A rotary pump can achieve vacuum pressures as low as 10−6 mbar (0.0001 Pa). This is possible with multi-stage rotary-vane vacuum pumps, which use two or more rotary-vane pump mechanisms to enhance the pressure. These pumps force the fluid through the stages, allowing for a more efficient vacuum.
The rotary vane pump is the most frequently used pump for achieving vacuum. It is a pre-vacuum pump that can directly evacuate atmospheric pressure. The pump has a rotating speed of approximately 1500 revolutions per minute and uses a radial, movable plug. The vacuum chamber of the pump is greased with oil, and a rough vacuum can be achieved with a double-stage pump.
It is important to consider the specifications of the pump when choosing the best one for your lab. Some of these specifications include motor speed (rpm), horsepower (hp), noise level (dB), hose barb (mm), and net weight (Kg). These factors can help determine the suitability of the pump for specific applications.
When selecting a lab vacuum pump, it is recommended to choose based on the needs of the application. Suction applications, filtration, and aspiration can be effectively performed at a few hundred Torr, which is within the range of dry pumps. However, for freeze dryers, molecular distillation applications, and Schlenk lines, vacuum levels of 10–3 Torr are typically needed, which can be achieved with oil-sealed, rotary vane pumps.
The evaporation rate in rotary evaporators depends on various factors such as the type of solvent, bath temperature, condensing power, size of the rotovap, and type of condensers. Different solvents may require different vacuum levels for efficient evaporation. Low boiling solvents can be evaporated using two-stage chemistry diaphragm pumps with a vacuum down to 7 mbar, while medium and high boiling solvents may require three- or four-stage chemistry diaphragm pumps with a vacuum to 1.5 mbar or less.
The volume and required pumping speed also play a role in selecting the appropriate pump. Benchtop rotary evaporators with volumes up to 5 litres typically require chemistry diaphragm pumps with pumping speeds of approximately 1-2 m3/h. On the other hand, large-scale rotary evaporators with volumes up to 20 litres may need chemistry diaphragm pumps with pumping speeds of around 3-5 m3/h.
In summary, rotary pumps can achieve vacuum pressures as low as 10−6 mbar. The selection of a vacuum pump depends on the specific application, including factors such as required vacuum level, volume, pumping speed, and the type of solvent being used.
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