Dumb yet technical question: Heatpipes

[GoStumpy]

How do they work?

I see that they work... I see that they channel heat... But really I doubt copper tubes would channel heat that well! Must be filled with something?


My limited researching (20 minutes?) has told me almost nothing...


When did they start being used in aftermarket coolers, anyhow? I remember when I overclocked (Duron 650) I don't remember ever seeing heatpipe coolers?

 

[beginner99]

20 min and did not check out Wikipedia?

heat pipe has a liquid in it. At the hot part liquid turns into vapor (which consumes heat, basic thermodynamics) and at the cold part it turns back into a liquid which gives of heat.

-> heat transfer

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

 

[velvetpants]

I was just repairing a computer yesterday where I noticed the guy had cut the top off all his cpu cooler heatpipes. Maybe he thought it would cool better if he got some air in there.

 

[BoomerD]

http://lmgtfy.com/?q=how+do+heatpipes+work%3F

 

[janas19]

20 min and did not check out Wikipedia?

heat pipe has a liquid in it. At the hot part liquid turns into vapor (which consumes heat, basic thermodynamics) and at the cold part it turns back into a liquid which gives of heat.

-> heat transfer

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

I think GoStumpy is referring to CPU coolers specifically, which do not have liquid in the heatpipes.

I'm not sure why the insides of the pipes would be hollowed out, tbh. Basically, the more surface area which a heatsink has, the more it is exposed to air. It is air which carries away excess heat. That is why air cooled cpu coolers have fins - lots of airflow.

But the inside of the pipes don't get airflow, so it seems they should be solid. *scratches head*

*Edit* On second thought, I honestly believe hollow cpu heatpipes have more to do with cheaper manufacturing, or marketing gimmicks, than any scientific reason.

 

[GoStumpy]

http://lmgtfy.com/?q=how+do+heatpipes+work?

Win.

 

[aigomorla]

actually its not liquid.. its a type of gas held under pressure.... due to the pressure it goes though liquid and gas stages at a very limited heat range.

Unless they completely avoided that and messed up the pipe.. which then its not a heat pipe but a heat transfer bridge which will blow on epic sizes.

 

[janas19]

actually its not liquid.. its a type of gas held under pressure.... due to the pressure it goes though liquid and gas stages at a very limited heat range.

Unless they completely avoided that and messed up the pipe.. which then its not a heat pipe but a heat transfer bridge which will blow on epic sizes.

I'm afraid GoStumpy may need to be more specific. When I think of cpu coolers/chipset coolers, typically, in that case "heat pipes" means the copper or aluminum pipes that don't contain liquid or gas and conduct heat away from a hot chip.

If you're talking about a pipe that does have gas/liquid change in it, that is called "phase change" cooling in computer applications. In that case, the pipes are not called "heat pipes," they would be referred to in typical HVAC terminology as either the "condensor coil" or "evaporator coil," depending on where they are located.

What happens in phase change cooling is you take a gas that has a low boiling point. It passes over the cpu and boils off, changing from liquid to gas. To change those molecules from a liquid to gas requires an energy transfer, which is what cools the CPU. It is the same exact principle of evaporation and energy transfer as the body sweating: when sweat evaporates, it cools you down.

The gas in a phase change cooler typically has a lot of heat/energy in it, and so simply using ambient air to cool it down may not be enough to change it back to liquid for the cycle to start over. That is why phase change coolers use an air compressor, to power the change from the gas to liquid with electricity. The pipes where this hot gas is cooled off then changed back to a liquid is called the "condensor coil. "

So to sum it up cpu coolers use "heat pipes" in passive/air type systems. In phase change cooling they are called "evaporators" or "condensors." Sorry for not making that point more clear in my earlier posts.

 

[PhoenixEnigma]

janas19, do you have anything you can cite for that? Specifically, that there's no working fluid in CPU/GPU heatsink heatpipes? That runs contrary to everything I've seen or read on the matter.

My understanding is that heat pipes, as used in computer heatsinks (and lots of other things, for that matter), are passive heat transfer devices that rely on the (generally large) enthalpy of vaporization to move energy, as opposed to thermal conduction through a material. Generally, this is a faster way to move more energy through the same cross section in the same amount of time (which is why we like them!)

