Anyone know what material is inside a typical heat pipe on a HS? I found a wikipedia article suggesting alcohol or ammonia. I know that all sealed systems eventually leak; I wonder what the predicted life span is of a "premium" HS like my Ultra-120 Extreme... and at the end of life, what will come out to drip all over my hardware
Further to the ability of tubes to contain a working fluid for long periods of time: Back at my parent's place, they have a fridge in the basement that's from the '50s. It still runs perfectly, though it pre-dates fridges with humidity control so it requires defrosting once in a while. The fridge runs, of course, on freon, which is now a banned/restricted fluid because of the damage it causes to the ozone layer (maybe other reasons). Anyway, the thing has never needed fluid replenishment.
If something as complicated as a fridge can hold the working fluid for 50+ years, then I'm sure that something as simple as heat pipes can be made to not leak over a very long lifetime.
I was under the impression that a heat pipe's fluid might be under pressure. Certainly the pressure will rise a great deal as the operating temperature increases. The internal pressure of the heat pipe must be close to the vapour pressure at the hot end. Otherwise, you must not be evapourating the liquid fast enough - so the liquid may not be wicking quickly enough or you may not have enough working fluid.
I've read in a computer mag (CPU I believe) that the working fluid is plain water. Also, the tube isn't under pressure, but a vaccuum. Not a total vaccuum, but enough of one that water will start to boil at a lower temp. (remember that water boils at room temp in a total vaccuum.) There should also be a "wick" that helps the water flow back to the heat source once it cools off enough.
Inside of a heatpipe is a liquid under low pressure (or vacuum) that boils into vapor when it absorbs heat. This vapor then condenses back into liquid at the cooler surfaces of the heatpipe and releases the heat. So the concept here is to draw the heat from the CPU into one end of the heatpipe, while putting the other end of the heatpipe in contact with a larger heatsink to expel the heat into the air.
I'm pretty sure the heatpipe isn't under pressure. The liquid might not be water, but I'm sure thats what the CPU mag said. (of course they can be wrong...) It might not be water, and I'm sure there is more then one way to do this.
I've had the opportunity in my job (actually an inadvertent, although - from the viewpoint of this discussion - happy accident involving some industrial-grade heatpipes on one of the reactors the other day) to find out what makes heatpipes tick.
After the fuss died down and I'd examined the wreckage out of natural/professional curiosity, I liberated one of those pipes, a monster which measured 10cm in diameter on my vernier after I'd dissected it - with an angle grinder. The inside had a braided steel wick that looked like a fat, silvery snake. It also stank of some volatile oil reminiscent of propane.
It hissed when I cut it, but whether it was under pressure or under vacuum, it pains me to admit that I'm only guessing here.
Because of the nature of the plant, we have two Chem. Eng. people running around, of whom one specialises in thermodynamics. So I went and interrogated him.
I learned the following bits of information, but the subject sadly managed to escape further interrogation (i.e. had to get back to the blown reactor).
On the one end of the scale you have the industrial class brutes. Like the one I 'confiscated' and chopped apart to see how it works. These are the kind that mpilchfamily described in his heatpipes 101 thread. The hefty kind, used in industrial applications (and by companies like Gigabyte to keep northbridges cool - unless that's just advertising). Some of these industrial brutes go up to 60cm or more in diameter, unless the chem. eng. bloke is bulling me - my sister and one of my uncles qualified in that field and all chemical engineers I've met are a little bit... odd. Must be all those chemicals.
On the other end of the scale you have the cheap heatpipe - go to your local hardware store to buy this one - 5mm copper tubing, and hey, it'll work. Only up to a point though...
What's interesting is that a copper tube will conduct more heat than a copper rod of the same diameter. Something to do with surface area, I think.
I'm more at home around logic systems, processes, integrated circuits, soldering irons, press frames, concrete, CAD applications, MIG welders, and A2 plotters that run off annoying USB to LPT adapters.
The most common types of wicks that are used are as follows:
This process will provide high power handling, low temperature gradients and high capillary forces for anti-gravity applications. The photograph shows a complex sintered wick with several vapor channels and small arteries to increase the liquid flow rate. Very tight bends in the heat pipe can be achieved with this type of structure.
The small capillary driving force generated by the axial grooves is adequate for low power heat pipes when operated horizontally, or with gravity assistance. The tube can be readily bent. When used in conjunction with screen mesh the performance can be considerably enhanced.
This type of wick is used in the majority of the products and provides readily variable characteristics in terms of power transport and orientation sensitivity, according to the number of layers and mesh counts used.
Not all heat pipes utilize a whick. Most motherboards that utilize heatpipes for cooling the chipset do not include a whick. So orientation of the board is important so as not to impair the cooling efficancy of the pipes. This is also a method used to cut cost.
Inside the container is a liquid under its own pressure, that enters the pores of the capillary material, wetting all internal surfaces. Applying heat at any point along the surface of the heat pipe causes the liquid at that point to boil and enter a vapor state. When that happens, the liquid picks up the latent heat of vaporization. The gas, which then has a higher pressure, moves inside the sealed container to a colder location where it condenses. Thus, the gas gives up the latent heat of vaporization and moves heat from the input to the output end of the heat pipe. by mpilchfamily, a THF thread