Liquid cooling is a time-proven approach to maintaining safe operating temperatures of high heat flux electronics. It’s effective, capable of rejecting high heat loads and well understood. Although liquid cooled approaches are active, meaning they consume power and have mechanical components, they are often the best solution when power and temperature requirements demand it. In some cases, the liquid cooling lines need to be routed close to the heat-generating components to maintain thermal goals. This can, however, present a significant risk to the system in the event of a leak. If there were to be a fluid leak in the system, it would result in catastrophic failure to the electronics system.
Risk Mitigation to mitigate fluid leaks
One way to mitigate the fluid leak scenario is by decoupling the ultimate heat sink (liquid) from the heat-generating devices. By adding heat pipes to transfer heat at low thermal resistance, you can increase reliability with a minimal performance impact. The added distance between electronics and coolant could lead to a thermal penalty.
In the example below, ACT designed, developed, and tested a thermal solution that is capable of transferring heat from within an electronics chassis to an external liquid cooling loop. This included the heat pipe heat spreader design as well as the series/parallel flow channel liquid cold plate that acted as the ultimate heat sink. The electronics assembly, referred as a slat, has a fixed thermal resistance that is set by the customer’s electronics design. This slat is mounted to a heat pipe embedded, or HiK™ heat spreader, which transfers the heat through a series of interfaces and aluminum conduction paths to the external liquid cold plates which are mounted on the top and bottom of the electronics chassis.
Each HiK™ heat spreader in this assembly transfers 180W. The power is split between the two cooling loops with each handing 90W of power. This is important to note since heat pipes work both gravity assisted, and against gravity. Since the heat pipes were designed to work in both of orientations, and have a fixed thermal resistance, the electronics were able to maintain symmetric thermal gradients using both the upper and lower HiK™ heat spreaders. There is a combination of 16 HiK™ plates within this system for a total heat load of 2,880W.
Customer Collaboration- What does the design process look like?
ACT began the effort with a detailed thermal analysis that mapped the thermal resistance network from the heat-generating components to the chilled water supply. Several design iterations were required to determine a cold plate design that met both temperature uniformity and pressure drop goals. The final solution was a serpentine tube-in-plate design to ultimately meet necessary thermal requirements. Each serpentine removed heat from four of the HiK™ plates, four serpentines were used in parallel to reject heat from the 16 HiK™ plates within the system as seen in the Figure 1 CAD drawing below.
Following the thermal analysis of the proposed system and the customers’ approval, the ACT team was ready to move forward with manufacturing the prototype. The first step in production was manufacturing the heat pipes, aluminum components and the manifold assembly. Commercial off-the-shelf cold plates were used in lieu of an all-aluminum assembly in order to simplify and expedite the prototype lead time. Following assembly of the prototype, heat loads were simulated using film heaters that matched the heat input footprint provided by the customer and chilled water was supplied using a commercial chiller. An image of the test setup is shown in Figure 2.
Results of the testing in comparison to the thermal predictions are shown in Figure 3. The max junction temperature was predicted to be 118.0°C whereas the assembly measured 118.2°C. The temperature drop highlighted in the red boxes on the left of Figure 3 represents the junction-to-case temperature drop within the electronics slot. This information was provided by the customer and is fixed in the comparison. There are minor differences between the predicted and measured interface materials. The predicted temperature drop of the cold plate was 11.7°C and measured a 14.5°C. One key temperature drop to highlight is for the HiK™ plate. The prediction estimated a 4.1°C temperature drop, whereas the assembly measured only 2.9°C.
It’s not uncommon for large chassis to have such high waste heat that it requires liquid cooling. While this is less reliable than conduction or air-cooled solution, there are methods to increase reliability by separating the liquid flow and potential leak points from the high-value electronics. By using heat pipes to transfer the heat from the source to the liquid-cooled sink, you can vastly reduce thermal resistance in this conduction path and keep the liquid loop a safe distance from the electronics. This hybrid approach can provide both high thermal performance and high reliability when compared to alternative solutions.
Discuss your application with ACT engineers today!
Pumping stations and their associated control stations are integral to a functioning municipal water system, therefore it is important that all control systems are designed to minimize downtime.