The basic design is pretty simple - take a hollow container, create a vacuum in it, put a little working fluid* in it, and seal it off. When you heat one end, you boil the working fluid off (consuming large amounts of energy), it flows to the cooler end and condenses (releasing all that energy), and the fluid flows back either via gravity or capillary action (depending on the design of the heatpipe - I'd expect capillary in a CPU heatsink). In practical applications, you mount a bunch of fins or something on the "cool" side and stick a fan on them, so you're able to dissipate that energy somewhere and keep it cool.

It is a phase change system, technically speaking, but it's sealed and tuned to operate passively in a specific range, and unlike an active phase change system with a compressor and such, isn't capable of below-ambient temperatures. If you want that, you need an external energy source of some kind (or a way to break thermodynamics quite badly.)


As to when they became popular in the PC world, I'm not entirely sure. I'm pretty sure they were reasonably new (or here, at least) around the time of the first Athlon X2's, but I'm not sure on that. Based on the OP's Duron-era experience, that puts it between 2000 and 2005. Anyone able to narrow it down more?

As a question of my own - does anyone know if heatpipes deteriorate over time, either by loss of working fluid or by loss of vacuum? I know aluminum is actually reasonably permeable to gasses over time, but I don't know how well copper fairs.

*Apparently plain water can (don't know if it is) used for computer-application temperature ranges, as the lower pressure in the heatpipe reduces the boiling point substantially. The enthalpy of vaporization of water is very high, and water is pretty cheap, so it would be an attractive option if it works.

 

[janas19]

My understanding is that heat pipes, as used in computer heatsinks (and lots of other things, for that matter), are passive heat transfer devices that rely on the (generally large) enthalpy of vaporization to move energy, as opposed to thermal conduction through a material. Generally, this is a faster way to move more energy through the same cross section in the same amount of time (which is why we like them!)

Yes I've researched this subject. No I can't give you a link atm, but I will tomorrow.

Think about what you just said though. You are saying, if I have it correctly, that a typical aftermarket cooler, manufactured by the likes of Coolermaster, Thermaltake, Zalman, etc, which is not a part of a water-cooled loop, is filled with a liquid or gas substance, that operates by the enthalpy of vaporization? Is that right?

I don't think that is the case at all. I think you maybe misunderstood these pipes my friend. If one of these aftermarket coolers is filled with a liquid that carries the heat away, how does the liquid move? If it evaporates, where does it condense back to liquid?

It doesn't heheh. All heat pipes rely on thermal conductivity (thank you for giving me the proper term though)

 

[PhoenixEnigma]

That's more or less what I'm saying, though I'd switch "contains" for "filled with", given there's not a whole lot of working fluid, really.

Perhaps a more convincing explanation, courtesy of a Los Almos press release:

A small amount of fluid within the heat pipe vaporizes at the hot end, condenses as it reaches the other, slightly cooler end, and returns to the hot end through a capillary wick, to repeat the process.

If you're question is "how does a hot gas move away from a boiling liquid?", I invite you to hold your hand over a boiling pot of water to find out*. More seriously - heat a gas and/or create more of it at the hot interface, pressure goes up locally, it expands to fill the rest of the heatpipe. In a heatpipe, you'll also be losing pressure at the cold end as gasses condense, so you get gas flowing one way through the center of the pipe, and liquid going the other along the inside walls.

If the question is "how does the liquid return to the hot interface", there's a couple ways. In a more controlled application, it's possible to just let gravity do all the work - put the hot end below the cold end, and fluid flows back down when it condenses. In something that might get stuck on it's side or upside down or in space or something (like a CPU heatsink), you'd line the inside of the heatpipe with something that's going to give you decent capillary action. This site actually shows a few of the options for that.

Condensation is going to happen at any 'cold' interface - anywhere the local temperature is below the boiling point, really. Obviously, condensing that gas back to a liquid releases a lot of heat (that's the point), and so warms that area up. In practice, this is going to be the area where you solder fins to the heatpipes to cool them.

To take a different look at things - if they were relying solely on thermal conductivity, the important factor would be total cross section, and solid copper rods offer a lot more cross sectional area for a given volume (and a given contact footprint with the CPU). Why don't we see high end air coolers using solid rods, then, and advertising better performance from it?


