thermosiphon Archives - Boyd | Trusted Innovation

Two-Phase Cooling – Ask an Expert Q&A

Boyd Blog — Tue, 07 Dec 2021 13:53:44 +0000

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 best-fit methods 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. Boyd 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.

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Thermosiphons vs Heat Pipes

Boyd Blog — Tue, 30 Nov 2021 13:53:44 +0000

In our previous blogs, we’ve gone over What Are Thermosiphons? and the various Thermosiphon Configurations and Applications. Here we’ll be answering one of the most commonly asked questions regarding Two-Phase Cooling: what is the difference between a Thermosiphon and a Heat Pipe?

The Main Distinction

While both are passive cooling systems that are based on evaporation and condensation of a working fluid, the distinction between a Heat Pipe and a Thermosiphon is the existence of a wicking structure within Heat Pipes that is absent from Thermosiphons. This wicking structure, usually a sintered powder, axially grooved, wire mesh, or screen wick, creates a capillary pressure that allows working fluid to return to the condenser, in any orientation including against gravity. For a Thermosiphon, the working fluid returns via gravity, meaning the heat source and evaporator need to be located below the condenser unit.

What Are Other Benefits and Drawbacks?

Qmax:

The maximum heat transfer capacity (Qmax) for a Thermosiphon is typically going to be greater than that of a Heat Pipe of an equal diameter and length. The wick structure restricts the amount of vapor space and the potential speed of liquid returning to the evaporator through the wick capillaries. In a Thermosiphon, fluid and heat can move more efficiently since it gravity removes the need for a wick.

Distance from Heat Source

Because Thermosiphons do not rely on a wicking structure to transfer fluid, the length that a Thermosiphon can transfer heat is much longer than that of a Heat Pipe. At Boyd, Thermosiphons have been created upwards of several meters in length. If the gravity is favorable, Thermosiphon length can be virtually unlimited.

Temperature Control:

Thermosiphons tend to allow for much tighter temperature control over multiple heat sources when compared to Heat Pipes. Since a Thermosiphon doesn’t rely on multiple individual tubes, the vapor pressure stays the same throughout the assembly. This means the temperature is going to be similar across the entire Thermosiphon assembly as heat is pulled from the heat sources.

Fewer Tubes and Lower Profile:

When using a Thermosiphon construction with a remote condenser unit, due to the higher heat transfer capacity, the number of tubes needed between the evaporator and condenser is much smaller than the number used for a similar Heat Pipe Assembly. Thermosiphon tubes also have a lower profile than Heat Pipes for similar heat transferability, meaning that they can potentially block less airflow through the system, leading to more efficient cooling.

Design and Complexity:

Thermosiphons are always custom designed for each specific application, based on a variety of factors. This can result in complex and involved design, planning, and development to bring a concept into a manufactured product. Heat Pipe Assemblies are also customized, but individual Heat Pipes have more off-the-shelf constructions readily available for integrating into an assembly.

Boyd has decades of experience creating both Heat Pipe and Thermosiphon assemblies for a wide variety of applications and industries. To learn more about our Two-Phase Cooling capabilities, visit our website or reach out to our experts.

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Thermosiphon Configurations and Applications

Boyd Blog — Tue, 23 Nov 2021 13:53:44 +0000

In our previous blog, we went over the basics of Thermosiphons and how they work. While they’re all passive, two-phase systems that contain three basic parts (an evaporator, a fluid loop, and a condenser), the way that they’re constructed can be different depending on how they’re used.

In this blog, we’ll be reviewing the types of Thermosiphon configurations and some common applications for each.

What Are The Different Thermosiphon Configurations?

At Boyd, Thermosiphons are generally broken down into four main categories:

3D Direct Contact Loop Thermosiphon:

Typical Industries and Applications: Industrial and Telecom (Power Inverters, Motor Drives, 5G Towers, Remote Radio Units, MIMO Radio)

3D Direct Contact Loop Thermosiphons dissipate heat from one or more heat sources mounted directly to the base of the Thermosiphon. These thermosiphons feature vapor supply and liquid return tubes in the base and the fins as well as manifolds that spread heat through the full 3D volume of the attached fins.

The working fluid absorbs heat and turns to vapor as it flows through the tubes in the base closest to the heat source and rises upwards from buoyancy. Manifolds lining the top and bottom of the assembly allow vapor and condensed fluid to distribute to each fin ensuring an isothermal 3D structure for consistent cooling. Natural or forced air flows through the near isothermal fin assembly to reject the heat to the surrounding environment with high efficiency. As heat is rejected from the system, the working fluid recondenses in the tubes in the fins, where it returns by gravity to the liquid manifold at the bottom and comes back to the evaporator for the process to be repeated.

Direct Contact Loop Thermosiphon:

Typical Industries and Applications: Industrial and Enterprise (CPUs, GPUs, Inverters, Servers)

One of the most common types of Thermosiphons, Direct Contact Loop Thermosiphon Assemblies feature separate condenser and evaporator components, which are spaced apart and connected by carefully oriented tubes. The evaporator base is mounted directly onto a heat source such as a CPU or GPU. Heat from the device vaporizes standing liquid within the evaporator, causing it to travel through vapor tube to the remote condenser unit.

