Vacuum pump issues

Selection of vacuum pumps When selecting a vacuum pump, the following points should be taken into consideration: 1. The operating pressure of the vacuum pump must meet the requirements regarding the ultimate vacuum level and operating pressure of the vacuum equipment. For example, if a certain vacuum drying process requires an operating vacuum level of 10 mmHg, the ultimate vacuum capacity of the vacuum pump selected must be at least 2 mmHg; ideally, it should be able to reach 1 mmHg. Typically, the ultimate vacuum capacity of the pump chosen should be half to an order of magnitude higher than the operating vacuum level of the vacuum equipment. 2. Select the operating point of the vacuum pump correctly. Each type of pump has a certain operating pressure range. For example, the operating pressure range of the 2BV series water ring vacuum pumps is from 760 mmHg to 25 mmHg (absolute pressure). Within such a wide pressure range, the pumping speed of the pump changes with pressure (for detailed variations, refer to the pump’s performance curve); its stable operating pressure range is from 760 mmHg to 60 mmHg. Therefore, it is more appropriate to select the pump’s operating point within this range, rather than allowing it to operate at 25–30 mmHg for an extended period. 3. At its operating pressure, the vacuum pump should be capable of removing all the gas generated during the process in the vacuum equipment. 4. Combine the vacuum pump correctly. Due to the selective pumping capability of vacuum pumps, sometimes a single pump is not sufficient to meet the pumping requirements; it is necessary to use several pumps in combination, so that they can complement each other to fulfill those requirements. For example, titanium sublimation pumps have a high pumping speed for hydrogen but cannot pump helium, while triode-type sputter ion pumps (or diode-type asymmetric cathode sputter ion pumps) have a certain pumping speed for argon; by combining the two, a better vacuum level can be achieved in the vacuum system. Additionally, some vacuum pumps cannot operate at atmospheric pressure and require pre-evacuation ； Some vacuum pumps have an outlet pressure lower than atmospheric pressure, requiring a pre-pump; therefore, these pumps need to be used in combination. 5. Requirements of vacuum equipment regarding oil contamination. When the equipment requires absolutely oil-free operation, various oil-free pumps should be selected, such as water ring pumps, molecular sieve adsorption pumps, sputter ion pumps, cryopumps, etc. If the requirements are not strict, a model equipped with an oil pump, along with some oil contamination prevention measures such as cold traps, baffles, and oil catchers, can also meet the requirements for a clean vacuum environment. 6. Understand the composition of the gas being sampled, whether it contains condensable vapors, particulate dust, or corrosive substances. When selecting a vacuum pump, it is necessary to know the gas composition and choose an appropriate pump for the gas to be evacuated. If the gas contains vapors, particles, and corrosive gases, it is advisable to consider installing auxiliary equipment such as condensers and dust collectors on the inlet pipeline of the pump. 7. What is the impact of the oil vapors emitted by vacuum pumps on the environment? If the environment does not permit pollution, an oil-free vacuum pump can be used, or the oil vapor can be vented outside. 8. Does the vibration generated by the vacuum pump during operation have an impact on the manufacturing process and the surrounding environment? If the process does not permit it, a vibration-free pump should be selected or vibration prevention measures should be taken. 9. The price of the vacuum pump, as well as the costs associated with its operation and maintenance. Selection of Water Ring Vacuum Pumps I. Determination of Pump Type The type of pump is primarily determined by the volume of air required for operation, the level of vacuum needed, or the exhaust pressure. 　　When the pump is in operation, attention should be paid to the following two aspects: • It is advisable to operate it within the high-efficiency range, that is, in the area of critical vacuum or critical exhaust pressure. • Operation near the maximum vacuum level or maximum exhaust pressure should be avoided. Operating in this area not only results in extremely low efficiency, but also leads to unstable operation, with frequent vibrations and noise. For vacuum pumps with a high degree of vacuum that operate in this range, cavitation often occurs as well; obvious signs of this phenomenon are noise and vibration inside the pump. Cavitation can cause damage to components such as the pump body and impeller, resulting in the pump being unable to function. 　　