In recent years, intense activity has been gone into improving the capabilities of cold plates. Specifically, the use of microchannels has provided great improvements in cold plate thermal performance. Regardless of a cold plate’s channel size, the following equations can be used for heat transfer coefficients when determining thermal performance. [1]
Where,
= Nussselt number
Dh = hydraulic diameter
= Reynolds number
ν = kinematic viscosity
Pr = Prandtle number
The pressure drop can also be calculated as:
Where,
P = density
f = friction factor
In recent years, microchannel cold plates have gained popularity due to their high performance. Webb shows that the best results can be achieved when the channel aspect ratio is about 7.4, and with a fin aspect ratio of 8. [2] Figure 1 shows a Fin-H copper microchannel with a channel hydraulic diameter of 0.49 mm. Due to the small size of the channels, the flow is generally considered to be laminar. The optimization resulted in a 25-mm wide and 20-mm long microchannel cold plate. [2]
Webb considered both single-pass and two-pass designs on the water side. The two-pass version was made to determine if there was any mal-distribution of the water from the single-pass case.
Figure 1. Copper Microchannel Fin-H Used in a Cold Plate. [2]
Figure 2 shows the thermal resistance of the Fin-H for the 1-pass and 2-pass designs as a function of flow rate.
Figure 2. Thermal Resistance of a Fin-H Cold Plate as a Function of the Water Flow Rate. [2]
This figure shows that the 1-pass version has a much better thermal resistance than the two-pass model for the same flow rate. It also shows that the flow has been distributed relatively uniformly. Figure 3 shows the pressure drop of the cold plate as a function of flow rate for the Fin-H and the Thermaltake Bigwater 735 cooler. [3] The figure shows the pressure drop of the 1-pass design is only 38% of the 2-pass design.
Figure 3. Pressure Drop of a Fin-H Cold Plate and a Thermaltake Cooler as a Function of the Water Flow Rate. [2]
Figure 4 shows the thermal resistance of the Fin-H cooler in the 1-pass design compared to the Thermaltake cooler [3]. At 2.28 l/min the Thermaltake’s thermal resistance is 0.106 K/W. The balance point of the Fin-H for 1-pass is with a thermal resistance of 0.07 K/W at a flow rate of 0.361 l/min. This is only 16% of the flow rate for the Thermaltake cooler.
Referring to Figure 3, the pressure drop is almost the same for both coolers. The major implication is that the microchannel cold plate requires a smaller pump compared to macrochannel cold plates, and provides a 50% increase in thermal performance.
Figure 4. Thermal Resistance of a Fin-H Cold Plate and a Thermaltake Cooler as a Function of the Water Flow Rate. [2]
Another innovative approach is the concept of forced-fed boiling (FFB). [4] Figure 5 shows a schematic of this process. It consists of a micro-grooved, thin copper surface with alternating fins and channels. The microgrooves have a hydraulic diameter of 28 microns, an aspect ratio of 15, and a fin density of 236 fins per cm.
There are feed channels on top of the micro-grooved surface. The fluid is forced through these channels into the microgrooves, which are located on top of the heated surface. The fluid vaporizes in the microgrooves and moves upward, while the liquid flows beneath the escaping vapor. This keeps the surface wet, resulting in an increase of the critical heat flux (CHF).
Figure 5. A Force-Fed Boiling Cold Plate. [4]
Figure 6 shows the heat transfer as a function of the temperature difference between the inlet fluid and the surface for various values of the flow rate for R245fa, a non-aqueous fluid for low pressure refrigeration applications. The figure shows that for heat fluxes of about 200 W/cm2 or less, heat transfer is independent of the flow rate, but this is not the case at higher heat fluxes. It also shows that the slope of the heat flux decreases with increasing temperature difference.
Figure 6. Heat Flux as a Function of the Temperature Difference for the FFB Cold Plate. [4]
Figure 7 shows an interesting trend for the heat transfer coefficient as a function of heat flux for the same fluid. At first, the heat transfer coefficient increases with the increase in heat flux. This indicates that by increasing the heat flux, a phase change process takes place which changes the single-phase flow to two-phase heat transfer. After reaching an impressive peak at 300 KW/m2K, the heat transfer coefficient starts to decrease. This is attributed to local dryouts from bubble generation, which also blocks the microchannels.
