Two-Phase Cooling encompasses several different cooling technologies, so we sat down with one of our thermal experts to answer some frequently asked questions.
In the first of our Ask an Expert series on our LinkedIn, we asked people to submit questions about one of our most popular thermal technologies: Two-Phase Cooling. We received several questions, so we spoke with one of our thermal experts, David Miller, to answer them below.
What Makes It “Two-Phase” Cooling?
Essentially, “Two-Phase” means that there is a phase change of a fluid from liquid to vapor that occurs. This change of phase causes a substantial transformation of heat, called latent heat of transformation. In a Heat Pipe, a very small amount of pure water (or other liquid) saturates the wick inside. When that heat pipe is exposed to heat in one area, that liquid evaporates, causing a phase change of the water to vapor. The changing of this water to a vapor quickly transports heat to cooler regions where it will condense. This moving of heat has a much higher effective thermal conductivity than conduction through a solid material such as copper.
What Are the Benefits of Two-Phase Cooling over Air-Cooling?
In any thermal management system, heat must move from hot to cold, so you need a way to move heat from one location to another. Heat Pipes, Vapor Chambers, and Thermosiphons are all components that accomplish this more efficiently than solid conventional metal conductors, such as copper or aluminum. Because of the phase change that occurs in these two-phase cooling methods, the thermal conductivity is 100-200 times that of copper. This reduces the total thermal resistance of the cooling system and the temperature difference (Δt) from one point to another.
With all of that said, you still need air. You can’t replace an air-cooled device with just a heat pipe; you use a heat pipe in conjunction with an extended surface area (fins) and some kind of air movement system (or by natural convection). But the lower the thermal resistance, the faster the heat will transfer, so for the same amount of power, the temperature difference from a device to the air will be lower with a two-phase enhanced cooling system.
When Would I Use Thermosiphons, Heat Pipes, and Vapor Chambers?
We have a lot of information about how each of these work on our website, which is a good starting point. Since most applications are unique, it would be best to contact Boyd and work with one of our application or design engineers to develop an optimized cooling solution for you.
What Are Some of the Challenges Involved in Implementing a Two-Phase Cooling System?
It depends on what the Two-Phase System is. The cost can vary substantially, depending on the technology that is used. There are also some limitations on the maximum length of a Heat Pipe; if you have to move heat across a very long distance, a heat pipe can potentially be limiting in length. The physical size of a Heat Pipe or Vapor Chamber can also be a challenge, as there are limitations in what you can manufacture based on what tooling is available. Finally, the environment can be a factor; environments that run below freezing can pose challenges since copper-water based Heat Pipes only operate above freezing temperatures.
Now, there are ways to get around each of these challenges, including using a Thermosiphon for longer distances, or by using methanol instead of water in a Heat Pipe for operating below freezing temperatures. Ultimately, the best thing to do is to send us your specific project requirements, and we can help determine the best method for cooling.
What Is the Difference Between a Heat Pipe and a Loop Heat Pipe?
Think of a conventional Heat Pipe as just a copper tube that has a wick and a specific amount of fluid inside. When you heat one end, the water inside evaporates and travels to the other end, where it cools. The liquid then condenses and travels back along the same pipe to the heated end by capillary action.
A Loop Heat Pipe runs as a continuous loop; it has a starting point (an evaporator) where the water turns to vapor. The vapor travels along a pipe to a condenser, where the liquid recondenses and travels along a separate line, returning it back to the evaporator. There is a lot of physics that drives the operation of a Loop Heat Pipe. A typical use for Loop Heat Pipes is cooling spacecraft applications, where heat must be moved longer distances (they typically use ammonia as the internal heat transfer fluid). If you have a fixed and known or favorable gravity orientation, a Loop Thermosiphon may be utilized in a similar manner while being more economical.
Is Two-Phase Cooling Always Liquid to Gas? Or Are There Solid to Liquid Solutions?
Yes, and they’re often referred to as “phase change materials (PCMs)”. One example of a PCM is paraffin wax, used for energy storage in conjunction with lithium-ion batteries in electric vehicles. The batteries can be embedded in wax, which gets heated and causes the wax to melt, which stores the heat. While the solid PCM is melting, the temperature stabilizes, allowing power spikes to be absorbed and then dissipated over a longer period of time. For any applications that are considering PCM or other Two-Phase Cooling techniques, reach out to us and our experts can help advise the optimal cooling method.
How Can I Simulate Heat Pipes Within My Application?
For a lot of people doing computational fluid dynamics (CFD) analysis, they use general rules of thumb to model the equivalent thermal conductivity of a Heat Pipe or Vapor Chamber. Geometrical limitations and orientation limit the ability to accurately model Heat Pipes, and the results do not take into account factors such as gravity, or bending and flattening of a Heat Pipe. You also cannot predict the limits of a Heat Pipe, or if it is overdesigned.
Aavid SmartCFD is the only software tool that accurately models complex Two-Phase Cooling components like Heat Pipes and Vapor Chambers, quantifies utilization capacity, and warns of dry-out. This software allows for all of those factors when simulating a Heat Pipe or Vapor Chamber. It helps predict the performance of a cooling system with a Heat Pipe or Vapor Chamber much more accurately than other CFD programs, so I’m a big proponent of SmartCFD.
What Are the Most Extreme Two-Phase Cooling Applications You’ve Worked On?
For extreme temperatures, we’ve made Heat Pipes for applications below 0°C before, which we were able to do using methanol as the fluid. On the high end, I’ve seen applications that go up to about 1000°C (though we generally try to use them in applications around or under 150°C for electronics cooling). In terms of extreme scale, I’ve developed a Thermosiphon for a geothermal application many meters in length.
We see very interesting applications that could be solved with Two-Phase solutions. We strongly encourage customers to contact us with their opportunities. We’re always happy to review the requirements and make recommendations.
What Is the Future of Two-Phase Cooling?
There are endless opportunities for Two-Phase solutions in thermal management, from the traditional enterprise and consumer applications to new uses in the eMobility, medical, and renewable energy industries. We’ve recently worked on making three-dimensional Heat Pipes and Vapor Chambers for very high heat flux applications, where you’d want a uniform temperature through an entire heat sink. Typically, you’d put a Heat Pipe or Vapor Chamber in the base of the heat sink that touches a hot device, heat spreads and transfers to fins, and dissipates via natural or forced convection. With a three-dimensional Vapor Chamber, you wouldn’t have to take the hit on thermal resistance in going from a heat pipe to a solid fin. Instead, thermal resistance is dramatically reduced since the vapor travels from the heated device directly into the fin volume.
Another area that is interesting is using ultra-thin Vapor Chambers, which offer exciting possibilities for cooling devices such as smartphones and tablets. Heat loads are continuously increasing, and graphite, which has been used conventionally, is reaching the limit as an effective thermal heat spreader. Our ultra-thin Vapor Chambers can move much larger amounts of heat very efficiently in very thin regions (as thin as 0.25mm thick).
We’d like to thank everyone who sent in questions! To learn more about our capabilities or to find out which Two-Phase Cooling solution is right for your application, reach out to our experts.