Oil-free rotary vane vacuum pumps. Screw vacuum pump DRYVAC from Leybold GmbH (Germany) The principle of operation of the pump VVN

Plunger (piston) vacuum pumps. Bypass devices. Harmful space

A plunger vacuum pump is a type of mechanical vacuum pump that is capable of compressing gases to atmospheric pressure. Such a device has a device similar to a double-acting piston compressor. The main difference is that the plunger vacuum pump has a higher compression ratio.

Left - initial stage, 2 positions in the center - intermediate stage, right - final stage

The plunger includes a cylindrical part that encloses the eccentric and a hollow rectangular part that moves freely in the hinge groove. When the flat part of the plunger pivots, the joint also pivots freely in the seat of the pump housing. This plunger is equipped with a channel through which gas enters the pumping chamber from the evacuated cavity. The counter flow of gas into the inlet of the pump is limited by the preliminary closing of the inlet when the spool moves. There is also the possibility of reducing the harmful space. The tightness of the contact of the rotor with the cylinder in pumps is ensured by the fact that a thick layer of oil forms in the wedge between the rotor and the cylinder.

Mechanical vacuum pumps pump out the volume starting from the atmospheric pressure level. Due to the fact that the pumped gas is discharged into the atmosphere, relative to mechanical vacuum pumps, such characteristics as the highest working pressure, as well as the highest start and release pressure, are not used. The key features of oil sealed mechanical vacuum pumps are:

ultimate residual pressure;
speed of action.

Mechanical vacuum pumps

A mechanical vacuum pump is a gas-removing unit that is used to obtain / maintain a pressure below atmospheric pressure in tanks, from where the working fluid is pumped out at certain intervals at a certain composition and value of the gas flow.

The operation of such a pumping unit is based on the fact that the gas moves as a result of the mechanical movement of the working parts of the pump, thereby performing a pumping action. The volume, which is filled with gas, is cut off from the entrance and moves to the exit. The gas is systematically moved to the outlet of the pumping unit as a result of a movement impulse, which is transmitted to the gas molecules.

In accordance with the design features and the mode of operation of this type of pump, seven types of pumps are distinguished (screw / diaphragm / piston / vane-rotary / spool / root / volute). In accordance with the type of working fluid, mechanical pumps can be molecular (they function due to the flow of molecules of the substance) and volumetric (they function due to the laminar flow of the substance). Mechanical vacuum pumps are differentiated according to the vacuum concentration level (high, low, medium). In addition, this type of pump is subdivided into those that can function without lubricant and with lubricant.

This type of pumping unit is used in a wide variety of industries: chemistry, metallurgy, electronics, food processing, medicine, astronautics. Mechanical vacuum pumps are also used in a wide variety of industrial installations, as well as in technical processes (for example, metal remelting, thin film deposition, simulation of space conditions, etc.).

Due to the growing demand for pumping units, mechanical vacuum pumps are constantly being improved and developed, pumping units with improved performance are being developed.

The operating speed of such pumps does not depend on the type of pumped gas. The residual pressure depends on the design of the pumping unit and the properties of the working fluid. The working fluid, as a rule, is oil, which has a list of required characteristics:

low acidity;
viscosity;
good lubricating properties;
low pressure of saturated vapors in the operating temperature range of the pump;
low absorption of gases and vapors;
stability of viscosity with temperature changes;
high strength of a thin (0.05-0.10 mm) oil film, capable of withstanding a pressure drop in the gap equal to atmospheric pressure.

The stability of the characteristics of mechanical vacuum pumps depends on the size of the gaps between the surfaces, the amount of these gaps, and the quality of the oil that lubricates the rubbing surfaces.

The plunger vacuum pump can be equipped with a bypass device to increase the efficiency. The bypass devices may differ in design. Their function is to equalize the pressure on both sides of the piston at the end of the piston stroke.

In the absence of these channels, the remainder of the compressed gas from the harmful space expands as the piston moves from left to right. In this case, the rest of the compressed gas has a pressure level p 2... Curve ea 1 up to suction pressure p 1 and p 1 and λ 0 = V 1 / V... In a vacuum pump at the extreme left position of the piston, the remainder of the gas moves into the right cavity of the cylinder, where the pressure is p 1... Harmful space pressure drops from p 2 before p in, and the rest of the gas expands along the curve fa... Suction starts at the very beginning of the piston stroke ( λ 0 = (V "1 / V)> λ 0). A similar process occurs with the piston stroke in the opposite direction (from right to left). As a result, the volumetric efficiency increases from 0.8 to 0.9. λ 0 .

