When the pump’s design is finalized and an example is built, the ultimate pressure and pumping speed are measured. By blanking the pump’s inlet with a pressure gauge, operating the pump for some time, and recording the pressure achieved, the ultimate pressure is measured.
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Vacuum Pumps Ultimate Pressure & Pumping Speed
There are many pressure units, so ultimate pressures will be quoted in Torr, millibar, Pascal, inches of mercury, etc. In formal terms, pumping speed is the ratio of throughput to the partial pressure of a gas at the pump’s inlet port.
This is the volume of gas (at any pressure) that is removed from the system by the pump in a unit of time. Basically, pumping speed is a measure of how fast the pump can remove gas. Depending on the size of the pump, it can be measured in liters per second (L/sec.), cubic feet per minute (cfm), or cubic meters per hour (m3/hr.).
When you compare pumps, you should keep several points in mind. First of all, pumping speed is measured under the same ideal conditions as ultimate pressure or minimum volume, right at the pump inlet, lowest possible outgassing rate, etc.
Secondly, Home Our VacuFIND… Company Vacuum Mart Division Materials Division Process Equipment Division Manufacturing Division Services & Support Pump Speed Chart Ballasts, Bubblers, and Inerts In a gas ballast, air or inert gas is added late in the pump’s operation cycle.
The idea is to increase the pressure in the gas segment caused by the exhaust. The larger gas flow sweeps away any remaining vapor. The benefits of using a gas ballast come at a cost: Solvent vapors ruin ultimate pressure, causing lubrication loss by dilution Water vapor does the same and rusts the internal parts.
Pumps with ballasts open experience a 10-fold increase in ultimate pressure. Gas bubbles don’t enter the pumping mechanism. As the oil casing is filled with oil, inert gas leaks into it. The bubbler’s main purpose is to dilute the nasty gas exiting the pump’s exhaust valve while purging dissolved gases from the oil.
An inert gas bubbler is a very smart idea when pumping spontaneously combustible gases like phosphine or arsine, or highly flammable gases like hydrogen or methane. By diluting gases, it increases mass flow toward the burn box.
By using a gas bubbler, you can achieve benefits without affecting the final pressure of the pump. A chemically inert fluid can be substituted for hydrocarbon oils (HC) in applications where the latter’s properties are not ideal, such as Reactive process gases will attack HC oils Oxygen forms explosive mixtures with hot HC vapors Spontaneously flammable gas ignites hot HC oil Only perfluoropolyether (PFPE) pump fluids are completely inert, completely unreactive, won’t explode, and won’t catch fire.
Fomblin® PFPE fluids are ideal for applications such as those listed above. In general, the pump’s manufacturer specifies its maximum pumping speed over the full operating pressure range. Following is a graph that illustrates the characteristics of root pumps, mechanical pumps, and high vacuum pumps.
Rotary Vane Vacuum Pumps
Vacuum Pumps Level: Coarse Vacuum or Rough Vacuum (depending on design)
Gas Removal Method: Gas Transfer
Pump Design: Oil-sealed (wet)
Rotary vane pumps come in two different types: those for coarse vacuum applications and those for rough vacuum applications.
There are various differences between coarse and rough rotary vane pumps based on their number of vanes, their tolerances, and their ability to trap exhaust oil vapors. A rotary vane pump traps gas in its inlet port between its rotor vanes and its body.
Gas enters the chamber through an inlet port. Eccentrically mounted rotors compress and sweep gas toward discharge ports. Gas is emitted from the exhaust valve when atmospheric pressure exceeds it. The vanes are lubricated, cooled, and sealed with oil.
The maximum pressure of single-stage rough rotary vane pumps is around 10-2 Torr, while the maximum pressure of two-stage rough rotary vane pumps is around 10-3 Torr. Depending on whether the pump is a coarse vane or rough vane, pumping speeds can range from 1–650 cfm.
The rough vane pump is primarily used in vacuum applications as a backing pump for roots or a high-vacuum gas transfer pump like a turbomolecular or diffusion pump. There are many applications for coarse vane pumps, including freeze drying, vacuum filtering, vacuum impregnation, materials handling, meat packing, and “house” vacuum systems.
