Beyond Performance: Addressing Reliability Concerns in Liquid Cooling

The Boyd Difference

Understanding the Roots of Coolant Leakage in Liquid Cooling Systems

In liquid cooling systems, coolant leakage poses a significant risk and can compromise performance and reliability. Understanding why coolant leaks is crucial for effective prevention. Corrosion and fluid interconnect issues are the primary factors that cause coolant leakage.

Fluid Interconnect Issues:

Ensuring Leak-Free Reliability in Liquid Cooling System Fluid Interconnect Joints, Fittings, and Seals

Our liquid cooling systems feature diverse joint technologies to meet varying customer needs while preventing leaks. We solder or braze joints to create strong, durable metal components and assemblies, perfected over decades of manufacturing experience. For barb fittings, we enhance security by pairing them with tube clamps, ensuring a tight and leak-free connection. To protect O-ring seals, we incorporate filters during testing and within our Coolant Distribution Units (CDUs) to remove larger particles that could damage the seals’ integrity. Boyd meticulously manufactures, handles, tests, and packages liquid cooling loop joints, fittings, and seals to ensure secure fluid interconnections before they reach the customer. This rigorous process maintains joint integrity and minimizes leak risks.

Erosion corrosion occurs when increased coolant velocity erodes metal surfaces, often caused by turbulent flow or sharp turns in the coolant path. This erosion weakens materials, heightening the risk of leaks and eventual system failure. Effective design solutions minimize flow velocity changes and ensure smooth coolant pathways to prevent erosion corrosion.

Boyd addresses these corrosion risks by implementing advanced seals, corrosion-resistant materials, precise manufacturing techniques, and rigorous testing in liquid cooling systems. These proactive measures not only mitigate fluid interconnect issues but also ensure reliable and efficient operation. They safeguard the integrity of high-value electronics, reduce downtime, and lower maintenance costs over the system’s lifespan.

Optimizing Leak Containment: Beyond System Shutdown

Optimizing Performance, Eliminating Leaks: The Boyd Advantage in Liquid Cooling

Boyd pioneers’ innovation through the design of leak-free solutions, advanced sealing techniques, and corrosion-resistant materials. Our engineering and material science expertise enables us to develop robust solutions that ensure the integrity and reliability of liquid cooling systems. Boyd tackles challenges such as fluid interconnect issues and corrosion risks through rigorous testing and precision manufacturing capabilities. Schedule a consultation with our experts to explore our comprehensive range of leak-free solutions tailored to meet your specific project needs and optimize system performance.

Water and water/glycol solutions are common heat transfer fluids used in cooling systems and recirculating chillers. Although the fluids are the lifeblood for your heat transfer applications, they can also cause corrosion within your systems.

Preventing Corrosion in Liquid Cooling Systems

Protecting your System from Leaks and Performance Degradation

This corrosion can result in a reduction in system thermal performance due to scaling on the heat transfer surface, decreased flow due to reduced pipe diameters from corrosion deposits, and ultimately the need for system component replacement due to corrosion damage.

Corrosion is the chemical or electrochemical reaction between materials, usually a metal and its environment, which results in deterioration of the metal and its properties. This article will cover chemical corrosion. (For more information on electrochemical or galvanic corrosion, please see our application note “Avoiding Galvanic Corrosion.”) Corrosion of metallic components is an inherent problem for water and water/glycol cooling systems because many metals naturally tend to oxidize in the presence of water. The dissolved oxygen in water accelerates most corrosion processes. In closed loop systems, dissolved oxygen is consumed over time and no longer poses a corrosion risk. For open loop systems, however, continued exposure to air allows oxygen to dissolve into the coolant. Therefore, open loop systems often suffer more corrosion problems compared to closed units.

