Molecular sieve adsorption pump Molecular sieve adsorption pump At low temperatures by the physical adsorption of molecular sieve to achieve vacuum pump. Molecular sieve adsorption pump simple structure, no oil pollution, no vibration and noise, is often used to obtain vacuum-free vacuum pump, often used as oil-free vacuum pump unit or ultra-high vacuum pump maintenance. All the atoms or molecules on the surface of the solid material are different from the atoms or molecules inside the solid. The internal attraction of atoms or molecules in a solid is symmetrically and uniformly distributed around the atoms or molecules, that is, in equilibrium with a saturated force field, while the forces exerted by atoms or molecules on the surface are asymmetric, that is, Solid surface has excess surface free energy, that is, the surface of the adsorption force exists, which is the solid surface of the gas or liquid adsorption. The surface of all solid substances have adsorption. But in fact only microporous material or very fine material due to the surface area is large, only a significant adsorption. Molecular sieve is this material. Molecular sieve (artificial zeolite) is a microporous structure of alkali metal aluminosilicate, the body has many cavity-shaped unit cells, during which microporous interlinked. At liquid nitrogen temperatures, gas or liquid molecules adsorb to the interior surface of the cell cavity through micropores. Molecular sieve has a large specific surface area (see Table molecular sieve properties table), so it can absorb a large amount of gas and liquid. In addition to molecular sieve adsorption, there are screening, ion exchange and catalytic role. Molecular sieve is the physical adsorption of gases, the process is reversible. Gas adsorbed at a low temperature gradually desorbs when the temperature rises. Molecular sieve can adsorb a large amount of water vapor when it is exposed to the air at room temperature. It can adsorb water vapor of 0.1-0.2 g per gram of 5A molecular sieve, equivalent to 1.3 × 10 -2.7 × 10-Pa gas per liter. Molecular sieve adsorption of large amounts of water vapor inhalation capacity will be greatly reduced. Molecular sieves and water molecules particularly strong affinity, and only after proper baking, adsorption of water vapor can be fully desorbed. Therefore, the molecular sieve adsorption pump should be baked after being exposed to the atmosphere or for a period of time. Usually, this treatment is to sieve the molecular sieve in an enclosed container and degassed for 1 to 2 hours at a temperature of 300 to 350 ° C. However, the heating temperature should not generally exceed 600 ℃, otherwise it will damage the molecular sieve. Molecular sieve sub-spherical shape, strip, columnar, flaky, granular and powder, the liquid nitrogen temperature, 5A can absorb nitrogen, oxygen, argon, carbon dioxide and water vapor; CnHn compounds can be adsorbed. Molecular sieve adsorption of gas has a strong selectivity. It can only adsorb gas and liquid air molecules whose molecular diameter is smaller than the diameter of the micropore. The diameters of the molecules are 2 to 4 angstroms (1 angstrom = 1 × 10 m) and the diameter of the oil molecules is 7 to 8 angstrom. Molecular sieve adsorption. Molecular sieve adsorption of different gases have different strengths, strong adsorption capacity of nitrogen, oxygen and argon. These three gases account for 99.9% of the air volume and the partial pressure is 10 Pa. However, zeolite adsorption capacity of neon, helium and hydrogen gas is very weak. The influence of temperature on the adsorption capacity of molecular sieve is very large. Under the temperature of liquid nitrogen (-196 ℃), 5A molecular sieve per gram can pump 50-100 cm from atmospheric pressure to 1 Pa. At the same time, Adsorbed nitrogen adsorption capacity of only 10.9 cubic centimeters. Therefore, bake degassing and cryogenic cooling are the necessary conditions to ensure the performance of molecular sieve adsorption pump. Structure molecular sieve adsorption pump is divided into two kinds of internal cooling (Figure 1 internal cooling molecular sieve adsorption pump) and external cooling (Figure 2 external cooling molecular sieve adsorption pump). The former has the advantages of small liquid nitrogen and electric energy consumption, large pumping rate and low ultimate pressure, but the processing is complicated. The latter is simple in structure and easy to process. However, the consumption of liquid nitrogen and electric energy is large, and a special liquid nitrogen- . If the molecular sieve adsorption pump is used as a fore-stage pump, a safety plug should be installed. Otherwise, the desorption of the adsorbed gas during the temperature rise may cause an explosion. If used as an ultra-high vacuum pump, due to the small amount of gas adsorption can not set safety plugs to reduce the deflation and leakage sources. Zeolite thermal conductivity is poor, the pump structural design needs to consider the heat conduction problems, so that when baking or cooling can quickly reach the predetermined requirements. Molecular Sieve adsorption capacity of the pump to determine one of the important factors is the amount of molecular sieve loading, often according to the molecular sieve load curve to decide. Figure 3 5A molecular sieve nitrogen, neon and helium load - equilibrium pressure curve for the 5A zeolite liquid nitrogen nitrogen, helium and helium three kinds of gas load - equilibrium pressure curve. The equilibrium pressure increased sharply when the ratio of the capacity of molecular sieve to be drawn was greater than 0.1, while the equilibrium pressure decreased little while the loading rate was less than 0.05, but the molecular sieve loading increased a lot. Therefore, taking the loading rate of 0.05 ~ 0.1 to determine the molecular sieve loading, you can take into account the performance of the pump and economic effects. Operating method Molecular sieve adsorption pump from the atmospheric pressure to start pumping, the ultimate pressure is mainly by the atmosphere of neon, helium partial pressure decision. There are two different methods of operation when using two molecular sieve adsorption pumps. First, only the first pump is charged with liquid nitrogen to cool it, and the vacuum system (excluding the second pump) is closed when it reaches the ultimate pressure (about 1 Pa). At this point the pressure of the vacuum system is in an equilibrium state, and the gas to be drawn is in a molecular flow state (molecular density is small). Along with the gas flowing to the neon in the pump, helium returns to the system again, causing the partial pressure of neon and helium in the system almost Not changed. Give the second pump plus liquid nitrogen cooling suction, vacuum system, the ultimate pressure can only reach 10 Pa. ② add liquid nitrogen to pump the first pump to pump the system (including the second pump) to 67 ~ 670 Pa off. From the beginning of pumping to the process of shutting down the pump, the molecular sieve continuously adsorbs the nitrogen, oxygen and argon in the drawn gas without adsorbing neon and helium, and the partial pressure of neon and helium in the pump rises continuously. Since the pressure in the vacuum system is always in an unbalanced state (the pressure in the pump is lower than the pressure in the container and the pipe), the drawn gas continuously flows into the pump, and the gas is in a viscous flow state (high molecular density) , Helium partial pressure, they are also difficult to return to the system was shut down in the pump, thereby reducing the system of neon, helium partial pressure. Then give the second pump plus liquid nitrogen cooling suction, the vacuum system can achieve lower pressure (about 10 Pa). The ultimate pressures achieved with different methods of operation can vary by an order of magnitude. In addition, it is also possible to repeatedly "flush" the vacuum system with dry pure nitrogen to "take away" neon and helium to lower the partial pressure of neon and helium in the system so that the system's ultimate pressure reaches 10 Pa. But this method is more cumbersome and consumes nitrogen. Can also be used oil-free mechanical pump or mechanical suction pump suction well vacuum, so that the molecular sieve adsorption pump to achieve the ultimate pressure is lower, but pay attention to pump oil pollution problems.
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While the focus of residential lighting is often on aesthetics, commercial lighting is task orientated. Commercial lighting systems are designed based on what the application is. For example, in an office-type setting, you may see task lighting, which illuminates specific areas where employees need concentrated light to be able to perform their jobs.
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