The condenser features an attached high-density fin stack, where ambient forced airflow removes heat from the system. As heat is removed, the working fluid recondenses, flowing back through a return tube into the evaporator.

Air-to-Air Loop Thermosiphon:

Typical Industries and Applications: Telecom, eMobility, and Industrial (Cabinets, Edge Compute, 5G Towers)

Air-to-Air Loop Thermosiphons work similarly to other air-to-air heat exchanger types, but use loop Thermosiphon technology instead of conduction or heat pipes to transfer heat from one air stream to another. An evaporator and condenser heat exchanger are connected by tubing with half of the system located within an enclosure and the other half outside of the enclosure.

Hot internal air from the enclosure evaporates the fluid on the evaporator coil, which rises through the vapor tube into the condenser coil. Forced external air flows through the condenser coil, recondensing the working fluid that then flows through the return tubes via gravity back down to the evaporator coil where the process repeats.

2D Thermosiphon Fin:

Typical Industries and Applications: Industrial and Telecom (5G, Remote Radio, MIMO Radio, Inverters, replacing or enhancing traditional extruded, cast, or bonded fin heat sinks)

Perhaps the most unique of the Thermosiphon categories, 2D Thermosiphon Fins are individual fins enhanced with Thermosiphon technology. Used primarily like standard fins to increase effective fin surface area for heat dissipation, Thermosiphons are embedded into each fin to increase the fin efficiency and overall performance.

These embedded Thermosiphons, available in a loop, honeycomb, or microchannel design, significantly improve the performance of standard aluminum fins. 2D Thermosiphon Fins reduce weight and can reduce heat sink volumes by optimizing the thermal performance and are commonly used due to how they can be mixed and matched with other traditional thermal technologies.

While these are the general categories for Thermosiphons at Boyd, each can be customized to suit a wide variety of applications. If you have questions about our Thermosiphons or which one is ideal for your next project, visit our website or reach out to our experts.

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What Is a Thermosiphon?

Boyd Blog — Mon, 15 Nov 2021 13:53:44 +0000

Thermosiphons are an efficient and versatile two-phase cooling solution that have grown in popularity over the last few years. In the first of our three-part blog series, we’ll be going over exactly what a Thermosiphon is and the key features that make it unique.

What Are Thermosiphons?

A Thermosiphon is a passive, two-phase cooling system that relies on gravity to circulate a fluid rather than a capillary wick structure often used in heat pipes and other heat transport devices. As with all passive two-phase cooling, the liquid and vapor exist within the self-contained envelope and contain no pumps or other moving parts.

While there are several different constructions for Thermosiphons, they generally all consist of three basic components: an evaporator, a fluid loop (or adiabatic section), and a condenser. The evaporator absorbs energy into the system, which causes the working fluid, usually a refrigerant or other dielectric fluid, to turn into vapor. The vapor then travels through the adiabatic section (vapor tube) due to the pressure difference between hot evaporator and cool condenser (buoyancy), where the heat is expelled from the system and the vapor condenses back into a liquid and returns to the evaporator via gravity. This process is repeated indefinitely as long as there is heat to reject from the system.

What makes Thermosiphons unique from other two-phase cooling systems is that they lack any kind of wicking structure. Where the wicking structure in a Heat Pipe creates a capillary pressure to return condensed fluid, Thermosiphons rely on gravity for the same process. This allows them to more efficiently carry higher heat loads across longer distances, but requires Thermosiphons to be designed and used in a specific gravity orientation. The thermosiphon evaporator must be located below the condenser so that gravity can return the working fluid.

Common Applications for Thermosiphons

Thermosiphons are useful for a wide variety of industries and devices. Thermosiphons are effective enterprise solutions for CPUs, GPUs, ASICs and FPGAs within servers, networking equipment, and cabinets. Industrial and power applications benefit from thermosiphons for cooling wind and solar inverter IGBTs. Power amplifiers in telecommunications equipment like Remote Radio Units and full cabinets leverage the high cooling capacity of thermosiphons.

The heat transfer coefficient of a thermosiphon is hundreds of times more than conventional materials like aluminum or copper and energy is transported with consistent temperatures throughout the unit. This makes Thermosiphons ideal for larger surfaces, heat transfer across multiple devices, or as a cost-effective option for any application where orientation with respect to gravity is known and fixed.

Thermosiphon Testing

At Boyd, Thermosiphons go through a variety of tests to ensure that they’re reliable and ready for use. Pressure testing, leakage testing, and thermal performance testing are just a few of the methods used at Boyd to ensure that each Thermosiphon meets specifications.

In our next blog, we’ll go into detail on the four types of Thermosiphon configurations at Boyd: Direct Contact Loop Thermosiphons, 2D Thermosiphon Fins, 3D Direct Contact Loop Thermosiphons, and Air-to-Air Loop Thermosiphons.

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