Corrosion is usually classified as either general or localized. General corrosion is the loss of metal uniformly distributed over an entire surface. It typically does not lead to rapid system failure because the rate of metal loss can be discovered before the metal ruptures. Localized corrosion, on the other hand, is not as predictable. It usually shows up in the form of pitting, which can penetrate the metal very quickly, forming cavities or holes. Another common form of localized corrosion is cavitation, which occurs when pockets of vapor form in a liquid. This process occurs when local pressure near the metal surface falls below the vapor pressure of the liquid. When these vapor bubbles collapse or implode, they generate large amounts of energy. This causes severe pitting to system components (such as pumps), generates a great deal of noise, and results in a decrease in pump efficiency.

Potential Corrosion Problems

Corrosion can lead to many problems, the most significant being perforation that may result in coolant leakage. Other problems may include reduced heat transfer caused by surface scaling, which occurs when metal reacts with oxygen, chloride, and/or inhibitors in the coolant and precipitates back to the metal surface, creating a layer that acts as a heat transfer barrier. Additionally, concerns include the clogging of particulate filters and damage to mechanical seals.

When copper corrodes, it is more often degraded by general corrosion than by pitting. General corrosion will often attack copper exposed to ammonia, oxygen, or fluids with high sulfur content. Another source of corrosion affecting copper is dissolved salt in the fluid, such as chlorides, sulfates, and bicarbonates.

For aluminum, pitting is the most common form of corrosion. Pitting is usually produced by the presence of halide ions, of which chloride (Cl-) is the most frequently encountered in liquid cooling loops. Pitting of aluminum in halide solutions open to air occurs because, in the presence of oxygen, the metal is readily polarized to its pitting potential and the naturally occurring protective oxide layer or film is penetrated. This film is stable in aqueous solutions when pH is between about 4.0 and 8.5. The film is naturally self-renewing and accidental abrasion or other mechanical damage of the surface oxide film is rapidly repaired. Boyd strongly recommends an inhibitor when using water with aluminum to maintain a clean heat transfer surface.

Stainless steel is typically used in corrosive environments but, as with aluminum, it is sensitive to high concentrations of chlorides (>100 ppm) in an oxidizing environment. Pitting remains among the most common and damaging forms of corrosion in stainless steel alloys, but it can be prevented by ensuring that the material is exposed to oxygen and protected from chloride wherever possible. Stainless steels high in chromium, and particularly molybdenum and nitrogen, are more resistant to pitting corrosion.

Corrosion Caused by Uninhibited Ethylene Glycol

Studies show that uninhibited ethylene glycol will degrade into five organic acids – glycolic, glyoxylic, formic, carbonic, and oxalic – in the presence of heat, oxygen, and common cooling system metals such as copper and aluminum. Copper and aluminum act as a catalyst in the presence of uninhibited ethylene glycol. These organic acids will then chemically attack copper and aluminum in as little as three weeks under extreme conditions (212°F and oxygen bubbling into the uninhibited ethylene glycol solution) to form metal organic compounds in the fluid, which can lead to clogging of pipes, pumps, valves, etc.

Literature references often state that copper and aluminum are compatible with uninhibited ethylene glycol, but usually those recommendations are based on a two-week chemical compatibility study of various metals at different temperatures. The study above indicates that uninhibited ethylene glycol typically does not begin to degrade until after three weeks under those extreme conditions. In conclusion, the reported data is based on ethylene glycol’s ability to dissolve the metal and ignores the concern of degraded, acidic uninhibited ethylene glycol and its effects on metals. The latter is much more corrosive towards metals.

Protecting Against Corrosion

In general, corrosion can be reduced through pH control and corrosion inhibitor use. The inhibitors attach to metal surfaces to passivate them and prevent corrosion. It is also important to maintain a stable water flow to avoid stagnant zones inside the cooling system, which can cause corrosion.

Quality of water also needs to be considered when trying to prevent corrosion. The corrosive effect of natural water can vary considerably depending on its chemical composition. As mentioned earlier in this article, chloride is corrosive and use of tap water should be minimized or avoided if it contains more than 100 ppm of chloride. Hardness of water also needs to be considered because it introduces calcium and magnesium, which form scale on the metal surfaces. Deionized water, demineralized water, or water that has been passed through a reverse osmosis process to remove harmful minerals and salts is highly recommended in order to avoid chloride and scale buildup. A suitable corrosion inhibitor must be used with deionized or demineralized water.

