When I was answering some questions in engineering section of "Yahoo Answers", I just came across this wonderful question.. sounds simple and one can answer easily blindly..Even though I studied thermodynamics in my engineering and knowing this answer better, i cannot provide a reason..thus this question made me to break my head and scratch all my old books...this is the question....for any technical argument comments are welcome...!!!
Does hot water expand in a closed system?
This is a question given to me by my thermodyamics professor. I know water expands when it is heated,however, does being inside a closed system impact this?
The full question is: In a closed tank with a volume of 15 L, we have 10 kg of water at 30 deg. C. Heat is added to the system. Will the liquid level eventually rise or drop?
my answer:
The full question is: In a closed tank with a volume of 15 L, we have 10 kg of water at 30 deg. C. Heat is added to the system. Will the liquid level eventually rise or drop?
my answer:
I am sure that you know what a boiler is..it is a pressure vessel used to produce steam.
now the boiler has water and burn fuel to produce steam. assume, you don consume the steam from the boiler. this is similar to your closed vessel. when you keep on heating it,
room temperature----water level is same
10 degree increase in temperature--- water expands..
20 degree more than room temperature---water expands
..
..
..
..
at 100 degree Celsius, water boils, steam is generated..but the steam is partially water & partially steam..called as wet steam...or thermodynamically it is called as "dryness fraction"...
from now on , if the temperature is increased,
wet steam is continuously generated, thus increasing the pressure of steam ...so the formation of steam leads to drop in water level...
keep on heating...
the steam is produced....drop in water level....
so it is clear that water becomes wet steam and then becomes dry steam.
but there exists a point,(assuming the closed container is strong enough to withstand high pressures)...
called "critical point" at which the water becomes directly dry steam...
the pressure at this point is called as "critical pressure"
critical temperature is the temperature above which the water cannot be in liquid state...
for water---critical temperature---374.15 Celsius and critical pressure----221.2 bar...
thus the water level will drop and finally at this point there will be no water....
a great question.....you made me think a lot....
hope my answer is clear...
thanx
now the boiler has water and burn fuel to produce steam. assume, you don consume the steam from the boiler. this is similar to your closed vessel. when you keep on heating it,
room temperature----water level is same
10 degree increase in temperature--- water expands..
20 degree more than room temperature---water expands
..
..
..
..
at 100 degree Celsius, water boils, steam is generated..but the steam is partially water & partially steam..called as wet steam...or thermodynamically it is called as "dryness fraction"...
from now on , if the temperature is increased,
wet steam is continuously generated, thus increasing the pressure of steam ...so the formation of steam leads to drop in water level...
keep on heating...
the steam is produced....drop in water level....
so it is clear that water becomes wet steam and then becomes dry steam.
but there exists a point,(assuming the closed container is strong enough to withstand high pressures)...
called "critical point" at which the water becomes directly dry steam...
the pressure at this point is called as "critical pressure"
critical temperature is the temperature above which the water cannot be in liquid state...
for water---critical temperature---374.15 Celsius and critical pressure----221.2 bar...
thus the water level will drop and finally at this point there will be no water....
a great question.....you made me think a lot....
hope my answer is clear...
thanx
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