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I was wondering about the main difference between using heat pipes and a fluid-based cooling system. In both cases the heat is transported away from the heat-generating area to a place where the heat is dissipated. What is now the advantage of using a fluid instead of heat pipes? Is it the lack of heat transfer points (a fluid has no heat exchange points, except of the radiator and thermal pad, while heat pipes have to be connected)?
Thus, when is it more beneficial to use a fluid cooling system instead of heat pipes?

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Heat pipes are very useful when you have limited space to work with or you want an entirely sealed and self contained unit.

An interesting example of this is that heat pipes are used to cool the legs of oil pipelines which run through permafrost areas, the cooling is required to stop the heat generate by friction in the pipe and solar heating from conducting through the support legs and melting the ground under them. Here the advantage is that they are small, simple and require no power supply and little maintenance.

Similarly heat pipes are now a pretty standard component in CPU coolers. For laptops they may be the only way to package adequate cooling for modern processors and even for desktops they make it more convenient to design and package high capacity coolers by making it possible to move the heat-sink away from the processor itself for greater size and better airflow.

Having said that water cooling for high performance desktop PCs is becoming increasingly mainstream so there is a certain amount of overlap.

Liquid cooling becomes more attractive as the heat loads get larger and you get beyond the limitations of needing to miniaturise pumps. It also has the advantage that it is reasonably simple to design a custom system using fluid cooling in terms of piping routes and radiator and fan capacities using fairly standard off the shelf components.

There is also the consideration that if you have large and geometrically complex components with large volumes it may be easier to have channels running through their existing internal structure than to try to find surfaces to attach heat pipes and you can cool a large and complex structure with a centralised pump and radiator eg an engine block. Similarly once you have the basic elements of a liquid cooled system it is very scalable.

Another advantage of fluid systems is that they work across a large range of temperatures, eg water cooling will work between 0C and 100C and that range can be further extended with additives and it is usually fairly easy to control the system to get a target temperature at various points in the system by the simple expedient of controlling the fluid flow-rate.

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  • $\begingroup$ Thanks for the answer! I would be interested in your last point in more detail. What is the optimal temperature range for heat pipes and water cooling? I have the impression that heat pipes are more efficient at lower temperatures, maybe 40-70C? And after a CPU hits ~80C, it gets more difficult to cool it with heat pipes, while water cooling should be able to remove that heat. $\endgroup$ – Peter Nemeth .malomsok. Jun 4 at 9:16
  • $\begingroup$ The key different is that heat pipes involve a phase change so the working fluid needs to evaporate at the hot side and condense at the cold side just based on the existing temperature differential. So you need to pick the right coolant for your application. On the other hand for liquid cooling as long as it never gets hot or cold enough to boil or freeze it's going to work. Water cooling also has the advantage that you can adjust the cooling effect simply by changing the flowrate of the liquid $\endgroup$ – Chris Johns Jun 14 at 19:51
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Heat pipes use a fluid that phase changes at a temperature between the two temperatures it is transporting between. It evaporates at one end, then condenses at the other, the fluid then runs via gravity or wicked via a membrane to the other end where it evaporates again.

They are low maintence as there are no moving parts, and when they are well designed, they can be quite effective. They are very effectivly used in evacuated tube solar thermal systems that actually just use water and propylene glycol as the working fluid for that temperature range. enter image description here

The down side to heat pipes is that they only work well around that designed phase change temperature. Above that, all the working fluid is gas and an insulator, below that and most of the working fluid is liquid that doesn't move.

"Pumped" or "Active" liquid cooling relies on the high heat capacity of a fluid (usually water or water with glycol). These systems have moving parts, but they can transfer a lot of thermal energy over a wider range of temperatures. Also for a given footprint (like under the hood of your car), active liquid cooling can transfer a lot more heat than an equivalently sized heat pipe system.

Consequently it is usually cheaper to use heat pipes in small or very repeatable applications, and active fluid cooling for large amounts of heat transfer and more custom situations.

Even evacuated tube solar collectors still use a pumped fluid system to transfer the heat to the hot water tank.

enter image description here

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It really depends on the application.

Heat Pipes are sort of a middle-ground between heat sink and liquid cooling technology. They take advantage of phase-changing a working gas/liquid to transfer thermal energy from a higher density of thermal energy (greater temperature) to a lower density. At any rate, no matter the method, you still are ultimately going to be transferring thermal energy to an ambient medium (namely air). Think about cars and motorcycles. Motorcycles are typically passively air-cooled and have engines with heat sinks built into the cylinders. There is little reason to employ a liquid cooling system here because they have plenty of exposure to air due to their smaller, open designs. Cars on the other hand, virtually always have a liquid cooling system with radiators mounted at the front of the vehicle, evacuating thermal energy from the closely packed engine cylinders to the more open and exposed front.

Because air (at 1 atmosphere) is such a poor conductor of thermal energy, how effectively you can get rid of heat is going to ultimately boil down to how much surface area is exposed to air, how much air you can move across the exposed surfaces, and the ambient temperature of the air.

When to use a Heat Sink: Most straight-forward option. Use a heat sink, add a fan and you're all set.

  • Simplest, generally most cost-effective Solution
  • Smallest Possible Footprint
  • High thermal capacity and tolerance
  • The greater the difference in temperatures (object to be cooled and ambient temperature), the more effective the heat sink becomes
  • Air flow is either available or can be readily controlled

When to use Heat Pipes: A little more planning may be involved, and space may get to be a concern, but all in all, heat pipes are a very effective means of cooling components within a specified range of temperatures.

  • Minimal Hassle and Maintenance
  • Reduced Weight/Portability (for same capacity heat sink or liquid cooling)

  • Component to be cooled has relatively low surface area by which to transfer heat (such as a computer cpu)

  • Vertical Mounting Available: Many heat pipes MUST be mounted so that gravity can properly transfer the liquid after it condenses at the top of the unit.

  • Specific temperature range (to work properly, working medium must phase change). If component and ambient temperatures are too low, the heat pipes will only function as a mediocre heat sink. If component and/or ambient temperatures are too high, the gas won't be able to condense and thermal handling will be reduced.
  • Use when space isn't an issue or heat pipes can be custom designed to fit --- for example, the Acer Predator 21 X uses heat pipes produced specifically to fit inside of the laptop. Stock heat pipe solutions otherwise tend to be somewhat bulky to accommodate the working medium (liquid and gas) and transfer heat through a radiator.

When to use Liquid Cooling: If you need to cool a component which is tightly packed amongst other components and otherwise has little access to air flow, liquid cooling is the way to go, so long as you have space elsewhere for a radiator and fan setup. Essentially, you can use this method to transfer heat from a space-restricted location to a more open location. Also, depending on the working liquid, liquid cooling will function optimally within a greater range of temperatures than heat pipes, but is also prone to more serious failures if temperatures become to low or high.

  • When Space is a serious issue --- a heat exchanger can take up very little space and plumbing can be flexible
  • When Air flow is a serious issue --- components to be cooled do not have ready access to air
  • Heat needs to be transferred directly away from components without building up
  • Excellent thermal capacity and heat exchange rate
  • When properly used, can maintain temperature closest to ambient
  • May be combined with other cooling options to lower temperature below ambient
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