There are different inhibitors for use with different metals, each with its advantages and disadvantages.

• Phosphate is an effective corrosion inhibitor for iron, steel, lead/tin solder, and most aluminum components. It is also a very good buffer for pH control. One disadvantage of phosphate is precipitation with calcium in hard water, which is one reason that deionized water is used for diluting a glycol/water coolant.
• Tolyltriazole is a common and highly effective corrosion inhibitor for copper and brass.
• Mercaptobenzothiazole also works for copper and brass, but it is not as stable as tolyltriazole.
• Nitrite is an excellent corrosion inhibitor for iron. At high concentrations, this inhibitor is corrosive to lead/tin solder.
• Silicate is an effective inhibitor for most metals but tends to form thick deposits in cooling systems. The rust inhibitors in automotive anti-freeze may cause premature failure of pump seals. Chromate and soluble oils have been used in the past, but their use has greatly diminished due to toxicity. Modern inhibitors have replaced them.

Liquid Cooling System Erosion-Corrosion

Ensuring Longer Lifetimes by Managing Erosion-Corrosion

Heat exchangers and cold plates are used in cooling applications to remove and transfer heat from one place to another using a heat transfer fluid such as water, ethylene glycol and water solution, oil, etc. There are thousands of combinations of fluids and fluid path materials used in these applications. One of the main criteria for selecting the fluid path materials in these components should be the materials’ ability to resist corrosion. Corrosion comes in many different forms, including “erosion-corrosion”. It is important to know fluid properties as well as material properties in order to minimize erosion-corrosion and optimize system performance and life.

What is Erosion-Corrosion?

Erosion-corrosion is acceleration in the rate of corrosion in metal due to the relative motion of a fluid and a metal surface. It typically occurs in pipe bends & elbows, tube constrictions, and other structures that alter flow direction or velocity. The mechanism for this type of corrosion is the continuous flow of fluid, which removes any protective film or metal oxide from the metal surface. It can occur both in the presence and in the absence of suspended matter in the flow stream. In the presence of suspended matter, the effect is very similar to sandblasting, and even strong films can be removed at relatively low fluid velocities. Once the metal surface is exposed, it is attacked by the corrosive media and eroded away by fluid friction. If the passive layer of metal oxide cannot be regenerated quickly enough, significant damage may occur.

Some materials are more resistant than others to erosion-corrosion under the same fluid conditions. Erosion-corrosion is most prevalent in soft alloys, such as copper and aluminum. Although increasing the flow rate of the fluid in your cooling application may increase its performance, it may also increase erosion-corrosion. Therefore, it is important to determine how great an impact increasing the flow rate will have on your thermal performance, as you may see minimal improvement in performance with a significant drop in the longevity of your heat exchanger or cold plate.

Table 1

Controlling Erosion-Corrosion

Some methods for minimizing erosion-corrosion include improving the flow lines within the pipe by deburring (i.e. – smoothing out irregularities), allowing bends to have larger angles, and changing pipe diameters gradually rather than abruptly. Other methods include slowing the flow rate (minimizing turbulence), reducing the amount of dissolved oxygen, changing the pH, and switching the pipe material to a different metal or alloy.

In addition to the fluid path material used, it is also important to consider your fluid’s temperature. At higher temperatures, flow rates should be lowered to minimize erosion-corrosion. For example, as a general rule, water flow velocities should not exceed 8 ft/sec for cold water and 5 ft/sec for hot water (up to approximately 140 °F). In systems where water temperatures routinely exceed 140 °F, flow velocities should not exceed 3 ft/sec. For maximum recommended water velocities in other typical tube materials, refer to Table 1. For other fluids, the maximum allowable fluid velocity can be calculated from:

Allowable velocity for given fluid] = [Allowable velocity for water] x [Density of water/density of given liquid] 1/2.

