Inside pressure of heat pipes and the associated saturation temperature of the work fluid

[DaoBrabo]

Hey guys.

I've been checking some manufacturer's website to learn more about solar collectors, and the evacuated tube collectors with heat pipes really grabbed my attention. It says there that heat pipes have water inside them vaporizing at about 30ºC. If that's true, then how come the potable water that exchanges heat in the heat pipe condenser gets heated way above that temperature? I mean, I don't the specs of the collector, but this is what I'm wondering:

If you design a heat pipe with a certain internal pressure associated to it, that's going to define the saturation temperature of the work fluid (water) in there. Can one consider that the inside pressure will increase considerably when the radiation heats up the pipe and therefore also increase the saturation temperature? It's important to remember the heat pipe is evacuated to a certain point.

On another perspective, I'm wondering if the fact that you only have a small amount of water mass in there makes it possible for the heat source to bring the vapor to a higher temperature and therefore heat the potable water to also higher temperatures, but I don't think that's true, since the thermal gradient along the longitudinal direction is negligible (we're dealing with a phase change device)and the vapor moves way too fast.

I hope someone can help me see this whole scenario correctly. Thanks a lot!!

 

[davefitz]

I am not sure how the described system is transferring heat to the potable water system, but if it is an indirect heat exchanger transferring heat to a liquid potable water supply, then the rate of heat transfer from the indirect HX to the potable water will be linearly proportional to the temperature difference between the heat pipe and the potable water. So, to transfer twice the heat, you will need to heat the heat pipe fluid to a higher temperature with a corresponding higher pressure.

For the case where the indirect HX is not working ( eg drained of liquid) the heat pipe will heat up to whatever temperature will cause the het pipe to burst, due to the combined effects of high pressure and temperature, so th system will need relief valves.

"Whom the gods would destroy, they first make mad "

 

[DaoBrabo]

The heat is transferred to the potable water supply through the condenser of the heat pipe, which acts as the heat exchanger. The potable water is the heat sink of the heat pipe. Sorry, my question was confusing. It is more about how heat pipes work than about the solar water heating system itself.

Assuming that a heat pipe has an initial inside pressure of about 0.614 psia (~4.23 kPa), its corresponding saturation temperature will be approximately 30ºC. I'm wondering if those will increase during the operation of the heat pipe, and if so, how much will they increase before the heat pipe fails (dry out). I know that probably depends on the design of the device, but I'm wondering in a general way if it would go up to something like 35ºC or like 80ºC.

By the way, thanks for your answer, davefitz, I appreciate your help!

 

[Compositepro]

our question is basically: "how does a heat pipe work?" Here you are:

http://en.wikipedia.org/wiki/Heat_pipe

 

[DaoBrabo]

Wow, that was super helpful Compositepro. Thanks, a had never heard of Wikipedia before. Great website.

I'm well aware of how heat pipes work in a BASIC level, I've worked a few weeks with them in a lab. My question is strictly about PRESSURE/SATURATION TEMPERATURE relationship and the input heat flux.

 

[MortenA]

Snipes aside, i think whats happening is that initially you may have a low pressure and fully condensed system, but as the sun heats the pipe the pressure builds inside the pipe

This link:

http://blogs.howstuffworks.com/brai...ors-work-very-efficient-way-to-heat-water.htm

shows some DIY solar pipe projects (two different principles i would say. The last is the one closest to the commercial "vacuum tube" types. Here the guy uses acetone, starts by replacing the air in the pipe by heating (liquid) acetone in the pipe and thus forcing air out, he then solders a cap on the pipe. Once the acetone cools a (partial vacuum will form). Now when the sun heats the pipe the pressure inside the pipe will go up. To further increase the efficiency the heat pipe is mounted inside an evacuated glass tube (and painted black). The first video shows the value of this. With this arrangement you can "move" energy out of the evacuated tube using only one penetration - makes it much easier. Also by using the pipe you will actually be able to make the primary circuit without any move parts - the end of the pipe will simply be within the primary heat storage . you then have flow by pump in your secondary (although i guess you knew that already).

So to answer your question i think that while the liquid may start to boil at 30 C it will constantly be at its boiling point but at a corresponding pressure. Somewhere i read the distilled water (perhaps added an anti freeze) is a more common liquid than acetone in the heat pipe (but acetone is probably easier to work with in a DIY set-up).

The last video is actually the best.