Once you’ve determined liquid cooling is the solution, do you know what heat transfer fluid to use? One of the most important factors when choosing a liquid cooling technology for your application is the compatibility of the heat transfer fluid with the wetted surfaces of each part of liquid cooling loop. This includes liquid cold plates, heat exchangers, CDUs, tubes, and any quick disconnects.

By selecting a compatible pairing of heat transfer fluid and wetted materials, you’ll minimize the risk of corrosion and optimize thermal performance.

Copper is compatible with water and glycol/water solutions.

Aluminum is compatible with glycol/water solutions, dielectric fluids, and oils.

Stainless steel is better for deionized water or other corrosive fluids because of its improved corrosion resistance over other metals.

Most cooling systems are compatible with water or glycol/water solutions but require specialized plumbing for deionized water or a dielectric fluid like polyalphaolefin (PAO).

Materials & Fluid Compatibility

Water as a Heat Transfer Fluid

Water is one of the best choices for liquid cooling applications due to its high heat capacity and thermal conductivity. It is also compatible with copper, which is one of the best heat transfer materials to use for your fluid path.

Facility Water or Tap Water

Water for liquid cooling comes from different sources. Tap water, for example, comes from a publicly owned water treatment facility or a well. The benefit to using facility or tap water is that it is readily available and inexpensive. What is important to note about facility water or tap water is that it is likely untreated and likely to contain impurities. Impurities can cause corrosion in the liquid cooling loop and/or clog fluid channels. Therefore, using good quality water is recommended to minimize corrosion and optimize thermal performance.

Water’s Potential for Corrosion

Water’s ability to corrode metal can vary considerably depending on its chemical composition. Corrosive chloride is commonly found in tap water. Facility or tap water should not be used in liquid cooling loops if it contains more than 25 PPM of chloride.

The levels of calcium and magnesium in the water also need to be considered. Calcium and magnesium can form scale on metal surfaces and reduce the thermal performance of the components.

Water Liquid Cooling Recommended Limit of Minerals

Deionized Water or Filtered Water

If your facility water or tap water contains a large percent of minerals, salts, or other impurities, you can either filter the water or purchase filtered or deionized water.

Corrosion Inhibitors: Phosphate, Tolyltriazole, and Organic Acids

We still recommend using a corrosion inhibitor for additional protection even if your facility or tap water is relatively pure and meets recommended limits. Phosphate is an effective corrosion inhibitor for stainless steel and most aluminum components. It’s also effective for pH control. One disadvantage of phosphate is that it precipitates with calcium in hard water. For copper and brass, tolyltriazole is a common and highly effective corrosion inhibitor. For aluminum, organic acids such as 2-ethyl hexanoic or sebacic acid offer protection.

Corrosivity of Deionized Water

However, as water’s resistivity increases, its corrosivity increases as well. Deionized water is approximately pH 7.0 but will quickly become acidic when exposed to air. The carbon dioxide in air dissolves in the water, introducing ions and reduces the pH to 5.0. Corrosion inhibitors are required when using virtually pure water. When using deionized water in a recirculating chiller or CDU, special high purity plumbing are a must. The fittings should be nickel-plated and the evaporators should be nickel-brazed. When using deionized water in cold plates or heat exchangers, stainless steel tubing is recommended.

Pros to Deionized Water in Liquid Cooling Systems

Tap water meets the needs of most liquid-cooling applications. However, deionized (DI) water has chemical and electrical properties that make it the optimal choice for cooling when the liquid circuit contains micro-channels or when sensitive electronics are involved.

DI water has an extremely low concentration of ions which defines important performance attributes. First, it eliminates mineral deposits which block coolant flow which degrade cooling efficiency and system operating performance. Second, it eliminates the risk of electrical arcing due to static charge build-up from the circulating coolant. Arcing can damage sensitive control electronics in the cooled equipment. No ions in DI water eliminates both problems.

How to use Deionized Water in a Liquid Cooling System

Applications that need DI water are found in industries such as:

• Medical equipment
• Laboratory instrumentation
• Pharmaceutical production and food processing
• Cosmetic
• Semiconductor manufacturing
• Lasers, plating, chemical and other industrial processing

Exercise care when using DI water. The lack of ions makes this coolant unusually corrosive. Called the “universal solvent,” deionized water is one of the most aggressive solvents known. It will dissolve everything to which it is exposed to varying degrees. Therefore all materials in the cooling loop must be corrosion-resistant.

Copper’s Non-compatibility with Deionized Water

Copper and many other common materials are not compatible with DI water and will contaminate it. When you design a system using DI water, be sure to specify DI-compatible materials such as stainless steel or nickel.

