Steam Production 3

[Nielse]

As water is heated it will gradually start to produce steam. I need some info on the amount of steam produced at temperatures aproaching the boiling point.

 

[pipehead]

Depends. There is not enough information. Heat input, pressure, surface area, unsteady state, steady state, too many varibles.

 

[Latexman]

Very little steam is produced as water is heated and temperature approaches the boiling point. Most commercial boilers create temperatures well above the boiling point of water, like 1000^oF or more.

Good luck,
Latexman

 

[Yorkman]

You won't produce any steam until the water reaches the saturation temperature for that given pressure. In an open vessel the "steam" that you see rising from the surface of the water as it comes up to the saturation temperature is, actually water vapor being produced by evaporation. True boiling will not occur until the vapor bubble formed at the heated surface generates enough pressure to overcome static pressure of the water and and the absolute pressure on/in the vessel. The amount of steam produced (Lbs/Hr)is as Pipehead mentioned a fuction of the thermal energy you throw at the vessel and its surface area. That is just a brief scratching of the surface.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[mbeychok]

Nielse:

Water or any other liquid has very many different boiling points at different pressures. For example, water boils at 100 deg C at a pressure of atmospheric pressure (101.3 kPa), or at 82 deg C at half of that pressure, or at 121 deg C at twice atmospheric pressure. In essence, a liquid has an infinite number of boiling point temperatures because it may exist at an infinite number of different pressures.

When we plot a graph of boiling point temperature versus pressure for any liquid, we obtain what is called the "vapor pressure graph" for that liquid.

Now, to answer your question. At atmospheric pressure, as we begin heating water, the "vapor pressure" of the water increases. That means that the amount of "water vapor" present in the air just above the surface of the water increases. In general usage, we don't refer to that "water vapor" as steam.

When the water temperature reaches the atmospheric pressure boiling point temperature of water (100 deg C), then and only then does "boiling" actually begin. Once boiling has begun, in general usage, we then refer to the evolved vapors as "steam".

In other words, "steam" is the vapor that evolves after the water boiling point temperature at the system pressure is reached and actual "boiling" begins. You asked for how much "steam is produced at temperatures approaching the boiling point" and the precise answer to that question is "none" ... because steam is not produced until the water is at its boiling point temperature (under the system pressure) and boiling actually begins. Before the that point, the amount of water vapor above the water surface increases ... but we don't call that steam. In other words, as the water temperature approaches the boiling point, the humidity of the air above the liquid is increased ... but that is not called "steam".


Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[Latexman]

Nielse,

You could use the advice one of my many bosses gave me years ago, "If you want the right answer, you have to ask the right question".

Good luck,
Latexman

 

[TBP]

Latexman - I would debate the point in your original response of "most commercial boilers...1000*F or more". Large utility boilers can have final steam temps of 1000 deg or more, but typically superheaters are involved. The boilers I've worked with had drum steam temps in the 650*F range, and achieved 1000 deg steam only after two stages of superheaters.

There are lots of low pressure residential steam heating boilers that produce steam very nicely at 2 PSIG or less.

 

[Latexman]

TBP,

When I wrote that I did not make clear that I was thinking about firebox temperatures. Still want to debate?

Good luck,
Latexman

 

[25362]

To my grasping Nielse's use of the word 'steam' is a translation from another language, in which there aren't differences between steam and water vapor, or water in its gaseous state. For example, in Spanish, vapor (de agua); in French, vapeur; in German, wasserdampf.

There are, however, differences between evaporation and boiling which may help to identify the conditions under which steam or water vapor are created, as explained above.

BTW, there are cases in which boiling doesn't take place without nucleation centers, and the water becomes 'superheated' (e.g.,: microwaves' heating).