Roots Vacuum Pump (abbreviated as: Roots Pump) refers to a variable capacity vacuum pump equipped with two leaf-shaped rotors that rotate synchronously in opposite directions within the pump. There is a small gap between the rotors, and between the rotors and the inner wall of the pump casing, without any contact.
Roots vacuum pumps have undergone long-term testing in devices such as petroleum, chemical, plastic, pesticides, turbine rotor dynamic balancing, aerospace space simulation, etc., and thus should be extensively promoted and applied domestically. They are also widely used in industries such as petroleum, chemicals, metallurgy, and textiles.
Roots pumps are vacuum pumps without internal compression, typically having a low compression ratio, which is why high and medium vacuum pumps require a fore-stage pump.
Roots pumps achieve suction by the synchronous and opposite rotation pushing action of a pair of leaf-shaped rotors within the pump chamber to move gases.
Roots vacuum pump refers to a mechanical vacuum pump with a pair of shoe-shaped rotors rotating synchronously at high speed. This pump cannot independently perform suction; it requires a preceding stage pump with an oil seal or water ring that can directly discharge to the atmosphere.
Its structure and working principle are similar to a Roots blower. During operation, its suction port connects to the vacuum chamber or the main suction pump of the vacuum system.
This type of vacuum pump ensures no contact between the rotors and between the rotors and the pump casing, with gaps generally ranging from 0.1 to 0.8 millimeters; it does not require oil lubrication. Rotor profiles may include arc, involute, and cycloid shapes.
Involute rotor pumps have high volumetric efficiency and easier-to-guarantee processing accuracy, which is why rotor profiles are mostly involute. Roots vacuum pumps can reach speeds of 3450 to 4100 revolutions per minute; pumping rates vary from 30 to 10,000 liters per second (1 liter = 10^-3 cubic meters); ultimate vacuum: single stage is 6.5×10^2 Pa, double stage is 1×10^3 Pa.
The ultimate vacuum of Roots pumps depends not only on the structure and manufacturing precision of the pump itself but also on the ultimate vacuum of the fore-stage pump. To improve the ultimate vacuum, Roots pumps can be used in series.
Similar to Roots blowers, as the rotors continuously rotate, the air being sucked in passes from the intake port into the space v0 between the rotor and the pump casing, and is then discharged through the exhaust port.
After suction, since the v0 space is completely enclosed, there is no compression or expansion of gas within the pump chamber.
However, when the top of the rotor passes the edge of the exhaust port, and the v0 space connects to the exhaust side, due to higher gas pressure on the exhaust side, some gas will backflow into the v0 space, causing a sudden increase in gas pressure. As the rotor continues to turn, the gas discharges out of the pump. Inside the pump chamber of a Roots pump, there are two "8"-shaped rotors mounted perpendicularly on a pair of parallel axes, driven by a pair of gears with a transmission ratio of 1, rotating synchronously in opposite directions. There is a certain gap between the rotors and between the rotors and the inner wall of the pump casing, allowing high-speed operation.
The arrangement of the two rotors inside the pump body determines the overall structure of the pump.
Generally, there are three structural layout schemes for domestic and international Roots vacuum pumps:
1. Vertical: The axes of the two rotors are horizontally installed, but the plane formed by the rotor axes is perpendicular to the horizontal plane. This structure allows the intake and exhaust ports of the pump to be horizontally set, facilitating assembly and piping connections. However, its drawback is the high center of gravity of the pump, which makes stability poor during high-speed operation. Therefore, except for smaller pumps, this structural type is not widely used.
2. Horizontal: The axes of the two rotors are horizontally installed, and the plane formed by the rotor axes is in the horizontal direction. In this structure, the intake port is on the upper part of the pump, and the exhaust port is at the bottom (or vice versa). The lower exhaust port generally leads horizontally outward. The intake and exhaust directions are perpendicular to each other. The exhaust port connects with a T-pipe, with one end connected to the exhaust duct and the other end sealed or connected to a bypass valve as needed. This structure's characteristics are low center of gravity and good stability during high-speed operation. This structural type is commonly adopted for medium and large pumps domes国内estic and abroad.