*Incidentally, heatpipes use the same principal that makes holding your hand just over boiling water more dangerous than immersion. Condensing that water back to a liquid nets you a lot of energy.

 

[janas19]

That's more or less what I'm saying, though I'd switch "contains" for "filled with", given there's not a whole lot of working fluid, really.

Perhaps a more convincing explanation, courtesy of a Los Almos press release:

Yes, I'm not saying that a passive enthalpy heat pipe can't be set up, I'm saying that the application that the OP referred to was CPU coolers. Currently there are none of these on the market, and the reason why is because they are much more costly than an air cooler or even water cooler to manufacture, and would not give you as good of results. The link you provided was to a NASA technology, we are referring to heat pipes in CPU coolers. The heat pipe described in that article would not be used on a processor.

*Edit* Google the phrase "Are heat pipes expensive?" The first result will link you to an FAQ of a true "heat pipe" company. Their products are used for aerospace applications. If you read that section, it will explain why the heatsinks used on computers are cast heatsinks.

I got confused on the terminology as well. A more accurate description of what is used on PCs should actually be "pipe-shaped cast heatsinks."

Wow, thanks for pointing this out to me, I've learned something.

 

[aigomorla]

A heat pipe is a type of phase change..

The liquid vaporizes and goes up to a fin section where its cooled.
The same principle as a compressor... only a compressor adds a lot more work to compress the gas back into a liquid with a much higher CoP vs a heat pipe.

In short they are similar if you have a real heat pipe..

The heat boils the liquid... then it goes up to the sink where the gas is allowed to condense into a liquid, then though capillary action, it is brought back to the heat source where it is vaporized once again, and repeated.

 

[PhoenixEnigma]

Yes, I'm not saying that a passive enthalpy heat pipe can't be set up, I'm saying that the application that the OP referred to was CPU coolers. Currently there are none of these on the market, and the reason why is because they are much more costly than an air cooler or even water cooler to manufacture, and would not give you as good of results. The link you provided was to a NASA technology, we are referring to heat pipes in CPU coolers. The heat pipe described in that article would not be used on a processor.

*Edit* Google the phrase "Are heat pipes expensive?" The first result will link you to an FAQ of a true "heat pipe" company. Their products are used for aerospace applications. If you read that section, it will explain why the heatsinks used on computers are cast heatsinks.

I got confused on the terminology as well. A more accurate description of what is used on PCs should actually be "pipe-shaped cast heatsinks."

Wow, thanks for pointing this out to me, I've learned something.

I assume you mean this Thermacore FAQ? From that page (bolding mine):

What are heat pipes used for?

Heat pipes are used for a wide variety of applications — anywhere heat must be transferred with a minimum thermal gradient, either to increase the size of the heat sink, to relocate the sink to a remote location or where isothermal surfaces are required. Typical applications include computer processor cooling, isothermal furnace liners and aerospace heat transfer.

Also, I decided to get a little firsthand information on this, since I had trouble finding anyone who had actually opened up a heatpipe on a shipping heatsink:


Started with an old Artic Cooling Freezer64 I had around:

For those who missed this gem, there's 6 heatpipes that run out the base, though I really only needed one:

As it was 1:30AM, the dremel probably would have annoyed the neighbors a little. Hacked a hole into the side with a file instead, and blasted it pretty good at point blank range with canned air:

Little hard to see there, here's a closer look with better lighting. That's not dust from cutting in there, the whole thing has a sintered metal lining. In this case, this is what's being used as a return path for the fluid. You'll also notice there's nothing dripping out of it - you don't need very much fluid at all (I wouldn't expect to see free liquid, just enough to wet both ends, which is why you don't hear sloshing in heatsinks), and between age and having being opened to the atmosphere and repressurised, there's none to see. Either they're very convincing and rather expensive fake heatpipes, or Artic Cooling was using the real deal in a $40 heatsink 5 years ago.

Either there's a massive conspiracy to make everyone think they have heatpipes in their PCs, or the real thing has become quite common.

Looking at the photos, you can also see how the heatpipes allow heat to be fairly evenly distributed and disapted over a substantially larger area than would have been possible otherwise (and this is actually quite a small heatsink by todays standards).

