Liquid Cooling and the Chemical Compatibility Imperative : CPC

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Power densities in electronic subsystems continue to increase, driving demand for more extreme cooling power – cooling power such as that available with liquid cooling. Now more than ever to optimize thermal management efficiency, sustainability and reliability, designers of systems that use liquid cooling are exploring innovative combinations of component materials, including advanced thermoplastics, specialized elastomers, metal alloys and engineered fluids.

Whether designing a closed-loop, single-phase immersion, two-phase immersion or direct-to-chip cooling system, component material compatibility is critical to performance. This tech guide provides general guidance regarding selecting the right ingredients for your liquid cooling solution.

Think Holistically When Selecting Components

A variety of subsystems and components make up the architecture that is critical to the successful and reliable operation of any cooling system.

Each system component has the potential for interaction with other component materials. Therefore, materials interactions and dependencies warrant detailed analysis during design and specification.

Coolants are of particular interest, not only as the primary conduit of thermal transfer, but because they are in contact with all wetted materials within a particular cooling system and essentially “connect” all components. Some fluids may promote corrosion or biofouling in the presence of certain materials, creating the potential for impedance in or failure of the cooling system. Therefore, it’s essential to understand what all the materials are and the interactions that they might have. Specifically, when assessing chemical compatibility, potential permeation or diffusive losses, identify critical points of connection – such as tubing junctions, manifold ports, and quick disconnect fittings – and evaluate each one for risks to reliability and performance.

Overall, when it comes to material selection, think holistically. Take into account all system components, and consider potential effects of the environment, working fluid, temperature, pressure, and mechanical loading, which might adversely impact performance.

In this tech guide, we’ll take a look at coolants commonly used in liquid cooling applications and provide an overview of the materials of construction. Finally, we’ll provide guidance regarding the potential compatibility of these fluids and materials when used together.

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Tapping Coolant Alternatives

Obviously, choosing a coolant is a focal point when designing a liquid cooling system. From a compatibility standpoint, it’s important to recognize that the fluid links virtually every component as it circulates through the liquid cooling system. See the Coolants table for a brief overview of a number of fluids typically used in cooling electronics.

Start by looking at operating and storage temperatures. Identify fluids with properties appropriate to your application environment, such as a boiling point that satisfies the thermal load and thermal efficiency needed without exceeding the critical heat flux. Check low temperature characteristics during storage and shipping, environmental exposures, particularly with engineered dielectrics such as fluorochemicals, as well as refrigerants. It’s often necessary to understand the environmental impact of the fluid throughout the life cycle – how it’s manufactured, the potential impact of it leaching into the facility or atmosphere during use, and end-of-life fluid reclamation requirements.

Coolants

Coolants — Fluids commonly used in electronics cooling applications
Water	Excellent heat transfer, low viscosity, non-flammable and low cost. Relatively narrow operational range and susceptible to freezing or boiling. Susceptible to biological fouling which inhibits heat transfer. May contain impurities with potential corrosive effects; deionization may reduce initially, though impurities may be extracted over time from wetted surfaces.
Ethylene glycol (EG)	Controls biological growth, lowers freezing point and elevates boiling point when used in solution with water, ranging from 10% to 90% EG. Lower cost than refrigerants or dielectrics. Water in solution may still promote corrosion, degrading coolant over time. Highly toxic, requires careful handling.
Propylene glycol (PG)	Controls biological growth when used in solution with water, ranging from 10% to 90% PG. Lower cost than refrigerants or dielectrics. Lower thermal conductivity and higher viscosity than EG. Water in solution may still promote corrosion, degrading coolant over time. Low toxicity for easier handling and disposal.
Mineral oil	Odorless, non-toxic and chemically inert. No evaporation or volatility. Enables immersion applications. Potential incompatibility with copper or some elastomers.
Refrigerants	Lightweight with excellent thermal transfer properties. Incompatible with some plastics and elastomers. Higher cost than water, EG, PG or mineral oil. Examples include R-1234yf and R-1336.
Dielectrics	Non-conductive engineered fluids that enable full immersion of electronics in single-phase, two-phase and direct-tochip applications. Low boiling points. High chemical stability. Higher cost. Potential incompatibility with thermoplastics or heavily plasticized elastomers.

When selecting fluids consider the ozone depletion and global warming potentials, in particular with regard to refrigerants and dielectrics. Over the last decade or so, the World Health Organization guidelines have increased emphasis on these parameters, prompting the development of greener alternatives, such as 3M Novec^™, HFE coolant, and more environmentally friendly, fourth generation hydrofluoroolefin refrigerants like R-1234 or R-1336.

In addition to thermal stability and chemical compatibilities, consider coolant toxicity, flammability, cleanliness requirements, environmental impact, and cost. And of course, when comparing coolant types and options, make sure to consider all materials that the fluid may come in contact with throughout the system.

Sizing Up Materials of Construction

Electronics cooling system components are, in general, comprised of three types of polymers – commodity plastics, engineered thermoplastics, and elastomers – and four types of metal alloys – aluminum, brass, copper and stainless steel. The information below provides a high-level comparison of liquid cooling system component materials of construction.

Polymer strengths

Polymer limitations

Elastomers

Elastomers can be engineered to meet a wide range of performance requirements. Elastomers are polymers that have the property of viscoelasticity – they’re rubbery and flexible – and are primarily used in components for fluid transport, such as tubing and hose, as well as sealing components such as O-rings and gaskets. To understand how elastomers tend to behave, we can look at how they’re made.

Vulcanization, or the process of curing, creates permanent cross-links in long polymer chains in elastomers. These chains ensure that when stresses are loaded and unloaded, the elastomeric component will return to its original position. In the case of an O-ring and a quick disconnect, for example, an elastomer will maintain its seal.

At a high level, specifying elastomers for use in a liquid cooling application requires detailed analysis and evaluation with the selected unique coolant to ensure compatibility and long-term reliability. For discussion purposes, some common material compound categories that might be seen in these applications…

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…can be identified as hydrogenated nitrile, ethylene-propylene or EPDM, and chloroprene. HNBR has great chemical resistance, excellent mechanical properties, including tensile strength, tear modulus to the wide temperature range and can be compounded for excellent resistance for high pressure applications. EPDM has excellent hot water and steam resistance characteristics. However, it’s got lower resistance to hydrocarbons so it’s not really well suited for any refrigerant type application.

Chloroprene, which is commonly known as neoprene, is very resistant to many chlorofluorocarbons or CFCs, that are used as refrigerant. It’s low cost but has moderate chemical resistance and limited temperature resistance.

Some additional things to consider when specifying elastomers are to consider the hardness (the durometer), the thermal resistance or robustness under both continuous and intermittent exposures, and certainly the compounding as it relates directly to chemical compatibility.

Metal alloys

Putting It All Together: Chemical Compatibility

With a foundational understanding of the fluids, plastics and metals that might be employed in a given liquid cooling application, we can assess potential chemical compatibility of system components, based on their make-up, to ensure reliable, long term operation.

While polymers and metals can be effective in any combination when appropriately specified, it is critical to distinguish…

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