On Tue, Jul 5, 2011 at 7:54 AM, Terry Blanton <hohlr...@gmail.com> wrote:
> All steam is dry steam when it leaves the surface of water by > definition. Where is this definition given? There are very clear, well-defined, concepts related to steam, dry steam, wet steam, and steam quality. A simple google search will help you understand them. Steam is water vapor at 100C. If there are suspended or entrained droplets of liquid, the steam is said to be wet; if not, dry. Steam quality represents the mass fraction of the fluid that is in the vapor state: 100% quality means there no liquid droplets present; 50% quality means that 50% of the water by mass is liquid, and 50% by mass is vapor. 0% quality means all liquid. Of course, 0% quality is just liquid if there are no other gases present, so no one would consider it steam. But note that 1% quality steam is already 94% steam by volume, and so all the liquid could conceivably be suspended as a fine mist in the gaseous steam. Especially if the steam is moving at a high speed. > Molecules of water must achieve sufficient kinetic energy > to overcome the intermolecular forces of liquid water. Statistically, > some molecules are able to achieve this at room temperature; so, water > will evaporate. Immediately upon leaving the surface of water; > however, those molecules begin to lose kinetic energy to the > surrounding air and begin to condense. Condensation occurs at the surface when favorable collisions from other water molecules, and favorable lack of collisions from the gas molecules allows them to break the bonds. The kinetic energy of the molecules can be less than the average at 100C. Once evaporated, the molecules can exist as a gas well below the boiling point. Condensation requires favorable collisions (glancing) with other water molecules (or droplets) so that the short range attractive force has time to bind them together. Condensation occurs continuously (considering the vapor molecules collectively), but so does evaporation, so at equilibrium, there is always some water vapor. If they leave with only > sufficient KE to depart the surface, ie 100 C, they will begin to > condense immediately. > A single molecule of water does not condense, so it is not clear what this means. The vapor, as I said, is continuously condensing. A single molecule has a certain half-life as a gas. However, molecules do not go walkabout together from the surface. Are you saying droplets cannot be formed directly from liquid? In a boiler, which is what you seem to be talking about, steam is formed under water, so there are bubbles, which rise to the surface, and produce small splashes. This turbulence produces droplets in a range of sizes. The smaller ones are carried into the air along with the rising water vapor (steam). Typically, in a boiler, the steam is about 5% liquid by mass, according to the site Storms linked to. Entrained liquid is very bad for pipes, and has been compared to sand colliding with the surfaces and fittings. That's why the question steam quality is extremely important. In a cool mister, there is no steam at all. Fine droplets are simply carried into the air. We don't know what happens in the ecat, but one possibility is that a small mass fraction of the water is vaporized, it occupies a large fraction of the volume, and entrains the liquid as a fine mist. Try to imagine what would come out of that hose if the ecat (in the Krivit test) were producing 1.5 kW. (Say it were replaced with an electric heater with exactly 1.5 kW power.) In that case, 600 W would raise the temperature to boiling, leaving 900W to produce steam. That's only enough to vaporize about 20% of the liquid by mass. You must therefore get a mixture of liquid and gas flowing. The gas would occupy about 99.7% of the volume in the conduit (before condensation). The liquid would occupy about 3 parts in 1000 of the volume. What would you expect to see coming out of the hose? Whatever it is, it's what people call low quality steam; also: wet steam. If > it's steam, it's dry. > > Wrong. Steam can be wet.