Stainless Steel Wetted Paths for Deionized Water

In a heat exchanger or cold plate, we recommend a stainless steel fluid path. A DI friendly recirculating chiller should contain a nickel-brazed evaporator, a stainless steel pump head, and nickel-plated fittings. Finally, to maintain DI water purity, a deionization cartridge must be included. As with all consumables the DI cartridge must be replaced periodically.

In summary, many types of equipment and applications require DI water-cooled systems. When properly designed and maintained, these systems can provide reliable cooling and leak-free operation for many years.

Inhibited Glycol and Water Solutions in Liquid Cooling

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 and Water

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 low viscosity, meaning reduced pumping requirements. Even though EGW’s thermal conductivity is not as high as water it provides freeze protection that can be beneficial during use or shipping.

Propylene Glycol and Water

Although EGW has more desirable physical properties than PGW, propylene glycol/water 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.

Automotive Ethylene Glycol and Water

Ethylene glycol is used in automotive antifreeze. However, automotive glycol should not be used in a cooling system or heat exchanger because it contains silicate-based rust inhibitors. These inhibitors can gel and foul, coating heat exchanger surfaces and reducing their efficiency. Silicates have also been shown to significantly reduce the lifespan of pump seals.

Choosing the Right Rust Inhibitor for Glycol and Water Solutions

While the wrong inhibitors can cause significant problems, the right inhibitors can prevent corrosion and significantly prolong the life of a liquid cooling loop. Inhibited glycols can be purchased from specialized fluid production companies and are highly recommended over non-inhibited glycols.

Glycol Concentration

As the concentration of glycol in the solution increases, the thermal performance of the heat transfer fluid decreases. Use the lowest possible concentration of inhibited glycol necessary to meet your corrosion and freeze protection needs. Dow Chemical recommends a minimum concentration of 25-30% EGW4. At this minimum concentration, ethylene glycol also serves as a bactericide and fungicide. With recirculating chillers, a solution of 30% ethylene glycol will result in only about a 3% drop in thermal performance over using water alone but will provide corrosion protection as well as freeze protection down to -15°C (5°F).

Water Quality in Glycol Solutions

The quality of the water used in the glycol solution is also important. The water should meet or exceed the limits specified in the Recommended Mineral Limit table, even if you’re using an inhibited glycol. Ions in the water can cause the inhibitor to fall out of solution, resulting in fouling and corrosion.

Adding Glycol into your Liquid Coolant

How Coolants Work in Chillers

In a recirculating chiller, the liquid coolant flows through the application, removing excess heat and raising the temperature of the liquid. This coolant then needs to be returned to set point temperature by flowing through a heat exchanger called the evaporator.

Learn more about Compressor-Based Refrigeration.

The heat exchanger transfers heat between the liquid coolant and the system’s refrigerant gas. The refrigerant’s temperature must be lower than the temperature of coolant liquid for heat to flow and for coolant temperature to be effectively returned to set point.

The temperature of the refrigerant is typically 5°C to 10°C lower than coolant temperature to allow heat to flow. Consequently, if the temperature set point is below 10°C (48°F), the refrigerant’s temperature can be close to, or even below the freezing point of water. If the coolant freezes, the evaporator can become obstructed, preventing water flow. Water expands as it freezes, which can permanently damage the evaporator.

Perfluorinated Carbons as a Heat Transfer Fluid

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 and therefore may be a better choice for some applications. However, water has a thermal conductivity of approximately 0.59 W/m°C (0.341 BTU/hr ft °F), while Fluorinert™ FC-77 has a thermal conductivity of only about 0.063 W/m°C (0.036 BTU/hr ft °F). Fluorinert™ is also much more expensive than deionized water.

Polyalphaolefin Liquid Cooling

PAO is a synthetic hydrocarbon used frequently in defense and aerospace applications 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. Boyd has PAO compatible recirculating chillers for testing cold plates and heat exchangers that will use PAO in their final application. PAO has a thermal conductivity of 0.14 W/m°C (0.081 BTU/hr ft °F). Although dielectric fluids provide low risk liquid cooling for electronics, they generally have a much lower thermal conductivity than water and most water-based solutions.

Selecting Your Heat Transfer Fluid

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’s 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 on liquid cooling technologies and the proper working fluid to use in your system, contact us.