Many municipal water authorities utilize booster pump stations to deliver properly pressurized water to homes, businesses, and factories. The pumping stations typically draw water from a remote reservoir or aquifer and monitoring enables correctly regulated water pressures. These booster pump stations are fitted with industrial programmable logic controllers (PLCs), pumps, and drives. An important feature of these stations is that they are connected to wireless internet networks, which allows technicians quick access to system performance information to provide faster servicing when a unit becomes inoperable. While being able to identify issues across large networks of booster pump stations is important, adding in redundancy and minimizing the risk of failure on equipment is critical in preventing unnecessary downtime.
- Internal Enclosure Temperatures: The original booster pump enclosures were not insulated, so any heat generated by the internal components was conducted through the interior of the enclosure to the outside metal surface of the enclosure. Depending upon outside temperatures the conductance of the metal enclosure to reject the internal heat load was not enough to keep the internal temperatures within operating range.
- Debris: These booster pump stations were located in residential areas, the enclosures were prone to mold due to weed whacking, leaf blowing, and grass cutting around the stations; these activities increase the number of particles in the surrounding air, and these particles can ultimately be drawn into nearby pump station enclosures. Without an enclosure cooling product that offers a NEMA-rated seal, the likelihood of mold and organic particles entering the pump station greatly increases. A sealed solution was necessary to neutralize the threat of contamination.
The solution for keeping the stations cool and sealed was offered by a CSIA-certified system integrator. The internal heat loads from the equipment were calculated along with the possible ambient outside seasonal temperatures. After utilizing ACT’s enclosure cooling selection tool, the integrator chose ACT’s HSC-45 above-ambient heat exchanger, which produces up to 900 watts of sealed cooling. The HSC-45 is thermostatically controlled by the PLC and only functions when needed to cool the station. Diagram 1 shows the HSC-45’s cooling method via fins that are thermally bonded to the center plate. Fans circulate air through the fins, thus providing the efficient heat transfer and cooling that the station’s interior required.
To avoid drawing unnecessary attention to the side-mounted unit, a requirement called for the ACT-HSC-45 to match the color of the station. Additionally, special tamper-proof mounting screws and rain hoods further protect and conceal the cooling unit. The ACT-HSC-45 heat exchanger fins and mounting plate are ElectroFin® corrosion-resistant powder-coated, and the housing is constructed of 316 stainless steel. These features environmentally rate the system for NEMA-4X installations. In total, these design elements provided the booster pump stations with a better performing and more secure enclosure solution.
The system integrator made the recommendation for ACT’s HSC series of units based on a self-conducted summertime rooftop test. Multiple enclosures by finish and color were included in the test. Each enclosure was internally monitored for temperature. The ACT-HSC unit had the best test results based on internal enclosure temperature.
Graph 1: Roof Top Test Results
Photo 1: Multiple enclosure test stand
Photo 2: ACT-HSC-22 shown mounted for testing
For more information about ACT’s cooling systems and customization, please visit https://www.1-act.com/enclosure-cooling/
This power electronics enclosure for a test setup is full of high-power electronic components generating massive waste heat. A customer found that the cabinet’s internal temperature was rising above acceptable temperatures and came to ACT because they were worried about overheating electronics causing downtime during testing.
The cabinet is set up inside a dusty warehouse with an average ambient temperature of 21°C. The 15 heaters inside of the cabinet are 100W each and create approximately 450W of waste heat. Before implementing a cooling solution, the internal cabinet temperatures were reaching 48°C, on average. This was putting the internal power electronics above their maximum operating temperature of 43°C and causing stress on the electronics; over time this can result in a power failure. The customer was proactive and reached out to ACT before the excess temperatures led to overheating of the components in the cabinet, potentially ruining the test setup. Adding a heat exchanger helped regulate the internal temperature, and the choice of an ACT Sealed Enclosure Cooler was ideal because NEMA rated sealing gasket ensures that the cabinet is not impacted by the dusty warehouse environment. An added benefit is that by choosing a NEMA 4X-compliant model, washdown operations are possible.
Impact of Installing an ACT-HSC-22
ACT’s HSC-22 sealed enclosure cooling unit provided the customer with a sealed cooling solution for their power electronics cabinet.
After installing the ACT-HSC-22, the average internal temperature of the power electronics cabinet dropped to 35°C, which is far below the maximum operating temperature of 43°C. After operating the test equipment for 2 minutes the internal cabinet temperature readout was 38°C, remaining the average temperature throughout the test.
The customer was pleased with this solution that provided the needed cooling while preventing worries about filters and regular cleaning inside of the cabinet. They also expressed satisfaction that the unit was quieter than an air conditioner.