There will always be a trade-off between thermal performance and reliability/longevity in any cooling system. Increasing fluid flow will give you more cooling or performance only up to a point. After that, increased fluid velocities may rapidly begin to erode and corrode the inside metal surface of the tubing. Designers should consider many different factors, such as the ones discussed above, in order to determine the best solution for their application.

Avoiding Galvanic Corrosion

How Corrosion Occurs in Liquid Cooling Loops

The galvanic corrosion rate depends on the electrical potential between the two metals. The Galvanic Series (fig. 2) orders metals based on the potential they exhibit in flowing seawater. The most reactive are at the top of the table and the least reactive at the bottom.

Elevated temperatures, which are likely in cooling loops, accelerate galvanic corrosion. A 10°C increase in temperature can approximately double the corrosion rate. Corrosion inhibitors can be added to the cooling water.

This slows, but does not eliminate, galvanic corrosion. Corrosion inhibitors bind with the ions in solution to neutralize them. The inhibitors are consumed in this process so they need replacing regularly. Non-aqueous coolants, such as oils, eliminate galvanic corrosion because they do not support ions. However, thermal performance is sacrificed, as the thermal conductivities of heat transfer oils are generally significantly lower than water-based coolants.

To avoid galvanic corrosion, we highly recommend using the same materials, or materials with similar electrical potential, throughout your cooling loop. You should ensure that plumbing, connectors and other components do not introduce a reactive metal into the system.

Using the same materials throughout your circuit does not mean that you must sacrifice performance. Boyd offers high performance heat exchangers and cold plates with aluminum, copper and stainless steel fluid paths.

Boyd’s application engineers are available to consult on component material compatibility. With careful design and component selection, you can ensure years of reliable, corrosion free service from your cooling loop.

Aluminum Corrosion Resistance for Liquid Cold Plates and Plate-Fin Heat Exchangers

When selecting components for your liquid cooling loop, you must consider their material compatibility as well as individual performance. Although an aluminum-tubed cold plate paired with a copper-tubed heat exchanger might meet your thermal requirements, it is not a reliable cooling circuit. Copper and aluminum have widely different electrochemical potentials, so when they are combined in a cooling system, galvanic corrosion is likely. Galvanic corrosion (also called dissimilar metal corrosion) erodes the metal, causing leaks over time.

Corrosion of Aluminum in Liquid Cold Plates

Aluminum is known for its corrosion resistance. Under the right conditions, aluminum rapidly forms a protective oxide layer. Generally, this occurs when oxygen is readily available and the surrounding medium has a moderate pH. There are two typical manifestations of aluminum corrosion: uniform corrosion and local corrosion. Uniform corrosion happens when the oxide layer is soluble in the corrosive medium. “The oxide film is soluble in alkaline solutions and in strong acids…but is stable over a pH range of approximately 4.0 – 9.0.” In uniform corrosion, the entire oxide layer is being stripped away faster than it can reform. Local corrosion, usually in the form of a pit, occurs when there is non-uniformity in the base metal or the surrounding environment. The metal may have a local concentration of alloying elements that creates a galvanic couple. Similarly, the surrounding environment may have a local concentration of active elements such as chlorides.

Liquid cold plates and heat exchangers are used with many different fluids and typically involve the recirculation of the same fluid. One fluid that should not be used in aluminum cold plates and heat exchangers is water. Tap water can contain active ions, such as copper, bicarbonates, chlorides, and/or other impurities that facilitate corrosion. Plus, the recirculation of the same fluid over time in a closed loop will cause the dissolved oxygen to come out of the solution. The resulting lack of oxygen will inhibit the formation of the oxide layer. Given enough time, aluminum will eventually corrode if isolated from oxygen and exposed to low quality water.

