Posts Tagged ‘boiling water’

Centrifugal Pumps Can Create Vacuums

Thursday, February 8th, 2018

    Last time we performed an engineering experiment that demonstrated how we can lower the boiling point of water inside a lidded pot without applying heat if we use a vacuum pump to lower the pot’s internal pressure.   We discovered that when pressure was lowered to 0.25 pounds per square inch (PSI), the water inside the pot turned to steam at a mere 59ºF, which initiated the cavitation process.   Today we’ll see how centrifugal pumps can also create vacuums to initiate cavitation.


Centrifugal Pumps Can Create Vacuums

Centrifugal Pumps Can Create Vacuums


    As we learned in a past blog, centrifugal pumps contain rotating impellers within a housing called a volute.   This housing has an inlet, known as an eye, where water flows into the pump from a pipe, and an outlet, known as a discharge, where water flows out of the pump.   The centrifugal pump creates a vacuum by mimicking the action of sucking soda through a straw.    The spinning impeller draws water into the housing by creating low pressure at the inlet, and if the pressure gets low enough, we’ll recreate what happened in our vacuum pump and pot experiment.   Water will boil at temps far lower than normal boiling point of 212 ºF.   Just as in our experiment, if pressure is lowered to 0.23 PSI, water present at the pump inlet will boil at 59ºF, causing thousands of tiny steam bubbles to form and the pump to cavitate.

    They’re just tiny bubbles, so what harm can they do? We’ll find out next time.

opyright 2018 – Philip J. O’Keefe, PE

Engineering Expert Witness Blog



Forms of Heat Energy – Boiling Water and Atmospheric Pressure

Sunday, July 21st, 2013

      If you’ve ever baked from a pre-packaged cake or cookie mix, you’ve probably noticed the warning that baking times will vary.   That’s because the elevation of the area in which you’re doing the baking makes a difference in the baking time required.   Living in New Orleans?   Then you’re at or below sea level.   In Colorado?   Then you’re above sea level.   Your cake will be in the oven more or less time at the prescribed temp, depending on your location.

      Last time we learned how the heat energy absorbed by water determines whether it exists in one of the three states of matter, gas, liquid, or solid.   We also learned that at the atmospheric pressure present at sea level, which is about 14.7 pounds per square inch (PSI), the boiling point of water is 212°F.   At sea level there are 14.7 pounds of air pressure bearing down on every square inch of water surface.   Again, I said sea level for a reason.

      The boiling point of water, just like cake batter baking times, is dependent upon the amount of pressure that’s being exerted on its surface from the surrounding atmosphere.   When heat energy is absorbed, it causes the water or cake batter molecules to move around.   In fact, the temperature measured is a reflection of this molecular movement.   As more heat energy is absorbed, the molecules move more and more rapidly, causing temperature to increase.

      When the water temperature in our tea kettle reaches its boiling point of 212°F at sea level, the steam molecules in the bubbles that form have enough energy to overcome the atmospheric pressure on the surface of the water.  They become airborne and escape in the form of steam.


      If we’re up in the Rockies at say an altitude of 7000 feet above sea level, the atmospheric pressure is only about 10.8 PSI.   There’s just less air up there.   That means there’s less air pressure resting upon the surface of the water, so it’s far easier for steam molecules to form into bubbles and leave the surface.   As a result the boiling point is much lower in the Rockies than it is at sea level, 196°F versus 212°F.

utility boiler expert

     So what if the water was boiling in an environment that had even higher pressures exerted upon it than just atmospheric?   We’ll see how to put this pent-up energy to good use next week, when we begin our discussion on how steam is used within electric utility power plants.