Figure 7. Heat Transfer Coefficient as a Function of Heat Flux for the FFB Cold Plate. [4]
While advances in cold plate performance have been incremental, their technology is still evolving. Improvements in microchannel manufacturing will open more opportunities in this field. Microchannel cold plates provide tremendous heat transfer coefficient capacities, but limitations prevent their broad deployment.
Fouling, dryout, and fabrication issues have been major negating factors for microchannel deployment in the broader market. Microchannel cold plates may have particular value in such applications as military, space, and high capacity computing, where service and maintenance are part of the deployment.
However, from the design and problem-solution standpoint, microchannel cold plates can be an effective part of a closed loop liquid cooling system.
For more information about ATS thermal management consulting and design services, visit www.qats.com/consulting or contact ATS at 781.769.2800 or [email protected].
Liquid cooling systems transfer heat up to four times better than an equal mass of air. This allows higher performance cooling to be provided with a smaller system. A liquid cooled cold plate can replace space-consuming heat sinks and fans and, while a liquid cold plate requires a pump, heat exchanger, tubing and plates, there are more placement choices for cold plates because they can be outside the airflow. [1]
One-time concerns over costs and leaking cold plates have greatly subsided with improved manufacturing capabilities. Today’s question isn’t “Should we use liquid cooling?” The question is “What kind of liquid should we use to help optimize performance?”
Figure 1. A Liquid Cooling System for a Desktop PC with Two Cold Plates. [2]
For liquid cold plates, the choice of working fluid is as important as choosing the hardware pieces. The wrong liquid can lead to poor heat transfer, clogging, and even system failure. A proper heat transfer fluid should provide compatibility with system’s metals, high thermal conductivity and specific heat, low viscosity, low freezing point, high flash point, low corrosivity, low toxicity, and thermal stability. [3]
Today, despite many refinements in liquid cold plate designs, coolant options have stayed relatively limited. In many cases, regular water will do, but water-with-additives and other types of fluids are available and more appropriate for certain applications. Here is a look at these coolant choices and where they are best suited.
Basic Cooling Choices
While water provides superior cooling performance in a cold plate, it is not always practical to use because of its low freezing temperature. Additives such as glycol are often needed to change a coolant’s characteristics to better suit a cold plate’s operating environment.
In fact, temperature range requirements are the main consideration for a cold plate fluid. Some fluids freeze at lower temperatures than water, but have lower heat transfer capability. The selected fluid also must be compatible with the cold plate’s internal metals to limit any potential for corrosion.
Table 1 below shows how the most common cold plate fluids match up to the metals in different cold plate designs.
Table 1. Compatibility Match-ups of Common Cold Plate Metals and Cooling Fluids [1]
The choices of cold plate coolants will obviously have varied properties. Some of the differences between fluids are less relevant to optimizing cold plate performance, but many properties should be compared. Tables 2 and 3 show the properties of some common coolants.
Tables 2 and 3. Comparisons of Properties of Typical Electronic Coolants. [4]
An excellent review of common cold plate fluids is provided by Lytron, an OEM of cold plates and other cooling devices. The following condenses fluid descriptions taken from Lytron’s literature. [5]
The most commonly used coolants for liquid cooling applications today are:
Water
Deionized Water
Inhibited Glycol and Water Solutions
Dielectric Fluids
Water – Water has high heat capacity and thermal conductivity. It is compatible with copper, which is one of the best heat transfer materials to use for your fluid path. Facility water or tap water is likely to contain impurities that can cause corrosion in the liquid cooling loop and/or clog fluid channels. Therefore, using good quality water is recommended in order to minimize corrosion and optimize thermal performance.
