Saturated, Multi-Phase Systems and Proof that a Watched Pot Does Actually Boil – Part 3

After looking at the diagrams in the last post and contemplating the title of the current string – Saturated, Multiphase Systems – you may think this all sounds pretty complicated, like something that is totally foreign to you. However, I would beg to differ with you on that.

Phase Change and the World We Live In

Consider the following pictures.

Cumulo Nimbus over the midwest 02

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Clouds apporaching Atlanta - Rainbow 01

All of these pictures show water that is in all three of its phases:

  • It exists in the solid phase in the ice crystals that make up the high altitude clouds.
  • It is in the liquid phase in the water droplets are falling from the clouds as rain and bending the light to make the rainbows.
  • It is in the vapor phase in the air that is all around the clouds.

The fact is that we walk around in a multi-phase, sometimes saturated system every day and as a result, are (at least subconsciously) aware of how such a system works, at least in practical terms.  We know, almost without thinking, that if we heat water, it will boil and change phase from liquid to vapor, which comes in handy if you like tea or coffee or steamed vegetables.  And we, we also know that if you cool it enough it changes phases the
other way and becomes a solid, a.k.a ice, which is also handy if you like cold drinks.

You may have even seen evidence of the volume change that occurs as water freezes if you have ever had ice spikes form in your freezer.

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Ice Tube

Ice spikes form because the water in an ice cube tray tends to freeze from the outside to the inside.  That means liquid water is trapped inside an ever-growing mass of solid water (just like the chewy center of a tootsie roll pop).  But as water transitions from a liquid to a solid, the molecules have to line up to form a crystal, meaning, as I understand it, that they need more room.

Since the ice tray constrains the expansion on one side of the freezing water, the liquid water is compressed, increasing its pressure until it pops a little hole through the thinner ice towards the center of the cube.  Water then flows out of the hole as the expansion continues, building up a little ice spike until the everything freezes solid.

If you thought that was interesting, you should visit the ice spike page on the SnowCrystals.com web site.   And while you are there, be sure to take a minute to visit the site’s home page and photo galleries. The site was created  by Kenneth Libbrecht, a professor of Physics at Caltech who, among other things, studies the physics of crystal
growth and takes absolutely beautiful pictures of snowflakes.

Phase Change and Energy

Phase changes involve storing and releasing significant amounts of energy when contrasted with the energy that is associated with simply changing the temperature of a solid, liquid, or vapor. For water:

  • Melting ice (a phase change from solid to liquid) uses up about 144 Btu/lb at atmospheric pressure. And, at atmospheric pressure, this phase change will occur at around 32°F.
  • Heating the liquid water to a temperature that is above 32°F, once all of the ice is gone, only requires about 1Btu/lb for every °F of temperature change.
  • Converting the liquid water to steam (a phase change from liquid to vapor) once the liquid has reached the saturation temperature (about 212°F at sea level, atmospheric pressure) uses up about 971 Btu/lb.
  • Superheating the steam above 212°F, once all of the liquid is gone, takes about 0.5 Btu/lb at atmospheric pressure. 
  • In Denver, Colorado, where atmospheric pressure tends to be lower than it is at sea level due to the elevation, all other things being equal, water will boil at a lower temperature; somewhere in the range of 200°F instead of 212°F.

As you can see from the preceding list, significantly more energy is used when changing phases as compared to changing the state of the water when it is in any particular phase. You will also notice that while the phase change is occurring, the temperature of the mixture (be it ice and liquid water or liquid water and steam) holds constant.  And, if you change the pressure at which the boiling takes place, the temperature at which it takes place changes too.

The specific characteristics I have mentioned above for water are also the general characteristics of a lot of substances; that is to say:

  • Phase changes will use more energy per unit mass than a change of state, like raising the temperature.
  • During a phase change (at a constant pressure) the temperature will remain constant as long as there is a mixture of liquid, vapor, and/or solid present.
  • The saturation temperature and pressure are related; in a saturated system with a pure substance, if you know one, then the other can be predicted by the equations of state for the substance, or by tables or thermodynamic diagrams for the substance (which where developed from the equation of state).

Phase Change and the World Around Us

In many ways, phase changes drive the world around us. The sun heats the oceans and the continents, but they react to that in different ways because of the differences in the way they absorb the energy. Those differences lead to temperature differences that cause the air to move (and the water too for that matter).  As the air moves, it picks up  (or loses) heat and moisture as a function of the land or water that it is moving over.

But its more complicated than that. Sometimes, things like mountains or differential heating or other air masses with different characteristics cause the air to be lifted. When a parcel of air is lifted, it tends to cool due to expansion. And at some point, the temperature of the parcel of air reaches the saturation temperature relative to the water vapor that it contains.

When that happens, the water vapor condenses. An in doing that, it releases energy. Specifically, it releases the energy that was keeping the water in a vapor state.  As a result, clouds might form. If the energy that is released causes the air to be warmer and less dense than the air around it, the atmosphere becomes unstable and the process continues to drive itself to produce the beautiful (but powerful) thunderstorms like the one pictured below and the one in the picture at the beginning of the post.

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That is one of the reasons atmospheric soundings and software packages like RAOB are such powerful tools in terms of understanding the environment we live in. By plotting data from weather balloons (radiosondes) on a type of thermodynamic chart called a skew-T/Log-P diagram, we can begin to understand the fairly complex phenomenon that are going around in the atmosphere and make predictions about what is likely to happen in
terms of the weather.

Phase Change and HVAC

We frequently take advantage of phase changes and saturated systems in HVAC processes.  For example, in a steam system, these phenomenon allow us to move a lot of energy around in smaller pipes over long distances relative to what it would take to do the same thing with hot water.  For an example of this, take a look at the third module in the class materials from the recent class on Steam and Hot Water systems at the Pacific Energy Center.

And, the volume change and related pressure changes that can occur in conjunction with a phase change tend to help move the steam to where it is needed the most, all with out the aid of a pump, at least not a pump in the line between the boiler and the load.  But this same phenomenon is one of the things that can make working with steam a bit more dangerous than working with hot water, especially hot water at temperatures below 212°F.  For some insight into that, spend a little time exploring Wayne Kirsner’s web site, especially the videos and forensic analysis of steam accidents.  It will give you a healthy respect for the energy represented by a phase change.

Refrigeration systems also leverage the energy transfer capabilities of changing a liquid to a vapor.  In the evaporator, liquid refrigerant boils to remove heat from what ever it is that we are trying to cool.  In the condenser, the heat is rejected by reversing the process. The temperature’s produced at the point of cooling is regulated by controlling the pressure inside the evaporator.  And the temperature of the heat sink that we use to reject the energy regulates the pressure at which the process occurs.

Finally, the energy associated with changing water to ice or changing the phase of eutectic salts is one of the mechanisms frequently used to store cooling energy or heating energy, be it to shift peak demand or to make solar energy available for times when the sun is not shining.

Bottom line is that we rely on phase change a lot, both to make the world a habitable place and to make our HVAC processes work.  And to understand that, we rely on the work of others who went before us and documented the relationships, developing the equations and charts that predict what is going on around us and in our systems.

Next, I will look at a little experiment I did that demonstrates some of the characteristics of water changing phase to steam.

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David Sellers
Senior Engineer – Facility Dynamics Engineering
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