Overview of the Solution
When Intel was looking for a thermal partner to help them develop the next generation cooling solution for their high performance computing products, they knew both the company and the technology they needed to work with. Advanced Cooling Technologies has been working on pumped two phase cooling for more than a decade and is a recognized technology leader. Pumped two phase cooling essentially uses the vapor produced from the working fluid’s phase transition to effectively and efficiently remove waste heat. To accommodate higher power density processors, Intel recognized the need for a higher performance and robust cooling solution.
The major challenges ACT had to overcome in developing a solution were the increased heat dissipation requirements within the very tight packaging footprint of Intel’s Coolant Distribution Unit (CDU). The CDU was designed for a total heat load of 4kW and for over 50,000 individual nodes in a 1U blade configuration, with a maximum weight of 12 lb. ACT utilized its wide array of thermal simulation tools to design and then prototype a pumped two phase cooling solution to meet these requirements. The resulting prototype, including the cold plate pictured above, used the available air cooling in the existing system, while increasing heat dissipation capability with an additional liquid cooling system. ACT’s solution provided even more space savings than originally anticipated, even with the addition of liquid cooling after the heat exchanger.
Dr. Devdatta Kulkarni, ACT’s main collaborator at Intel, authored a white paper on the project, “Experimental Study of Two-Phase Cooling to Enable Large-Scale System Computing Performance,” presented at ITHERM 2018. The paper includes compelling test result data, and an acknowledgement of the team at ACT for their contribution.
Advanced Cooling Technologies is a recognized leader in thermal management solutions, including pumped two phase cooling. When faced with challenging thermal requirements, ACT is the one you want to turn to. Contact the thermal experts today to discuss your application.
Single-phase cold plates are commonly used to dissipate waste heat from electronics components. To accommodate the varying footprint and output power of electronics components available, cold plates are being developed with custom internal structures to maximize the efficiency of heat rejection from the electronics components. Cold plates with these custom internal structures require thermal and hydraulic performance evaluations to ensure they meet target operating conditions before being deployed in the end application. Advanced Cooling Technologies, Inc. developed the custom test apparatus, shown in Figure 1, for a customer that enables the thermal and hydraulic performance of multiple cold plate designs to be evaluated and compared under equivalent operating conditions in order to identify the optimum cold plate design.
This test apparatus was designed with an integrated data acquisition (DAQ) panel to read and record instrument responses from thermocouples and pressure transducers, easy connection to an external chiller, and a custom manifold that enabled cold plates of various internal geometries to be connected and removed from the test apparatus. The test apparatus was outfitted with resistance temperature detectors (RTDs) at the inlet and outlet ports and differential pressure transducers such that the temperature rise and pressure drop of the cold plate can be measured.
Cold plates to be tested are prepared with heat sources and outfitted with surface mount thermocouples and incorporated into the test apparatus. The heat sources are energized with an external power supply and once steady-state is achieved, the thermal resistance of the cold plate is calculated from Equation 1 where Rth is the thermal resistance of the cold plate, Ts is the surface temperature of the heat source, Tout is the outlet temperature of the cold plate, Tin is the inlet temperature of the cold plate, is the mass flow rate of coolant, and Cp is the specific heat capacity of the coolant.
For any given cold plate configuration, there is a pumping power required to remove a certain amount of heat. The coefficient of performance (COP) of a cold plate is a dimensionless number that provides a measure of the amount of pumping power required to remove a given amount of heat and provides an equivalent comparison for different cold plate configurations. Thus, in addition to the thermal resistance of the cold plate, the COP of the cold plates can be evaluated from Equation 2, where ΔP is the pressure drop of the cold plate and is the volumetric flow rate of coolant through the cold plate.
Thus, this single test apparatus, designed, developed, and fabricated by ACT, can provide a full thermal and hydraulic performance evaluation to enable the down selection of the optimal cold plate for a given set of operating conditions.
One of ACT’s customers was interested in using spray and jet impingement cooling to remove high heat fluxes (1000W/cm2) from the surface of a CPU. A test section was designed capable of taking single jet impingement heat transfer data using Intel’s supplied 34980A thermal test platform. Figure 5 shows an overall view of the test section with the Sil-clamp bolted to the base plate. The Thermal Test Vehicle is also shown in place. The 8 bolts hold the Sil-clamp in a precise location above the base plate and also provide even pressure around the O-rings to the acrylic cylinder. Figure 17 shows the flow exiting the nozzles at the projected angle as well as the projected impact of the spray with the die. In the design process the 3-D CAD software was used to verify no flow interactions. The test apparatus was designed to test at various inlet temperatures from -10°C to 125°C and work with three different fluids (HFE-7000, Methanol, and Water). The apparatus successfully measured heat transfer coefficients at various locations along the die, providing the customer with the precise measurements required for their application; see Figure 5.
