What is the principle of the post-treatment equipment for removing water from air compressors?

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Compressed air is a process gas source with multiple uses in China at present, and it is also the second-largest energy source after electricity. Compressed air dehumidification devices are used to further treat air that has been processed by air compressors and is therefore high in temperature and pressure, in order to remove the gaseous water contained within it and achieve better performance in its use. This article provides a brief principle analysis of the dryers that are currently dominant in the market. As is well known, the atmosphere contains a certain amount of gaseous water, and due to factors such as season, geographical location, and climatic conditions, the amount of gaseous water is constantly changing. Water vapor in compressed air can be removed by methods such as pressurization, cooling, and adsorption, while liquid water can be removed by methods such as heating, filtration, and mechanical separation. Compressed air processed by air compressors is usually saturated, with a relative humidity of 100%, and it contains various impurities; such compressed air cannot be used directly in industrial applications and requires further drying using post-treatment equipment. Three methods have been commonly used in industry to dry compressed air: ① Utilizing the property that the partial pressure of water molecules in compressed air is determined by temperature to carry out cooling, dehydration, and adsorption ； ②Dehydration and drying by utilizing the property of adsorbents to absorb water molecules from compressed air ； ③Dehydration drying is carried out by utilizing the deliquescence properties of certain chemicals. Of the three types of compressed air equipment mentioned above, the third type has been phased out by the market, while the first and second types are being widely used. Based on the relevant properties in physics, we can understand that the amount of water vapor in compressed air is determined by temperature. At a constant pressure, the lower the temperature, the less water vapor there is in the compressed air. 1 Refrigerated air dryer: It lowers the temperature of compressed air so that excess water vapor condenses into a liquid and is removed. The refrigerated compressed air dryer (hereinafter referred to as refrigerated dryer) operates based on this principle. Therefore, refrigerated dryers are equipped with a refrigeration system, which can be divided into water-cooled and air-cooled types depending on the principle involved. This article focuses on the water-cooled system. A refrigeration system consists of four main components: a compressor, a condenser, an expansion valve, and an evaporator. The compressor transforms the cold medium gas at low temperature and low pressure into a gas at high temperature and high pressure. This gas then flows through pipes to the condenser, where the cooling water circulating within the condenser converts the cold medium from a gas at high temperature and high pressure into a liquid at low temperature and high pressure ； The high-pressure liquid, after being cooled by the condenser, passes through an expansion valve, which converts it into a low-pressure liquid ； The refrigerant liquid at low temperature and low pressure flows through the evaporator, where it exchanges heat with the air inside the evaporator, thereby lowering the temperature of the compressed air and removing excess water vapor. At this point, the refrigerant changes from a low-temperature, low-pressure liquid to a gas, flows through the compressor once again, and enters the next cycle. As shown in Figure 1, this is the working principle of a cold dryer: moist air first enters the pre-cooler, where it undergoes its first heat exchange with the cold air that has been cooled by the evaporator. From there, the moist air moves to the evaporator, where it undergoes a second heat exchange with the refrigerant. After two heat exchanges, the excess water is removed in the gas-liquid separator; at this point, the compressed air discharged through the pre-cooler can be used directly. It is worth noting that a pre-cooler is not an essential component of a refrigerated dryer, but nowadays all refrigerated dryers are equipped with pre-coilers. This is because the precooler primarily recovers the waste heat contained in the compressed air that has been cooled by the evaporator, and uses this heat to cool the high-temperature gas containing a large amount of water vapor, **thereby reducing the load on the refrigeration system. At the same time, the cold air is warmed up in the pre-cooler, preventing condensation from forming on the outer wall of the exhaust pipe due to low temperatures. https://pic.ouryao.com/data/attachment/forum/202112/11/130323epr84415q4111xpp.png.thumb.jpg 2 Adsorption-type compressed air dryers. Adsorption-type compressed air dryers utilize the capillary action of the adsorbent itself to absorb water vapor present in compressed air. Drying machines generally have a twin-tower structure: one tower absorbs moisture under higher pressure, while the other tower carries out dehydration and regeneration under lower pressure, operating in a continuous cycle. Drying machines are classified into heatless regeneration and heat-regenerated types based on the regeneration method, and this article focuses on heatless regeneration. Figure 2 shows the working principle diagram of the cold dryer. Taking adsorption in tower A and regeneration in tower B as an example, wet air enters tower A through valve 15 for adsorption and then exits through check valve 1. The finished product is taken at the outlet and enters Tower B through control valve 5, flow restrictor 7, and check valve 4 for dehydration and regeneration. The airflow