Based on the above principles, when the vacuum level or gas pressure required by the pump is not high, a single-stage pump should be preferred. If the vacuum level or exhaust pressure is high, a single-stage pump often fails to meet the requirements. Alternatively, if it is necessary for the pump to handle a large volume of gas at high vacuum levels, that is, if a relatively flat performance curve at high vacuum levels is required, a two-stage pump can be used. If a vacuum level of above –710 mmHg is required, a water ring-atmospheric pump or a water ring-Rotary vane vacuum unit can be used as the vacuum pumping device. II. Selecting the vacuum pump based on the air volume required by the system After initially determining the type of pump, it is also necessary to choose the specific model of the vacuum pump according to the air volume required by the system. Facing various types of water ring vacuum pumps and compressors, we have compiled their respective features below to assist users in making selection decisions. Code, Main Features, Ultimate Vacuum Level in mmHg, Operating Vacuum Level in mmHg, Suction Volume Range in m3/min, Sealing Type: SK – A domestically designed single-stage water ring vacuum pump with a simple structure and easy maintenance. Currently, low-end water ring vacuum pumps are gradually being replaced by 2BV and 2BE series products. -700 -300~-650 0.15~120 Gaskets, mechanical seals; 2SK – a two-stage water ring vacuum pump designed in China, equivalent to two SK water ring pumps connected in series. It offers higher vacuum levels and a greater pumping speed at high vacuum conditions compared to single-stage water ring pumps. Currently, low-end water ring vacuum pumps are gradually being replaced by 2BV and 2BE series products. -735 -300~-700 1.5~30 Packing, mechanical seal 2BV. Utilizing Siemens’ advanced technology, it features a coaxial design for pumps, resulting in a compact structure, high efficiency, high vacuum level, and stable performance. It will gradually replace the SK and 2SK series of water ring vacuum pumps with a pumping capacity of 0.4~6 m3/min. -735 -300~-700 0.45~8.33 Mechanical seal 2BE1 utilizes Siemens’ advanced technology; it features high efficiency, high vacuum levels, and stable performance, and will gradually replace the SK and 2SK series of water ring vacuum pumps with a pumping speed of 6~120 m3/min. -735 -640 -300~-700 -300~-600 5~120 Packing, mechanical seal 2BE3 utilizes advanced Siemens technology; it features high efficiency, high vacuum levels, and stable performance, and will gradually replace the SK series water ring vacuum pumps with a pumping speed of 80~600 m3/min. -610 0~-600 80~600: Gaskets, mechanical seals. Based on Soviet technology from the 1950s; low efficiency and high energy consumption. These were phased out in the early 1980s and are mainly used by existing customers. -640~-700 -300~-650 1.5~27 Packing SZB: Soviet technology from the 1950s; low efficiency and high energy consumption. It was phased out in the early 1980s and was mainly used for water intake in pumps, having now been replaced by SK series water ring pumps. -600 – 300 – 550, 0.33 – 0.66; packing. Thirdly, the water temperature has a significant impact on the performance of water ring vacuum pumps, and the performance curves of these pumps are all determined at a water temperature of 15°C. Therefore, when selecting a water ring vacuum pump in practice, it is necessary to adjust its pumping speed accordingly. The actual pumping speed of the water ring pump can be calculated as a correction value according to Qt=Q15K, K=(P1-Pt)/(P1-P15), where: Qt-The gas volume m3/min when the water temperature is t℃ Q15-The gas volume m3/min when the water temperature is 15℃ P1-The suction pressure of the water ring pump mmHg Pt-The saturated steam pressure mmHg when the water temperature is t℃ P15-The saturated steam pressure mmHg when the water temperature is 15℃ K-Gas volume correction coefficient The K value of the water ring vacuum pump at various temperatures and suction pressures can refer to the table below Water temperature ℃ Suction pressure mmHg 25 30 35 40 50 60 80 100 120 150 200 250 300 400 500 600 760 0 1.685 1.486 1.376 1.307 1.225 1.177 1.124 1.096 1.078 1.061 1.045 1.035 1.029 1.022 1.017 1.014 1.011 2 1.627 1.445 1.345 1.281 1.206 1.162 1.114 1.088 1.071 1.056 1.041 1.032 1.027 1.02 1.016 1.013 1.01 4 1.561 1.398 1.308 1.252 1.184 1.145 1.102 1.079 1.064 1.05 1.037 1.029 1.024 1.018 1.014 1.012 1.009 6 1.487 1.345 1.268 1.218 1.16 1.126 1.088 1.068 1.055 1.043 1.032 1.025 1.021 1.015 1.012 1.01 1.008 8 1.403 