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Last time we learned how the thermodynamic properties of water contribute to the phenomenon of cavitation, and how liquids exist in three states, solid, liquid, or vapor, depending on temperature and surrounding air pressure. Our example of an open pot of water being heated on a stove demonstrated that once water temperature rose above 212ºF, it changed to steam, which initiated the cavitation process. Today we’ll see how decreasing pressure contributes to cavitation.
How Decreasing Pressure Contributes to Cavitation
Cavitation can occur without a heat source. In our pot example, we can start the cavitation process by simply decreasing the pressure of the air resting on top of the water, thereby also decreasing the water’s pressure. Normally atmospheric pressure on Earth exists at around 15 pounds per square inch (PSI). But if we introduce a vacuum pump to an enclosed space, we can create an internal pressure which is lower than the surrounding atmospheric pressure outside the pot. In other words, we create a vacuum. A vacuum is any air pressure lower than atmospheric pressure. This vacuum environment produces an entirely new set of circumstances under which cavitation can occur. In fact, creating a vacuum makes it possible to boil water without using any heat! As we learned in a past blog on the different forms of heat energy, the boiling point of water varies depending on the location of the stovetop, whether it’s in a place of low altitude, like New Orleans, or higher altitude, like Denver. But if we apply a tight lid to the pot and isolate its internal atmosphere from surrounding atmospheric pressure, you create a closed environment. This allows us to manipulate the pot’s internal pressure. When we attach a vacuum pump to remove air, we reduce the air pressure bearing down on the water inside. With much of the air removed, pressure inside the pot drops below normal atmospheric pressure existing outside the pot, and we discover that at 0.25 PSI water turns to steam at a mere 59ºF and cavitation can begin. That’s right, you can boil water without using heat. Next time we’ll apply our knowledge of water pressure and temperature to an industrial setting and see how cavitation occurs inside pumps. opyright 2018 – Philip J. O’Keefe, PE Engineering Expert Witness Blog ____________________________________ |
