How to Select Rotary Vane Vacuum Pumps and Systems
Rotary vane vacuum pumps create regions of low pressure by moving fluid using a rotating vane assembly in the pumping chamber. Typically there are two or more rotating vanes that move the gas or fluid from inlet to outlet. Rotary vane pumps can be employed universally throughout the entire low and medium vacuum ranges.
Pump Design and Operation
Rotary vane vacuum pumps are mechanical vacuum pumps that operate via rotary positive displacement. Pumping systems consist of a housing, an eccentrically installed rotor, vanes that move radially, and an inlet and outlet. Larger pumps may have additional impeller vanes.
Image Credit: Pfeiffer Vacuum
As the rotor turns, the ends of the vane barely touch the housing, creating a seal from inlet to outlet. The inlet and outlet are often perpendicular, however for vacuum service applications two inlets may be used; one for air supply and the other for the pumped media. The rotor and vanes divide the working chamber into two separate spaces having variable volumes. As the rotor turns, gas flows into the enlarging suction chamber until it is sealed off by the second vane. The enclosed gas is compressed until the outlet valve opens against atmospheric pressure, pushing gas molecules out the port.
This video provides a visual depiction of a two-stage rotary vane vacuum operation.
Video Credit: TheVacuumator - Youtube user
There are a number of different types of specifications listed by manufacturers to describe rotary vane vacuum pumps and systems.
The most important specifications to consider for rotary vane vacuum pumps are ultimate operating vacuum and pumping speed.
Ultimate (maximum) operating vacuum or ultimate pressure is the lowest pressure which the vacuum pump can generate (typically within a set time). Buyers should note the conditions or assumptions used to obtain this value, since manufacturers may provide this rating using assumptions that are not realistic under normal operating conditions (e.g. ignoring the pressure of condensable gases like water vapor).
Pumping speed or vacuum flow is the volumetric rate at which gas is emptied from the vacuum chamber, typically given in ft3/min (cfm), m3/s, L/min, or gal/min (gpm). A pump's maximum achievable pumping speed (over its entire pressure range) is always referred to as its rated pumping speed, and pumping speeds listed by manufacturers are typically referenced to STP (standard temperature and pressure). Pumping speed needs to be matched according to the needs of the application, which are dependent on the system's chamber volume, desorption, and process gas loads. Pumping speeds for rotary vane pumps vary between 1 and 650 cfm (cubic feet per minute) based on the design and application.
Selection Tip: Keep in mind that the speed of the pump itself is seldom the actual pumping speed in the system's chamber. Rated pumping speeds are typically defined under the same conditions as the ultimate pressure, which should be noted when comparing performance specifications to the needs of the application.
Rotary vane vacuum pumps are typically powered by an electric source, either alternating current (AC) or direct current (DC).
AC power is the most common source for rotary vane vacuum pumps and systems. Single phase AC motors are well established and are typically cheaper than three phase AC motors, but are also bigger and less efficient (for equivalent horsepower).
DC power sources use DC power from a battery.
Rotary vane vacuum pumps and systems can be either dry (oil-less) or oil-sealed based on the pump design.
Most rotary vane pumps are oil-sealed, meaning they use oil as an operating fluid to perform multiple functions within the pump. Oil is used to lubricate all moving parts, seal the outlet valve, compress the gap between vanes and the working chamber, and ensures an optimal temperature balance. Rotary vane pumps may use one of three specific lubrication styles.
The splash lubrication style (the most common) involves splashing oil onto components from an oil bath.
The oil-flooded lubrication style involves the heavy application of oil for moving parts. Oil contamination is likely in pumps incorporating this lubrication style.
A less common style called positive pressure involves maintaining constant oil pressure for the highest level of lubrication
Other rotary vane pumps are designed dry or oil-less, meaning they use permanently sealed bearings or other isolation technology to eliminate oil in the fluid train. The moving parts of the pump typically still use oil, but in these designs oil is not used to seal valves or spaces in the working chamber, eliminating the possibility of fluid contamination. Dry pumps are typically more expensive than oil-sealed pumps.
Rotary vane vacuum pumps and systems may incorporate one or more stages in order to generate a vacuum. A stage consists of the working portions of the pump, specifically the working chamber, rotor, and vanes. For many applications, multi-stage pumps may be more efficient than using separate pumps in series.
Rotary vane vacuum pumps are most commonly single stage pumps. Single-stage pumps move gas molecules directly from the evacuated chamber into the atmosphere. They typically have ultimate pressures around 10-2 Torr.
Some rotary vane pumps are two stage pumps, which evacuate gas molecules in two stages for obtaining lower ultimate pressures up to 10-3 Torr.
In rare cases, rotary vane pumps may be designed with three stages for increased ultimate vacuum. It may also involve a system in which a one or two-stage rotary vane pump is used in combination with another type of pump.
Rotary vane pumps with four or more stages typically exist as systems in which a one or two-stage rotary vane pump is used in combination with other types of pumps. These systems are employed in applications requiring extremely high vacuums.
Rotary vane vacuum pumps come in two types: those used to generate coarse vacuum levels (760 to 1 Torr) and those used to generate rough vacuum levels (760 to 10-3 Torr). Design of these pump types varies primarily based on the number and tolerance of vanes in the pumping chamber. Rough rotary vane pumps, which are commonly used as backing pumps for high vacuum pumps, have more vanes than coarse pumps and operate with more exact tolerances and higher efficiencies. Coarse rotary vane pumps are used in applications such as freeze drying, vacuum filtering, vacuum impregnation, materials handling, meat packing, and "house" vacuum systems, most of which do not require tight tolerances.
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