There are two "8-shaped” rotor mounted perpendicularly to each other on a pair of parallel shafts in the pump chamber of roots pump, these two rotors are drove by a pair of gears (transmission ratio=1) to keep synchronous s rotation movement in opposite direction. Also, there are certain gaps between two rotors, and between the rotor and inner wall of the pump casing to ensure the high revolving speed operation.
Since the Roots pump is a vacuum pump with no internal compression and usually has a very low compression ratio, high and medium vacuum pumps need backing pumps. The ultimate vacuum of roots pump depends on the structure and manufacturing accuracy of the pump itself, as well as the ultimate vacuum of the backing pump. In order to increase the ultimate vacuum of the pump, roots pump can be used in series. The Roots pump works like a Roots blower. Due to the continuous rotation of the rotor, the gas is sucked from the intake port into the space between the rotor and the pump casing, and then it is exhausted through the exhaust port. Since the space is completely closed after inhalation, there is no compression and expansion of the gas in the pump chamber.
However, when the top of the rotor turns over the edge of the exhaust port and the space between the rotor and the pump casing communicates with the exhaust, due to the high gas pressure at the exhaust side, part of the gas is returned to the space, which makes gas pressure suddenly increase. As the rotor continues to rotate, the gas exits the pump.
Working Principle of The Rotary Vane Pump
Rotary vane vacuum pump (referred to as rotary vane pump) is a kind of oil seal mechanical vacuum pump. It can be used alone or as a backing pump for other high vacuum pumps or ultra-high vacuum pumps. It has been widely used in metallurgy, machinery, military industry, electronic, chemical, light industry, petroleum, pharmaceutical and other production and research departments.
Rotary vane pump can pump the dry gas out of the sealed container. And it can also pump a certain amount of condensable gas out if a gas ballast device is attached. However, it is not suitable for the removal of oxygen-rich gases, as well as the gas that is corrosive to metals, chemically react with pump oils, and contain particulate dust.
As one of the most basic vacuum obtaining devices in vacuum technology, rotary vane pumps are mostly small and medium sized pumps. Rotary vane pumps are available in single and double stages. The so-called two-stage is the structure of the two single-stage pumps in series. The rotary vane pumps are generally two-stage, in order to obtain a higher degree of vacuum.
The relationship between the pumping speed and the inlet pressure of rotary vane pump is defined as below: Under the inlet pressures of 1333 Pa, 1.33 Pa, and 1.33×10-1 (Pa), the pumping speed values shall respectively not be lower than 95%, 50% and 20% of the pump's nominal pumping speed.
The rotary vane pump is mainly composed of pump body, rotor, rotary vane, end cap, spring and so on. A rotor is installed eccentrically in the cavity of the rotary vane pump. The outer circle of the rotor is tangent to the inner surface of the pump cavity (there is a slight gap), and two rotors with springs are mounted in the rotor slot. When rotating, the top of the rotary vane is kept in touch with the inner wall of the pump chamber by the centrifugal force and the tension of the spring, and the rotor rotates to drive the rotary vane to slide along the inner wall of the pump chamber.
Two rotors divide the crescent-shaped space surrounded by the rotor, the pump chamber and the two end caps into three parts A, B and C. When the rotor rotates in the direction of the arrow, the volume of the space A that connects with the intake port is gradually increasing, it is in the suction process. At the same time, the volume of the space C that connects with the exhaust port is gradually reduced and it is in the exhaust process. The volume of the centered space B is also gradually reduced and it is in the process of compression. Since the volume of the space A is gradually increased (expanded), the gas pressure is reduced, and the external gas pressure at the inlet of the pump is higher than the pressure in the space A, so the gas is sucked in. When the space A is insulated from the suction port, it is turned to the position of the space B, the gas starts to be compressed, the volume gradually decreases, and finally the exhaust port is connected. When the compressed gas exceeds the exhaust pressure, the exhaust valve is pushed open by the compressed gas and the gas passes through the oil layer in the tank to the atmosphere. By the continuous operation of the pump, continuous pumping can be achieved. If the exhausted gas passes through the air passage and transferred to another stage (low vacuum stage), it will be pumped away from the low vacuum stage, compressed by the low vacuum stage, and discharged to the atmosphere, which means a two-stage pump. At this time, the total compression ratio is borne by two stages, thereby the ultimate vacuum can be increased.
Note: Most vacuum pump that equipped on vacuum package machine are rotary vane pumps.