One thing this doesn't really show off (but the FAQ linked above explains) is how heatpipes allow the fins and fan to be removed some distance from the heatsouce - you see this a lot in laptops. Since the primary method of heat transport is by fluid and not direct thermal conductivity, the effective thermal conductivity gets higher just about in proportion to length, as the thermal gradiant stays pretty close to the same.

-Summer Glau

-Edit: Hrmm, no way to hide long blocks behind a button? Alas.

 

[Meghan54]

Yes I've researched this subject. No I can't give you a link atm, but I will tomorrow.

Think about what you just said though. You are saying, if I have it correctly, that a typical aftermarket cooler, manufactured by the likes of Coolermaster, Thermaltake, Zalman, etc, which is not a part of a water-cooled loop, is filled with a liquid or gas substance, that operates by the enthalpy of vaporization? Is that right?

I don't think that is the case at all. I think you maybe misunderstood these pipes my friend. If one of these aftermarket coolers is filled with a liquid that carries the heat away, how does the liquid move? If it evaporates, where does it condense back to liquid?

It doesn't heheh. All heat pipes rely on thermal conductivity (thank you for giving me the proper term though)

And if what you posted is the understanding from all your "research" on heatpipes used in cpu coolers, you are completely wrong in your assessment.

 

[janas19]

And if what you posted is the understanding from all your "research" on heatpipes used in cpu coolers, you are completely wrong in your assessment.

False.

 

[monkeydelmagico]

Combo deal. Copper alone conducts heat well. Combine that with aigomorla's very clear explainaition of phase change vis-a-vis thermodynamics and you have the basis for a modern "air" cooler.

I'd be curious what liquid is inside. Solar heat pumps i think use trace amounts of purified water. There are probably some different liquids that could enhance the effect. I'm going to cut open my heat pipes and replace the water with some high-octane moonshine. Fun project.

 

[janas19]

I assume you mean this Thermacore FAQ? From that page (bolding mine):
Also, I decided to get a little firsthand information on this, since I had trouble finding anyone who had actually opened up a heatpipe on a shipping heatsink:


Started with an old Artic Cooling Freezer64 I had around:

For those who missed this gem, there's 6 heatpipes that run out the base, though I really only needed one:

As it was 1:30AM, the dremel probably would have annoyed the neighbors a little. Hacked a hole into the side with a file instead, and blasted it pretty good at point blank range with canned air:

Little hard to see there, here's a closer look with better lighting. That's not dust from cutting in there, the whole thing has a sintered metal lining. In this case, this is what's being used as a return path for the fluid. You'll also notice there's nothing dripping out of it - you don't need very much fluid at all (I wouldn't expect to see free liquid, just enough to wet both ends, which is why you don't hear sloshing in heatsinks), and between age and having being opened to the atmosphere and repressurised, there's none to see. Either they're very convincing and rather expensive fake heatpipes, or Artic Cooling was using the real deal in a $40 heatsink 5 years ago.

Either there's a massive conspiracy to make everyone think they have heatpipes in their PCs, or the real thing has become quite common.

Looking at the photos, you can also see how the heatpipes allow heat to be fairly evenly distributed and disapted over a substantially larger area than would have been possible otherwise (and this is actually quite a small heatsink by todays standards).

One thing this doesn't really show off (but the FAQ linked above explains) is how heatpipes allow the fins and fan to be removed some distance from the heatsouce - you see this a lot in laptops. Since the primary method of heat transport is by fluid and not direct thermal conductivity, the effective thermal conductivity gets higher just about in proportion to length, as the thermal gradiant stays pretty close to the same.

-Summer Glau

-Edit: Hrmm, no way to hide long blocks behind a button? Alas.

Glad you posted this.

I made a mistake myself in my earlier posts, because I admit myself I wasn't entirely clear on the concept.

Here is a better explanation: in circuitry cooling there are such things as "heat pipes," and there are cast heat sinks. A "heat pipe" as used in aerospace circuit cooling has a working liquid in it, and the liquid is under a vacuum. True.

In personal computers there are cast heat sinks to cool circuits/processors. These cast heat sinks may be formed in the shape of a pipe, and hence called "heat pipes" since they are pipe-shaped and carry away heat. However, one shouldn't confuse these two. A heat pipe in conputers is NOT a heat pipe in aerospace applications since it does NOT have a working liquid in it, operate under vacuum, or work by enthalpy (cast heat pipes work by thermal conductivity).