If you determine that your facility water or tap water contains a large percent of minerals, salts, or other impurities, you can either filter the water or can opt to purchase filtered or deionized water. [5, 6]
Deionized Water – The deionization process removes harmful minerals, salts, and other impurities that can cause corrosion or scale formation. Compared to tap water and most fluids, deionized water has a high resistivity. Deionized water is an excellent insulator, and is used in the manufacturing of electrical components where parts must be electrically isolated. However, as water’s resistivity increases, its corrosivity increases as well. When using deionized water in cold plates or heat exchangers, stainless steel tubing is recommended. [5, 7]
Inhibited Glycol and Water Solutions – The two types of glycol most commonly used for liquid cooling applications are ethylene glycol and water (EGW) and propylene glycol and water (PGW) solutions. Ethylene glycol has desirable thermal properties, including a high boiling point, low freezing point, stability over a wide range of temperatures, and high specific heat and thermal conductivity. It also has a low viscosity and, therefore, reduced pumping requirements. Although EGW has more desirable physical properties than PGW, PGW is used in applications where toxicity might be a concern. PGW is generally recognized as safe for use in food or food processing applications, and can also be used in enclosed spaces. [5, 8]
Dielectric Fluid – A dielectric fluid is non-conductive and therefore preferred over water when working with sensitive electronics. Perfluorinated carbons, such as 3M’s dielectric fluid Fluorinert™, are non-flammable, non-explosive, and thermally stable over a wide range of operating temperatures. Although deionized water is also non-conductive, Fluorinert™ is less corrosive than deionized water. However, it has a much lower thermal conductivity and much higher price. PAO is a synthetic hydrocarbon used for its dielectric properties and wide range of operating temperatures. For example, the fire control radars on today’s jet fighters are liquid-cooled using PAO. For testing cold plates and heat exchangers that will use PAO as the heat transfer fluid, PAO-compatible recirculating chillers are available. Like perfluorinated carbons, PAO has much lower thermal conductivity than water. [5, 9]
Conclusion
Water, deionized water, glycol/water solutions, and dielectric fluids such as fluorocarbons and PAO are the heat transfer fluids most commonly used in high performance liquid cooling applications.
It is important to select a heat transfer fluid that is compatible with your fluid path, offers corrosion protection or minimal risk of corrosion, and meets your application’s specific requirements. With the right chemistry, your heat transfer fluid can provide very effective cooling for your liquid cooling loop.
For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit www.qats.com/consulting or contact ATS at 781.769.2800 or [email protected].
Thanks to faster boot-up times and enhanced reliability and performance, solid-state drives (SSD) have grown in popularity with consumers in the past decade. From laptops to portable hard drives to telecommunications applications, solid-state drives are eclipsing hard disk drives, particularly as the price of SSD technology lowers, making it more cost-effective for system designers.
What makes the SSD different? Solid-state drives have no moving parts. There is no mechanical arm to read and write data. Instead SSD use embedded processors to control the processes related to storing, retrieving, caching, encrypting, and cleaning up data.
Apple is one of many laptop companies that have turned to solid-state drives for laptops because of improved boot times and better battery efficiency. (Wikimedia Commons)
As explained by Storage Review, “Conversely, a hard disk drive uses a mechanical arm with a read/write head to move around and read information from the right location on a storage platter. This difference is what makes SSD so much faster. As an analogy, what’s quicker? Having to walk across the room to retrieve a book to get information or simply magically having that book open in front of you when you need it? That’s how an HDD compares to an SSD; it simply requires more physical labor (mechanical movement) to get information.” [1]
Consumers and engineers alike are turning to SSD and that has made a number of companies jump into the market, although, according to a recent report that counted global sales through 2016, Samsung (21 percent) and Kingston (16) percent remain the largest retailers of SSD in the world. All the other companies listed, including Intel, SanDisk, and Toshiba, all had percentages in the single digits. [2]
The benefits of SSD are well-known: System boot times that are typically 1/3-1/4 of HDD, half of the power draw for longer battery life, much larger storage capacity, reduced noise during use, and greater mean time between failure (MTBF).
One area of importance for SSD, which is another byproduct of no moving parts, is that they generally produce less heat than HDD. For instance, you are less likely to burn your lap while working on your laptop if it has a solid-state drive. Thermal issues remain for SSD, as they do for any electronic device, but compared to HDD they have fewer cooling requirements. [3]
A recent study out of Carnegie Mellon University (Pittsburgh, Pa.) in collaboration with Facebook, Inc. analyzed the reliability of flash-based SSD. One of the external factors that the researchers considered was temperature. In examining three distinct groups of SSD in a Facebook data center, the study described similar failure rates at a range of 30-40°C, but the failure rates varied greatly as temperatures increased beyond that operating range. Failure rates are explained in the following tables. [4]
This chart from researchers at Carnegie Mellon demonstrates the instability that temperature spikes cause in SSD performance. [4]
The researchers concluded, “In general, we find techniques like throttling, which may be employed to reduce SSD temperature, to be effective at reducing the failure rate of SSDs. We also find that SSD temperature is correlated with the power used to transmit data across the PCIe bus, which can potentially be used as a proxy for temperature in the absence of SSD temperature sensors.”
