Below are the questions asked during the event, along with their respective answers.

Q: Are there any temperatures where a heat pipe will not work?
A: A heat pipe operates along the saturation curve of its working fluid. In order to change the temperature scale, one would change the working fluid. However; there is a point that is essentially a heat pipe dead zone where there is not a great option for a compatible working fluid and envelope material. This is roughly between 300 and 450 degrees C.

Q: What is the farthest distance a loop thermosyphon can transfer heat?
A: Loop thermosyphons have been designed and fielded with over 50 feet of transport length. Operational considerations should be evaluated for each design to determine maximum transport distance.

Q: Are there any safety concerns when using a heat pipe?
A: Since a heat pipe is passive you do not have to worry about moving parts breaking down. But there are internal pressures to consider. When the temperature reaches the top of the saturation curve the internal pressure will be extremely high and will cause the heat pipe to burst. ACT has designed a safety release where the pinched end will open and release the vaporized working fluid. A well designed thermal management system will not experience this type of challenge.

Q: Can a Pumped Two-Phase cooling solution be integrated into a single-phase cooling loop?
A: In many cases, P2P replaces the single-phase cooling option; however there are instances where it makes sense to integrate P2P with cooling loops already available in the system. For example, P2P can provide specific value to smaller sub-sections of the system such as (1) isothermalizing battery cells or (2) providing di-electric cooling to critical electronics, and then exchange heat with the existing single-phase cooling loop. Another example is coupling P2P with vapor compression loops which can provide sub-ambient capability.

Q: Can you use an air-to-air heat exchanger since the enviro is at 25C?
A: The imaging equipment used a liquid cooling line as the primary coolant, that was the 25 C sink temperature referenced. In that application, the detectors/sensors were internal to the environment and the power was significant enough that air cooling to the environment would have resulted in too large of a delta T.

Q: Is TIM used in data center cooling system?
A: TIMs are often used between components and thermal solutions to avoid tolerance issues (poor contact) and CTE mismatch. In certain cases they can be avoided, but it’s on a case by case basis.

Q: What are the most common fluids used in two-phase cooling such as heat pipes?
A: The fluid selection is typically made based on system requirements, like temperature range, rather than “type” of heat pipe. However in general, water heat pipes and vapor chambers are most common because they operate well against gravity (good surface tension), are a strong fluid near the critical temperature of electronics, and are compatible with copper.

Q: Do thermosyphons typically use different fluids than heat pipes and vapor chambers?
A: Refrigerants are often used in thermosyphon applications where pumping against gravity isn’t a large design parameter, as they have the advantage of being di-electric which is preferred for Power Electronics type applications.

Q: Are there any Heat Pipe like cooling (non Pump solution) which can work where the Evaporator is Gravitationally Higher?
A: Most heat pipes must operate in variable orientation and are designed such that they can work with the evaporator above the condenser. The wick structure is designed to passively “pump” the fluid against gravity in these scenarios. There is a physical limitation in terms of pumping height depending on the wick structure but a general guideline is 10-12″ against gravity as a maximum length.

Q: Is a loop thermosyphon used with a Peltier module better for a heat sink and how feasible is it?
A: Heat pipes, loop thermosyphons, etc are often coupled with Peltier coolers to control temperature at a specific location. For instance, if you need to go sub-ambient or hold a consistent temperature, you can use the Peltier module to do that function and utilize the heat pipe/thermosyphon to move heat from the backside of the Peltier. That being said, Peltier coolers are not overly efficient and cannot handle very large waste heat dissipation so there are practical design considerations to take into account.

Q: What is the latest cooling technique for CPU thermal management? Can thermosyphon be used for cooling very high power CPU’s?
A: CPU thermal management varies based on the power and heat flux and can include all the technologies covered during the webinar. Thermosyphons and specifically loop thermosyphons provide the highest power option for passive cooling and should be considered for high power CPUs but ultimately it’s a case by case basis on what technology should be considered. If you have a specific case you’d like to discuss, please send us the details and we’d be happy to provide you more precise feedback.

Q: What is the cost difference between using water versus a refrigerant-based solution?
A: The consideration for water vs refrigerant is not typically done on cost, but rather operating temperature or system-level considerations (di-electric, etc.). The cost difference is not substatial.

Q: What is the efficiency difference in comparison of passive and active 2-phase?
A: Passive options should always be considered until they reach practical limitations. They have a long life and require no energy or maintenance, so the operating cost and energy efficiency are unmatched. As power increases, active solutions would need to be considered.

