Recently, diaphragm pumps have becoming 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. 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.
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 can not 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.
Diaphragm pumps offering such a low ultimate pressure are suited as backing pumps for turbomolecular pumps with fully integrated Scroll 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.
Fig. 2.1 Schematic on the design of a diaphragm pump stage (Vacuubrand)
Fig. 2.2 Principle of operation for a two-stage diaphragm pump (Vacuubrand)