Tuesday, January 28, 2003

Steam tables

Anybody can blog about interesting stuff. But if you want a challenge, find something as geeky as steam tables. No, I don't mean those things in cafeterias that keep food warm. I'm talking about those little books full of numbers with the big folding Mollier chart in the back that have served generations of mechanical and other engineers. Nowadays I suppose they're going the way of slide rules and trig tables.

OK, what is a steam table? It is a listing of the properties of saturated and superheated steam, typically including entropy, enthalpy, and specific volume under various pressure and temperature conditions. Is that better?

You might think you know a little about steam, but it has many surprises. For one, there comes a point at a certain temperature and pressure (called the critical point) beyond which you can no longer tell steam from liquid water. You can have steam at temperatures well below 212°F and you can suppress it at temperatures far higher. And if you think of steam as just water vapor, you can even have steam without having liquid water first. And we engineers take advantage of all of these facts.

So why don't we see this stuff everyday? Because we live in a narrow range of atmospheric pressures, and a fairly narrow range of temperatures. Outside that range water shows us some new tricks. For instance, the pressure is elevated inside an about-to-freeze soft drink container. If you open it then, you might see some of it instantly turn to ice as the pressure is lowered. Or if you wonder where all that steam came from when you lift the lid of a pot, that's because in doing so you've just slightly reduced the pressure at the surface of the water of the pan - the steam wasn't in that state until you lifted the lid.

What if the pressure is higher? Then steam cannot form until the temperature gets higher - the steam table will tell you that temperature (the saturation temperature). This is significant because steam and water hold different quantities of energy per unit mass (the enthalpy) and behave differently in transferring their heat. So pressure and temperature states are carefully monitored in places like pressurized water nuclear reactors (where there must be no steam), inlets to steam turbines (where there must be no liquid), and pressure cookers (which cook faster, but will blow up and scald you if you don't respect their strength limits).

Now what if you lower the pressure below atmospheric? Then the steam shows up at a lower temperature. Those of you from high mountains know what this does to your cooking - the food dries out quicker even from that small change in atmospheric pressure. Having steam at lower pressures can help in sterilizing substances which might be damaged by higher temperatures.

What if you really lower the pressure? Then you can see sublimation, in which water passes directly from the solid state to a vapor. This is nothing new if you've seen dry ice, but about any pure substance will do this under the right conditions. And now you know what is done to "freeze-dry" foods. (Actually ice can turn to vapor at higher pressures, which is why your food can get "freezer burn").

Steam tables apply to water specifically, but other substances have had their properties cataloged extensively. Chief among these are refrigerants such as ammonia or Freons.

The steam tables are a bit of an idealization in that they apply to pure water. This doesn't happen very often. But they are close enough for many applications.

This PDF has some graphics and a deeper discussion of the properties of a pure substance right out of a thermodynamics book. This page defines some terms.

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