This difference may be hard to grasp based on what I've said alone, so with any luck I will put up better links and pictures later today to illustrate the difference between pipe-shaped cast heatsinks and true heat pipes.

*Links* (will add later)

 

[pitz]

Good lord, is there actually people here who believe that the completely passive cooling systems used in PC's actually have 'heatpipes' filled with liquid? That makes no sense whatsoever.

Basically the idea of a heatsink is that convective heat transfer is proportional to surface area. Larger surface area (ie: lots of fins) = lots of thermal 'flow'.

 

[janas19]

Good lord, is there actually people here who believe that the completely passive cooling systems used in PC's actually have 'heatpipes' filled with liquid? That makes no sense whatsoever.

Basically the idea of a heatsink is that convective heat transfer is proportional to surface area. Larger surface area (ie: lots of fins) = lots of thermal 'flow'.

Pitz, it does sound funny but it actually makes sense, because manufacturers of aftermarket, cast heat sinks call the pipe-shaped heatsink 'heat pipes." That's why people get confused, because marketers use the exact same terminology as the aerospace industry. Technically, one should be called "cast heat pipes" or "pipe-shaped conductive heatsinks" and the other "liquid-filled enthalpy" heat pipes. But what do you know, they both are called "heat pipes." Go figure.

 

[janas19]

*Update*

Ok, so I apologize to you guys. My earlier posts were not very clear because I was confused myself. Hopefully, THIS post will clear up the distinction between what exactly is a heat pipe, and what is called a "heat pipe" by marketers but is actually not (further explanation follows).

Link #1 1: Heat Pipe Technology: Passive Heat Transfer for Greater Efficiency
http://www.thermacore.com/thermal-basics/heat-pipe-technology.aspx

"Heat pipes are the most common passive, capillary-driven of the two-phase systems." Note this manufacturer says that heat pipes have "TWO-PHASE" systems. This is important.

"Key Components of a Heat Pipe

The three major components of a heat pipe include:

A vacuum tight, sealed containment shell or vessel
Working fluid
Capillary wick structure"

OK, so in order to be a true heat pipe, it must have 3 components right? Those are: A VACUUM TIGHT, sealed containment shell. A working FLUID. And a CAPILLARY wick structure. These 3 are the components of a heat pipe.

Link #2: Passive Two-Phase Devices
http://www.thermacore.com/products/two-phase-heat-transfer-devices.aspx

Remember, Thermacore states that heat pipes are two-phase systems. So this link takes us away from the theoretical and shows us the actual two-phase heat pipe devices for sale. The one we want is under " Heat Spreader Solutions: Remote Dissipation"

"Remote heat pipe heat sinks, using cost-effective coolants like methanol or water, can move hundreds of watts of heat and stand up to extreme variations of temperature. A Thermacore remote dissipation cooling system for Navy radar cooling performed successfully after 200 cycles between -40° C and 70° C, and its simple, rugged construction stood up to heavy shock tests without faltering."

So this heat pipe is used in Navy radar cooling, stands up to hundreds of watts in extreme variations in temperature.

Link #3:http://www.enertron-inc.com/enertron-products/heat-pipe-selection.php

On this page what you want to look at is "Table 1: Typical Operating Characteristics of Heat Pipes." For ranges between 5-230 degrees celsius, typically the working liquid is WATER, under a vacuum tight shell.

So what I'm getting at, is that in order to be a true heat pipe, it must be a two-phase change device, with liquid inside, and under a vacuum.

Now, the question becomes: is what cpu cooler manufacturers call a "heat pipe" a true heat pipe?

In order to be yes, it would have to contain a liquid inside. Also, I can't imagine how they would decrease the pressure inside the shell enough so that water boils around 70 celsius (70 the average temp of a processor) without having some sort of valve there, can you? So I would think it would be logical to look for some valve to "charge" the heat pipe and apply a vacuum pump, otherwise how could they apply vacuum?

Finally, lets take a closer look at some cpu coolers and what are called "heat pipes."

http://www.pcper.com/reviews/Cases-...0-HP-600W-Modular-Heat-Pipe-Cooled-PSU-Review

This is a typical marketing phrase that shows a product described as "heat pipes." The reviewer in fact makes a blatant error, in that he describes the science of "two-phase system" heat pipes, but doesn't provide any evidence that said product contains them. No liquid shown. No evident valve or any mechanism for charging or applying vacuum.

