Overview of various vacuum pump principles

A vacuum pump is a device that uses various methods to create, improve and maintain a vacuum in a closed space. A vacuum pump can be defined as a device or equipment that uses mechanical, physical, chemical or physical-chemical methods to pump a container to obtain a vacuum. With the development of vacuum applications, the types of vacuum pumps have developed many kinds, and their pumping speeds have risen from a few tenths of a second to hundreds of thousands or millions of liters per second. The ultimate pressure (ultimate vacuum) ranges from coarse vacuum to extremely high vacuum above 10-12Pa.

Division of vacuum area

Classification of vacuum pumps

According to the working principle of vacuum pumps, vacuum pumps can basically be divided into two types, namely variable volume vacuum pumps and momentum transmission pumps.

Variable volume vacuum pump is a vacuum pump that uses the periodic change of the volume of the pump chamber to complete the suction and exhaust to achieve the purpose of pumping. The gas is compressed before exiting the pump chamber.

The momentum transfer pump (molecular vacuum pump) relies on high-speed rotating blades or high-speed jets to transfer momentum to the gas or gas molecules, so that the gas is continuously transferred from the inlet to the outlet of the pump. (Introduction in separate paragraph)

Variable volume vacuum pump is divided into: reciprocating, rotary (rotary vane, slide valve, liquid ring, Roots, spiral, claw rotor), other types.

Working pressure range of various vacuum pumps

Specification and model representation of vacuum pump:

The models of various mechanical vacuum pumps made in China are usually represented by the Chinese phonetic alphabet. The Chinese phonetic alphabet indicates the type of pump; the number before the letter indicates the number of stages of the pump, and the "1" is omitted for single stage; the number after the horizontal line after the letter indicates the pumping speed (L/S) of the pump.

Domestic vacuum pump model comparison table

Commonly used vacuum pump technology

steam jet pump

Wet pumps (liquid ring vacuum pumps, rotary vane pumps)

Dry pump (Roots pump, screw pump, claw pump)

1. Steam jet pump

The working principle of the jet vacuum pump: the steam jet vacuum pump has a vacuum pump equipment with a certain pressure. The steam passes through the Laval nozzle to reduce pressure and increase speed. The potential energy of the steam is converted into kinetic energy and injected into the mixing chamber at supersonic speed. The mixed gas enters the diffuser, decelerates and pressurizes, and converts the kinetic energy into pressure energy. In order to reduce the pumping load of the post-stage pump, a condenser is configured, through the convection of two media with a certain temperature difference, heat exchange is carried out to achieve the purpose of condensing high-temperature media and discharge to atmospheric pressure. The working principle is shown in the figure below:
Commonly used vacuum pump technology

2. Wet pump: liquid ring vacuum pump

In the circular pump body (A), an impeller (B) is mounted on an eccentric shaft with respect to the center line of the pump body. The centrifugal action of the rotating impeller forces the working fluid through the channel (D) towards the periphery of the cavity to form a circulation (C).

When the pump is active, the gas mixture to be delivered enters the impeller through the suction port (H) on the intermediate plate (E), and a vacuum is formed at the pump inlet. The gas mixture is charged into the impeller cavity between the inner diameter of the liquid ring and the roots of the impeller blades. As the impeller rotates, the extent to which the impeller blades are submerged in the liquid ring increases and the volume between the liquid ring and the impeller blade roots decreases. The result is that the gas mixture is compressed until it reaches the discharge opening (J) in the intermediate plate (K). The gas mixture is discharged through a discharge port.

During the compression cycle, heat is transferred to the liquid ring. In order to keep the temperature below the evaporation point of the working liquid, cooling is necessary. Cooling is achieved by continuously adding cold working fluid to the liquid ring. The amount of working fluid added is equal to the amount of working fluid discharged through the discharge port together with the compressed gas mixture. The gas mixture and working fluid are finally separated through the pump outlet.

Liquid ring pump according to different structure can be divided into the following types:

(1) single-stage single-acting liquid ring pump single-stage refers to only one impeller, single-acting refers to each rotation of the impeller, suction, exhaust each time. The ultimate vacuum of this pump is higher, but the pumping speed and efficiency are lower.
(2) single-stage double-acting liquid ring pump single-stage refers to only one impeller, double-acting refers to each rotation of the impeller, suction, exhaust each for two times. Under the same pumping speed conditions, the double-acting liquid ring pump is greatly reduced in size and weight than the single-acting liquid ring pump. Since the working chambers are symmetrically distributed on both sides of the pump hub, the load acting on the rotor is improved. The pumping speed of this pump is larger and the efficiency is higher.

(3) two-stage liquid ring pump is mostly a single acting pump in series. Essentially, the impellers of two single-stage single-acting liquid ring pumps are connected by a common spindle. Its main feature is that it still has a large pumping speed and stable working condition under high vacuum.

(4) The atmospheric liquid ring pump is actually a unit of the atmospheric ejector series liquid ring pump. The liquid ring pump in front of the series atmospheric pump is to improve the ultimate vacuum and expand the scope of use of the pump.

Wet pump: rotary vane pump

The rotor is within a stator having a cylindrical bore.

The rotor has two blades that slide in opposite directions in the slot.

When the rotor is in operation, the blade ends are in contact with the oil lubricated stator wall.