Presence of harmful space is the reason why the piston vacuum pump is not able to create an absolute vacuum and has a theoretical limit of this value, which corresponds to a certain residual pressure p pr... The magnitude p pr in the absence of a bypass, more than in the presence of one.

If the vacuum pump operates continuously, then the volume of sucked gas is equal to the volume of process gases emitted into the atmosphere and the volumes that are sucked in from the outside through leaky areas do not change over time. The shaft power rating of the vacuum pump is also unchanged. It should be noted that this parameter is several times higher for cars equipped with a bypass, because the work of expanding the bypassed amount of compressed gas is lost.

In various spheres of human activity, the creation of a vacuum is required. This term characterizes the state of the gas phase, the pressure of which is below atmospheric. It is measured in millimeters of mercury or pascals. The rarefaction of gases occurs when the substance is forcibly removed from devices with a limited volume. A technical device designed for this purpose is called a vacuum pump. It can be used on its own or as part of more complex systems.

Vacuum is widely used in various technical devices. It allows you to reduce the boiling point for water or chemical liquids, to remove gases from materials that require increased uniformity of composition, to create sterile conditions for processing and storage. With a small size and economical energy consumption, modern vacuum pumps allow you to quickly reach a deep degree of vacuum. They are used in a wide variety of processes and areas of activity:

in the oil refining and chemical industries to maintain the necessary conditions for the reactions and separation of the resulting mixtures;
when degassing metals and other materials to create parts with a homogeneous structure and the absence of pores;
in the pharmaceutical and textile industry for quick drying of products without increasing the temperature;
in the food industry for the packaging of milk, juices, meat and fish products;
in the process of evacuating refrigeration and other equipment with increased requirements for the absence of moisture;
for the normal functioning of automatic conveyor lines using vacuum suction cups as grippers;
when equipping production and research laboratories;
in medicine during the operation of breathing apparatus and dental offices;
in the printing industry for fixing thermal films.

How vacuum pumps work

A vacuum is created by mechanical removal of matter from an enclosed space. Technically, this is done in a variety of ways. Principle of operation jet type vacuum pump It is based on the entrainment of gas molecules by a stream of water or steam escaping at a high speed from the ejector nozzle. Its scheme provides for the connection of a side pipe, in which a vacuum is created.

The advantage of this design is the absence of moving parts, and the disadvantage is the mixing of substances and low efficiency.

In technology, the most widespread are mechanical units... The work of a vacuum pump with a rotating or moving reciprocating main part consists in periodically creating an expanding space inside the housing, filling it with gas from the inlet pipe, and then pushing it out through the outlet. In this case, the design of a vacuum pump can be very diverse.

The main varieties of vacuum pumps

In the manufacture of devices for creating a vacuum, metal and plastic materials are used that are resistant to the chemical attack of the pumped medium and have sufficient mechanical strength. Much attention is paid to the accuracy of fitting the assemblies and the tightness of the contact surfaces, excluding the backflow of gases. Here is a list of the main types of vacuum pumps, which differ in design and principle of operation.

Liquid ring

A liquid ring vacuum pump is one of the options for liquid ring units, using to create a vacuum circulation of clean water... It has the form of a cylinder with a bladed rotor rotating on an off-center shaft. Before starting work, it is filled with liquid.

When the engine is started, the impeller accelerates the water along the inner walls of the housing. A crescent-shaped vacuum area is formed between it and the rotor. Gas rushes into it from the pump inlet. Moving vanes move it along the shaft and eject it through the outlet. Aggregates of this type are often used also for partial gas cleaning due to its intense contact with water.

The use of a liquid as a working body offers many advantages.

The water rotating in the space between the rotor and the pump casing eliminates the possibility of backflow of gases, replacing the seals and reducing the requirements for the accuracy of parts manufacturing.
All rotating parts of the pump are constantly washed with fluid, which reduces friction and improves heat dissipation.
Such devices rarely require repair, have a long service life and consume a minimum of electricity.
Working with gases containing water droplets and small mechanical impurities does not adversely affect the technical condition of the equipment.

The latter circumstance is important when using such pumps for pumping air from containers containing moisture. They are used for air conditioners and other refrigeration units when evacuating the system before filling them with freon.

Rotary vane

Such pumps have a cylindrical casing with a carefully polished inner surface and a rotor located inside it. Their axes do not coincide, so the side clearance has different values. The rotor includes special movable plates, which are pressed by springs to the body and divide the free space into sectors of variable volume. When the engine is turned on, the gases are set in motion so that a vacuum is always created in the inlet pipe, and overpressure in the discharge pipe.

