Selection of vacuum pumps

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Selection of Vacuum Pumps I. 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, vacuum coating requires a vacuum level of 1×10-5 mmHg; therefore, the vacuum pump used must have a vacuum level of at least 5×10-6 mmHg. Typically, the vacuum level of the pump should be half to an order of magnitude higher than that 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 diffusion pump operates in the range of 10-3 to 10-7 mmHg. Within such a wide pressure range, the pumping speed of the pump changes with pressure, while its stable operating pressure range is 5×10-4 to 5×10-6 mmHg. Therefore, the operating point of the pump should be selected within this range, and it should not be allowed to operate at 10-8 mmHg for an extended period of time. For example, a titanium sublimation pump can operate at 10-2 mmHg, but it is preferable for its operating pressure to be less than 1×10-5 mmHg. 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. Since vacuum pumps have selective pumping capabilities, sometimes a single pump is not sufficient to meet the pumping requirements; it is necessary to combine several pumps, with each complementing the others, in order 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. If the equipment requires absolutely no oil, 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 vacuum pump can be used, along with various oil contamination prevention measures such as cold traps, baffles, and oil traps, to 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 vapor discharged by the vacuum pump on the environment? If the environment does not allow 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. II. 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: 1. It is necessary to operate it as much as possible within the high-efficiency range, that is, within the range of critical vacuum or critical exhaust pressure. 2. 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. If it is to be used only as a vacuum pump, a single-acting pump is a better choice. This is because single-acting pumps have a simple structure, are easy to manufacture and maintain, and exhibit good cavitation resistance in high-vacuum conditions. If it is to be used only as a compressor with a large capacity, a double-acting pump is more suitable. Because double-acting pumps have a large air displacement, are small in size and light in weight, the radial forces can be automatically balanced, the shaft is less prone to fatigue fracture, and thus the pump has a longer service life. (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 amount of air required by the system. For the selection of the pumping speed and the calculation of the pumping time for vacuum pumps, refer to: Vacuum calculation formulas. Common formulas for vacuum: 1. Boyle’s law: Volume V, pressure P; P•V = constant. For a gas of constant mass, when the temperature remains unchanged, the pressure of the gas is inversely proportional to its volume. 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℃ 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 from 5 to 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 pre-stage pump 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. 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); 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, t is the time required to reach P预, and Pa is the atmospheric pressure in torr.
13. Selection of the pumping speed for the pre-stage pump: For transfer pumps such as diffusion pumps, oil-enhanced pumps, Roots pumps, and turbomolecular pumps, whose exhaust pressure is below one atmosphere, a pre-stage pump is needed to keep the pressure before them below a critical value. The pre-stage pump 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 pre-stage pump in l/s, Pn is the critical pre-stage pressure of the main pump (the maximum exhaust pressure) 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 scale marks by which the oil column rises in the dropper (marks); t – the time required for the oil column to rise by n marks (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 scaled portion of the dropper (mm); n – the number of scale marks on that portion (marks); Υ0 – the specific gravity of the oil (grams/cm3); Υm – the specific gravity of mercury (grams/cm3); Pa – the local atmospheric pressure (torr); ΔVt – the volume corresponding to one scale mark on the dropper (liters/mark). 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 rotary 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), and 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. III. Common terms related to vacuum 1. Definition of vacuum: A vacuum system refers to a state in which the gas density is lower than that of the atmospheric pressure in that area. 2. Vacuum level: It indicates the degree of thinness of the gas in a vacuum state, and is usually expressed as “high vacuum level” or “low vacuum level”. A high vacuum level indicates a \"good\" vacuum degree, while a low vacuum level indicates a \"poor\" vacuum degree. 　　 3. Vacuum unit 　　 It is usually measured in Torr, but in recent years Pa has been used as the unit internationally. 　　 1 torr = 1/760 atmosphere = 1 millimeter of mercury. 4. Conversion between torr and pascals: 1 torr = 133.322 pascals, or 1 Pascal = 7.5×10-3 torr. 5. Mean free path: The average distance that a gas particle in random thermal motion travels between two successive collisions, denoted by the symbol “λ”. 　　 6. Flow rate 　　 The amount of gas that passes through any cross-section per unit of time; denoted by the symbol “Q”, with units of Pa•L/s or Torr•L/s. 　　 7. Conductance 　　 Represents the ability of a vacuum tube to allow gas to pass through it. The unit is liters per second (L/s); in a steady state, the flow conductance of a pipe is equal to the pipe flow rate divided by the pressure difference across the pipe ends. The symbol is denoted as “U”. 　　 U = Q/(P2 – P1) 8. Pressure or force per unit area: The force exerted by gas molecules on the walls of a container, denoted by “P”. 　　 9. Standard atmospheric pressure: A pressure of 101,325 dynes per square centimeter, symbol: (Atm). 　　 