3. Vertical Axis: In some foreign Roots pumps, the axes of the two rotors are vertically installed relative to the horizontal plane. This structure makes it easy to control assembly gaps, facilitates rotor assembly, occupies minimal floor space, but makes it inconvenient to disassemble gears and other transmission mechanisms, and the lubrication setup is more complex. Once the overall structure is determined, the pump body's structure and shape are similarly decided.
4. Roots Pump with Overflow Valve: To prevent accidents from overloading, Roots pumps have a relatively reliable safety device, namely an overflow valve installed on the bypass line. The overflow valve remains closed when the exhaust manifold pressure is within specified limits. If the pressure exceeds those limits, the overflow valve opens automatically to release excess pressure and then closes when pressure returns to normal. An overflow valve serves as an automatic pressure-differential control, allowing the pump and its fore-stage pump to operate continuously across various pressure ranges. Using this design can shorten vacuum chamber evacuation times by 30-50% when in a rough vacuum state. For larger pumps, the overflow valve is mounted externally on the bypass line, whereas smaller roots 被pump models have the valve inside the pump casing.
5. Roots Pump with Steam Condenser: In scenarios requiring steam extraction, pumping systems must be designed with condensers to cool the steam, which are installed either before or after the pump rather than directly on the Roots pump body. In some cases, sublimation cooling by the condenser reduces heating in the Roots pump. If dual condensers are used, appropriate solvents can be utilized for cleaning during maintenance, ensuring smooth steam flow in the conduits.
High pumping speed over a wide pressure range;
Quick start-up and immediate operation;
Not sensitive to dust and water vapor in the extracted gas;
Rotors do not need lubrication; no oil in the pump chamber;
Low vibration, good rotor dynamic balance, no exhaust valve;
Low driving power and minimal mechanical friction loss;
Compact structure and small footprint;
Low operating and maintenance costs.
Hence, Roots pumps are widely applied in industries such as metallurgy, petrochemicals, paper manufacturing, food, and electronics.
The working principle and performance characteristics of Roots pumps are essentially similar to cam pumps, but the rotors are of the Roots type. They can transport liquids with viscosities up to several tens of thousands of centipoise.
Roots pumps mainly comprise two rotors with opposite rotation directions situated inside the pump body, driven by a pair of synchronous gears. Inside the pump body, Roots pump rotors engage with each other but maintain a gap. The gap size primarily depends on the liquid viscosity. If the viscosity exceeds a certain range, the gap must be adjusted accordingly.
Regularly check the oil level position of the Roots pump. If it does not meet the requirements, adjust it to the specified level. The standard oil level during Roots pump operation is at the center of the oil gauge.
The oil change interval for the Roots pump should be based on actual usage conditions and whether it meets performance requirements, decided by the user. Generally, for new Roots pumps extracting clean and dry gases, it is recommended to change the oil after approximately 100 hours of operation. Once no black metal particles are visible in the oil, the interval can be extended appropriately.
Regularly check the oil quality of the Roots pump. If the oil deteriorates, replace it with new oil promptly to ensure normal pump operation.
Under normal circumstances, the Roots pump should undergo maintenance after 2000 hours of operation. This includes checking the aging of seals, inspecting for cracks in the exhaust valve plate, and cleaning any deposits from the valve plate and exhaust valve seat. Clean all parts inside the Roots pump chamber, such as rotors, blades, and springs, typically using gasoline and drying them thoroughly. For rubber components, wipe them dry with a cloth. Handle parts carefully during cleaning and assembly to prevent damage.
Add bearing lubricating oil to the bearing housing and observe that the oil level is at the oil gauge's centerline. Replace or replenish the lubricating oil in a timely manner.
Inspect the Roots pump pipeline and connections for any looseness. Manually rotate the rotor to check for smooth operation.
After reassembling the Roots pump, conduct a trial run. Typically, let it run idle for 2 hours and replace the oil twice, as volatile substances might remain inside the pump after cleaning. Proceed with normal operation if everything runs smoothly.
Start the motor and, once the Roots pump operates normally, open the outlet pressure gauge and intake Roots pump appropriately. Gradually open the gate valve once the pressure reaches the desired level and check the motor load condition. Try to regulate the flow and pressure of the Roots pump within the ranges specified on the nameplate to ensure the pump operates at its highest efficiency point for maximum energy savings.