Types of Vacuum Pumps – An Overview
[Vacuum Pumps]
Oil pumps and dry pumps are two different types of pumps. During the pumping process, gas may be exposed to oil or water. Wet pump designs use oil or water for lubrication and/or sealing and this fluid can contaminate the displaced (pumped) gas.
There is no fluid in the swept volume of dry pumps, so the pumping mechanism is separated from the gas by tight clearances between rotating and static parts, dry polymer (PTFE) seals, or a diaphragm. Although dry pumps may use oil or grease in the pump gears and bearings, it is sealed from the swept gas.
Dry pumps reduce the risk of system contamination and oil disposal compared to wet pumps. By simply changing the pump from a wet to a dry style, you cannot easily convert vacuum systems from wet to dry.
As a result of the wet pump, the chamber and piping can become contaminated and must be thoroughly cleaned or replaced to avoid contaminating the gas in the future.
Following is an introduction to the most commonly used vacuum pump types by function.
Oil Sealed Rotary Vane Pump (Wet, Positive Displacement)
[Vacuum Pumps]
Gas enters the inlet port of the rotary vane pump and is trapped by an eccentrically mounted rotor. As soon as atmospheric pressure is exceeded, the spring-loaded valve allows gas to discharge.
The vanes are sealed and cooled with oil. With a rotary pump, the pressure is determined by the number of stages and their tolerances. In a two-stage system, the pressure can reach 1×10-3 mbar. With a pumping speed of 0.7 to 275 m3/h (0.4 to 162 ft3/min), it pumps water at a rate of 0.7 to 275 m3/h.
Liquid Ring Pump (Wet, Positive Displacement)
[Vacuum Pumps]
In the liquid ring pump, gas is compressed by a vaned impeller located eccentrically within the pump housing. A cylindrical ring is formed against the inside of the pump casing by centrifugal acceleration.
Liquid rings create compression chambers between the impeller vanes by creating seals. The volume enclosed by the vanes and the ring varies cyclically because of the eccentricity between the impeller’s axis of rotation and the pump housing. Through a port in the housing, gas is compressed and discharged.
There are only two moving parts in this pump, making it simple and robust. The machine tolerates process upsets very well, and it can handle a wide range of capacities. The pressure of water can reach 30 mbar at 15°C (59°F), while the pressure of other liquids can be lower. 15 to 17,700 feet/minute (15 to 30,000 m3/h) is possible with this pump.
Diaphram Pump (Dry, Positive Displacement)
[Vacuum Pumps]
A diaphragm is rapidly flexed by a rod riding on a cam rotated by a motor, allowing gas to transfer from one valve to another. Low maintenance and a compact design make it ideal for compact spaces.
A diaphragm and valve have a typical lifespan of 10,000 operating hours. Small compound turbo-molecular pumps are backed by diaphragm pumps in clean, high vacuum applications. For sample preparation, this pump has a small capacity and is commonly found in R&D laboratories.
Using a diaphragm pump to back a compound turbo-molecular pump, you can achieve an ultimate pressure of 5 x 10-8 mbar. Pumping speeds range from 0.6 m3/h (0.35 ft3/min) to 10 m3/h (0.35 ft3/min).
Scroll Pump (Dry, Positive Displacement)
[Vacuum Pumps]
In the liquid ring pump, gas is compressed by a vaned impeller located eccentrically within the pump housing (Fig. 4). A cylindrical ring is formed against the inside of the pump casing by centrifugal acceleration.
Liquid rings create compression chambers between the impeller vanes by creating seals. The volume enclosed by the vanes and the ring varies cyclically because of the eccentricity between the impeller’s axis of rotation and the pump housing. Through a port in the housing, gas is compressed and discharged.
BOOSTER PUMPS
Roots Pump (Dry, Positive Displacement)
[Vacuum Pumps]
Roots pumps are primarily used as vacuum boosters to remove large volumes of gas. By meshing two lobes together and counter-rotating them, the pump continuously transfers gas in one direction.
A primary/backing pump’s performance is boosted by approximately 7:1 and its ultimate pressure is increased by approximately 10:1. It is possible for roots pumps to have two or more lobes. Combined with primary pumps, ultimate pressure can be achieved at a typical value of 104 Torr. A pumping speed of 100,000 m3/h (58,860 ft3/min) is possible.