Temperature is an increasing factor for SSD. Like the rest of the electronics industry, engineers are designing SSD to handle more chips, more channels, more cores, and more controllers to handle a greater level of processing capability. A study from the Computer Architecture and Memory Systems Lab at the University of Texas – Dallas (UT Dallas), presented at HotStorage 2014, reported that there were 64 times as many chips and channels in SSD as there were just 12 years before. Just like the Carnegie Mellon study, the UT-Dallas researchers determined that “device-level protection mechanisms dynamically reduce heat output.” [5]
This chart from UT-Dallas shows the performance degradation that comes from overheating of SSD. [5]
The UT-Dallas study concluded that overheating led to malfunctions in the SSD and that devices with larger data sizes reached the overheating point quicker. According to the report, there was “significant performance degradation at the overheating points” and that overheating and its requisite power throttling “hinder SSD from integrating more resources.” This problem, the study concluded, was “holding back state-of-the-art SSD from achieving potential performance gains.”
As SSD continues to gain a stronghold in the market, including Intel’s recent announcement that it was going to accelerate the deployment of its SSD technology throughout its product line to “enhance user productivity and mobility while reducing IT total cost of ownership,” [6] it is obvious that thermal characterization of SSD and thermal management of systems with SSD are primary concerns for the industry.
There is also a real cost to bad data and SSD are not removed from that risk. From the CPU to the PCB to SSD storage, inaccuracies and outright errors adversely impact device reliability and system design. One model that is commonly used in the characterization of SSD and other data storage products is the Arrhenius Model:
where:
This reliability model has temperature as a key component. These models can be used via pen and paper, computer modeling or on a spreadsheet. But is the Arrhenius model a good model for NAND (negative-AND) flash memory?
Data presented at the Flash Memory Summit in 2014 by IBM showed that reliability models such as Arrhenius are not necessarily accurate for characterizing SSD. Indeed, such a model can end up creating a correct match for just two data points. The presentation recommends that acceleration models be validated. In addition, it is recommended that it is best to test a full device and that doing so will best allow measurement of the total behavior. [7]
The Storage Networking Industry Association (SNIA) also released a test methodology, test suite, and reporting format for SSD to ensure the accuracy and reliability of the data being reported. [8]
Measuring the thermal characteristics of SSD is similar to the process of characterizing a standard semiconductor. A typical test setup for characterization would include a closed-loop wind tunnel, preferably with heater, sensors, thermocouples or RTD and an analog-to-digital capture system or hot wire anemometer. A closed-loop wind tunnel with heater provides an environment for controlled temperatures from ambient to 85°C or more (although most testing will take place lower than 70°C to avoid damaging the device). Thermocouples or sensors will give important data about the junction temperature of the machine as it is in operation within the system and instruments and sensors can be incorporated to see how the SSD reacts to external factors.
It is critical that thermal management is considered not only in the design phase, but that the products are tested to determine the impact of temperature on date storage to avoid errors, lost information and device failure. To make sure that the data is accurate and reliable and to save time and money in the long-run, it is imperative to use research-quality instruments during the test phase.
As an article from Qpedia Thermal eMagazine explained, “Small errors in temperature and air flow measurements can have a significant effect on reliability predictions. The origin of these errors lies in the measurement process or the use of inaccurate instruments.”
The article continued, “Accurate and high-quality instruments are not only essential for any engineering practice, their absence will adversely impact reliability predictions of a product at hand. No company wants to have its products returned, especially because of thermally induced failures.” [9]
The ATVS-2020™ Automatic Temperature & Velocity Scanner is a patented, multi-channel hot wire anemometer system for single or multi-point measuring of air temperature and velocity. (Advanced Thermal Solutions, Inc.)