To reduce friction, the plates are made of antifriction materials or special low-viscosity oils are used. Pumps of this type are capable of creating a sufficiently strong vacuum, but they are sensitive to the purity of the pumped liquid or gas, require regular cleaning and contaminate the product with traces of grease.

Diaphragm piston

The working body of the pumps of this principle of operation is flexible membrane associated with the linkage. It is made of modern composite materials that are resistant to mechanical stress. Its edges are firmly fixed in the body, and the central part bends under the action of an electric or pneumatic drive, alternately reducing and increasing the space of the inner chamber.

The change in volume is accompanied by the suction and expulsion of incoming gases or liquids. When the two membranes work together in antiphase, a continuous pumping mode is ensured. The valve system regulates the correct distribution and direction of flows. The mechanism has no rotating or rubbing parts in contact with the pumped product.

TO advantages of such pumps should include:

no contamination of the product with grease or mechanical impurities;
complete tightness, excluding leaks;
high efficiency;
ease of flow control;
long-term operation in dry mode, which does not damage the structure;
the ability to use a pneumatic actuator to work in an explosive environment.

Screw

The principle of operation of screw pumps is based on displacement of liquid or gas along the rotating screw. They consist of a drive, one or two helical rotors and a correspondingly shaped stator. High precision manufacturing of parts does not allow the pumped medium to slip back. As a result, excess pressure is formed at the pump outlet, and vacuum at the inlet.

Such equipment is not cheap because of the high quality requirements. It cannot be kept in dry mode for a long time.

The main advantages of such pumps:

uniformity of consumption;
low noise level;
the ability to pump liquids with mechanical impurities.

Vortex

Vortex vacuum pumps by their design resemble centrifugal equipment... They also have a vane impeller that rotates on a central shaft. The fundamental difference lies in the location of the inlet pipe on the outer circumference of the body, and not in the area of the central axis.

The minimum clearance between the impeller and the casing ensures stable movement of the pumped liquid in the required direction. Units of this type are capable of generating a sufficiently high discharge pressure and have a self-priming effect. These pumps are easy to operate, easily repaired and have proven themselves excellent when pumping gas-liquid mixtures, but they have low efficiency. They are sensitive to the ingress of mechanical impurities that can lead to rapid wear of the impeller.

Self-made vacuum pump

If you are not willing to bear the costs of purchasing factory equipment, try making your own vacuum pump. For pumping air from a container of small volume, it may be suitable a medical syringe or a slightly modified hand bicycle pump.

Advice! With frequent use and evacuation of large vessels, it is more convenient to use devices with an electric drive.

Consider the option of manufacturing a vacuum unit from the compressor of an old refrigerator. It is already designed for pumping gas and, with minimal repairs, will be able to create a vacuum. Your actions will be extremely simple:

at some distance from the compressor, cut off two copper pipes suitable for it with a hacksaw;
dismantle the compressor together with the power supply circuit or replace it together with the starting relay with a new one, by analogy with the old one;
on the copper pipe that came from the condenser, put on a durit hose of a suitable diameter and connect it with the other end to the evacuated container;
for the tightness of the connection, you can use a standard clamp or use a twist of steel wire;
connect the vacuum pump to the electrical network and after starting on the air outlet from the second copper pipe make sure that it is working correctly.

Important! The compressor of the refrigerator is not intended for operation in a humid environment, therefore it must be ensured that no water gets on it.

Today, a lot of physical and chemical processes are carried out in a vacuum environment. To create it, vacuum pumps of various types and types are used. They are divided according to the type of work, technical capabilities, and functional purpose. Today, manufacturers of vacuum technology produce positive displacement and non-displacement pumps.

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Volumetric mechanical installations pump air through the action of moving working elements. They gradually compress air while decreasing the volume of the chamber. This type of pump includes installations with diaphragm, vane-rotary, liquid ring, cam and spiral working elements. Typically, they are used to create low to medium vacuum, which is 10-2 mm Hg. Art. Some installations are capable of generating high pressures.

The rest of the pumps use a non-mechanical principle of operation, in which gases are exposed to low temperatures or other phenomena that contribute to the creation of a vacuum. Pumps of this type are used to create high and ultra-high vacuum. These include diffusion, oil-steam, multi-charged, getter, ion getter and other pumps. However, most of these pumps work in conjunction with foreline pumps to provide the required pressure. They are necessary to create a pre-vacuum and are available in all types of mechanical pumps.