1.286 1.221 1.181 1.132 1.104 1.073 1.056 1.046 1.036 1.026 1.021 1.017 1.013 1.01 1.008 1.007 10 1.308 1.219 1.169 1.138 1.101 1.08 1.056 1.043 1.035 1.027 1.02 1.016 1.013 1.01 1.008 1.006 1.005 12 1.201 1.143 1.111 1.09 1.066 1.052 1.037 1.028 1.023 1.018 1.013 1.01 1.009 1.006 1.005 1.004 1.003 14 1.081 1.057 1.045 1.036 1.027 1.021 1.015 1.011 1.009 1.007 1.005 1.004 1.003 1.003 1.002 1.002 1.001 16 0.946 0.962 0.97 0.976 0.982 0.986 0.99 0.992 0.994 0.995 0.996 0.997 0.998 0.998 0.999 0.999 0.999 18 0.795 0.855 0.888 0.908 0.933 0.947 0.963 0.971 0.977 0.982 0.987 0.989 0.991 0.994 0.995 0.996 0.997 20 0.627 0.735 0.795 0.833 0.878 0.904 0.932 0.948 0.958 0.967 0.976 0.981 0.984 0.988 0.991 0.992 0.994 21 0.536 0.671 0.745 0.792 0.848 0.88 0.916 0.935 0.947 0.959 0.97 0.976 0.98 0.985 0.988 0.99 0.992 22 0.439 0.602 0.692 0.748 0.816 0.855 0.898 0.922 0.936 0.95 0.963 0.971 0.976 0.982 0.986 0.988 0.991 23 0.337 0.53 0.636 0.703 0.783 0.829 0.88 0.907 0.925 0.941 0.957 0.966 0.972 0.979 0.983 0.986 0.989 24 0.23 0.454 0.577 0.655 0.747 0.801 0.86 0.892 0.912 0.932 0.95 0.96 0.967 0.976 0.981 0.984 0.987 25 0.117 0.374 0.515 0.604 0.71 0.772 0.84 0.876 0.899 0.921 0.942 0.955 0.962 0.972 0.978 0.982 0.986 26 0 0.289 0.449 0.55 0.671 0.741 0.818 0.86 0.886 0.911 0.935 0.948 0.957 0.968 0.975 0.979 0.984 27 - 0.2 0.38 0.494 0.63 0.708 0.795 0.842 0.872 0.9 0.926 0.942 0.952 0.964 0.972 0.977 0.982 28 - 0.106 0.308 0.435 0.587 0.674 0.771 0.824 0.857 0.888 0.918 0.935 0.946 0.96 0.968 0.974 0.979 29 - 0.008 0.231 0.372 0.541 0.638 0.746 0.804 0.841 0.876 0.909 0.928 0.941 0.956 0.965 0.971 0.977 30 - - 0.151 0.307 0.493 0.601 0.719 0.784 0.824 0.863 0.899 0.921 0.934 0.951 0.961 0.968 0.975 31 - - 0.066 0.238 0.443 0.561 0.692 0.762 0.807 0.849 0.889 0.913 0.928 0.946 0.957 0.965 0.972 32 - - - 0.166 0.39 0.519 0.662 0.74 0.788 0.835 0.879 0.904 0.921 0.941 0.953 0.961 0.97 33 - - - 0.089 0.334 0.475 0.631 0.716 0.769 0.819 0.868 0.896 0.914 0.936 0.949 0.958 0.967 34 - - - 0.01 0.276 0.429 0.599 0.691 0.749 0.804 0.856 0.886 0.906 0.93 0.945 0.954 0.964 35 - - - - 0.215 0.381 0.565 0.665 0.727 0.787 0.844 0.877 0.898 0.925 0.94 0.95 0.961 36 - - - - 0.15 0.33 0.53 0.637 0.705 0.77 0.831 0.867 0.89 0.918 0.935 0.946 0.958 38 - - - - 0.012 0.221 0.453 0.579 0.657 0.732 0.804 0.845 0.872 0.905 0.925 0.937 0.951 40 - - - - - - 0.102 0.369 0.514 0.605 0.691 0.774 0.821 0.852 0.891 0.913 0.928 0.943 42 - - - - - - - 0.277 0.443 0.547 0.646 0.74 0.795 0.831 0.875 0.9 0.917 0.935 45 - - - - - - 0.122 0.324 0.45 0.57 0.685 0.751 0.795 0.848 0.879 0.9 0.921 50 - - - - - - - - 0.086 0.257 0.419 0.574 0.664 0.723 0.794 0.836 0.864 0.893 55 - - - - - - - - - 0.018 0.233 0.438 0.556 0.634 0.728 0.784 0.821 0.859 60 - - - - - - - - - - 0.004 0.27 0.424 0.524 0.647 0.72 0.767 0.817 70 - - - - - - - - - - - - -0.18 0.068 0.23 0.429 0.546 0.624 0.704 80 - - - - - - - - - - - - - - - - 0.115 0.297 0.417 0.542 100 - - - - - - - - - - - - - - - - - - - - - 0 Commonly used vacuum calculation formulas 1, Boyle's law 2, Guy-Lussac's law 3, Charles' law 4, mean free path 5, pumping speed 6, conduction 7, vacuum pumping time 8, maintenance pump selection 9, diffusion pump pumping speed estimation 10, Roots pump front pumping speed 11, leakage rate 12, rough pump pumping speed selection 13, backing pump pumping speed Select 14. Diffusion pump pumping speed calculation formula 15. Rotary vane vacuum pump geometric pumping speed calculation formula 16. O-shaped rubber groove depth 17. Square rubber groove depth Return to homepage 1. Boyle's law Volume V, pressure P, P·V = constant For a certain mass of gas, when the temperature remains unchanged, the pressure of the gas is inversely proportional to the volume of the gas. That is, P1/P2 = V2/V1. 2. Gay-Lussac’s law: When the pressure P remains constant, for a given mass of gas, its volume V is proportional to the absolute temperature T: V1/V2 = T1/T2 = constant. When the pressure remains constant, for a given mass of gas, an increase (or decrease) in temperature of 1°C results in a corresponding increase (or decrease) in volume of 1/273. 3. Charles’s Law: When the volume V of a gas remains constant, for a given mass of gas, the pressure P is proportional to its absolute temperature T; that is, P1/P2 = T1/T2. At a constant volume, for a given mass of gas, every increase (or decrease) of 1°C in temperature results in an increase (or decrease) in pressure of 1/273 relative to the original value. 4. Mean free path: λ = (5×10^-3)/P (cm)
5. Pumping speed: S = dv/dt (liters/second), or S = Q/P. Where Q is the flow rate (torr·liters/second), P is the pressure (torr), V is the volume (liters), and t is the time (seconds).
