Laboratory vacuum pumps are one of the most ubiquitous pieces of laboratory equipment, and also one of the most difficult to choose. Vacuum pumps come in many different designs with an array of features, and it is important to carefully consider the applications for which the pump will be used before selecting a particular model.
The first decision to be made when purchasing a laboratory vacuum pump is whether a rotary vane (oilsealed) pump or diaphragm (oil-free) pump is needed:
- Rotary vane pumps are the traditional vacuum pump and are still widely used. However, they are only needed in applications where a greater vacuum is required.
- Diaphragm pumps are appropriate for most laboratory applications. They are easier to maintain and, if pumps with chemical-resistant diaphragms and valves are selected, may be used for corrosive solvents without the use of a cold trap.
For most laboratory applications, the oil and excess vacuum of a rotary vane pump creates greater service demands and a greater need for cold trap coolant to protect the oil. Moderating the excess vacuum may be accomplished by introducing air in order to reduce the vacuum to required levels. However, this practice increases noise, releases a malodorous, hazardous oil mist into the lab, and reduces the cold trap's ability to protect pumps, leading to an increased risk of pump corrosion.
Diaphragm (oil-free) vacuum pumps provide a vacuum in a range needed for most laboratory applications. Even high boiling-point solvents, such as DMF, can be evaporated at near roomtemperature (<100°F), or DMSO at about 125°F, with a good diaphragm vacuum pump.
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Having decided on a diaphragm pump, the purchaser should decide whether a chemical-resistant pump is needed, or whether a pump suited only for non-corrosive applications is sufficient (remembering to consider possible future uses). Corrosion-resistant components significantly increase the cost of a vacuum pump.
Next, the buyer should consider the depth of vacuum required to meet his or her application requirements.
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Rotary Vane Pumps
Rotary vane pumps should only be used for demanding evaporation, filtration and drying processes that cannot be supported by diaphragm pumps (<2 mbar). This type of pump is especially useful for the evaporation of higher boiling-point solvents with temperatures at or below ambient. However, when the pump is used for evaporation or sublimation, the oil becomes contaminated with the vapor. The oil must therefore be changed regularly to protect against pump damage, and a cold trap chilled with dry ice or liquid nitrogen should be installed to limit oil contamination and to protect the pump.
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Scroll pumps are an excellent alternative to rotary vane pumps where oil-free pumping is desirable. This technology is suitable for noncorrosive liquids and delivers low ultimate pressure at high speed. Dry scroll pumps are oil-free, durable, reliable and easy to maintain.
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