what drives boiling in a P-v diagram?

[neilfaraway]

studying for my first paper in thermofluids, I am puzzled by how a mass of water will completely turn to vapour once it reaches saturation pressure. My textbook says during the saturated mixture phaze both temp and pressure remain constant. What drives the boiling?

To take a parallel, added heat drives boiling in a T-v diagram.

thanks
Neil. [sig][/sig]

 

[socrates]

Neil: that's correct; once you have reached saturation pressure the vapor pressure of the fluid exactly equals the atmospheric pressure. As to temperature that will not change untill all the available fluid has vaporized, then the temperature will continue to rise. For any given Temp. or Press. at which occurs a phase change the corresponding Pressure or Temp. must stay the same; if for example the temperature at which water boiled increased as the water boiled then in effect it would never boil. Don't forget also that as the fluid boils the volume of vapor is increasing while that of the fluid is decreasing thus the total volume is very much increasing especially the specific volume; since the ratio of volume to mass for a gas is much greater than for a fluid for obvious reasons.
Hope this helps DS

 

[whitefish]

Neil, Your question about what drives boiling, I think it is the addition of energy that drives the boiling if the pressure and temperature are constant. This is what happens in the simple case of an open pot over a fire.

On the other hand if the sytem is closed and the energy in the system remains constant, a change in pressure can drive boiling, this is what happens when saturated water passes through a valve (such as a steam trap) and is discharged to a lower pressure. The boiling action results in "flash steam". It is very common in steam heating systems.

 

[HotSteam]

The specific state of a fluid can be deturmined by it's properties. How ever temperature is discontionus along the saturation line.

Starting with a liquid as you add heat(enthalpy) the temperature increases. When the boiling point is reached and you continue to add heat to the fluid the temperature remains constant while the heat energy produces a change of phase.

Boiling is a very complex process. Depending on the temperature differance of the heating component you can have different types of boiling. From just mild surface evaporation to agated supperheated bubles froming on the heating ellement. usuall the supperheat would be given up as the bubbles travel up through the fluid to teh surface.

Knowing the enthalpy, total volume of the fluid and pressure you can deturminan howmuch is vapor and how much is liquid.

As in a previous post. Decreassing the pressure will result vaporation of some or all of the fluid.

Andy

 

[imok2]

neilfaraway
If you look at Mollier Diagram on any refrigerent you will see how the process takes place I used refrigerent because I'm familer. A diagram of water will look the same

 

[25362]

At equal P,T vapour contains more energy per unit mass than the liquid. Therefore, to convert liquid to vapour energy (enthalpy) must be added, either from external sources or taken from the liquid itself, by reducing the partial pressure by a stripping effect (temperatures would, in this case, come down, remember perspiration) or by a reduction in total pressure, also with subsequent cooling.

 

[lubes]

Energy (heat) is what drives the vaporization in those instances. If you heat up water up at constant pressure up to its boiling temperature that will not be enough to vaporize the water. You still need an additional push, need to supply the heat of vaporization or latent heat to effect the change of state. That is why water boiling in a pot over a fire will not vaporize all of a sudden at 100 deg C. It will need some time to get all vaporized (the time required to supply all the heat of vaporization).