6. Conductance: C = Q/(P2 – P1) (liters/second)
7. Vacuum pumping time: For pumping from atmospheric pressure to 1 torr, the formula for the pumping time is t = 8V/S (an empirical formula). Here, V is the volume and S is the pumping rate; typically, the value of t ranges between 5 and 10 minutes. 8. Selection of the maintenance pump: S維 = S前/10
9. Estimation of the pumping speed of a diffusion pump: S = 3D² (D = diameter in cm)
10. Pumping speed of the forepump for a Roots pump: S = (0.1–0.2)S罗 (l/s)
11. Leakage rate: Q漏 = V(P2 – P1)/(t2 – t1); Q漏 represents the system’s leakage rate in mmHg•l/s, where V is the volume of the system in liters, P1 is the pressure in the system when the vacuum pump is stopped in mmHg, P2 is the pressure reached in the vacuum chamber after time t in mmHg, and t is the time it takes for the pressure to rise from P1 to P2 in seconds.
12. Selection of the pumping speed for a roughing pump: S = Q1/P预 (l/s); alternatively, S = 2.3V•lg(Pa/P预)/t. Here, S is the effective pumping speed of the mechanical pump, Q1 is the leakage rate of the vacuum system in torr•liter/second, P预 is the desired pre-vacuum level in torr, V is the volume of the vacuum system in liters, and t is the time required to reach P预. Pa represents the atmospheric pressure in torr.
13. Selection of the pumping speed for the forepump: For transfer pumps such as diffusion pumps, oil-enhanced pumps, Roots pumps, and turbomolecular pumps, whose exhaust pressure is below one atmosphere, a forepump is needed to keep the pressure before them below a critical value. The forepump must be capable of removing the maximum amount of gas produced by the main pump. Based on the principle that the flow rate at each section of the pipeline remains constant, we have: PnSg ≥ PgS or Sg ≥ Pgs/Pn. Here, Sg is the effective pumping speed of the forepump in l/s, Pn is the critical forepump pressure for the main pump (the maximum exhaust pressure) also in l/s, Pg is the highest operating pressure in the vacuum chamber in torr, and S is the effective pumping speed of the main pump at pressure Pg. (l/s) 14. Formula for calculating the pumping speed of a diffusion pump: S = Q/P = (K•n)/(P•t) (liters/second). Where: S – the pumping speed of the pump under test (l/s); n – the number of divisions the oil column rises by in the dropper (divisions); t – the time required for the oil column to rise by n divisions (seconds); P – the pressure measured near the pump outlet (torr); K – the dropper coefficient (torr•liters/second). K = V0•(L/n)•(Υ0/Υm) + Pa△Vt. Where V0 – the original volume of the dropper and vacuum tubing (liters); L – the length of the graduated part of the dropper (mm); n – the number of divisions on the graduated part of the dropper (divisions); Υ0 – the specific gravity of the oil (grams/cm³); Υm – the specific gravity of mercury (grams/cm³); Pa – the local atmospheric pressure (torr); △Vt – the volume corresponding to one division on the dropper’s scale (liters/division). 15. Formula for calculating the geometric pumping speed of a rotary vane vacuum pump: S = πZnLKv(D2-d2)/(24×104) (l/s). Where: Z is the number of vanes; n is the rotational speed (revolutions per minute); L is the length of the pump chamber; D is the diameter of the pump chamber; d is the diameter of the rotor (cm); Kv is the volume utilization factor (usually taken as 95%). 16. For an O-shaped rubber groove, the depth B = 0.7D, where D is the diameter of the rubber; the width C = 1.6B. 17. For a square rubber groove, the depth B = 0.8A, where A is the side length of the square rubber piece; the width C = 1.67B