, which significantly reduces the relative humidity of the compressed air at the compressor’s outlet. For instance, when the system pressure is 0.7 MPa and the relative humidity of the intake air is 80%, although the compressed air discharged from the air compressor is saturated at that pressure, when converted to atmospheric pressure conditions prior to compression, its relative humidity is only 6–10%. In other words, the moisture content of the compressed air has been reduced to d d. However, as the temperature gradually drops in the gas pipelines and gas-using equipment, a large amount of liquid water continues to condense out of the compressed air. How is oil contamination in compressed air caused? The lubricating oil of air compressors, oil vapors and suspended oil droplets in the ambient air, as well as the oil used for lubricating the pneumatic components in the system, are the main sources of oil contamination in compressed air. For the air compressors currently in use, aside from centrifugal and diaphragm compressors, almost all air compressors (including various oil-free lubricated compressors) will introduce some degree of contaminated oil (oil droplets, oil mist, oil vapor, and carbonized fragments) into the air supply pipes. The high temperature in the compression chamber of the air compressor causes about 5%–6% of the oil to vaporize, crack, and oxidize; these substances settle on the inner walls of the air compressor’s pipes in the form of carbon and a sticky film. The lighter fractions are carried into the system by the compressed air as steam and tiny suspended particles. In short, for systems that do not require lubricants during operation, any oils and lubricants present in the compressed air used can be considered as oil-contaminated materials. For systems that require the addition of lubricants in operation, any rust inhibitors or compressor oil present in the compressed air are considered to be oil-contaminated impurities. How do solid impurities get into compressed air? The main sources of solid impurities in compressed air are as follows: ① The surrounding atmosphere contains various types of impurities with different particle sizes. Even though air filters are installed at the intake of air compressors, impurities in the form of \"aerosols\" with a size of less than 5 μm can still enter the compressor along with the inhaled air. During the compression process, these impurities mix with oil and water and end up in the exhaust pipes. ②During the operation of an air compressor, the mutual friction and collision between various components, the aging and detachment of seals, as well as the carbonization and decomposition of lubricating oil at high temperatures can all introduce solid particles such as metal particles, rubber dust, and carbon-based decomposition products into the air supply pipelines. What is an air source equipment? What are they? The source device is the apparatus that generates compressed air – the air compressor. There are many types of air compressors; the common ones include piston-type, centrifugal-type, screw-type, vane-type, and scroll-type. The compressed air output from air compressors contains numerous contaminants, such as moisture, oil, and dust. It is necessary to use purification equipment to properly remove these contaminants, thereby preventing them from adversely affecting the normal operation of the pneumatic system. Gas source purification equipment is a general term for multiple devices and apparatuses. Gas purification equipment is also commonly referred to as post-treatment equipment in the industry, and typically includes gas storage tanks, dryers, filters, etc. ● Gas storage tank: The function of a gas storage tank is to eliminate pressure fluctuations. It relies on adiabatic expansion and natural cooling to further remove moisture and oil from compressed air, thereby storing a certain amount of gas. On one hand, it alleviates the contradiction where the gas consumption in a short period of time exceeds the output capacity of the air compressor; on the other hand, it maintains gas supply for a brief period in cases of air compressor failure or power outage, thereby ensuring the safety of pneumatic equipment. ● A dryer, or compressed air dryer, is, as the name implies, a device used to remove water from compressed air. The commonly used types are freeze-dryers and adsorption dryers; there are also types such as deliquescence dryers and polymer diaphragm dryers. Freeze dryers are the most commonly used devices for removing water from compressed air, and they are typically employed in situations where only standard requirements are placed on the quality of the air supply. Refrigerated dryers utilize the characteristic that the partial pressure of water vapor in compressed air is determined by the temperature of the compressed air, to achieve cooling and dehumidification. Compressed air refrigerated dryer, commonly referred to in the industry as a “cold dryer”. Its main function is to reduce the moisture content in compressed air, that is, to lower the \"dew point temperature\" of compressed air. In general industrial compressed air systems, it is one of the essential devices for drying and purifying compressed air (also known as post-treatment). 1 Basic principles: Compressed air can have its water vapor removed through methods such as pressurization, cooling, and adsorption. A freeze-dryer utilizes cooling methods. We know that the air compressed by air compressors contains various gases as well as water vapor, so it is wet air. The moisture content of moist air is generally inversely proportional to pressure; that is, the higher the pressure, the lower the moisture content. When the air pressure increases, the water vapor in the air that exceeds the permissible amount will condense into water (in other words, the volume of the compressed air decreases, and it can no longer hold the same amount of water vapor). This means that, compared to the air taken in originally, its moisture content is lower (here it refers to the comparison with the compressed air returning to its uncompressed state). However, the exhaust from the air compressor is still compressed air, with its water vapor content at its maximum possible level; it is thus in a critical state between gas and liquid phases. At this point, the compressed air is said to be in a saturated state; therefore, with just a little more pressure applied, water vapor immediately changes from a gaseous state to a liquid state—that is, water condenses out. Assume that air is a moist sponge that has absorbed water, and its moisture content is the amount of water it has absorbed. If some water is forced out of the sponge, then the moisture content of that piece of sponge decreases relatively. If left to recover on its own, the sponge will naturally be drier than before. This thus achieves the purpose of removing water and drying through pressure. If, while squeezing the sponge and water is continuously flowing out, force is no longer applied once a certain level is reached, the flow of water will stop – this is the saturated state. Continuing to increase the pressure of squeezing still results in water flowing out. Therefore, the air compressor itself has the function of removing water, and the method used for this is pressurization; however, this is not the purpose of the air compressor, but rather an unwanted complication. Why isn’t “pressurization” used as a method for removing water from compressed air? This is mainly due to cost considerations, to increase the pressure by 1 kilogram. It is quite uneconomical to consume about 7% of the energy. On the other hand, “cooling” for water removal is relatively economical; freeze-dryers achieve this goal using a principle similar to that of air-conditioning dehumidification. This is because the density of saturated water vapor has a limit; at pneumatic pressures within the range of 2 MPa, it can be assumed that the density of water vapor in saturated air depends only on temperature, and not on air pressure. The higher the temperature, the greater the density of water vapor in saturated air, and thus there is more water; conversely, the lower the temperature, the less water there is (this can be understood from common sense – it’s dry and cold in winter, while it’s humid and hot in summer). By cooling compressed air to the lowest possible temperature, the density of the water vapor it contains is reduced, resulting in the formation of \"dew.\" These tiny water droplets are collected and removed, thereby achieving the goal of eliminating moisture from the compressed air. Since this process involves dew condensing into water, the temperature must also be above the \"freezing point\"; otherwise, freezing will prevent effective drainage. Typically, the nominal \"pressure dew point temperature\" of freeze-dryers is between 2 and 10°C. For example, the \"pressure dew point\" of 0.7 MPa at 10°C converts to a \"atmospheric pressure dew point\" of -16°C. It can be understood that when used in an environment with a temperature of not lower than -16°C, no liquid water will form when compressed air is discharged into the atmosphere. All methods of removing water from compressed air only achieve a relatively dry state, meeting a certain required level of dryness. It is impossible to remove all moisture completely, and striving for a level of dryness that exceeds what is necessary for use is also very uneconomical. 2 Working Principle: A compressed air refrigeration dryer reduces the moisture content in compressed air by cooling it, causing the water vapor present in the compressed air to condense into liquid droplets. The condensed droplets are discharged outside the machine through an automatic drainage system. Secondary dew formation will not occur as long as the ambient temperature of the piping downstream of the dryer outlet is not lower than the dew point temperature at the evaporator outlet.