Two pumping cycles per revolution-relatively small unbalanced forces.

The main problems of wet pumps: sealing liquid supply, pollution, waste water treatment, corrosion, steam pressure limitation (especially liquid ring pumps), stability, operating costs.

3. Dry pump

Dry vacuum: no pollution of industrial production flow;

Deep vacuum, more flexible operation;

No waste water, no pollution, (less or) no waste disposal costs;

Lower maintenance costs;

Recycle the solvent immediately at the pump discharge;

Consumption of water, electricity, gas and other costs are lower-operating costs are lower.

Technical comparison:
Cost of Dry Pump

Higher investment costs, but lower operating costs.

Extended service intervals and bearing life.

In the operating range, the power consumption is low. Therefore, the energy saving effect is better in the capacity ratio (M3/hr/kW).

No contaminated oil is to be disposed of (except for the gearbox).

Dry pump technology: pump type: screw pump, claw pump;

Screw pump:

Screw pump is the use of gear transmission synchronous reverse rotation of the left screw and the right screw without contact with each other for high-speed rotation, the use of pump shell and intermeshing spiral groove is divided into a number of spaces, forming a number of stages, the gas in the equal groove (cylindrical pitch) transmission movement, but no compression, only the spiral structure at the end of the screw has a compression effect on the gas. A pressure gradient can be formed between the stages of the screw to disperse the pressure difference and increase the compression ratio. Each clearance and pump speed have a great influence on the performance of the pump. In the design of the screw each part of the gap, to consider the expansion, processing and assembly accuracy and working environment (such as pumping dust gas, etc.).

Screw pump structure and characteristics

Cooling by deceleration or gas injection-reduced capacity and reduced hazard

Twin-screw operation

Stainless steel rotor

rotor meshing matching

cantilever shaft

single stage compression

produce excessive heat

have good liquid compatibility

Maintenance is very easy

Complex screw structure

 

claw pump
Claw pump structure characteristics

High gas compression ratio

Effective gas compression

Process material can pass through the pump (good dust handling)

Good steam delivery and recovery

Can withstand heating load

Evacuate to transfer with diffusion pump

Direct support for industrial diffusion pumps

momentum transfer vacuum pump

A vacuum pump that uses high-speed rotating blades or high-speed jets to transfer momentum to the pumped gas or gas molecules, so that the gas is continuously transferred from the inlet to the outlet.

Molecular pump: traction molecular pump, turbo molecular pump and compound molecular pump three categories.

Jet vacuum pump: water jet pump, gas jet pump and steam jet pump three categories

Diffusion pump: oil or mercury vapor as the working medium. The mercury diffusion pump does not have a fractionation device, and the oil diffusion pump has a fractionation device.

Jet Vacuum Pump

Jet vacuum pump is a low vacuum pump that uses a high-speed jet of liquid or gas to carry the pumped gas to obtain a certain degree of vacuum in the pumped container.

According to the working medium can be divided into water jet pump, atmospheric jet pump and water vapor jet pump

Jet pump features:

Simple structure, stable and stable work;

Can remove the gas containing water vapor, dust, flammable and explosive gases and corrosive gases;

Large pumping capacity, wide application: metallurgy, chemical industry, food manufacturing and other fields;

Disadvantages: low pumping efficiency and large energy loss.
Water jet pump is water as the working medium, through the high-speed jet to eject the pumped gas, so that the container to achieve a certain degree of vacuum of the rough vacuum equipment.

The ultimate pressure of the single-stage pump is 3.3kPa, and the two-stage series connection can obtain a better vacuum degree, which is mainly limited by the saturated vapor pressure of water.

It can be used alone or as the front stage of other vacuum pumps.

The role of the nozzle is to convert the pressure of water into kinetic energy, and its structure has a great influence on the performance of the pump.

The suction chamber is generally cylindrical with a cross-sectional area 6-10 times the nozzle outlet area.

The diffuser consists of a tapered section, a throat pipe and a divergent section. The tapered section makes the gas smoothly enter the throat (15 ~ 30o); the throat makes the liquid and gas evenly mixed for mass migration and energy transfer; the gradual expansion section converts the kinetic energy of the gas-liquid mixed medium into pressure energy to compress the pumped gas. (5 to 8o)
The diffusion pump oil in the boiler is heated and boiled to form steam;

The oil steam enters the umbrella nozzle through the guide tube to form a supersonic steam flow;

Due to the density gradient, the pumped gas above the oil vapor jet diffuses into the interior of the vapor jet, gains momentum and is carried on the pump wall;

The oil vapor condenses on the wall of the water-cooled pump, releases the gas, and is pumped away by the front-stage pump;

The condensed oil droplets flow back to the boiler along the pump wall.

The working pressure of the vacuum pump should meet the limit vacuum and working pressure requirements of the vacuum equipment.

Correctly select the operating point of the vacuum pump.

Under its working pressure, the vacuum pump should be able to discharge all the gas generated in the process of vacuum equipment.

Understand the composition of the pumped gas, whether the gas contains condensable vapor, whether there is particle dust, whether there is corrosion, etc.

Requirements for oil contamination of vacuum equipment. If the equipment is strictly required to be oil-free, various oil-free pumps should be selected.

Vacuum pump price, operation and maintenance costs.