Domestic vacuum pumps

Domestic vacuum pumps, unlike foreign installations, have large dimensions, are made of high quality materials, are highly efficient and reliable. They can be used in various industrial fields as well as in agriculture. Domestic samples of the same series have similar designs, while they have many modifications. Most of the pump elements are suitable for other models, so they have a high maintainability.

The most common models that are produced in our country include installations of the HBR and VVN series. They are widely used in various systems, but differ significantly in their design. These models have many modifications that differ in size, main performance indicators, residual pressure. In HBP units, mineral and semi-synthetic vacuum oils are used, which are designed to seal gaps. In VVN pumps, additional lubricating elements are not used due to the fact that this function is performed by the working fluid, which, as a rule, is represented by water.

Vacuum pumps HBR

HBP vane vacuum pumps are used to create low medium and high vacuum. A wide range of installations allows them to be used in industrial, agricultural, woodworking, food and other enterprises. The units are distinguished by the fact that they are able to create a vacuum with a high residual pressure in a short time. HBP pumps are versatile, as they can perform tasks of various types.

The model range is represented by such units as NVR-0.1D, 2NVR-0.1D, 2NVR-0.1DM, NVR-1, NVR-4.5D, 2NVR-5DM, 2NVR-5DM1, 2NVR-60D, 2NVR-90D , 2НВР-250Д. The units can have a one-stage and two-stage type of action, they can be modified with a gas ballast valve and have different capacities. Units of this type can only be efficiently pumped if the vacuum system is completely free of dust, dirt and condensation.

VVN vacuum pumps

Vacuum pumps of the VVN model series differ significantly from other pumps in that liquid is used in the system when performing an operation. As a rule, water is used in this capacity. Pumps have a narrower functionality, but at the same time they are indispensable in many areas of activity.

The main advantages of VVN liquid ring vacuum pumps:

are able to clean the pumped out mixture;
applicable in systems with mechanical impurities;
ecological cleanliness;
lack of vacuum oil in the system;
ease of use and maintenance;
low power consumption;
maintainability;

VVN vacuum pumps are used in the food, chemical, medical, pulp and paper, microbiological, agricultural, woodworking, pharmaceutical and perfume industries.

Vacuum pumps for industrial furnaces

In industrial furnaces, vacuum pumps are used to speed up the operations of annealing, normalizing, quenching, as well as to improve the quality of the material. In a vacuum space, all chemical and physical processes are carried out quickly and efficiently.

Vacuum pumps can be used in industrial furnaces of arc, induction, thermal, hydrogen type. Often, diffusion furnaces are used to ensure low residual pressure, which have a non-volumetric type of action.

In order to effectively carry out heat treatment in an industrial furnace, pumps must be used that provide a sufficient pumping speed. It also allows you to count on high performance. An equally important indicator is the residual pressure, but it can differ significantly in different furnaces from the type of operation being performed.

Vacuum pumps for climatic chambers

Climatic chambers are equipment that is necessary to study the qualities of various materials and units. For efficient and fast operation, the units use vacuum pumps.

In order to use the pump in a climatic chamber, it is necessary that it:

withstood increased / decreased temperature indicators;
high humidity;
created a sufficient level of vacuum;
had the ability to create and hold the necessary pressure.

Rotary vane vacuum pumps

Rotary vane pumps are excellent for industrial applications. A wide range of models allows for various types of operations. Installations with a high residual pressure and speed are used for climatic chambers and heat treatment furnaces.

The installations have high reliability, durability, maintainability. They can be classified as universal means of creating a vacuum. At the same time, to ensure their operation, it is necessary that the vacuum system be cleaned of mechanical impurities and moisture. To work in climatic chambers, pumps made of stainless steel are used.

Vacuum pumps for degassing chambers

Degassing is a process that cannot take place without the participation of a vacuum pump. But it performs the main task of pumping gases and gas mixtures from various materials. To carry out the evacuation of gases and vapors from dense materials, as a rule, two-stage vacuum pumps are used.

Two stage vacuum pump

The two-stage vacuum pump is an upgraded model of the single-stage pump with a higher capacity. This type of installation is widely used in production areas where it is necessary to create a higher pressure. At the same time, they are reliable and can be used with various types of gases.

In two-stage vacuum pumps, the chambers are dependent on each other. This helps to synchronize and therefore increase productivity. Every year they are gaining more and more popularity due to the fact that they practically do not have large dimensions, but at the same time provide the best technical performance.

Dry vacuum pump

Dry vacuum pumps are becoming increasingly important because they are able to pump out the system without contamination. Unlike other units, they do not use an oil seal.

They have lower performance, in contrast to analog installations, but at the same time they are quite reliable. For efficient and proper operation, it is periodically necessary to carry out maintenance with the replacement of plates, which can wear out during operation.