Claw Pump (Dry, Positive Displacement)
[Vacuum Pumps]
There are two counter-rotating claws on the claw pump which transfer gas axially instead of top-to-bottom, like the Roots pump. There are usually several Roots-claw stages combined on one shaft of a Roots-claw pump, which is a Roots-claw primary pump combination.
High flow rates and a rugged design make it suitable for harsh industrial environments. The ultimate pressure is typically 1 x 10-3 mbar. Pumping speeds range from 59 to 472 feet per minute (100 to 800 m3/h).
Screw Pump (Dry, Positive Displacement)
[Vacuum Pumps]
Screw pumps use two rotating screws, one left-handed and one right-handed, that mesh without touching. Gas is transferred from one end to the other by rotation. A smaller space between the screws reduces the pressure at the entrance end as the gas passes along, and it becomes compressed as it passes along.
There are several advantages to this pump, including high throughput capacity, efficient liquid handling, and the ability to withstand dust and harsh environments. You can achieve an ultimate pressure of approximately 1 x 10-2 Torr. The pump can pump up to 750 m3/h (440 ft3/min).
Turbomolecular Pumps (Dry, Kinetic Transfer)
[Vacuum Pumps]
By using high-speed rotating, angled blades that propel the gas at high speeds, turbomolecular pumps transfer kinetic energy to gas molecules. The blade tip speed is typically 250 – 300 m/s (670 miles per hour). By transferring momentum from the rotating blades to the gas, they provide a greater probability of molecules moving toward the outlet.
The pressure is low and the transfer rate is low. There is a typical ultimate pressure of less than 7.5 x 10-11 Torr. Pumping speeds range from 50 to 5000 liters per second. Bladed pumping stages are often combined with drag stages to enable turbomolecular pumps to exhaust at higher pressures (> 1 Torr).
Vapor Diffusion Pumps (Wet, Kinetic Transfer)
[Vacuum Pumps]
The vapor diffusion pump transfers kinetic energy to molecules of gas by dragging them from the inlet to the outlet using a heated oil stream. These pumps feature an older technology that has been largely replaced by dry turbomolecular pumps.
Despite their low cost, they are highly reliable and have no moving parts. The ultimate pressure is typically less than 7.5 x 10-11 Torr. There is a range of pumping speeds between 10 and 50,000 liters per second.
Cryopump (Dry, Entrapment)
[Vacuum Pumps]
A cryopump captures gases and vapors rather than transferring them through the pump (Fig. 11). Cryogenic technology is used to freeze or trap gas on a very cold surface (cryocondensation or cryosorption ) at 10°K to 20°K (minus 260°C).
Despite their effectiveness, these pumps can only store a limited amount of gas. Regeneration is the process of removing collected gases/vapors from the pump by heating the surface and pumping them away through another vacuum pump.
It is necessary to use a refrigeration compressor in order to cool the surfaces of cryopumps. These pumps can achieve a pressure of 7.5 x 10-10 Torr and pump at a speed of 1200 to 4200 l/s.
Sputter Ion Pumps (Dry, Entrapment)
[Vacuum Pumps]
The sputter ion pump captures gases through the processes of gettering (the chemical combination of active materials with gases) and ionization (the electrical conductivity of gas molecules).
High magnetic fields combined with high voltage (4 to 7kV) create a cloud of electron-positive ions (plasma) which are deposited onto a titanium cathode and sometimes a tantalum cathode as a secondary cathode. As a result of the cathode capturing gases, a getter film is formed. Sputtering describes this phenomenon.
Cathodes need to be replaced periodically. As low as 7.5 x 10-12 Torr can be achieved with these pumps, as they contain no moving parts, are low maintenance, and require only minimal maintenance. A maximum flow rate of 1000 liters per second is achieved by them.
Summarizing…
In order to fully comprehend the advantages and limitations of each vacuum pump technology, a more detailed discussion is necessary.
In vacuum furnaces, vacuum pumps are one of, if not the most critical set of components. The processes we run and the quality we achieve is a function of how these systems perform.