10. Ultimate vacuum: After the vacuum vessel has been thoroughly evacuated, it stabilizes at a certain level of vacuum, and this level is known as the ultimate vacuum. Typically, the vacuum vessel must undergo 12 hours of gas purification, followed by another 12 hours of vacuum pumping; during the final hour, measurements are taken every 10 minutes, and the average value of these 10 measurements is taken as the ultimate vacuum value. 　　 11. Pumping rate: At a certain pressure and temperature, the amount of gas removed from the pump’s inlet per unit of time is referred to as the pumping rate, or simply the flow rate. That is, Sp = Q/(P – P0). 12. Thermocouple vacuum gauge: A vacuum gauge that measures the degree of vacuum by taking advantage of the principle that the potential of a thermocouple is related to the temperature of the heating element, and that the temperature of the element in turn is related to the heat conduction of the gas. 　　 13. Ionization vacuum gauge (also known as thermionic ionization gauge) consists of a cylindrical collector, a grid, and a filament located at the center of the grid, with the cylindrical collector being outside the grid. A hot cathode emits electrons that ionize gas molecules; the ions are collected by a collector, and a vacuum gauge measures the gas pressure based on the amount of ion current collected. 　　 14. Compound vacuum gauge 　　 Composed of a thermocouple vacuum gauge and a hot-cathode ionization vacuum gauge, with a measurement range from atmospheric pressure to 10-5 Pa. 　　 15. Cold cathode ionizer: A pair of cathode plates are located at both ends of the anode tube; under the influence of an external magnetic field, a Penning discharge is generated within the anode tube to produce ions. This vacuum gauge measures the gas pressure based on the amount of ion flow collected by the cathode plates. 　　 16. Resistance vacuum gauge: A vacuum gauge that measures the degree of vacuum by using a bridge circuit, based on the principle that the resistance of a heating element is related to temperature, and that the temperature of the element is related to gas conductivity. 　　 17. McLeod vacuum gauge (compression vacuum gauge): An absolute vacuum gauge that compresses the gas to be measured using mercury (or oil) to a very small volume, then compares the difference in liquid column heights between the open and closed tubes, and uses Boyle’s law to directly calculate the gas pressure. 　　 18. B–A type: This is a thermocathode ionization gauge with the cathode and collector reversed. The collector is a thin filament placed at the center of the grid, while the filament is located outside the grid; this reduces the influence of soft X-rays, extends the lower limit of measurement, and enables the detection of ultra-high vacuum conditions. 　　 19. Water ring vacuum pump: A water ring is generated as the impeller rotor of the pump rotates. A mechanical vacuum pump that achieves pumping by the periodic change in volume between the water ring and the blades, resulting from the eccentric rotation of the rotor. 　　 20. Reciprocating vacuum pump 　　 A mechanical vacuum pump that uses the reciprocating motion of a piston to evacuate air. 　　 21. Oil-sealed mechanical vacuum pumps: Mechanical vacuum pumps that use oil to maintain sealing; they can be classified into fixed-vane type, rotary-vane type, slide-valve type, epicyclic type, etc. 　　 22. Roots vacuum pump: A mechanical vacuum pump that features a pair of shoe-shaped rotors rotating at high speeds in synchronization; this pump cannot evacuate air on its own, and it requires a vacuum pump equipped with oil seals or water rings that can discharge air directly into the atmosphere as a preceding stage. 　　 23. Turbomolecular vacuum pump: It has a high-speed rotating impeller; when gas molecules collide with the rapidly rotating turbine blades, they are directed toward the exhaust port and then removed by the pump in the previous stage. 　　 24. Oil diffusion vacuum pump A high-speed steam stream is ejected from the nozzle of the diffusion pump. Under molecular flow conditions, gas molecules continuously diffuse into the vapor stream and are carried by the vapor to the pump outlet, where they are compressed stage by stage before being expelled by the preceding pump. 　　 25. Cryogenic vacuum pump: A vacuum pump that uses low-temperature surfaces below 20K to adsorb gases. 　　 26. Cold trap (water-cooled baffle) – A device placed between the vacuum vessel and the pump, used to absorb gases or capture oil vapors. 　　 27. Air purge valve: A small hole is made in the compression chamber of an oil-sealed mechanical vacuum pump, and a control valve is installed there. When the valve is opened and the amount of air introduced is adjusted, the rotor moves to a certain position, allowing air to enter the compression chamber through this hole. This reduces the compression ratio, so that most of the vapor does not condense and can be expelled from the pump along with the introduced air. The valve that serves this purpose is called an air purge valve. 　　 28. Vacuum freeze-drying 　　 Vacuum freeze-drying is also known as sublimation drying. The principle involves freezing the material so that the water contained within it turns into ice, and then using vacuum to cause the ice to sublimate, thereby achieving drying. 　　 29. Vacuum evaporation 　　 Heating a material in a vacuum environment and coating it onto a substrate is known as vacuum evaporation, or vacuum coating. 　　 30. Vacuum drying 　　 A method of drying items by taking advantage of the low boiling point in a vacuum environment. 　　 31. Common names for vacuum systems 　　 (1) Main pump: In a vacuum system, it is the vacuum pump used to achieve the desired level of vacuum in order to meet specific process requirements; for example, the oil diffusion pump in vacuum coating machines is the main pump. 　　 (2) Front-stage pump: A vacuum pump used to maintain the pressure in front of another vacuum pump below its critical front-stage pressure. Pumps such as vane pumps or slide valve pumps installed in front of a Roots pump serve as pre-pumps.