Designing with liquid-immersion cooling systems

09/21/2016


Selecting a liquid

The liquid properties impact major facets of the design and should be reviewed in detail. The mechanical industry is accustomed to working with typical liquids, such as water, glycol solutions, and refrigerants; deviations associated with unique liquids can create challenges. Properties such as kinematic viscosity, dynamic viscosity, specific heat, density, thermal conductivity, the coefficient of thermal expansion, and heat capacity can influence the design. Because the liquid is typically proprietary, the properties are not available in standard design guides or catalogs and are provided by the cooling technology provider.

Figure 3: This rendering shows liquid-immersion cooled IT cabinets (sealed configuration). Courtesy: Environmental Systems Design Inc.Liquid properties have a direct bearing on heat-transfer characteristics and greatly impact the selection of heat-transfer equipment, such as coils, heat exchangers, and fluid coolers. As discussed, standard catalog data cannot be used for this purpose. However, major manufacturers are capable of providing estimated performance for unique liquids.

System pressure drop calculations can also be challenging. One option is to use the underlying principles of fluid mechanics. For example, the Darcy-Weisbach equation can be used to estimate the pressure drop through pipes when circulating Newtonian fluids. For sealed-immersion applications, the pressure drop through the ITE enclosure is typically supplied by the technology provider. When selecting pumps for liquid circulation, properties like density and viscosity will impact the brake horsepower, head, flow capacity, and efficiency of the pump. Major manufacturers can provide pump performance when circulating unique liquids. Another option is to estimate performance by using correction factors, equations, or charts developed by organizations, such as the Hydraulic Institute, and applying them to standard pump curves developed for water.

When the loop design is being established, the compatibility of materials that are expected to be in direct contact with the liquid should be reviewed. For example, pump seals and valve seats are frequently constructed of ethylene propylene diene monomer (EPDM), a type of synthetic rubber. However, EPDM is not compatible with petroleum-based liquids.

Similarly, the use of inert pipe materials, such as stainless steel and copper, and use of mechanical joints in lieu of welding, soldering, or brazing should be discussed with the technology provider. Any contamination of liquid due to incompatibility with materials of construction can have serious repercussions and can lead to catastrophic failure of the ITE.

Requirements for fluid maintenance should be discussed with the technology provider. Petroleum-based liquids, such as mineral oil, are susceptible to biological and water contamination over time. Liquid degradation can negatively impact the heat-transfer properties and can lead to premature failure of the ITE. The infrastructure should incorporate suitable means for fluid maintenance if deemed necessary.

For sealed immersion configurations, pressure limitations of the ITE enclosures must be considered. For a particular application, the pressure rating was less than 10 psig. The requirement impacts the elevation of mechanical infrastructure relative to the ITE, as the static head imposed on ITE needs to be kept to a minimum. Similarly, the pressure drop through the circulation loop, hence the pump head requirement, should be minimized. Pressure-relief valves or other means should be incorporated to prevent accidental overpressurization.

The right solution?

When dealing with extremely dense cabinets, immersion cooling is worthy of consideration. It is suitable for deployments ranging from a few kilowatts to several megawatts. Due to improved heat-transfer performance as compared with an air-cooling system, liquid-supply temperatures higher than 100° F are feasible. Higher liquid temperatures increase the hours of economization, offer the potential for heat recovery, and in certain climates can eliminate the need for chillers completely. The elimination of internal ITE fans reduces energy consumption and noise. In addition, pump energy for circulating liquid is typically lower than fan energy.

Despite the mechanical advantages, there are reasons for caution when deploying liquid-immersion cooling in data centers. The impact on infrastructure, such as structural, electrical, fire protection, and structured cabling, should be evaluated. In a typical data center, air-cooling systems are still needed as certain ITE, such as spinning drives, cannot be liquid-cooled. Immersion cooling is still in its nascent stage, and long-term statistical data is needed for detailed evaluation of ITE and infrastructure reliability, serviceability, maintainability, and lifecycle costs.


Saahil Tumber is a senior associate and lead mechanical engineer at Environmental Systems Design Inc., and is responsible for the overall design of HVAC systems for data centers, trading areas, and other mission critical facilities requiring high availability. His data center experience spans both enterprise and colocation projects.


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