Roots vacuum pump

Roots vacuum pump (abbreviated as Roots pump) refers to a variable volume vacuum pump that is equipped with two blade shaped rotors that rotate synchronously in opposite directions inside the pump. There are small gaps between the rotors and between the rotors and the inner wall of the pump casing, but they do not touch each other.

Roots vacuum pumps have been tested for long-term operation in equipment such as petroleum, chemical, plastic, pesticide, and turbine rotor dynamic balancing, so they should be vigorously promoted and applied in China. It is also widely used in industries such as petroleum, chemical, metallurgical, and textile. Vacuum pump accessories are used as vacuum pump mufflers for noise control of vacuum pumps.

Working principle

Roots pump is a vacuum pump without internal compression, usually with a very low compression ratio, so high and medium vacuum pumps require a front-end pump.

A vacuum pump that relies on the synchronous and reverse rotation of a pair of blade shaped rotors inside the pump chamber to move gas and achieve pumping.

Roots vacuum pump refers to a mechanical vacuum pump with a pair of synchronous high-speed rotating sole shaped rotors. This pump cannot pump air separately, and the front stage needs to be equipped with oil seals, water rings, etc. to directly exhaust the atmosphere.

Its structure and working principle are similar to that of a Roots blower, and its suction port is connected to the main vacuum pump of the evacuated container or vacuum system during operation. The rotors of this vacuum pump are not in contact with each other, and the rotors are not in contact with the pump casing; No need for oil lubrication. The rotor profile includes circular arcs, involutes, and cycloids. The volume utilization rate of involute rotor pumps is high, and the machining accuracy is easy to ensure, so the rotor profile is often an involute profile.

Structural composition

The arrangement of the two rotors of a Roots vacuum pump in the pump body determines the overall structure of the pump. There are generally three overall structural arrangements for Roots vacuum pumps both domestically and internationally:

1. Vertical: The axes of the two rotors are installed horizontally, but the plane formed by the axes of the two rotors is perpendicular to the horizontal plane. This structure allows the inlet and outlet of the pump to be set horizontally, making assembly and connecting pipelines more convenient. But its disadvantage is that the center of gravity of the pump is too high, and its stability is poor during high-speed operation. Therefore, except for small-sized pumps, not many of them adopt this structural type.

2. Horizontal: The axes of the two rotors are installed horizontally, and the plane formed by the axes of the two rotors is in a horizontal direction. The inlet of this structure of the pump is above the pump, and the outlet is below the pump (there are also the opposite). The exhaust outlet below is usually connected horizontally, so the intake and exhaust directions are perpendicular to each other. The exhaust port is connected to a three-way pipe that opens in two directions, with one end connected to the exhaust pipe and the other end connected to a bypass valve. The characteristic of this structure is a low center of gravity and good stability during high-speed operation. Large and medium-sized pumps both domestically and internationally often adopt this structural type.

3. Vertical shaft type: Some Roots pumps abroad have their two rotor axes installed perpendicular to the horizontal plane. The assembly clearance of this structure is easy to control, the rotor assembly is convenient, and the footprint is small. However, the assembly and disassembly of transmission mechanisms such as gears are inconvenient, and the lubrication device is also complex.

4. Roots pump with overflow valve: In order to prevent accidents caused by overload, a reliable safety protector is installed on the Roots pump, that is, an overflow valve is installed on the bypass pipeline. When the exhaust port is at the specified pressure, the relief valve is closed. When the exhaust pressure exceeds the specified pressure, the valve of the overflow valve is automatically pushed open to produce overflow. After the exhaust pressure returns to normal, the overflow valve closes on its own. It can automatically adjust and is also the allowable pressure difference device of the pump, so the biggest advantage of the relief valve is that it allows the Roots pump to operate continuously in various pressure ranges together with the preceding pump.

5. Roots pump with steam condenser: When steam needs to be pumped, the pumping unit must be designed with a condenser that will condense the steam. This condenser can be installed before or after the pump, rather than on the pump body of the Roots pump. In some cases, condensation and heat absorption can reduce the heating of Roots pumps.

Structural characteristics

(1) Has a larger pumping speed within a wider pressure range;

(2) The rotor has good geometric symmetry, resulting in low vibration and smooth operation. There are gaps between the rotors and between the rotor and the housing, which do not require lubrication and have low friction loss. This can greatly reduce the driving power and achieve higher speeds;

(3) The pump chamber does not require oil sealing and lubrication, which can reduce the pollution of oil vapor to the vacuum system;

(4) There is no compression in the pump chamber and no exhaust valve. Simple and compact structure, insensitive to dust and water vapor in the extracted gas;

(5) Low compression ratio results in poor hydrogen extraction efficiency;

(6) The surface of the rotor is a complex curved cylindrical surface, which is difficult to process and inspect. Roots vacuum pumps have experienced rapid development both domestically and internationally in recent years. It has been widely used in industries such as smelting, petrochemicals, electrical engineering, and electronics.  

Performance characteristics

The characteristics of Roots vacuum pump are: fast start-up, low power consumption, low operation and maintenance costs, high pumping speed, high efficiency, insensitivity to small amounts of water vapor and dust contained in the pumped gas, high pumping rate in the pressure range of 100-1 Pa, and the ability to quickly remove suddenly released gas. This pressure range is exactly between the oil sealed mechanical vacuum pump and the diffusion pump. Therefore, it is often connected in series between a diffusion pump and an oil sealed mechanical vacuum pump to increase the pumping volume in the intermediate pressure range. At this point, it is also known as a mechanical booster pump.

Application

Roots vacuum pumps are widely used in smelting, degassing, rolling in vacuum metallurgy, as well as in vacuum distillation, vacuum concentration, and vacuum drying in the chemical, food, and pharmaceutical industries. Vacuum pump accessories are vacuum pump silencers used for noise control of vacuum pumps.