When water is the preferred choice for a heat transfer system, distilled water is usually combined with a glycol to reduce its freezing point and increase its boiling point. For the reasons stated above, it is critical that corrosion inhibitors be used. Corrosion inhibitors are controlled amounts of active ions (usually phosphates) that take over the role of oxygen in forming a corrosion resistant layer. Since these inhibitors depend on a chemical reaction with the aluminum, using low quality water such as tap water would reduce the inhibitors’ effectiveness.

Designing for Corrosion Resistance

Alloy selection is a key factor in high corrosion resistance. For example, braze sheets, which separate the fluid passages in plate-fin heat exchangers, consist of an internal core and external clad layer that usually represents about 10% of the overall sheet thickness. The clad layer is a brazing alloy that joins the braze sheet to both the hot and cold fins and the braze sheet to the side bars. Vacuum brazing alloys use silicone and other elements to lower the melting point of the alloy. Since the braze alloy is more anodic than the core, the braze alloy provides cathodic protection, and thus corrosion protection.

Cathodic protection is a concept that has been used in the ship building business for decades. For hulls made of steel, a plug made of an active element, like zinc, is used to protect the hull. Because zinc is more active than steel, the zinc corrodes faster that the steel. Among the alloying elements of aluminum, the alloys with a minimum of copper and iron have the best corrosion resistance. “3xxx series alloys are generally among those having the highest general corrosion resistance…The 6xxx alloys also have high resistance.”

There are other considerations in cold plate and heat exchanger design. Internal fluid static pressure and external stresses put the core components under stress. These stresses often require that high strength alloys (6xxx series) be used for braze sheets and/or fins. Braze sheet thickness is a tradeoff between performance, weight, and corrosion protection. A thick braze sheet is heavy and reduces thermal performance. A thin braze sheet has less strength to withstand stresses and offers less corrosion protection. If a corrosive environment is present, thin braze sheets will withstand an attack for less time than a thicker sheet.

Cold Plate and Heat Exchanger Leak Testing

During their manufacturing process, cold plates and heat exchangers may be hydraulically tested with pure water. However, water should not remain in the unit longer than is needed to conduct the testing. A thorough drying process is critical to eliminate the possibility of water corrosion. “Bubble testing,” or pressurizing a unit with a gas and submerging it in water, is used widely throughout the industry. This practice requires that the external surfaces be dried after testing. Learn More about Boyd testing services and procedures.

Liquid Cold Plate and Heat Exchanger Operation

When operating a water/glycol cold plate or heat exchanger, it is important to have a maintenance plan. The typical maintenance activity is flushing and refilling the system with the proper mixture of inhibited ethylene glycol and water. This should be done on a periodic basis at an interval determined through system level testing during the operational evaluation phase. Periodically, the fluid pH and refractive index should be measured. These measurements will change over time. From these measurements, a flushing frequency could be determined.

During deployment, it is common for coolant systems to be “topped off”. This practice should not harm the cold plate or heat exchanger as long as the glycol concentration is not diluted to the point of making the inhibitor ineffective. Inhibitor effectiveness is a function of top-off water quality, other metal types in the fluid loop, and the age of inhibitor in the system. If “topping off” is employed, it is advisable to monitor the pH of the fluid. If the pH falls below 4.0 or rises above 9.0, a system flush/fill should take place as soon as possible.

Corrosion resistance begins with cold plate or heat exchanger design. It is also important to develop maintenance procedures that will maximize the life of the aluminum cold plate or heat exchanger.

Corrosion Prevention Summary

Although we can’t stop corrosion all together, there are ways to significantly limit it. By selecting proper fluid path materials, monitoring solution chemistry (specifically pH levels and water quality), and choosing appropriate inhibitors, you can minimize the cost impact due to corrosion and ensure the effective operation of your liquid cooling loop for years.