Learn more about the instruments that ATS has to offer for SSD thermal characterization in the video below:
A cold chain is a series of packaging, shipping, and distribution steps, all conducted at controlled temperatures. A successful cold chain keeps products within their required temperature ranges even when shipping between continents or hemispheres. Cold maintenance preserves the optimal shelf lives of produce, seafood, frozen food, pharmaceuticals, and other products that must be kept constantly chilled or frozen to maintain their quality.
Fig. 1. Components in a KoolTemp Insulated Container from Cold Chain Technologies. [1]
About 70% of all food consumed in the United States is handled by cold chains. Without proper cooling or freezing, most of this food would show signs of perishing before reaching its end user and could potentially be inedible and unsafe.
For pharmaceuticals, consider a vaccine supply shipped to a third world country without a cold chain infrastructure. Excess exposure to heat could make the vaccine inactive and, even worse, this may not be discovered until after the shots have been distributed.
It is critical for shipping parties (food companies, drug companies, et al.) to have sufficient scientific knowledge of their products and the environments they will travel through before reaching the end users. With that knowledge in hand, the cold chain can accommodate almost every product that must ship under cold temperatures.
Fig. 2. There Are Four Temperature Range Standards Commonly Used to Designate Optimal Transport Temperatures in the Cold Chain. [2]
Cold transportation and storage standards have been developed by the U.S. as well as countries in Europe and Asia. Guidelines are also provided by the World Health Organization (WHO). The most common temperature standards in the food cold chain are “banana” (13°C), chill (2°C), frozen (-18°C) and deep frozen (-29°C); each is related to specific product groups. These standards are mainly used in the produce (agricultural) industry.
For vaccines and other pharmaceuticals, the cold chain must be compatible with labeled instructions such as “Store in a refrigerator, 2°C to 8°C (36°F to 46°F),” or “Store in a freezer, -25°C to -10°C (-13°F to 14°F).”
Food and pharma providers apply science to make products that will better withstand warmer temperatures. But most of these products must still be kept cold or frozen once they are packaged and shipped out. Poor temperature conditions or delays of an in-shipment food product or a drug can damage that product enough so it loses any market value or utility. [3]
Fig. 3. An Insulated Box Liner from IPC Helps Keep Produce Cool During Shipment. [4]
The pharmaceutical industry factors in potential temperature fluctuations during transit and storage. For many of pharma products, there is an MKT (mean kinetic temperature). This is a “thermally equivalent” temperature that degrades the same amount of a drug as degraded by the different temperatures during a particular period of time. [5]
MKT is a complex calculation with many data points. Per Wikipedia, the mean kinetic temperature can be expressed as shown in Figure 4:
Fig. 4. The Mean Kinetic Temperature (MKT) Expresses the Effect of Temperature Fluctuations During Storage and Transit of Perishable Goods. [6]
Here is a simple analogous example of working out an MKT:
A dozen eggs sat:
In a 20°C room for two hours.
In a 2°C refrigerator for four hours.
And on a 25°C loading dock for one hour.
Using MKT, a company can calculate that the temperature profile of the eggs was “thermally equivalent” to storing them at 10.096°C for seven hours. [7]
Cold Chain Packaging
Ensuring that a shipment will remain within a temperature range for an extended period of time comes down largely to the type of container and the refrigeration method. Duration of transit, the size of the shipment, and the outside temperatures experienced are all important in deciding the type of packaging. Examples of packaging used in shipping range from small insulated boxes that require dry ice or gel packs, rolling containers, or a 53-foot truck with its own refrigeration unit.
