With the modern equipment available, it is not a difficult task to create vacuum. As a result, vacuum equipment becomes more and more popular in various industries. Industrial vacuum equipment is subdivided into vacuum pumps, vacuum units and vacuum plants. Another term for vacuum pumps is the evacuation equipment because these apparatus are used to evacuate air, gases and vapor-gas mixtures.
The two key parameters of a vacuum pump are:
The pumping speed (S) is expressed in litres per second and demonstrates the amount of pumped medium evacuated during a time unit. The vacuum amount is characterized by the residual pressure in the reservoir with a rarefied atmosphere. The units of measurements for this quantity are Pa, kPa, mm Hg and % of the atmospheric pressure.
Vacuum pumps are subdivided into the types as follows:
The parameters described below are used to assess the vacuum pump adequacy to the expected operation conditions:
The pump operation principle depends on the nature of the gas flow, i.e. on the rarefication level.
During the initial gas evacuation, it flows at high speed, and swirls exist in the flow (i.e. the gas flow mode is turbulent). Pressure drop results in elimination of swirls, and gas flow becomes inertial (because the flow depends on the inertia of gas medium). If the gas flow speed drops further, the flow mode becomes viscous.
If the gas flow can be described as viscous, inner flow layers move actively, as opposed to the layers near the pump walls (these layers are practically immovable). So, the particle speed is maximal at the center of the gas flow. The flow movement type depends on the gas viscosity. When the pressure parameters are low, molecules move freely, and practically there is no interaction between them. With the internal friction disappearing, the flow movement becomes molecular.
This pumping plant may be of one of three types: high-vacuum pump, booster unit or forevacuum unit (depending on the pressure range for which the pump capacity is maximal). These pumps are widely applied in metallurgy. In accordance with their operation principle, they are subdivided into two groups:
Vacuum oil is used to prevent gas leakages through the gaps in the vacuum pumping plant parts affected by friction. Vacuum oil seals the gaps and acts as a lubricant. So, mechanical pumps with oil used as a seal and a lubricant are oil pumps. The pumps in which no oil used are dry pumps.
Vacuum pumps eliminate gases, vapor and air from the pump’s operating chamber volumes that are closed and sealed. With gases, vapors and air gradually removed, the volume of cavities is changed; as a result, molecules of the substance to be evacuated are redistributed in a desirable direction. The details of the vacuum pump operation principle depend on the unit type.
The major part of vacuum pumping plants operates by displacement; this is also the case for volumetric pumps (except for oil-steam pumps and steam jet ejector apparatus). The resulting vacuum amount directly depends on the quality of sealing of the working space created by the operating elements of the pump, such as vanes, slide valves and impellers in combination with the liquid. Vacuum pump operation must meet two key requirements: pressure in the closed space must be reduced to Pвmin by way of gas medium intake from the closed space (volume), and this process must be implemented within the specified time interval. If a pump is capable to intake the appropriate volume of gas medium but fails to reach the required pressure reduction, a forevacuum pump shall be used to provide additional reduction of the gas medium pressure.
Each group of vacuum pumping plants has its advantages due to the specific elements of design, operation principle, operating liquid type and other factors. For example, water-packed ring pumps are high-strength, operable at high temperatures and heavy-duty conditions, and capable to evacuate polluted vapors. Sliding vane rotary pumps are especially resistant to water steam; also, they are small-sized, reliable, and demonstrate high pumping speed and low energy consumption. Roots vacuum pumps demonstrate high capacity, quick operation, uniformity of pumping, and no oil in the compressed gas. Membrane and volute pumping plants are applicable for aggressive media if all parts are covered with special coating (for membrane pumps, in addition, a membrane must be made of rubber). The advantages of screw pumps include no oil consumption, no condensers used, and low power consumption.
Vacuum pumps are used to evacuate air, vapor, vapor-gas mixtures and non-aggressive gases (free of mechanical impurities and moisture) from sealed operating reservoirs of fixed plants installed indoors. This type of plants is also widely applied for initial vacuum generation in high-vacuum plants.
Vacuum pumps are actively used in a wide range of industries, processes and technical applications:
Vacuum pumps are subdivided into basic types as follows:
Water-packed (or liquid-packed) units. Usually, water is used as an operating liquid; however, in accordance with the technology characteristics, other liquids can be used, such as oil, some acids, alkalis or coolants. A rotor with vanes moves water away; as a result, water ring is created against a stator walls.