Considerations for Making a Cold Plate Specifically for Your Application

Cold Plate Technologies

Normalized Performance Curves

Channeled Cold Plates

Key Thermal Considerations During Liquid Cold Plate Design

Cold Plate Thermal Specifications

In addition to four types of cold plate technologies, there are also four scenarios of thermal requirements, which are listed below:

• Uniform Heat Flux, Fixed Flow Rate, 1 Maximum Pressure Drop, 1 Maximum Surface Temperature – With thermal scenario one, there is uniform input heat flux, a fixed flow rate, one specified maximum pressure drop that is limited at a fixed flow rate, and one specified maximum surface temperature where the surface temperature does not need to be uniform.
• Same as 1, but Non-Uniform Heat Flux – Thermal scenario two has the same specifications as scenario one, but heat loads vary instead of being uniform. The heat loads are concentrated in several locations under components or under specific areas
• Same as 1, but Surface Temperature Maximum Varies – Thermal scenario three also has the same specifications as scenario one, but thermal scenario three has specified maximum surface temperatures that vary across the cold plate, usually at the individual components.
• Same as 1, 2, or 3, but Surface Temperature Uniformity Required – With thermal scenario four, the thermal specifications may be the same as with thermal scenarios one, two, or three, but with the additional requirement that the maximum surface temperature must be uniform across the entire cold plate or under specific components. For example, if there are two types of components mounted on the cold plate, each component type may require temperature uniformity of the common components, but the two types may have different maximum surface temperatures.

Cold plate scenarios 2 and 3 are the ones most commonly encountered in custom cold plate design. Scenarios 1 through 4 are listed in order of increasing complexity and cost.

When designing custom cold plates to any specification, the logical steps most thermal experts take are defining the thermal map, generating the liquid circuiting concept, calculating temperature rise and pressure drop, and rerouting the liquid circuit if necessary.

Defining the Thermal Map

With several possible thermal scenarios, step one in custom cold plate design is to define the thermal map in detail. To create a thermal map, an engineer needs the dimensions, locations, and heat loads of the components to be cooled. The maximum allowable cold plate surface temperature(s); the coolant composition, its flow rate, and inlet temperature; and available pressure drop are needed as well. Also, heat flux must be calculated for each component which includes thermal spreading, if necessary.

Key Thermal Considerations During Liquid Cold Plate Design

Generating the Liquid Circuit Concept

The next step is to generate the first iteration on a liquid circuit concept. The liquid circuit must provide the required performance to cool the component with the highest heat flux and each component after it on the liquid circuit. In addition, it must do so with the specified flow rate and with an acceptable pressure drop. Sometimes techniques such as uneven widths of liquid series passes, different fin densities under individual components, and varying fin heights and types can be used to satisfy the competing requirements of performance and pressure drop. The fin’s geometry and height determine the “fin efficiency”, or how well it transfers heat to the liquid.

Sometimes the shape of high heat flux components (e.g. – a large round footprint) requires a change from the natural uniform flow distribution over the pass width to force non-uniformity, which can be achieved by using different lengths of fin or different fin densities over the pass width. Before the next component, some liquid equalizing pools (i.e. – mixing pools) should be designed in. Another fluid distribution challenge is the need for islands in the fluid path to accommodate component mounting. Any complication mentioned above can increase the cost of the cold plate due to the additional number of fin pieces, multiple depths in a cavity, multiple fin-forming equipment set-ups, and EDM cutting needed.

Calculating Temperature and Pressure Drop (Detailed Design Stage)

After the liquid circuit concept is outlined, the thermal map should be verified by calculating the maximum surface temperature under each component and calculating the total pressure drop. All the critical thermal areas must be modeled. If any one of the requirements is not met, the liquid circuits must be reworked and the calculations rerun.

Rerouting the Liquid Circuit

If the cold plate requires a varying maximum surface temperature (as in thermal scenario three) and normal liquid circuiting does not meet the specifications, the liquid circuit should be rerouted to deliver the coolest liquid to critical devices first or to by-pass part of the liquid directly to these components.