Read the full technical write-up on this project
With support from the National Science Foundation, ACT has developed a pumped two-phase cooling system for high heat flux electronic components and laser diodes and is now working on packing the system in a compact, user friendly, stand-alone platform. The two-phase system efficiently handles fluxes on ~ 300-500W/cm2, requires little pumping power, maintains device temperatures below operating limits and provides for a high degree of isothermality over the heated surface. Meeting these requirements is important in many applications including lasers whose emission wavelengths are temperature-dependent.
Unlike single-phase cooling, two-phase systems take advantage of the phase change (latent heat) of the coolant which enables it to handle higher heat fluxes for a given heat load. Two-phase cooling systems can however be more complex and prone to flow and thermal instabilities. As such, techniques to effectively manage instabilities have been developed and characterized. These include the application of engineered microporous coatings on the heated surface(s), which enhance boiling performance by increasing the number of nucleation sites together with a capillary-driven resupply of the coolant to the heated surface, which prevents or postpones dry-out (extends CHF).
A schematic of a pumped two-phase cooling system is shown in Figure 1. Key components include a pump, preheater, surge tank, evaporator (the heat sink), condenser and accumulator. The surge tank and the preheater differentiate this system from a traditional liquid cooling loop. The surge tank consists of vapor and liquid at saturation; by controlling the pressure in the tank, the saturation (boiling) temperature of the working fluid can be controlled. The preheater heats the subcooled liquid exiting the condenser to a temperature close to the saturation temperature before it enters the evaporator. This is important as boiling heat transfer is most efficient at saturation (minimal subcooling).
Figure 1: Schematic of a pumped two-phase cooling system
Also shown in Figure 2 is a representative copper minichannel heat sink coated with a microporous coating.
Figure 2: Minichannel heat sink with porous sintered power coating
The pumped two-phase cooling system shown in Figure1 was fabricated and evaluated. The heat transfer coefficient (HTC) [W/m2K] and the Incipient Wall Superheat [K] were determined as a function of the input heat flux and coolant mass flux [kg/m2s] using refrigerant R134a and others. Representative results for the HTC for coated and uncoated minichannel heat sinks are shown in Figure 3. Clearly, the HTC is higher for the coated heat sinks and the CHF is extended.
Figure 3: Heat Transfer Coefficient (HTC) associated with Minichannel Heat Sinks noting the increase in CHF for coated heat sinks.
The incipient wall superheat is also shown in Figure 4 as a function of input heat flux on coated and uncoated heat sinks. Again, the microporous coating enhances the thermal performance of the heat sinks as evident by lower values of incipient wall superheat; in other words, a heat-generating device mounted on a two phase heat sink with microporous coating will be maintained at a lower temperature compared to one that is mounted on an identical uncoated heat sink.
Figure 4: Incipient wall superheat decreases with the application of microporous coatings
In addition to improving the heat transfer, the thin copper coatings help suppress two-phase flow instabilities. Figure 5 shows flow instabilities with a bare copper cold plate, where the vapor flow occasionally reverses and enters the input plenum. In contrast, instabilities are suppressed in the cold plate with a thin porous coating, see Figure 6.
Figure 5. High heat fluxes with uncoated mini- and micro-channel systems can have unstable flow. Note the intermittent vapor flow back into the inlet plenum.
Figure 6. The maximum heat flux in two-phase systems can be increased by adding a thin porous layer. An additional benefit is that the flow becomes more stable.
In short, pumped two phase cooling systems have been developed. Flow and thermal stability issues were well managed with the use of porous coatings, which increase the heat transfer coefficient and extend the CHF. Pumping power required is minimal and the application of the coating does not increase the pressure drop in a measurable way as the coating thickness is very small compared to the channel dimensions. Additional work on flow boiling heat transfer in minichannel heat sinks is ongoing with a focus on maximizing performance and understanding the parameters (i.e., the coating properties – thickness for a given particle size, etc.) that affect performance for specific applications.
For more information on Pumped Two Phase Cooling, contact The Thermal Experts at ACT.