How does a diaphragm pump work?
Diaphragm vacuum pumps are oscillation displacement pumps. They belong to the gas transfer vacuum pump family. The range they provide means that they fall into the category of rough vacuum. They are important in laboratories where pressure of only a few mbar is needed. This leads to their usage in chemical laboratories for pharmaceutical or medical processes, among other applications.
Operating principle
Diaphragm vacuum pumps are single or multi-stage dry compressing vacuum pumps (diaphragm pumps having up to four stages are being manufactured). Here the circumference of a diaphragm is tensioned between a pump head and the casing wall (Fig. 2.1). It is moved in an oscillating way by means of a connecting rod and an eccentric. The pumping or compression chamber, the volume of which increases and decreases periodically, effects the pumping action. The valves are arranged in such a way that during the phase where the volume of the pumping chamber increases it is open to the intake line. During compression, the pumping chamber is linked to the exhaust line. The diaphragm provides a hermetic seal between the gear chamber and the pumping chamber so that it remains free of oil and lubricants (dry compressing vacuum pump). Diaphragm and valves are the only components in contact with the medium which is to be pumped. When coating the diaphragm with PTFE (Teflon) and when manufacturing the inlet and exhaust valves of a highly fluorinated elastomer as in the case of the DIVAC from Leybold, it is then possible to pump aggressive vapors and gases. It is thus well suited for vacuum applications in the chemistry lab.
Fig 2.1 Schematic on the design of a diaphragm pump stage.
1) Casing lid 2) Valves 3) lid 4) Diaphragm disk 5) Diaphragm 6) Diaphragm support disk 7) Connecting rod 8) Eccentric disk
Benefits of diaphragm pumps
Recently, diaphragm pumps have become ever more important, mainly for environmental reasons. They are alternatives to water jet vacuum pumps, since diaphragm pumps do not produce any waste water. Overall, a diaphragm vacuum pump can save up to 90% of the operating costs compared to a water jet pump. Compared to rotary vane pumps, the pumping chamber of diaphragm pumps are entirely free of oil. By design, no oil immersed shaft seals are required.
Limitations of Diaphragm pumps
Due to the limited elastic deformability of the diaphragm only a comparatively low pumping speed is obtained. In the case of this pumping principle a volume remains at the upper dead center - the so called “dead space” - from where the gases cannot be moved to the exhaust line. The quantity of gas which remains at the exhaust pressure expands into the expanding pumping chamber during the subsequent suction stroke thereby filling it, so that as the intake pressure reduces the quantity of inflowing new gas reduces more and more. Thus, volumetric efficiency worsens continuously for this reason. Diaphragm vacuum pumps are not capable of attaining a higher compression ratio than the ratio between “dead space” and maximum volume of the pumping chamber. In the case of single-stage diaphragm vacuum pumps the attainable ultimate pressure amounts to approximately 80 mbar. Two-stage pumps such as the DIVAC from Leybold can attain about 10 mbar (see Fig. 2.2), three-stage pumps can attain about 2 mbar and four-stage diaphragm pumps can reach about 5 ·10-1 mbar.
Fig 2.2 Principle of operation for a two-stage diaphragm pump. Opening and closing of the valves, path and pumping mechanism during four subsequent phases of a turn of the connecting rod (a-d)
Compatibility with other pumps and applications
Diaphragm pumps offering such a low ultimate pressure such as the three head and 4 head diaphragm pumps are suited as backing pumps for turbomolecular pumps with fully integrated molecular drag stages (compound or wide range turbomolecular pumps). In this way a pump system is obtained which is absolutely free of oil, this being of great importance to measurement arrangements involving mass spectrometer systems and leak detectors. In contrast to rotary vane pumps this combination of pumps for leak detectors offers the advantage that naturally no helium is dissolved in the diaphragm pump thereby entirely avoiding a possible build-up of a helium background. However ultimate vacuum for oil sealed rotary vane pump or scroll pump is considerably better than that of even the 4 head diaphragm pumps.
