If the same water is _theoretically_ supposed to boil at the same precise temperature at a given pressure, I just don't understand how water can _theoreticallly_ survive as a liquid drop while surrounded by steam which is above the boiling point. In other words, the theory that the same water always boils at the same _precise_ temperature for a given pressure is an idealisation and an approximation.
My conclusion is consistent with Prof. Hasok Chang (Cambridge university) experimental finding that the same water does not always boil at the same precise temperature for a given pressure. In particular he has shown the surface characteristics of a boiler can lower the boiling point by two or three degrees. He also says such anomalous behaviour is well known among people who work with with steam but it has been ignored or dismissed by the academy. Harry On Sat, Nov 19, 2011 at 3:39 PM, Alan Fletcher <a...@well.com> wrote: > Boiler Efficiency and Steam Quality: The Challenge of Creating Quality Steam > Using Existing Boiler Efficiencies > http://www.nationalboard.org/index.aspx?pageID=164&ID=235 > > ... > > Lower Pressure Increases Entrainment > > As a steam bubble rises through the water and reaches the surface, it > finally breaks through the final layer of water and enters the steam space. > This final act of leaving the water causes water entrainment in several > ways. > > Initially, the bursting of the steam bubble or the rupture of the thin layer > of water surrounding it produces an initial rush of high-velocity steam that > carries a small amount of that thin water layer into the steam space. Then, > the loss of the steam bubble from the water surface briefly creates a crater > on the water surface. Water rushes in to fill this crater, colliding with > water rushing from the other sides of the crater, and produces a tiny splash > near the center of the crater. The water droplets from these splashes are > then easily entrained in the rising steam. > > The size of the bubbles is directly related to steam pressure. Low-pressure > operation requires a larger volume of steam to carry the required heat > energy. This low-pressure operation produces more and larger steam bubbles > and creates greater turbulence on the water surface. These bubbles produce > more craters and larger craters, as well as more and larger splashes as they > leave the water surface. In addition, low-pressure operation results in a > higher vapor velocity which, when combined with the high turbulence of > low-pressure operation, tends to carry water droplets into the steam systems > rather than allowing them to fall out by gravity. > > The solution is to operate the boiler at its maximum design pressure and use > pressure-reducing valves at the point of use where required. > > ... > > (Lots of related articles ... eg "How to Destroy a Boiler" and "Anatomy of > a catastrophic boiler accident". ) > > No specific numbers on the range of steam quality from a kettle boiler. > > > >