Oil Free Vacuum Pump

Oil-free vacuum ones are used in enterprises where it is necessary to ensure the cleanliness of the operation. Very often they are used in laboratory studies, where it is necessary to create a sufficient level of residual pressure in a short time. The units are highly reliable and maintainable.

In the manufacture of pumps of this type, designers perform careful calculations, since it is important that there are sufficient clearances between the elements that will avoid friction, but will not be so large as to allow a significant decrease in performance.

High vacuum vacuum pumps

The creation of a high vacuum, as a rule, takes place using several pumps, including a fore-vacuum and a high-vacuum unit. The foreline pump, represented by one of the volumetric units, performs a preliminary vacuum, pumping out up to 97% of the gases, and the high-vacuum pump does the rest, reaching its limit values.

The following can be used as high vacuum pumps:

turbomolecular;
diffusion;
ionic;

Turbomolecular pumps

Turbomolecular pumps are a far cry from other high pressure pumps. They are able to independently create a high vacuum, since they have a mechanical principle of operation. The settings operate in the range of 10-2 - 10-8 Pa. The main working mechanism is represented by a stator and a rotor with discs that are located at a certain angle.

Molecules of gas mixture, being in a turbomolecular pump, significantly increase the speed of movement due to collision with each other. The rotor rotates at a speed that exceeds 10,000 revolutions, which is the main reason for the high pressure.

Vacuum ion pump

Ion or ion getter vacuum pumps were widespread before the advent of other high vacuum pumps. With their help, a pressure of 10-6 mbar is created. Today they are used less often, but all equal find their consumer. Pumps of this type are environmentally friendly and an advantageous method of obtaining ultra-high vacuum.

In the installation, molecules are captured and bound by gases or a getter layer, and then retained in the volume of the installation. They are able to hold a vacuum even when they are inoperative. The main element of the pump is the chamber and other stationary elements. The ion pump consumes a small amount of electricity and has a low noise level.

Oil-free (dry) rotary vane vacuum pumps are positive displacement pumps that allow to obtain a vacuum of medium depth with the complete absence of oil exhaust in the discharged air. The depth of the achieved vacuum is from 90 to 400 mbar residual pressure, depending on the model. Which is 9 to 40% of atmospheric pressure.

It is quite difficult to create a good oil-free rotary vane pump, so the number of manufacturers in the world is not so large. Basically, they are made in Europe (, and). And only low-capacity pumps are produced in the USA, China and Taiwan. Among the latter, Taiwanese pumps are in greatest demand.

Operating principle

Dry rotary vane pumps have in general the same principle of operation as. They also use an eccentrically mounted rotor with blades that can slide freely in their slots.
Animation 1: the principle of the rotary vane pump

However, there are some differences. Dry pumps do not use oil to seal the clearance between the blades and the housing, nor to lubricate moving parts, nor to cool. Therefore, the blades of dry pumps are made not of metal, but of a graphite composite. Graphite creates much less friction than metal and therefore does not require much cooling. In addition, the graphite vanes quickly rub against the surface on which they slide, ensuring good sealing of the gaps between the housing and the vanes.

On the one hand, the design of oil-free pumps is simpler: there is no oil separator and no oil channels. On the other hand, the lack of lubrication increases the demands on the quality of the surface finish.

Pros and cons of oil-free rotary vane vacuum pumps (versus oil)

There are two main reasons why you should choose a dry rotary vane pump: relatively clean air at the outlet and the ability to work with a coarse vacuum for a long time. In addition, there is no need to constantly monitor the oil level and worry about dehumidification of the pumped gas.

All the advantages of dry pumps are a mirror image of the disadvantages of oil-lubricated models: if it is preferable for oil to work in a high vacuum support mode, then a dry pump can work for a long time with a coarse vacuum at the inlet. Also, a situation often arises when the evacuated air remains in the same room where people work. Having passed through the oil-lubricated model, the air is inevitably saturated with oil vapors, which not only smell unpleasant, but are not particularly useful for those around them. Exhaust line filters solve this problem to some extent. But there are no perfect filters.

On the other hand, after passing through an oil-free rotary pump, the air, although it does not remain perfectly clean, in this case, instead of oil, particles of graphite dust enter the air. This dust, firstly, is emitted much less than oil. And secondly, graphite does not smell, and it is much easier to filter it out. Therefore, an oil-free pump is a good choice for areas where people work.

