PHASE CHANGE

The three common states of matter are, of course, Solid, Liquid and Gas.  Many compounds can be forced to change their state, or phase, from solid to liquid or liquid to gas, by applying heat.  Materials “come down” from the higher state when they are in a cool environment to which they can give up that heat.

Water undergoes phase change at temperatures that are familiar to us.  We already know that water boils at 212°F and freezes at 32°F.  However, the BTU transfer that is involved during phase change is not so familiar.  For example, while it takes only those 180 BTUs to raise a pound of water from its melting point to its boiling point, it takes over 900 BTUs to convert that same pound of water to steam, without increasing its temperature.

Now, the resultant steam has, in effect, stored those BTUs, and when it condenses inside a cold radiator, it releases close to 1000 BTUs per pound!  Although hard to control, and out of favor for home use, phase change is a very efficient way to transport heat without having to circulate a ton of hot water, or enough hot air to fill Snoopy II.  Steam heat is still widely used in food processing and other industrial processes.

POWER PLANTS:

Phase change plays a major role in the operation of Electric power plants, where fossil fuels generate steam to drive turbines.  Here, converting water to water vapor is important, because the explosive 20-to-one expansion delivers a powerful mechanical thrust to the turbine blades.   That steam now stores the zillion BTUs that were needed to heat the water and drive it through phase change.  To return to water, those BTUs must be removed, and the cooling process wastes about 60% of the energy provided by the fossil fuel, as explained in Power Generation!

When that waste is added to the pollution at both ends of the process, particularly if coal is the fuel, we have reason to hope that there is another way to do the job.  There is!  Take a look at Windmills.

REFRIGERATION:

Phase change is a critical part of Refrigeration, also.  First developed to keep freight cars cool enough to transport fruit and vegetables, the Refrigeration Cycle is at the core of Air Conditioning.

Refrigeration uses a special refrigerant, a fluid that boils at a point well below room temperature, leveraging the number of BTUs that the fluid will carry away.

CONDENSATION:

There is a form of phase change that occurs all around us all of the time and that is the condensation of water vapor on cooling surfaces.  It can be a plague on our houses.  Unwelcome condensation often means, among other things, wasted heat energy.  See CONDENSATION.