Improving Heat Transfer Between Surfaces and Overall Thermal Performance

This thermal resistance can be expressed as Rja, where:

• Rja – Thermal resistance from the device junction to ambient air or water
• Rjc – Thermal resistance from the device junction to the package case, determined by the electronic device manufacturer (designer has no direct influence)
• Rcs – Thermal resistance from the package case to the heat sink or cold plate, determined by the size and quality of the contact areas between the electronic device and the heat sink or cold plate, the materials used, and contact pressure
• Rsa – Thermal resistance from the heat sink or cold plate to ambient air or water, determined by the heat sink or cold plate design (material and geometry)

Therefore, one way to reduce Rja is to reduce Rcs, the contact resistance between the electronic device case and ambient-cooled, finned heat sinks or liquid-cooled cold plates. There are several factors that impact Rcs, including surface flatness, surface roughness, contact force or clamping pressure, surface cleanliness, and interface materials.

Surface Flatness and Surface Roughness

Table 1:

A mounting surface flatness of 0.001 in/in is generally required for satisfactory contact between the electronic device and the heat sink or cold plate. The surface roughness should be equivalent to that of the electronic device, where 32-64 µin is usually adequate. Finer finishes add unnecessary cost with little or no improvement in thermal performance. Surface flatness is typically much more critical than surface finish in achieving a good thermal interface.

Contact Force

Surface Cleanliness

Thermal Interface Material

Contact Thermal Resistance Factors Review

Contact conditions encompass a number of areas including surface flatness, surface roughness, surface cleanliness, contact pressure, and interface materials. There are many technologies and techniques available for optimizing the thermal path from the electronic device junction to the heat sink. It’s important to minimize the thermal resistance in order to maintain the electronic device temperature below its maximum rated value and increase the end product reliability.

Heat Spreading

Heat spreaders offer cost-effective, reliable high thermal conductivity and efficacy with almost no moving parts.

A heat spreader is an effective solution for dealing with heat sources that have a high heat-flux density (high heat flow per unit area) and where the secondary heat exchanger in itself is not an effective method of dispersing heat (due to space limitations, energy use, cost, etc.). Heat spreaders can allow designers to use an air-cooled, rather than liquid-cooled, secondary heat exchanger.

Boyd offers a variety of heat spreading technologies that provide significant improvement in solution effective thermal conductivities. These heat spreading technologies include:

• Advanced Solid Conduction (k-Core®/ Graphite Technologies)
• Embedded Heat Pipe Cold Plates
• Vapor Chamber Assemblies

Heat Transfer

Heat Pipes

The four common, commercially produced heat pipe wick structures are grooves in the internal tube wall, wire or screen mesh, sintered powder metal and fiber/spring. Different wicks have varying capillary limits (the capillary pumping rate at which the working fluid travels from condenser to evaporator).

Loop Heat Pipes

A loop heat pipe (LHP) is also a two-phase heat transfer device that uses capillary action to remove heat from a source and passively move it to a condenser or radiator. LHPs are similar to heat pipes but have the advantage of being able to provide reliable operation over long distance (up to 75 meters) and the ability to operate against gravity (high g environments).

In a loop heat pipe, the wick structure is only in the evaporator and the vaporized fluid is separated from the liquid and travels in a loop through the condenser back to the evaporator. Boyd has developed and manufactured different designs of LHPs ranging from powerful, large size LHPs (>2000W) to miniature LHPs (<100W) that have been successfully employed in a wide range of aerospace and ground based applications.

Working Fluids, Operating Temperature Ranges, Orientation and Forming

The type of working fluid also influences heat pipe performance. A heat pipe or loop heat pipe only functions when the working fluid temperature is above its freezing point. When the temperature is above the vapor condensation point of the working fluid, the vapor will not condense back to liquid phase, and no fluid circulation – and no cooling – occurs. Working Fluid selection is based on the operating temperature range of the application.
Boyd has designed and developed heat pipes and loop heat pipes for operating temperature ranges from Cryogenic (<-250°C) to High Temperature (>2000°C). Water is the most common working fluid due to its favorable thermal properties and operating temperature range of 5°C to 250°C.

Boyd has designed, developed and manufactured heat pipes using over 27 different working fluids.

The orientation of a heat pipe relative to gravity, combined with its wick structure, also plays an important role in its performance. For example, the groove wick has the lowest capillary limit but works best under gravity-assisted conditions, where the evaporator is located below the condenser. Loop heat pipes are less sensitive to orientation and rely on a high capillary pumping wick in the evaporator to drive performance.

Heat pipes can be formed (flattened or bent) for integration into an assembly. If a heat pipe is flattened or bent, it will reduce the maximum amount of heat that can be transported. Avoiding this limitation is a design consideration.

Heat Pipe Applications

For moving heat in industrial, electronic, aerospace and other applications, heat pipes and loop heat pipes are generally integrated into a thermal subsystem to transport heat from the heat source to remote areas. Heat pipes are effective in carrying heat away from heat sources and heat-sensitive components to a finned array or a heat sink in another location.