Another significant drawback of oil-lubricated pumps is the need for constant monitoring of the oil level. This level can both increase, due to the appearance of condensation, and decrease, for example, when working with a coarse vacuum or when the temperature is exceeded. Any of these scenarios is disastrous for a vane oil pump: if there is not enough oil, it will overheat and burn, and if there is a lot of condensate in the oil, the pump will quickly rust. An oil-free pump is initially devoid of these drawbacks: there is no need to constantly monitor it, it is enough to check the thickness of the blades every 2-3 thousand working hours.

In general, for residual pressures above 400 mbar, an oil-free pump is a good choice. But for creating a deeper vacuum, it is no longer suitable. The most advanced models from our catalog can provide only 100 mbar residual pressure. Another limitation is the service life. Oil-filled models can produce the same performance for years (only occasionally adding oil is required), which is what many laboratories use, maintaining a stable vacuum in the laboratory cabinet day and night. A dry rotary vane pump can also operate 24/7, but as the blades wear out, its performance will decrease. Therefore, it is recommended to turn on such a pump exactly when it is needed, and turn it off at the end of the shift.

Wear of working plates

As you can see from the animation above, the plates are constantly moving along special slots in the rotor. Escaping under the action of centrifugal force, they adhere tightly to the walls of the chamber and divide the free space of the working chamber into several isolated volumes.

The rotor of the pump rotates at a high speed (usually 1400-1500 rpm, since 4-pole electric motors are used), therefore, the problem of friction of the plates on the inner surface of the working chamber arises. In pumps with oil lubrication, this problem is not acute, so the working plates (blades) can be either composite or more durable metal. However, in dry pumps, only carbon vanes are available. Graphite itself is a good lubricant - graphite plates slide over the working chamber without overheating. But at the same time, graphite wears out relatively quickly. Moreover, not only its length is reduced from friction against the pump casing, but its thickness is also reduced from friction against the rotor.

Image 1. Three types of wear of graphite blades of rotary vane pumps.

The wear of the graphite vanes (plates) leads to air leaks and a decrease in vacuum depth and pump performance. What is the average life of oil-free pump blades? Most manufacturers bashfully do not indicate this term. However, we have some information.

Stairs Vacuum Taiwanese indicate the need to replace the vanes after 8,000 - 10,000 hours. However, they note that the performance of any oil-free rotary vane pump begins to decline after 3,000 hours of operation.

The Italians DVP write about the life of the records of 10,000 hours. Once an engineer came to our office who had an SB 16 pump from this Italian company. He said that the pump worked for them for 20,000 hours (though in compressor mode, but this does not change the essence), after which it stopped working normally (it was about the wear of the blades, and not about the breakdown of the pump). At the same time, the exhaust hoses inside were covered with a thin layer of graphite dust. This example says that the manufacturer indicates the minimum guaranteed service life of the blades, in practice they can work even more, but with a decrease in operating parameters.

The Germans Becker VX, KVX series hold the record for blade service life (alas, for the price of pumps too) - at least 20,000 hours, in practice, from 20,000 to 40,000.

Image 2. A graph of the decrease in performance of dry rotary vane pumps due to wear of the blades.

At what depth of vacuum does the efficiency of rotary vane vacuum pumps become the highest?

The efficiency of oil-free vane pumps is not fixed, but depends on the operating point (vacuum depth). At an inlet pressure close to atmospheric (at rough vacuum), the pump efficiency is very low and becomes acceptable (40% and higher) at a vacuum depth of 300 mbar (700 mbar residual pressure). The efficiency reaches its maximum (almost 60%) at a vacuum of 600-700 mbar (300-400 mbar absolute pressure), and then again begins to decrease to 40% as the vacuum deepens.

Picture 3. Comparison of efficiency of dry rotary vane vacuum pump and vortex single-stage blower.

If we compare, for example, an oil-free rotary vane vacuum pump and a single-stage vortex blower operating in vacuum mode, it turns out that these two devices do not compete with each other, but complement each other. In the range of created pressures from -100 to -300 mbar, the vortex blower shows the best efficiency values, and in the range from -300 to -900 mbar, the vane-rotor device already works much more efficiently.

Vacuum pumps are widely used in a wide variety of industries and science. The main application of vacuum pumps is to remove air or gas from a hermetically sealed volume and create a vacuum in it. We will consider the most common types, characteristics of vacuum pumps, their principle of operation and main applications.

Vacuum pumps are classified according to the operating pressure range into:

primary (foreline) pumps,
booster pumps
secondary pumps.

In each pressure range, different types of vacuum pumps are used, differing from each other in design. Each of these types has its own advantage in one of the following points: possible pressure range, performance, price and frequency, and ease of maintenance.

