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I am looking to design a diaphragm pump for a device. I know that I would need to achieve a (negative) vacuum pressure of ~ 350-400 mmHg. I know from standard rotary pump design the pump rate should be dictated by stroke volume and speed, and assume this is the same for a diaphragm pump, with stroke length perhaps.

How can I calculate the ultimate achievable vacuum pressure? I am looking for an explanation of the idealized equations for calculating this property of a diaphragm pump.

This would be for pumping a water based fluid, although slightly more viscous.

Air
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daFireman
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1 Answers1

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Since you are looking for a vacuum that's less than full vacuum, What you are looking for is the NPSH - Net Positive Suction Head.

Essentially, when pumping matter in the liquid state to a vacuum pressure, the liquid state can boil at the reduced pressure to form gas. These "bubbles" are known as cavitation, and they are known for destroying mechanical equipment. At the vacuum pressure you are looking for, the water will boil at 70-80 Celsius, so as long as your system is under that temperature you should be fine.

When looking over the pump diagram, you should see two diagrams - the normal pressure v. flow diagram you are used to, and the NPSH diagram. This will tell you if your pump can withstand the vacuum requirements.

Ultimately, due to cavitation, you can't achieve full vacuum using water. You would need to start with a gas. And yes, vacuum pumps that go to as close as possible to full vacuum exist. However, you can't use them to pump liquid. Instead, you can use them like a vacuum sewer system, where the vacuum pump pumps the air out of a large pressure vessel that is connected to the water you want to move.

Mark
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