A high-capacity power electronics cooler is an example of a thermal solution where space is often insufficient for mounting a finned heat sink directly adjacent to the heat source. Instead, high-capacity heat pipes move the heat to the finned array, which dissipates heat energy using forced convection. Hundreds of watts can be dissipated this way.

Benefits of Heat Pipes and Loop Heat Pipes

The integration of heat pipes and loop heat pipes into a thermal solution delivers many benefits, including.

• High effective thermal conductivity (>5000 W/m•K)
• Long distance heat transport
• High reliability
• No moving parts
• Cost-effective
• Passive — do not require moving parts and other similar potential maintenance challenges

In addition, heat pipes and loop heat pipes can be designed for a variety of external environmental factors such as mechanical shock, vibration, force impact, thermal shock/cycling, and corrosive environment that can affect heat pipe life.

Dispersing Heat

Using thermal solution technologies from Boyd such as heat sinks, heat pipes, vapor chambers, loop heat pipes, k-Core®, liquid cold plates, and heat exchangers, designers can choose to dissipate waste heat to air (natural or forced convection), to liquid (water, water/glycol, PAO), or radiate to space.

Dissipating Heat to Air

In many applications, the preferred method of thermal management is convection cooling to air, especially in electronics cooling applications. With Boyd’s heat sink, heat pipe assemblies, and heat spreader technologies, waste heat is typically absorbed from a heat generating device (e.g., an electrical component within an electronics system — i.e. computers and data centers) and then moved or spread for dissipation into the ambient air through either natural or forced (using a fan air mover). Thermal technologies from Boyd such as remote heat pipe assemblies and vapor chambers allow the designer to move heat from high heat flux components to a location with a larger surface area (typically plate fins or folded fins) and lower heat flux for dissipation into the ambient air.

Dissipating Heat to Liquid

Applications with large heat loads such as military radars or power electronics often require waste heat to be dissipated into the liquid coolants (water, water/glycol, PAO) of a secondary system for ultimate heat dissipation. Boyd’s heat pipe cold plates and liquid cold plates allow designers to move heat from a heat generating device into a coolant being circulated from a secondary system.

Dissipating Heat through Radiation

As satellites are packaged with more electronics, the challenge of rejecting heat through the limited surface area becomes greater. Boyd’s low temperature, axially grooved heat pipes (ammonia/aluminum, ethane/aluminum) and loop heat pipe technology make it possible to reject heat through radiator panels that are stored for launch, then deployed from the satellite when the satellite achieves orbit. Our low temperature axially grooved heat pipes spread heat out from the satellite electronics to the radiator panels, dissipating waste heat to space. And our loop heat pipe technology is capable of transporting and rejecting heat loads from hundreds of W to greater than 2,000 W.

The first step in selecting the right product for your cooling system application is to determine the heat load, or the amount of heat generated by your system. This article explains how to establish heat load for any liquid cooling application. The same process can be adapted for air cooling systems as well.

Rough Estimate for Heat Load

Formula for Specific Heat

• Q = heat load (W [BTU/hr])
• m = mass flow rate (kg/s [lb/hr])
• Cp = specific heat (J/g-K [BTU/lb °F])
• ΔT = change in temperature (°C [°F])

What Are Thermocouples?

Thermocouples are a sensor built from two dissimilar metals that generate an electrical charge based on the temperature at the joint between those two materials. Thermocouples are a crucial element in thermal testing.

Accurate measurements rely on placing the thermocouple junction as close to the point under test. If a material is in the way, thermal resistance and thickness can also help determine the temperature at a specific point but decrease the overall accuracy of your measurement.

Thermocouples Measurement Accuracy

Thermocouple Calibration

How to Measure Liquid Flow Rate Without a Flow Meter

If a flow meter is not available, measure the constant flow rate of the system with a graduated container and a timer. Collect the fluid in the graduated container over a measured period. Divide the amount of fluid by the amount of time that has elapsed. A constant flow rate is essential when measuring the flow in this manner. The density of the fluid should be used to convert the volumetric flow rate to mass flow rate.

These methods of determining heat load are generic to any liquid cooling application and can be used when sizing a CDU, recirculating chiller, cold plate, or heat exchanger.

Now I Know my Heat Load, What Next?

Once you’ve calculated the heat load of your system, you can start determining the amount of cooling you require. This piece of information combined with the amount of volume allowable for a cooling system will help thermal engineers select or develop a liquid cooling system that will meet your project needs.

Need help getting a pulse on your system? Contact our engineering team or learn more about our Testing Services.

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