Temperature Uniformity

If the cold plate requirements specify maximum surface temperatures and temperature uniformity as in thermal scenario four, the design process is even more complex. The simplest solution to provide uniformity of maximum surface temperatures of identical components is to position the components on similar points of similar parallel liquid passages. The result should be a circuit that delivers liquid with a common temperature at sufficient flow rates to these components. Another technique that is used to provide a more uniform surface temperature across the entire cold plate is to use a counterflow arrangement (Figure 2). In a number of parallel channels, on a surface or on both sides of the plate, each second channel has flow in the opposite direction. For a one-side loaded or very thin cold plate, such an approach may significantly reduce surface temperature gradient. A similar effect may be delivered by organizing two separate layers of liquid.

Reducing Complexity & Costs

Certain thermal or mechanical requirements may force an illogical pass of the liquid circuit, resulting in greater complexity and a higher cost cold plate. For example, applications frequently have predetermined mounting hole locations that the liquid circuit must navigate around and/or components and fluid inlet and outlet locations that are fixed, significantly limiting the options for the liquid circuit. Generally, the more flexible the design is, the easier the cold plate will be to engineer and the more savings you’ll realize. By working closely with a printed circuit board designer or electrical engineer, the thermal engineer can provide input on the spacing and positioning of components to ensure they are designed with electrical as well as thermal requirements in mind. This may significantly simplify the cold plate design and reduce cost. For more information on cold plate costs please see our application note “Cold Plate Manufacturing Cost Drivers”.

It’s important to understand the various design techniques that allow a custom cold plate solution to meet the most challenging thermal and mechanical requirements. With thousands of permutations for a custom cold plate design, skilled engineering is key. Flexibility with the location of inlets and outlets, proper fluid circuit routing, and the use of fin or channels can help to create a thermal solution that provides great value for the application. As heat loads become more and more concentrated and the space allocated for cooling becomes smaller and smaller, custom cold plates will be used more and more to meet applications’ unique liquid cooling needs. Aavid, Thermal Division of Boyd Corporation has decades of experience designing and manufacturing custom cold plates for printed circuit boards and other electronics and ensuring their high thermal performance requirements and cost limits are met or exceeded.

Learn more about our different Liquid Cold Plate solutions in our Liquid Cold Plate Section.

Calculating the Thermal Resistance of a Liquid Cold Plate

To select the best cold plate for your application, you need to know the cooling fluid flow rate, fluid inlet temperature, heat load of the devices attached to the cold plate, and the maximum desired cold plate surface temperature, Tmax. From these you can determine the maximum allowable thermal resistance of the cold plate.

First, calculate the maximum temperature of the fluid when it leaves the cold plate, Tout. This is important because if Tout is greater than Tmax, there is no solution to the problem.

Alternatively, you can use the heat capacity graphs found in our Thermal Reference Guide in the technical library. These graphs describe the change in temperature, ΔT, that occurs along the fluid path. To find Tout, add ΔT to the inlet temperature, Tin.

Assuming Tout is less than Tmax, the next step is to determine the required normalized thermal resistance (θ) for the cold plate using this equation:

Any cold plate technology that provides a normalized thermal resistance less than or equal to the calculated value will be a suitable solution.

Cold Plate Performance Comparison

We present cold plate performance data using local thermal resistance – the surface temperature versus the local liquid temperature. This methodology enables more precise thermal analysis for high heat loads. See full details on thermal resistance calculations and how to select a cold plate technology.

Normalized Performance Curves

Notes

Thermal resistance is inversely proportional to area. To find the thermal resistance of a 25 square inch cold plate, divide the normalized performance by 25.

Our CP30 standard cold plate is designed for prototyping purposes. It has a thick surface plate for machining. We show two traces – before machining (0.5″ / 13 mm) and after machining (0.05″ / 1.3 mm). The performance of a custom vacuum-brazed cold plate is usually significantly better than this standard part.

For comparison purposes, the performance of all cold plates is shown using water as the coolant. Treated water is recommended with aluminum (CP20 & CP30) cold plates.