Sliding vane rotary pumps (with oil sealing). These pumps has a cylinder-shaped casing with a rotor rotating in it. The rotor is provided with grooves with vanes moving reciprocally in these groves. While the rotor rotates, vanes are pressed against the chamber walls and are sealed by oil film; as a result, the volume is divided into sectors. When the rotor turns, the volume of sectors becomes larger and, when they become connected with the suction pipe, gas is sucked. Then, the volume of sectors is reduced, they are disconnected from the suction pipe, resulting in gas compression. As a result of a specific movement, sectors butt against the discharge pipe, resulting in reduction of their volume, and gas flows into this pipe.
Oil is fed into the cavity to provide lubrication for parts affected by friction.
Sliding vane rotary oil-free pumps. Their operation is similar to pumps with oil sealing but they do not use lubrication.
Double-rotor pumps. These pumps have a casing (with two identical rotating rotors) and two end covers, bearings, gaskets and a timing gear. The operating chamber moves, without compression, from the suction cavity into the discharge cavity; as a result of this movement, gas is removed and compressed. The operating chamber is communicated with the discharge; as a result, pressure in the chamber is boosted by gas flowing from the discharge window.
Membrane pumps. In these pumping plants, oil-free gas removal is implemented due to the membrane changing its shape (it is deflected). In small pumps of this type (micropumps), the membrane drive is activated by the crank mechanism using a rod; in similar large pumps, the drive is activated hydraulically by the piston pump.
Volute pumps. These pumps include gas ballast devices for vapor pumping, reaching the preset suction pressure. If condensation arises, the pumping plant shall be installed vertically to resolve this problem.
Screw pumps. These pumps operate non-lubricated. The drive RPM is controlled by the frequency changer, making it energy-effective. Operation of screw pumps is pulsation-free.
Turbomolecular pumps. Their operation is implemented by a rotor imparting additional speed to the molecules of evacuated substance towards their movement direction. The rotor is a system of disks; it must be balanced properly. The shaft may be vertical or horizontal. For long trouble-free operation, holding parts (bearings and collars) must be precise and quick.
The plunger vacuum pump is a type of mechanical vacuum pumps that can compress gases to reach atmospheric pressure. This apparatus, in terms of its design, is similar to the double-acting piston compressor. The major difference is higher compression provided by plunger vacuum pumps.
Initial stage (left), intermediate stage (two middle figures), final stage (right)
A plunger includes a cylinder-shaped part enveloping an eccentric and a hollow rectangular part moving freely in the hinge’s grooves. When the flat part of the plunger turns, the hinge also turns freely in the pump’s casing socket. This plunger is provided with a channel used for gas flowing from the evacuated cavity into the pump chamber. The slide valve moves to close the inlet preventing the gas counter-flow from entering the inlet part of the pump. Also, noxious space can be reduced. Thick oil layer is accumulated in the wedge-shaped area between the rotor and the cylinder, making the contact between them tight.
Mechanical vacuum pumps evacuate the volumes, starting from the atmospheric pressure level. Because the evacuated gas is released into the atmosphere, such characteristics as the maximum operating pressure, the maximum inlet pressure and the maximum discharge pressure are not applicable to mechanical vacuum pumps. The key characteristics of oil-sealed mechanical vacuum pumps are as follows:
A mechanical vacuum pump is a unit evacuating the gas used to reach or to maintain pressure below the atmospheric pressure level in reservoirs from which the operating liquid is pumped out during the specified intervals under the specified gas flow composition and amount.
This pumping plant operates by gas movement resulting from mechanical movement of the operating parts in the pump; as a result, evacuating impact is implemented. The volume filled with gas is separated from the inlet and driven towards the outlet. Gas firmly moves towards the pumping unit discharge due to the momentum imparted to the gas molecules.
In accordance with the details of design and operation, these pumps are subdivided into seven types: screw pumps, diaphragm pumps, piston pumps, sliding vane rotary pumps, slide valve pumps, Roots pumps, volute pumps. In accordance with the operating liquid type, mechanical pumps are subdivided into molecular pumps (operated by the substance molecule flow) and volumetric pumps (operated by the laminar substance flow). Mechanical vacuum pumps are also subdivided in accordance with the vacuum concentration (high, low, moderate vacuum). Also, the pumps of this type are subdivided into those operable lubricant-free and those operable with lubricants.
This type of pumping plants is applied in a great variety of industries: chemical industry, metallurgy, electronics, food industry, medicine, space exploration. Mechanical vacuum pumps are also used in a great variety of industrial plants and processes (metal remelting, thin film application, space environment simulation etc.).
Due to growing demand for pumping plants, mechanical vacuum pumps are continuously improved and modified, and new pumping plants with improved characteristics are designed.
