Ask A Scientist© General Science Archive

 


> >  name       brian
> >  status     student
> >  age        17
> >  EMAIL
> >  Question - I have gotten in a heated debate with a friend of mine.
> > Please tell me if it is possible to have "DRY" steam, and why it is or is
> > not possible. This arugment stems from a discussion we had over the fact
> > that a "can of steam" is impossible to have. Thank you very much.
>
>
>Yes, you can have "dry" steam. A measure of the "wetness" of steam is
>called quality. Quality varies from 0 to 1, with a quality of 0 being a
>saturated liquid and 1 being a saturated vapor. When steam has a quality of
>1, it is considered "dry" steam. If the quality falls between 0 and 1, it
>is considered "wet" steam. You can have a can of steam because of the ideal
>gas law, PV=nRT. At a given pressure (P), volume (V), and temperature (T),
>a certain mass (n is the number of moles of a substance) can be in just
>about any state. Tip the pressure one way, and the state changes. Same
>goes for volume and temperature. The trick to get a dry can of steam is to
>know the volume of your can and the amount of H20 in the can. Assuming the
>can to be a constant volume and that the H2O cannot escape, you pick the
>temperature and pressure needed to get "dry" steam. There is a chart of
>steam tables in just any thermodynamics book that can tell you this
>information on what to choose. Change anyone of the variables slightly
>thought, you'll loose your can of "dry" steam.
>Hope this clears up the debate and keeps the friendship strong.
>Chris Murphy
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Hi Brian!
>Look if one takes the expression "dry steam" literally
>it is impossible because "steam" means  "water vapour"
>so dry steam would be inconsistent.
>But...there are nowadays industrial equipments that
>use that expression meaning something else.
>So "dry steam" describes a system that produces
>a high temperature with "low moisture vapor". This
>vapor contains only 5-6% water and is much less
>dense than the air we breathe..
>And thanks for asking NEWTON!
>Mabel
>(Dr. Mabel Rodrigues)
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 >   name        brian
 >   status      student
 >   age         17

 >   Question -  I have gotten in a heated debate with a friend of mine.
 > Please tell me if it is possible to have "DRY" steam, and why it is or is
 > not possible. This arugment stems from a discussion we had over the fact
 > that a "can of steam" is impossible to have. Thank you very much.


>Brian--I am not an expert in thermodynamics, but as I recall from the
>subject many years ago, I believe your friend may be correct. Steam is the
>vapor phase of water, and you may be thinking that since steam came from
>water that it is intrinsically "wet". If I remember from a "thermo" class,
>you can have superheated steam and it has very little moisture. In fact when
>you design a turbine engine that is powered by steam (as some power plants
>have), you want very little moisture in the steam being fed to the turbine
>due to the deleterious effects of the entrapped moisture on the turbine
>blades. The drier the steam, the better.
>
>  As the steam gives up its energy in the turbine application and the steam
>begins to cool, it will get wetter due to the normal process of condensation
>and therefore you have drain lines from a turbine to capture the condensate
>and return it to the feed water line that feeds whatever mechanism is
>producing the steam (i.e., a boiler, etc.). Steam does have mass, and it is
>possible to have a "can of steam" if you displace the air in the can with
>the steam that you feed to it. I would not try holding the can, though,
>because by definition the steam has to be at least 212 degrees Fahr. (the
>boiling point of water). This is why if you hold your hand over a water
>kettle on the stove, you may burn your hand.
>
>This is very basic stuff here, but if you need a more technical answer you
>can get it from any good college level text book on thermodynamics in the
>reference section of your library. You can also do a search on "steam" via
>the Internet by using any good search engine such as Lycos, Alta Vista,
>Yahoo, etc. and going to their Reference materials section. Hope this helps
>you.
>
>JS
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