Regardless of the design of the vacuum pumps, the basic operating principle is the same. The vacuum pump removes air and other gas molecules from the vacuum chamber (or from the outlet of a higher pressure vacuum pump if connected in series).

As the pressure in the chamber decreases, the subsequent removal of additional molecules becomes exponentially more difficult. Therefore, industrial vacuum systems must cover a large pressure range from 1 to Torr. In the scientific field, this indicator reaches torr or less.

The following pressure ranges are distinguished:

Low vacuum:> atmospheric pressure to 1 torr
Medium vacuum: 1 torr to 10-3 torr
High vacuum: 10-3 torr to 10-7 torr
Ultra-deep vacuum: 10-7 torr to 10-11 torr
Extreme high vacuum:< 10-11 торр

Compliance of vacuum pumps with pressure ranges:

Primary (foreline) pumps - low vacuum.

Booster pumps - low vacuum.

Secondary (high vacuum) pumps: High, ultra deep and extremely high vacuum.

Classification of vacuum pumps according to the principle of operation with gas

There are two main technologies for working with gas in vacuum pumps:

Gas pumping
Gas capture

Pumps operating on gas transfer technology are subdivided into kinetic pumps and positive displacement pumps.

Kinetic pumps operate on the principle of transferring momentum to gas molecules from high-speed blades to ensure a constant movement of gas from the pump inlet to the outlet. Kinetic pumps usually do not have sealed vacuum chambers, but can achieve high compression ratios at low pressures.

Positive displacement pumps work by mechanically capturing a volume of gas and moving it through the pump. In a sealed chamber, the gas is compressed to a smaller volume at a higher pressure and after that, the compressed gas is displaced into the atmosphere (or into the next pump).

Typically, kinetic and volumetric operate in series to provide higher vacuum and flow rates. For example, very often a turbomolecular (kinetic) pump is supplied assembled in series with a screw (positive displacement) pump into a single unit.

Gas capture technology pumps capture gas molecules on surfaces in a vacuum system. These pumps operate at lower flow rates than transfer pumps, but can create ultra-high torr and oil-free vacuums. Trap pumps operate using cryogenic condensation, ionic reaction, or chemical reaction and have no moving parts.

Types of vacuum pumps depending on the design

Depending on the design, vacuum pumps can be divided into oil (wet) and dry (oil-free), depending on whether the gas is exposed to oil or water during pumping.

A wet pump design uses oil or water to lubricate and / or seal. This liquid can contaminate the pumped gas. Dry pumps have no liquid in the flow path and depend on the sealed gaps between the rotating and static parts of the pump. The most commonly used seal is polymer (PTFE) or a diaphragm to separate the pump mechanism from the pumped gas. Dry pumps reduce the risk of oil system contamination compared to wet pumps.

The following designs, described below, are most often used as primary (foreline) pumps.

Primary foreline pump. Principle of operation. Design options

Oil Filled Rotary Vane Pump

(wet, voluminous)

In a rotary vane pump, gas enters the inlet and is captured by an eccentrically mounted rotor, which compresses the gas and transfers it to the outlet valve. A spring-loaded valve allows gas to escape when atmospheric pressure is exceeded. The oil is used to seal and cool the blades. The pressure achieved with a rotary pump is determined by the number of stages. The two-stage design can provide a pressure of 1 × 10-3 mbar. The capacity ranges from 0.7 to 275 m3 / h.

Liquid ring vacuum pump. Design and principle of operation

(wet, voluminous)

The liquid ring pump compresses the gas by means of a rotating impeller located eccentrically inside the pump housing. The liquid is fed into the pump and forms a cylindrical moving ring by means of centrifugal acceleration. This ring creates a series of seals in the spaces between the impeller vanes, which are the compression chambers. The eccentricity between the axis of rotation of the impeller and the pump casing leads to a decrease in the volume between the impeller blades and thus to the compression of the gas and its release through the outlet pipe. This pump has a simple, robust design as the shaft and impeller are the only moving parts. The liquid ring pump has a large power range and can provide a pressure of 30 mbar when using water with a temperature of 15 ° C. When using other fluids, lower pressures are possible. The range of available capacities is from 25 to 30,000 m3 / h.

Diaphragm vacuum pump

(dry bulk)

Diaphragm pumps use a flexible diaphragm that is connected to the stem and moves alternately in opposite directions so that gas enters the space above the diaphragm and fills it completely. Then the intake valve closes and the exhaust valve opens to release the gas.