after regeneration passes through valve 14 to the silencer and is ultimately released into the atmosphere. Since the airflow from the dehydration and regeneration process is ultimately discharged into the atmosphere, the desiccant dryer has air consumption. https://pic.ouryao.com/data/attachment/forum/202112/11/130334v1n9hooztx9x9xoz.png.thumb.jpg 3 Blower-heat regenerated adsorption-type compressed air dryer. The blower-heat regenerated adsorption-type compressed air dryer is also a type of adsorption-type compressed air dryer; it utilizes the affinity between adsorbents and water molecules to remove excess moisture. However, there are some slight differences compared to other types. The forced-air heat absorption dryer uses a blower to draw in ambient air, which is then heated to regenerate the adsorbent in the drying tower, **reducing the consumption of product gas. As shown in Figure 3, the working principle of the forced-air heat absorption dryer can be roughly divided into three steps: the adsorption regeneration stage, the cold blowing stage, and the pressure equalization stage. Since the pressure equalization process merely ensures that the pressures in the two adsorption towers are equalized after the cold blowing is completed, and it takes place in a very short period of time, no detailed explanation is provided in this paper. https://pic.ouryao.com/data/attachment/forum/202112/11/130513zh8s3aghaohts4tg.png.thumb.jpg (1) Adsorption and regeneration stage. Wet air flows from the inlet, passes through valve No. 1 to tower A for adsorption, and then exits through valve No. 5. At the same time, the blower draws in ambient air, which flows through valve 10 to the electric heater; the air, now heated to a high temperature, passes through valve 8 to regenerate Tower B. The moisture contained in the adsorbent is then discharged via K12, while the regenerated air flow is released into the atmosphere through valve 4. During the adsorption and regeneration phases, the regenerant gas is drawn from the environment by a blower and finally discharged into the atmosphere; thus, there is no gas consumption, resulting in maximum energy savings. (2) Cold blowing stage. The adsorption process in Tower A is the same as that in the adsorption and regeneration stages; thus, no further elaboration is necessary here. The difference is that some of the finished gas is taken at the outlet and flows through valve No. 9 to the electric heater (which is closed at this time and functions merely as a pipe); thereafter, it passes through valve No. 8 to perform cold blowing on Tower B, and after the cold blowing process, the gas is discharged into the atmosphere via valve No. 4. Although this process involves gas consumption, the amount consumed is much less than that of adsorption-type compressed air dryers. It is suitable for applications where low regeneration gas consumption is required. 4. Heat-regeneration adsorption type compressed air dryer: The heat-regeneration adsorption type compressed air dryer also falls under the category of adsorption dryers, but it differs from them in terms of structure. Its main difference from adsorption dryers is that it first uses the high-temperature gas discharged by the air compressor to heat and regenerate the adsorbent in one of the towers, thereby removing excess moisture from the adsorbent, followed by cold blowing and pressure equalization. Based on whether there is consumption of product gas, they are divided into compression heat regeneration adsorption dryers with gas consumption and those without gas consumption. The functional principle of the compression heat desiccant dryer is also roughly divided into three steps: adsorption regeneration stage, cold blowing stage, and pressure equalization stage. (1) Heating and regeneration stage. As shown in Figure 4, the high-temperature gas discharged from the air compressor flows through valve 9 to Tower A, where it is first heated for regeneration in order to remove excess moisture ； It then enters the rear cooler and gas-liquid separator through valve 3 for cooling and water removal, and flows toward Tower B via valve 10. Tower B carries out adsorption treatment on the compressed air, and the air is discharged through valve 12 to produce the final product gas. (2) Cold blowing stage. The compressed air passes through Valve 7 and directly enters the aftercooler and gas-liquid separator, where it is cooled down to remove excess moisture. It then goes through Valve 10 to enter Tower B for adsorption treatment, and finally is discharged via Valve 12. Unlike the heat regeneration phase, during the cold blow phase, a portion of the finished gas stream must be taken at the outlet via Valve 17, then passed through Valve 13 to cold-blow Tower A. The cold blow gas is discharged through Valve 14. As shown in Figure 5, which is the schematic diagram of the zero-gas-consumption compression heat adsorption type compressed air dryer, the most direct difference compared to those that consume gas is that this dryer has two coolers and a separator. Taking the regeneration of Tower A by the adsorption of Tower B as an example: (1) Heating regeneration stage. The high-temperature gas discharged from the air compressor enters Tower A through Valve 1 for heating and regeneration; it then passes through Valves 2 and 3 to reach the cooler and separator, where excess moisture is removed. After that, it enters Tower B through Valve 4 for adsorption treatment, and is finally discharged through Valve 5. (2) Cold blowing stage. The hot air first enters the cooler and separator for cooling, then is used to cool Tower A through Valve 6 and Valve 2. It passes through Valve 7 to enter a second cooler and separator where it is cooled again, subsequently entering Tower B through Valve 4 for adsorption treatment, and is finally discharged through Valve 5.