A Theoretical Question

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Carrdo
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A Theoretical Question

Post by Carrdo »

You have a jet of dry steam at 100 degrees centigrade at atmospheric pressure.

You stick your thumb into the jet of dry steam (you know it is going to hurt).

The jet of dry steam condenses on the skin of your thumb giving up all of its latent heat of vaporization to become water at 100 degrees centigrade.

The question.

Theoretically, can the release of all of this latent heat due to condensation of the dry steam at 100 degrees centigrade into very hot water at 100 degrees centigrade raise the temperature of your skin to more than 100 degrees centigrade?

Is the answer yes or no and give the reasons why.
pat1027
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Re: A Theoretical Question

Post by pat1027 »

No, because before you skin reaches 100C you will jerk it away saying "That was stupid!". The volume of the jet and distance play a part. A couple feet away from the blow downs on my locomotive I can put my hand into the stream of water and once the water is blown out the steam with no ill effect. The steam is hotter and I suspect dilution with air plays a part. But I have not put any science to it.
John Hasler
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Re: A Theoretical Question

Post by John Hasler »

Carrdo writes:
Theoretically, can the release of all of this latent heat due to condensation of the dry steam at 100 degrees centigrade into
very hot water at 100 degrees centigrade raise the temperature of your skin to more than 100 degrees centigrade?

Heat must flow from the condensing steam to your thumb for the steam to be able to condense. That can't happen if your thumb is hotter than the steam.

BTW what you describe is saturated steam. Dry steam is always at least a bit superheated.
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Bill Shields
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Re: A Theoretical Question

Post by Bill Shields »

struggling a bit with the concept of dry steam at 100 C travelling through the air to someone's thumb...

or am i missing something...been a bad year....
Too many things going on to bother listing them.
pat1027
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Re: A Theoretical Question

Post by pat1027 »

To your question would the condensed water collect on your thumb raising it to 100C. Perhaps a small amount of condensation would occur. I suspect the jet of steam would blow the droplets away and the steam would be largely responsible for raising the temperature of you thumb. Consider when you put a cold lid on a pot of boiling water the lid collects condensate.
Rich_Carlstedt
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Re: A Theoretical Question

Post by Rich_Carlstedt »

Simple answer- No
You cannot "raise" the temperature above the steam's temperature, because the steam would then become a "Cooling" agent and absorb
the surplus heat energy.

Rich
Carrdo
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Re: A Theoretical Question

Post by Carrdo »

I am not advocating you try anything like the example I gave.

I have always had trouble applying (and I guess understanding) the basic laws of thermodynamics and heat transfer and this is what I was trying to get at.

When the "dry" steam at 100 degrees centigrade condenses to water at 100 degrees centigrade it gives up all of the latent heat of vaporization it formerly had. But all that heat doesn't just disappear so where does it go?

I presume it is transferred (and by what mechanism(s)(?) (largely, mostly?) to the adsorbing or absorbing surface which I just happened to pick as your thumb. So even if the temperature of this surface cannot exceed 100 degrees centigrade, the heat transfer (can, will?) heat up the adsorbing or absorbing surface really fast or what happens here?
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Fender
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Re: A Theoretical Question

Post by Fender »

I think the short answer is that the latent heat can only be transferred to something that is at less than 100C temperature. If the object (finger or otherwise) is at or above this temperature, it would not absorb any heat from the steam.
Dan Watson
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kcameron
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Re: A Theoretical Question

Post by kcameron »

Another issue is that would only be at the point of the jet. The steam jet is expanding outward, kind of like light from a bulb. That's why the blast from the blowdown will feel cool at a distance. The rapid expanding has released that original heat into the velocity of expansion, meaning the latent heat has turned into kinetic energy in the water droplets. I don't think I worded it right but that's another thing in this picture to keep in mind.
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Bill Shields
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Re: A Theoretical Question

Post by Bill Shields »

Like I said.. dry steam @100c moving through the air?
Too many things going on to bother listing them.
RET
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Re: A Theoretical Question

Post by RET »

Hi Don,

The simple answer is that the change between dry steam at 100 degrees C and atmospheric pressure to water at the same temperature and pressure is a reversible condition and which way it goes depends entirely on whether heat is being added to or removed from the system. If heat is being removed, the amount of water will increase, if it is being added, the amount of steam will increase, but everything will stay at the same temperature as long as both water and steam are present and the pressure is unchanged.

By the way, we are talking about "the latent heat of vaporization," which is 512 calories per gram which is quite significant. It only takes 100 calories per gram to raise water from the freezing point to the boiling point, but more than 5 times that amount is required to turn the water into steam.

This is the theory, but if you can run an experiment where all the variables are tightly controlled, you will see that it is borne out in practice. You will also find that this experiment can be very hard to do!

Hope this helps a bit.

Richard Trounce.
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