The diaphragm vacuum pump is compact and very easy to maintain. Diaphragms and valves typically have a service life of over 10,000 operating hours. A diaphragm pump is used to support small turbomolecular pumps in a clean, high vacuum. It is a low power pump widely used in research laboratories for sample preparation. Typical ultimate pressure 5 × 10-3 mbar. Capacity 0.6 to 10 m3 / h (0.35 to 5.9 cfm).

Spiral vacuum pump

(dry bulk)

The main elements of the pump are spiral rotor and stator. The expanded gas enters large circular spaces that taper when it reaches the center of the spiral rotating rotor. A PTFE polymer seal provides a seal between the pump scroll elements without using oil in the pumped gas. The attainable pressure is 1 × mbar. Productivity from 5 to 46 m3 / h.

Booster pumps

Double rotor vacuum pump

(dry bulk)

Twin-rotor pumps are mainly used as booster pumps and are designed to remove large volumes of gas. The two rotors rotate without touching each other to continuously transfer gas in one direction through the pump. This increases the performance of the primary / foreline pump by increasing the pumping rate to approximately 7: 1 and improving the final pressure to approximately 10: 1. Booster pumps can have two or more rotors. Typical ultimate pressure<10-3 Торр может быть достигнуто (в сочетании с первичными насосами). Производительность составляет подобных агрегатов может достигать около 100 000 м3/ч.

Cam-gear pump

(dry bulk)

The lobe-toothed pump has two lobes that rotate in opposite directions. The operation of a vacuum pump is similar to that of a rotary pump, except that the gas is conveyed axially and not from top to bottom. Very often a lobe pump and a twin-rotor pump are used in combination. Rotor stages and cam stages are installed on one common shaft. This type of pump is designed for harsh industrial conditions and provides high performance. Typical ultimate pressure 1 × 10-3 mbar. The productivity is from 100 to 800 m3 / h.

Screw pump

(dry bulk)

The main working parts of the unit are two rotating screws that do not touch each other. Rotation carries gas from one end to the other. The screws are designed in such a way that as the gas passes through them, the space between them becomes smaller and the gas is compressed, thereby causing a reduced inlet pressure. This pump has a high performance. The PCP can handle fluids and impurities and also works well under harsh conditions. Typical ultimate pressure is about 1 × 10-2 Torr. The capacity can reach 750 m3 / h.

Secondary (high vacuum) pumps

Turbomolecular pump

(dry, kinetic)

Turbomolecular pumps work by transferring kinetic energy into gas molecules using high-speed rotating angled vanes that propel the gas at high speeds. The rotation speed of the blade tip is usually 250-300 m / s. Receiving momentum from the rotating blades, the gas molecules move to the outlet. Turbomolecular pumps provide low pressure and low performance parameters. Typical ultimate pressure is 7.5 x 10-11 Torr. Performance range from 50 to 5000 l / s. Pumping stages are often combined with stagnation stages, which allows the turbomolecular to reach higher pressures (> 1 torr).

Diffusion steam oil pumps

(wet, kinetic)

Steam diffusion pumps transfer kinetic energy to gas molecules using a high speed heated oil stream that moves the gas from the inlet to the outlet. This provides a reduced inlet pressure. This design is rather outdated. To a large extent they are being supplanted in the market by the more convenient dry turbomolecular pumps. Oil diffusion pumps have no moving parts and are highly reliable. This vacuum pump has a low price. Ultimate pressure less than 7.5 x 10-11 Torr. Performance range 10 - 50,000 l / s.

Cryogenic pump

(dry, gas recovery technology)

Cryogenic pumps work by capturing and storing gases and vapors, rather than pumping them through themselves. This type of pump uses cryogenic technology to freeze or trap gas on a very cold surface (cryocondensation or absorption) at a temperature of 10 ° K to 20 ° K (minus 260 ° C). These pumps are very efficient but have limited gas storage capacity. The collected gases / vapors must be periodically removed from the pump, heating the surface. They are pumped out using another vacuum pump. This process is also known as regeneration. Cryogenic pumps require the installation of an additional compressor cooling system to create cold surfaces. These pumps can reach pressures of 7.5 x 10-10 Torr and have a capacity range of 1200 to 4200 l / s.

Major manufacturers of vacuum pumps

The vacuum pump can be purchased from the following manufacturers

BUSCH www.buschvacuum.com

Becker www.beckerpumps.com

Elmo Rietschle http://www.gd-elmorietschle.com/en

NASH http://www.gdnash.com/liquid_ring_vacuum_pumps/

Robuschi http://www.gardnerdenver.com/en/robuschi/products/vacuum-pumps

Pfeiffer Group group.pfeiffer-vacuum.com

Samson Pumps www.samson-pumps.com