Fig. 5. Canadian Vaccine Storage and Handling Guidelines for Immunization Providers. [8]
The major cold chain technologies used for providing a temperature controlled environment during transport involve a range of materials and vehicles. Below is a quick summary [9]
Dry ice – Solid carbon dioxide is about -80°C and is capable of keeping a shipment frozen for an extended period of time. It is widely used for the shipping of pharmaceuticals, dangerous goods, and foodstuffs and in refrigerated unit load devices for air cargo. Dry ice does not melt, instead it sublimates when it comes in contact with air. [10]
Gel packs – Large shares of pharmaceutical and medicinal shipments are classified as chilled products. This means they must be stored in a temperature range of 2-8°C. The common method to provide this temperature is to use gel packs, or packages that contain phase-changing substances that covert from solid to liquid and vice versa to control an environment. Depending on the shipping requirements, these packs can either start off in a frozen or refrigerated state. Along the transit process they melt to liquids, while at the same time capturing escaping energy and maintaining an internal temperature. [11]
Eutectic plates – These are also known as cold plates. The principle is similar to gel packs, but the plates are filled with a liquid and can be reused many times. Eutectic plates have a wide range of applications, such as maintaining cold temperature for rolling refrigerated units. They can also be used in delivery vehicles to keep temperature constant for short periods of time. [12]
Liquid nitrogen – An especially cold substance at about -196°C, it is used to keep packages frozen over a long period of time. Liquid nitrogen is commonly used to transport biological cargo such as tissues and organs. It is considered a hazardous substance for the purpose of transportation. [13]
Quilts – These are Insulated pieces that are placed over or around freight to act as a buffer against temperature variations and to maintain a relatively constant temperature. Using quilts, frozen freight will remain frozen for a longer time period, often long enough to make the usage of more expensive refrigeration devices unjustifiable. Quilts can also be used to keep temperature sensitive freight at room temperature while outside conditions can substantially vary (e.g. during the summer or the winter). [14]
Reefers – Their name derived from ‘refrigeration’, reefers are temperature controlled, insulated vans, small trucks, semi-trailers or standard ISO containers. They are specially designed to allow temperature-controlled air circulation maintained by an attached and independent refrigeration plant. A reefer is therefore able to keep the cargo temperature cool and even warm. The term reefer increasingly applies to refrigerated 40-foot ISO containers with the dominant size being 40 high-cube footers (45R1 being the size and type code). A reefer carries around 20-25 tons of refrigerated cargo and is fully compatible with the global intermodal transport system, which implies a high level of accessibility to markets around the world. [15]
The first reefer ship for the banana trade was introduced in 1902 by the United Food Company. This enabled the banana to move from an exotic fruit that had a small market, because it arrived in markets too ripe, to one of the world’s most consumed fruit. Its impact on the reefer industry was monumental.
Conclusion
It takes time and coordination to efficiently move a shipment and every delay can have negative consequences, notably if this cargo is perishable. The greater the physical separation, the more likely freight can be damaged in one of the transport operations involved. Some goods can be damaged by shocks while others can be damaged by undue temperature variations.
For a range of goods labeled as perishables, particularly produce, quality also degrades with time. Ensuring that cargo does not become damaged or compromised in shipment, businesses in the pharmaceutical, medical and food industries are increasingly relying on the cold chain.
A recent industry forecast sees the global cold chain market growing by 7% every year, reaching $340 billion by 2025. [16]
For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit www.qats.com/consulting or contact ATS at 781.769.2800 or [email protected].
On Jan. 6, 2000, champion race car driver Sam Schmidt crashed his vehicle at the Walt Disney World Speedway in Orlando, Fla. The accident severely injured his spinal cord, leaving him paralyzed from the neck down and with doctors telling him that he would never walk again, let alone get behind the wheel of a car.
ATS partnered with ARROW Electronics to devise a thermal solution for the computer system in the semi-autonomous car that allowed Sam Schmidt to get back behind the wheel. (Advanced Thermal Solutions, Inc.)
Colorado-based neurosurgeon Dr. Scott Falci had other ideas and enlisted the aid of several technology companies, including ARROW Electronics, the Air Force Research Laboratory (AFRL), and Ball Aerospace and Technologies Corp., to make his dream of helping Schmidt drive come to fruition 17 years after the accident.
The result was the SAM Car. Using infrared sensors, cameras, on-board GPS and other next-generation technologies, the team created a semi-autonomous vehicle that Schmidt could power by simply moving his head. Leaning right or left would steer the car, tilting his head back would cause the car to accelerate, and biting down on a special mouthpiece would cause the car to break.
Watch this CNBC video with Jay Leno to learn more and see the car in action:
ATS engineers Bahman Tavassoli, Vineet Barot, and Anatoly Pikovsky are proud to have collaborated with these other innovative pioneers to provide Mr. Schmidt with the ability to get back behind the wheel where he belongs.
For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit www.qats.com/consulting or contact ATS at 781.769.2800 or [email protected].
For information on ATS Cold Plates, visit our Cold Plate page at 
