Suppose I have a heat pipe that is set to have vapor liquid transition
of 30 degrees F. If I then put that heat pipe about half way into an
insulated container of water and then put the whole assembly in a
freezer at 0 degrees F, will the heat pipe stop "working" once the water
turns to ice at 30 degrees F?
In other words, if I want to cool something with a heat pipe down to
temperature X, do I have to use a heat pipe that has a vapor liquid
transition temperature of X?
When the temperature of the heat pipe reaches 30f, the thermal
conductivity will go down dramatically. However, the walls of the
heat pipe will continue to conduct heat. So, the water will continue
to cool, although at a much slower rate.
That's what I thought, thanks for the confirmation. Given all this I am
pretty sure I know the answer to my next question. If the temperature
in the freezer goes above 30F, even if the water is at an even higher
temperature, there isn't going to be much heat movement either, is there?
That makes me wonder though. If I use CO2 in my heat pipe and have it
set so that when the temperature of the heat pipe is at 30F the pressure
inside the heat pipe is what it needs to be for CO2 to be either liquid
or vapor, when the temperature of the whole system drops below 30F the
pressure in the heat pipe will be lower and the CO2 will vaporize at a
lower temperature. Doesn't that mean that it will continue to function
as a heat pipe even if the temperature of the ice is below 30? I guess
that would only work if the graph of Vapor pressure vs Temperature is a
Mechanical Magic wrote:
In a heat pipe there is some liquid with its vapor only. So (in
equilibrium) the pressure is always at vapor pressure which means the
system is always at boiling point, no matter what the temperature is. So
I don't understand what you mean by vapor liquid transition at 30 deg F.
There is always a dynamic transition of molecules from liquid to vapor
state and vice versa, at any temperature. If one side is hotter than the
other more liquid will evaporate than vapor condenses, the vapor
pressure rises and vapor will condense at the cold end. A wick or
gravity will then transport liquid from the cold to the hot end.
So, unless all the liquid in your heat pipe evaporates or freezes it
will keep working. Heat transport in the water will drop once it is
frozen because convection is more efficient, but heat will still make it
to the heat pipe through conduction.
Efficiency will also drop when the vapor pressure get's low in the heat
pipe. Say, a heat pipe with mercury as working medium will work better
at 400/500 degree C (2.1/8.2 bar) than at 0/100 deg C (2,7E-7/3,7E-4
bar) because there is much more mercury evaporating/condensing per time
unit than at lower temperatures. However, a heat pipe with pentane will
not have such a high drop in vapor pressure when operating at 0 deg F
instead of 30 deg F.
Vapor pressure for pentane is:
270 K (26 deg F) 21.1 kPa
255 K (-1 deg F) 10.0 kPa
Ohh, ohh, I just got an idea, what you mean by "Suppose I have a heat
pipe that is set to have vapor liquid transition of 30 degrees F." There
must, of course be a proper amount of liquid in the heat pipe, which
varies with temperature. If the temperature get's too high, there may be
too little if any liquid left to cool the hot side. However, if you go
to lower temperatures there will be enough liquid (unless it's frozen)
and it will hardly be too much, since there is usually not much matter
in the vapor to condense.