For these pumps, operation speed does not depend on the type of pumped gas. The residual pressure depends on the pumping plant design and on the operating liquid properties. Usually, an operating liquid is oil demonstrating the necessary characteristics:
Stability of characteristics of mechanical vacuum pumps depend on the size of gaps between the surfaces, the number of these gaps and the quality of oil used to lubricate the friction surfaces.
A plunger vacuum pump can be equipped with a bypass to improve its efficiency. The bypasses may differ in terms of their design. Their purpose is to equalize pressure at both sides of the piston at the end of the piston stroke.
If there are no such channels, the residual compressed gas from the noxious space expands while the piston moves from left to right, with the residual compressed gas pressure p2. The curve ea1 is until the pressure p1 reached, and p1 and λ0=V1/V. When the vacuum pump piston is in the leftmost position, the residual gas flows into the right cavity of the cylinder, were the pressure is p1. The pressure in the noxious space drops from p2 to pв, and the residual gas expands in accordance with the curve fa. Suction starts at the beginning of the piston stroke (λ0=(V'1/V)>λ0). The similar process exists when the piston travels backwards (from right to left). As a result, the volumetric efficiency grows from 0.8 to 0.9 λ0.
It is the noxious space that makes the piston vacuum pump incapable to create absolute vacuum and results in theoretical limit for this value, with the residual pressure pпр. For systems with no bypass, pпр is higher than for those provided with a bypass.
If a vacuum pump operates continuously, the volume of the gas evacuated by suction is equal to the volume of process gases discharged into the atmosphere, and the amounts suctioned from the environment through non-tight areas remains invariable in time. The vacuum pump shaft power factor also remains invariable. It should be noted that this parameter is several times higher for machines with a bypass due to the loss of work of expansion of the bypassed compressed gas amount.
Rotary compressors are used to compress gases an to rarefy them, i.e. as vacuum rotary pumps. Forevacuum pumps have some differences in design terms and are used for work with diffusion pumps and molecular pumps.
Rotary vane pumps have a cylinder-shaped casing (with an inlet pipe and a discharge pipe) and a rotor positioned eccentrically. The vanes are in the grooves of the rotor. Two, four or more vanes may be used in vacuum pumps of this type.
The diameter of a rotor is small; as a result, centrifugal force is insufficient to press the vanes firmly against the inner surface of the casing. So, springs are used to press the vanes. The pumping plants of this type generate the residual pressure 7…13 Pa; for operation with two stages, as low as 1.3 Pa; for three stages, 0.13 Pa.
Rotary vane pumps with low operation speed (≤1 litres/s) run in an oil bath; as a result, joints in the pumping plants are sealed, and friction-related losses are reduced. The design of rotary vane pumps with high pumping speed (≤103 litres/s) is in accordance with Figure 4, Diagram 6. There is no oil bath in these pumps. Race rings are used to reduce friction losses; these rings are rotated by vanes. Evacuated gas flows through the holes in the race rings. In some designs with vanes made of antifriction materials, no race rings are used.
The major drawback of vane vacuum pumps is the volumetric efficiency reduction resulting from small wear of vanes. Even small wear of these parts results in gas leakage through the gaps from the discharge side to the suction side. High compression ratio contributes to significant growth of gas temperature. With the residual pressure reduction, the volumetric efficiency falls.
Great number of vanes in the rotor makes the machine less sensitive to wear of vanes and less susceptible to the volumetric efficiency reduction. However, the pump becomes more sophisticated in terms of design, and its noxious space volume grows. To reduce the impact of noxious space, bypasses are used in vacuum pumps of this type. The shaft power factor is calculated as follows: ηмех=0.8…0.9.
Water-ring packed vacuum pumps (compressors with a liquid ring) are widely used to generate vacuum. These apparatus are able to generate vacuum as high as 98%. However, low efficiency (0.40…0.45) shall be considered as a drawback.
These machines are provided with an impeller mounted eccentrically. Vanes are mounted on the impeller. Liquid is inside the casing; during rotation, it is pressed against the casing walls by the centrifugal force, resulting in a water ring. Cavities of various sizes arise between the pump vanes and the liquid ring. Initially, cavities grow, and gas flows into the pump through the suction inlet. At the next stage, the volume of cavities is reduced, and compressed gas leaves the pump. For these machines, the ultimate pressure is (2…3)*103 Pa. This type of pumps operates, starting from the atmospheric pressure; in the compressor mode, the available pressure is up to 2*105 Pa. The operation speed is 25…500 litres/s. The major drawback of this type of pumps is high specific power consumption, about 200 W/(litres/s). This parameter results from the necessity to move liquid in the pump.
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