Calculating the Thermal Resistance of a Heat Exchanger

Hot process fluid temperature TH has dropped out of the formula because liquid temperature has been removed from the equation, we do not have to calculate flow rates and heat capacities of the liquid. We are just left with the desired surface temperature of the cold plate, as well as the temperature of ambient air cooling the heat exchanger, and the performance is fully characterized by the thermal resistances of the cold plate and heat exchanger. Therefore, we no longer have to analyze the individual components of the system. Instead we determine the thermal resistance of the entire system. Note that the effect of flow is not excluded from the results because it is already incorporated within thermal resistance values.

A customer wants to use a CP12 a 12″ (30.48 cm) cold plate (plate side), to remove 1200 W of heat from a 12″x5″ (30.48 cm x 12.70 cm) electronic device. The coolant is 1 gpm (3.785 LPM) of water and room temperature is 20°C. The customer wants the smallest heat exchanger that will remove 1200 W of heat generated by this device, while maintaining a maximum surface temperature of 80°C.

Step 3:

Table 1 shows the resistance and flow rates of the CP12 cold plate and two different heat exchanger/fan combinations:

By looking at the system as a whole, we start to see trade offs between the components, including how flow rate can impact heat exchanger selection. At low flow rates, cold plate thermal resistance increases. This requires a larger heat exchanger with more thermal capacity, and therefore lower thermal resistance. At higher flow rates, it is possible to use a smaller heat exchanger.

Liquid-to-air heat exchangers and cold plates are often combined in a fluid circuit, so it is important to understand how to select the components simultaneously to optimize your system’s performance. With accurate specifications and a simplified equation, selecting the components in your liquid cooling loop can be relatively straightforward. In addition, by selecting components from the same thermal vendor, you use components that are tested in a similar manner and are more likely to work well as a system.

Cabinet Cooling Thermal Calculations

How to Calculate the Required Thermal Resistance for a Cabinet or Enclosure

Cabinet cooling applications use the heat exchanger in the opposite configuration – cold water flows in the liquid circuit and warm air from the cabinet is cooled as it passes across heat exchanger fins. In cabinet cooling applications, you usually need to know the temperature of air as it enters the cabinet, and the maximum temperature that air in the cabinet will reach. Neither of these can be read directly from heat exchanger performance curves.

The usual way to calculate the temperature change of air is to use the mass flow rate calculation.

This can be time-consuming and prone to error.

To avoid these calculations, Boyd developed charts to quickly estimate temperature rise in common heat transfer media at various heat loads. Graphs are available for air, water, oil, and 30/70 ethylene glycol-water (EGW). To calculate temperature change, simply select the appropriate graph, look up your flow rate and heat load, and read off the temperature change. In our technical library under thermal reference you can view a pdf or our temperature change graphs.

When used in conjunction with product performance curves, these offer a quick and simple way of calculating the temperature of cold air entering the cabinet, and maximum air temperature in the cabinet.

Example Cabinet Cooling Calculation

You are evaluating a 6310 heat exchanger with an Ostro fan for cooling an electronics cabinet. The water entering the heat exchanger is at 20°C and a flow rate of 1 gpm. The heat load, Q, is 2400W.

What is the temperature of cooled air entering the cabinet (i.e. the temperature of air leaving the heat exchanger) and what is maximum temperature in the cabinet (i.e. the temperature of warm air entering the heat exchanger)?

First check the performance curve of 6310 in the catalog. You will see that with a 1gpm water flow and the Ostro fan which supplies approximately 250 cfm, its performance is 80W/°C.

To determine the outgoing temperature of water we use the ‘Water Flow’ chart.

At 1 gpm and 2400 W, this shows that the change in temperature is approximately 9°C. Therefore, the outgoing water temperature is 20°C + 9°C = 29°C.

Graphs for air, water, oil, and EGW are available in downloadable PDF format. These are helpful for sizing heat exchangers and cold plates and are also useful in a variety of other temperature change calculations.