1. Overview

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

• Coarse vacuum 105~103 Pa

• Low vacuum 103~10 -1 Pa

• High vacuum 10-1~10 -6 Pa

• High vacuum 10-6~10 -10 Pa

• Extremely high vacuum <10-10Pa

 

1. Classification of vacuum pump

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.

 

Variable volume vacuum pump

• Reciprocating

• Rotary type (rotary vane, slide valve, liquid ring, roots, screw, claw rotor)

• Other types

 

2. Working pressure range of various vacuum pumps

3. 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

2. Common 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

Schematic diagram of steam jet pump

Principle of steam jet pump

Working steam with pressure ρ0 and velocity ω0 flows through the Laval nozzle (see Laval nozzle). The velocity of the jet at the outlet of the nozzle rises to supersonic ω1, while the pressure drops to ρ1. Therefore, it will be sucked by the exhaust gas, collide with the high-speed steam flow, mix and exchange energy, and flow to the diffuser. In the diffuser contraction section, the steam and the pumped gas are further mixed to become a uniform mixed gas. In this process, the velocity of the mixed gas begins to decrease continuously, while the pressure increases continuously, producing a positive shock wave near the diffuser throat. The velocity of the gas mixture drops sharply from supersonic to subsonic, while the pressure rises sharply. At the outlet of the diffuser, when the pressure rises to atmospheric pressure or the inlet pressure of the subsequent ejector, the mixed gas is discharged.

Advantages:

No mechanical moving parts; stable and durable; fast vacuuming; compact, small footprint; safe work; low investment cost; strong adaptability; simple structure and light weight.

Disadvantages:

Moving fluid supply; wastewater treatment; process contamination; difficulty in fault finding; interstage cooling; often multistage; freezing below 400PaA

 

2. Wet pump

liquid ring vacuum pump

Working principle of 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.

 

Basic types of liquid ring vacuum pumps

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.

 

Advantages:

The structure is simple, the manufacturing precision is not high, and it is easy to process.

The structure is compact, the pump has a high number of revolutions, and can generally be directly connected to the motor without a deceleration device. Therefore, with a small structural size, large exhaust volume can be obtained, and the area occupied is also small.

The compressed gas is essentially isothermal, I .e. the temperature of the compressed gas process changes very little.

Because there is no metal friction surface in the pump cavity, there is no need to lubricate the pump, and the wear is very small. The sealing between the rotating member and the stationary member can be accomplished directly by a water seal.

The suction is uniform, the work is stable and stable, and the maintenance is convenient.

Disadvantages:

Low efficiency, generally around 30%, better up to 50%.

The vacuum degree is low, which is not only limited by the structure, but also limited by the saturated vapor pressure of the working liquid. Using water as working fluid, the ultimate pressure can only reach 2000~4000Pa. With oil as working fluid, up to 130Pa.

Sealing liquid circulation and pollution

 

rotary vane pump

 

Principle

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.

 

oil seal rotary vane pump

Advantages

Good ultimate pressure; low noise; wide application range

 

Disadvantages

Oil seal liquid pollution; very sensitive to corrosion; high maintenance costs; pumping speed range is limited; not suitable for pumping in addition to high oxygen, explosive, the pump seal oil from the chemical role, and containing particles of dust gas.

 

The main problem of wet pump

Sealing liquid supply; pollution; wastewater treatment; corrosion; steam pressure limitation (especially liquid ring pumps); stability; operating costs.

 

3. Dry pump

Dry vacuuming:

No contamination of industrial process streams.

Deep vacuum, more flexible operation.

No waste water, (less or) no waste disposal costs.

Lower maintenance costs.

Recover 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

Lower operating costs

Extended service intervals and bearing life.

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

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

 

Dry Pump Technology: Types of Pumps

Roots pump, screw pump, claw pump

 

Roots pump

 

screw pump

Screw pump principle

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.).

 

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

 

Principle of claw pump

 

Claw pump compression ratio

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

 

3. Summary

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.

 

Dry pump has a competitive advantage:

No contamination of industrial process streams

Higher vacuum and more flexible operation

No wastewater, (less or) no wastewater treatment expenditure

Lower maintenance costs

Lower expenses such as water and electricity → lower total operating costs