More openings:

1. Post by Phoenix: "You'll also notice there's nothing dripping out of it." Notes there is no liquid inside the pipe.
2. http://www.frostytech.com/articleview.cfm?articleID=2616. Cuts open a Zalman "heat pipe" cooler. Note there is no liquid inside.

So, in conclusion, there is a lot of confusion surrounding the term "heat pipe" in both the enthusiast and aftermarket community. CPU coolers are called "heat pipes," but they do not contain the components of a true "two-phase change heat pipe." What they are in fact are heatsinks, which may be "pipe-shaped," or even hollow. But they are certainly not phase change, and rely on thermal conductivity instead.

Further reading on phase change cooling, the concept and implementation: http://www.overclockers.com/build-your-own-phase-change-pc-cooling-system/

 

[aigomorla]

the contents inside a heat pipe are subjected to pressure.

This is why heat pipes are sealed.

The moment u cut open a heat pipe, that pressure is brought to 1ATM standard.
At that pressure the liquid is converted into Gas.

No one will use a liquid which stays liquid at 1atm because those liquids arent as efficient as the ones which are used inside heat pipes.

As pressure increases, a gas will turn to liquid, and u can get your "heat pipe" action working.

Your all under the assumption that the liquid inside the heat pipe stays liquid at standard pressure, when it doesnt. It only stays in liquid form as long as its subjected to pressure inside a sealed heat pipe.

And well, the moment u cut open one, yes, your not going to see liquid because that liquid no longer has the pressure to stay liquid and will escape as gas.

 

[96Firebird]

Isn't air a fluid? So does it need a liquid, or a fluid? I see water is the typical fluid, but couldn't air be used as well?

 

[Smoblikat]

I think GoStumpy is referring to CPU coolers specifically, which do not have liquid in the heatpipes.

I'm not sure why the insides of the pipes would be hollowed out, tbh. Basically, the more surface area which a heatsink has, the more it is exposed to air. It is air which carries away excess heat. That is why air cooled cpu coolers have fins - lots of airflow.

But the inside of the pipes don't get airflow, so it seems they should be solid. *scratches head*

*Edit* On second thought, I honestly believe hollow cpu heatpipes have more to do with cheaper manufacturing, or marketing gimmicks, than any scientific reason.

I think theyre hollow because its easier to heat up a thin sheet of metal than an entire block of it.

 

[PhoenixEnigma]

Air wouldn't work very well, for a couple reasons. The efficency of heatpipes comes largely from the working fluid changing state, and regular air is bad for that in a couple ways. First, it's not going to be a liquid at the sorts of temerature we're interested in, so state change isn't going to happen. Second, air is a mix of gasses, so you don't get a single boiling point to design around - you'd have situations where the less volatile components are lying around taking up space, interfering with the ones that are actually working.

If you wanted to run a cryo-temperture heatpipe, though, you'd probably use something that's a gas at STP. Nitrogen and argon are both cheap and fairly nonreactive, so I'd expect they're likely candidates.

As to vacuum charging - take a closer look at the photos of that Freezer64 I posted. Notice how each heatpipe (there's three, really, bent into U shapes) has a machined copper end and a crimped-and-metal-coated end? It's most visible for the front set because of how I took the photo, but you can see it on all of them. I'll give you three guess why that is. Hint: it's not a stylistic choice.

Again, either manufactures across the board are putting in needlessly complicated fake heatpipes that have everything but a working fluid in them and claiming they're the real deal, or they're using real heatpipes. If you believe the former, I'd look into turning those aluminum fins into a nice hat.

As well, to repeat another point that wasn't addressed - solid copper rods would (and do!) transfer heat than hollow copper tubes (not heatpipes) - cross sectional area is key. Do you really think there wouldn't be someone using that for market differentiation? Or someone selling actual heatpipe based designs, for the same reason?

As an interesting bit of an aside, you can see the effect aigomorla is talking about in reverse sometimes when you open a bottle of warm pop, and a bunch of condensation appears in the air in the bottle. It's quite a noticable change, even though the pressure drop isn't actually too great.