Q: How is a heat pipe typically attached to the semiconductor devices?
A: In many cases, heat pipes are mounted to an evaporator block or embedded into an existing heat spreader that is mechanically attached to the device. A Thermal Interface Material (TIM) is often used in between the metal mounting plate and device to account for tolerance stack up and CTE mismatch.

Q: At -40C how with the heat pipes start working with orientation? What wick do you use?
A: This question is difficult to answer without more information. If the operation is needed below 0 we would use a fluid that can operate in that range. These fluids typically have lower surface tension therefore are limited in their ability to operate in any orientation. In any case with heat pipes, it’s important to understand the system operating details (power, temperature, geometry, etc.) extremely well to recommend a solution that will work.

Q: Being sized for a certain point of temperature or pressure at which the phase changes, do heat pipes have limitations when used in a very wide range of operation (-80ºC to %2B80ºC)?
A: In most cases, the fluid selected is based on the critical operating temperature. For instance, in your example of -80 C to +80 C, the heat pipes are typically designed for the higher end of the temperature range, because that is where you need the optimal performance to keep your electronics cool. In this case, water is optimal. Water will freeze below 0, but will not cause damage and will operate again once the temperature exceeds 0. In almost all applications, cooling at the low levels is not required and the system will operate without heat pipe performance. There are lower temperature fluids that will have higher performance in the subzero temperature range if that’s preferred as well. The primary goal is to understand your system and application to advise on the best choice of working fluid.

Q: Automotive EV vehicles need to operate reliably for 10%2B years, what long-term degradation mechanisms (contamination, property changes, etc) are known to exist for fluids and contactable internal surfaces?
A: This depends on the solution you select. For heat pipes, they are designed and typically implemented in systems that operate for 20+ years. They operate passively, have no moving parts, and can offer very long life. Phase Change Material (PCM) is very similar. For an active pumped solution (single or two-phase), it’s important to understand the life of all individual components going into the system.

Q: Can heat pipes be used in smartphones?
A: Heat pipes are currently being used in smartphones; they are a good choice as they can be flattened and bent to numerous geometries.

Q: How about Graphite as a linear heatpipe?
A: Graphite has strong thermal conductivity, so it can be considered for 1D or 2D heat spreading. In many cases, graphite is encapsulated into another structure to overcome it’s mechanical/structural limitations. In most geometries and temperature ranges, heat pipes will offer equal or better thermal conductivity at a lower cost than encapsulated graphite.

Q: Do you have solutions for the automotive electronics controllers? The controller usually has a few ICs (~ 20W) with a closed housing. This kind of controller is not EV battery pack.
A: Yes, 20 W is fairly low power and we do a lot of applications in cooling these levels in a sealed housing.

Q: What is the operating pressure of water heat pipes in general?
A: The internal pressure follows the fluids saturation pressure; therefore, if you know your operating temperature you can determine internal pressure by finding that point along the saturation curve.

Q: How good is the jet impingement heat transfer?
A: Jet impingement heat transfer is a very good method because it uses high velocities and direct contact. The challenge is that it’s difficult to implement in practical applications where you need to keep the surface clean and/or do not have an unlimited source of fluid to spray (difficult to collect and recycle).

Q: Are thermosyphons used in automotive battery cooling applications?
A: Loop Thermosyhons are traditionally used in stationary systems where you can guarantee that the condenser will be above the evaporator. Therefore, they could be used in automotive battery cooling, however, you would need to account for all driving conditions and pitch changes in the design process.

Q: How is the energy density of an electric vehicle with a heat pipe as thermal management as compared with other cooling systems?
A: Heat pipes are strong passive options for high energy density cooling; in the case of EVs, it’s important to understand the entire system and operational requirements to comment if they are the ideal solution compared to other technologies. If the primary challenge is heat spreading or transport, they are typically a good option to consider.

Q: 2-phase systems; do you use Novec 7000 in automotive battery applications?
A: We have used a variety of fluids in past systems, and start new designs by considering all fluids and selecting the fluid that is most appropriate based on operating conditions.

Q: Any maintenance needed for liquid cooling systems for telecom products?
A: Yes, with any active system, components will have a finite life. The pump is one of the major components to understand the mean time between failure (MTBF) and service appropriately.

Q: How are heat pipes helpful in CT scan?
A: In the application of CT scanners, there are many detectors located along a given surface, and temperature uniformity is critical to the overall image quality. Heat pipes can help keep all those detectors at a very uniform temperature compared to basic metallic heat spreading.