Diaphragm pumps - also known as membrane pumps - are positive displacements pumps used in the rough vacuum range from 0,5 to 1000 mbar. A membrane (rubber, Viton ® or Teflon® ) is moving up and down in the pump body compressing and exhausting gas coming from the inlet. Inlet and exhaust check valves are actuated by the pressure difference.
A simple animation of the principle can be found below:
Working principle of a mordern diaphragm pump. Courtesy of KNF, Freiburg, Germany.
Diaphragm vacuum pumps represent an ecological replacement for water jet pumps in laboratory use. Pumps utilise a dry compression process avoiding waste, water or oil. With a single pump chamber ('pump head') ultimate pressures of 50 mbar are achieved. This ultimate pressure is limited due to the remaining dead volume between pump head and diaphragm. Two pump heads in series can reach 3 mbar and three in series even 0,5 mbar. In order to rationalise the production, many manufacturers produce pump chambers and diaphragms of the same size in large quantity. This is assembled in series for lower ultimate pressure or in parallel for higher pumping speed. Membranes of Teflon® are resistant to solvents, thus suitable in chemical processes.
Pumping speeds from 0,1 to 5 m³/h are available on the market. The higher pumping speeds are covered then by scroll pumps. Some pumps can be operated with 24V-DC-motors enabling them to be incorporated in mobile instruments. Some have variable speed motors to reduce pumping speed (and noise) if not needed and to extend the service interval.
Figure 1: 'cutaway view of a single stage diaphragm pump. Courtesy of KNF, Freiburg, Germany.
Figure 2: parts subject to wear: diaphragm, valves and seals
The chemical resistant versions are widely used in chemical laboratories. Examples are rotary evaporators, filtration, and solid phase extraction. Standard pumps are used medical or pharmaceutical processes, drying and freeze drying, gel drying, vacuum cushions and vacuum mattresses, or small vacuum ovens.
Figure 3: chemical resistant rotary evaporator system with pressure controller
Pumps with ultimate pressures of 3 mbar or better are frequently used in small high vacuum pumping systems backing the wide range turbomolecular pumps.
Figure 4: Turbomolecular pump system with diaphragm pump
Most modern diaphragm vacuum pumps today offer a maintenance interval of more than 10,000 hours. The parts that wear are the membranes, the valve plates and the gaskets. In most cases the user observes a poor ultimate pressure due to leaking or ruptured membranes or valves. Simple, low cost spare part kits are available, and the maintenance can be done by the user itself. No special skills or tools are required - simply see the manuals and watch the videos on the web.
In order to be satisfied with these pumps for long time, avoid pumping dust and liquids. If this happens the pump must be opened and cleaned. If liquid has entered the pump, it must be turned on the side that the liquid can escape through the exhaust.
Diaphragm pumps have become the work horse in laboratories where smaller pumping speeds and ultimate pressures above a few mbars are sufficient. They have replaced the water polluting water jet pumps. They are also found in many high vacuum systems as small dry backing pumps today. With the advanced design in the membrane structure today, trouble-free operation for far more than 10,000 hours is possible followed by an easy service.
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