I speculated that the steam temperature could be marginally higher than 100C due to the raised pressure within the steam bubbles. Spingears seems to disagree with that and perhaps he is right. For the sake of argument, lets presume my speculation is correct. A thermometer bulb held in the steam can rise to the same temperature as the steam, provided that the rate of heat transfer to the bulb can match the rate of heat loss by conduction up the thermometer. In that case the bulb will remain dry and you are correct. If the rate of heat transfer cannot match this however, steam will condense onto the bulb giving you a maximum temperature of 100C.
Very succinct! Put better than I did.
Don’t you mean that the only way that steam can be produced from boiling water at higher than 100C is under higher pressure? Can’t I have an open-bottomed hot-steam balloon, filled with steam at 200C and 1 bar, kept hot by a hydrogen-oyxygen flame?
Whew. I was quite glad Spingears finally came in with a right answer. Lots of misinformation ahead of that. A final clarification - Steam produced soley by adding heat to boiling water is NEVER hotter than the water that produced it. If the steam is produced in a closed pressurized vessel, the steam is hotter than 212F (100C) but the water that produces it is also at that exact same hotter temperature. Steam produced by boiling water is called saturated steam. At a given pressure, to have steam hotter than the water, heat must be added directly to the steam. The steam is then called superheated.
If you could somehow produce this situation, then you would be adding heat directly to the steam, keeping it superheated. You could not have created this situation just by heating liquid water.
You are correct. “Saturated” is the term for steam that is in contact with water. The temperature of saturated steam will be essentially the same as the temperature of the water; it will not be above 100C unless the pressure is higher than 1ATM.
“Dry” or “superheated” steam (steam that isn’t in contact with water) doesn’t share this limitation - it can be raised to most any temperature, regardless of the pressure. Barring some strange tricks, it obviously cannot be cooled below the temperature at which (given the ambient pressure) water boils (and steam condenses).
Now, here’s another one for you: clean water in a clean glass vessel or test-tube can be superheated, because of the extra energy required to nucleate steam bubbles. When the degree of superheating is high enough, a large quantity of water tranforms to steam at once causing the water to “bump.” Since this is a scalding hazard, anti-bumping pellets can be added that provide nucleation sites and prevent superheating.
Question - is it possible to produce superheated steam from superheated water in this way? I’m following up my earlier speculation that steam from a boiling pan could be fractionally hotter than the water. Having thought about that some more I doubt there would be any sensible difference, but I’m wondering if my reasoning was sound or not.
A hot (air) baloon filled with steam? Have to give that some thought.
In the condenser of a power plant the steam is less dense than air which rises to the top where an ejector or vacuum pump can remove it. Not sure about the case of saturation density of steam compared to air. In the case of the steamer the hot steam is less dense than the surrounding cooler air and rises.
Suppose you trapped saturated steam in a closed envelope (balloon) and introduced a flame nozzle burning oxygen and hydrogen which will add heat and water vapor to the trapped vapor, voila, superheated steam which without pressure to confine becomes saturated steam like the rest of the volume. This assumes the envelope is elastic, if inelastic it bursts or if elastic it stretches.
NOTE: The introduction of a measuring device, i.e. a thermometer, into a stable system results is some small distortion of the conditions due to its very presence.
The smaller the measuring device the less the disturbance.
Every now and then I heat a cup of coffee in the microwave and add creamer in powdered form. Never all at once as sometimes I overheat it and it boils vigorously as the creamer is added. The vapor is NOT superheated, except perhaps for a micro-second or so. It is at atmospheric pressure and therefor at saturated steam temperature and quickly cools below that by contact with the surrounding air. Superheated water is unstable and dangerous, not a thing to play with casually.
Superheating water is one thing, superheating steam is quite a different matter!
The last time I counsulted the Steam Tables, Superheated vapor can only exist at pressures higer that the saturation pressure for a given temperature. Conversely superheated vapor can only exist at temperatures higer that the saturation temperature for a given pressure.
Lets go the other way. How cold can steam be. Lower than 212 at high altitude, I know, but what about in a vacuum? Do I recall correctly that ice can go directly to steam in space? Or vise-versa, is it.
The steam tables I found won’t play on my Mac. I probably couldn’t read them anyway.
At -40 deg. F water is a solid and the saturation pressure is 0.0019 psi absolute which is a pretty good vacuum by ordinary standards. The evaporation of ice to vapor is termed sublimation. I can’t say positively but my considered opinion, lacking a reference, is that below -40 deg.F the vapor pressure drops to or approaches zero and sublimation ceases. Therefor steam (water vapor) cannot exist at lower temperatures.
You are right in that (water) ice in space will sublimate depending on temperature. Sunlight impinging on an ice mass in space will cause sublimation. Lacking heat input the ice remains in a static state.
PS I’m using “Thermodynamic Properties of Steam” by Keenan & Keyes, John Wiley & Sons, London, 1936. There have been no signifigant changes since then save for corrections of typos, etc.
No, if steam at 1 atm pressure is heated more, it becomes superheated steam at 1 atm. The likliehood of being able to add enough heat to a balloon to keep it superheated while floating in the air is very low, but there are no physical laws against it.
Not quite. For a given temperature, steam becomes superheated if the pressure is BELOW the saturation pressure. Note that this does not mean that if you take steam and reduce the pressure you will get hotter steam. As the pressure goes down, the boiling (saturation) temperature also goes down. So if you take freshly boiled water at 1 atmosphere of pressure with a temp of 100C and reduce it to 0.5 atm of pressure, the steam will now be superheated (much hotter than you could get out of just boiling it at 0.5 atm), but it will still only be just slightly below 100C.
OK I stand corrected. There will be some short lived superheated steam in the vicinity of the O2/H2 flame. A steam ballon would not be very reliable as the steam would condense rapidly leading to a rapid descent. I’ll stick to hot air ballons, thank you.
Restated
Superheated vapor can only exist at temperatures higer than the saturation temperature for a (given) corresponding pressure. {delete ( ) }
Superheated vapor can only exist at pressures (higher) lower than the saturation pressure for a (given) corresponding temperature. {delete ( ) }
BoringDad, Question: How are you going to reduce the pressure from 1 to 0.5 atm? And when that is done will not the 100C water boil vigorously until it reaches equilibrium at saturation temp corresponding to .5 atm?
Dare I mention:
The triple point where water, ice and vapor coexist?
The critical point above which the liquid is indistinguishable from the vapor? or
Sub-cooled water which can freeze with an accompanying rise in temperature?
I can’t help about steam, but I strongly suspect that the “sheep” you brother is refering to is actually a domesticated specie related to the wild “dahu” (rupicapra vacca montanus) a caprine living in french mountains. There are two subspecies of dahus, the dextrogyre (which has shorter legs on the right side) and the levogyre dahu (shorter legs on the left). In case it wouldn’t be obvious, the dextrogyre dahus are brachydextro-dolicholevopodes (which means they move clockwise along hillsides)
Both are endangered species, and dahu hunting is strictly restricted. Generally only people who have a strong interest in wildlife are issued permits. Mostly zoologists, but also sometimes tourists. Due to the interest of allowing children to discover the wonders of local wildlife, summercamps are generally issued permanent dahu hunting licenses (safari-like hunting, which doesn’t include the actual killing of dahus, of course). Due to the shyness of this animal, such hunts are always organized at night. The traditionnal weapons used are saucepans or whistles, depending on the regions. Actually being able to see a dahu at short distance is quite rare, but will eventually happen if you’re patient enough, as proven by the fact that, by the time they reach their teens, most childs report having observed them.
Though there’s ample documentation available on the french speaking internet (like this page about its reproduction, or this one , which include a very rare photograph) since the specie is poorly known and rarely studied outside our boundaries, it was quite difficult to dig up a cite in english (for incredulous people like the OP). I eventually found one, though, here (at the bottom of the page. The first article, in french, has been published in the paper “Le Monde”) an unfortunately very short abstract of an article from the peer-reviewed “Science”. It includes a description of one of the traditionnal hunting methods.
How are you going to reduce the pressure? Well, that is an important question and will affect the answer. Also how much extra water in the container makes a difference. I was assuming an isenthalpic expansion (no work done) of 100% saturated steam (no unboiled water in the container.)
If you are assuming there is extra water, then yes, a portion of that will boil off as the pressure is dropped, but possibly not all of it, and the result will be saturated.
Noooo!!! Do not dare! Such things are unspeakable! Plus talking about them would require me to open a book. Engineers at small power plants rarely deal in such arcane topics.