An Introduction to Liquid Cooling for Rack Enclosures

Power densities in electronic subsystems continue to increase, driving demand for more extreme cooling power. System engineers are increasingly exploring liquid cooling to optimize thermal management efficiency, sustainability and reliability.

Liquid cooling is replacing air cooling for several reasons: 3,500 to 4,000 times more efficient at transferring heat; noise reduced due to slower fan speed; 85% reduction in carbon footprint; and significant cost savings results from more efficient cooling because less energy is required. Read more about IBM’s experiment from 2010 in Scientific American.

Fluids commonly used in cooling applications

• Water – excellent heat transfer, low viscosity, non-flammable and low cost. Susceptible to freezing or boiling and biological fouling.
• Ethylene glycol (EG) – controls biological growth, lowers freezing point and elevates boiling point when used in solution with water. Lower cost that refrigerants or dielectrics. Highly toxic and requires careful handling.
• Propylene glycol (PG) – controls biological growth when used in solution with water. Lower cost than refrigerants or dielectrics. Lower thermal conductivity and higher viscosity than EG. Low toxicity for easier handling and disposal.
• Mineral oil – odorless, non-toxic and chemically inert. No evaporation or volatility. Potential incompatibility with copper or some elastomers.
• Refrigerants – lightweight with excellent thermal transfer properties. Higher cost (R-1234yf and R-1336)
• Dielectrics – non-conductive engineered fluids that enable full immersion of electronics in single-phase, two-phase and direct-to-chip applications. Low boiling point and higher chemical stability. Higher cost. Potential incompatibility with thermoplastics or elastomers.

Factors to consider when choosing a coolant for your liquid cooled rack or enclosure

Choosing a coolant is a focal point when designing a liquid cooling system.

Start by looking at operating and storage temperatures. You’ll want to identify a fluid with properties that are compatible with your application’s environment, such as boiling point of the thermal load and thermal efficiency needed without exceeding the critical heat flux. It’s also critical to check low temperature characteristics during storage and shipping or other environmental exposures.

We recommend considering the ozone depletion and global warming potentials when selecting a fluid. Over the last decade, the World Health Organization guidelines have increased emphasis on these parameters, prompting the development of greener alternatives, such as 3M Novec (will be discontinued in 2025), FHE coolant and fourth generation hydrofluoroolefin refrigerants.

In addition to thermal stability and chemical compatibilities, consider coolant toxicity, flammability, cleanliness requirements, environmental impact, and cost.

Component construction materials commonly used

Polymers

• Commodity plastics – includes HDPE, POM, PP, PS and PVC. Relatively low cost and readily available. Potential flammability in high-temperature applications or thermal degradation and shrinkage in some environments.
• Engineered thermoplastics – includes PEEK, PEI, PESU, PPSU, PSU. Improved mechanical and thermal properties, with a higher cost than commodity plastics.
• Elastomers – includes CR, EDPM/EPM, FKM, HNBR, silicone. May be modified to enhance flame retardance, durability or chemical resistance. Some types may leach into fluids during thermal cycling or exposure to certain solvents, negatively impacting coolant performance.

Metal alloys

• Aluminum – durable, lightweight metal with strong thermal properties. Potential for galvanic corrosion, especially with water-based coolants and in presence of copper. Anodization increases corrosion resistance.
• Brass – durable and strong thermal properties. Relatively low cost and often plated with nickel or chrome for improved corrosion resistance.
• Copper – durable and strong thermal properties. Galvanic corrosion potential, especially with water-based coolants.
• Stainless steel – Highest in durability and stability, but lower thermal conductivity and higher costs. Passivation increases corrosion resistance.

Material and coolant compatibility

When considering components in a liquid cooling system, the following combinations are:

• A = recommended. Little to no potential for chemical reaction or corrosion.
• B = good options. Minor potential for chemical reaction or corrosion, with limited affect on system performance.
• F = not recommended. Mild to severe chemical or corrosive reactions likely. May impede system performance.

Connectors for a liquid cooled rack enclosure

Connectors are critical to the safe and reliable operation of liquid cooling systems. A well-design fluid connector should easily facilitate connection, disconnection and rerouting of fluid; support 100% uptime during installation, reconfiguration and maintenance; and allow secure, efficient, reliable and leak-free management of fluids within the cooling system.

Connector type

• Quick disconnects (QDs) – increasingly used because they’re easier to install/uninstall than other connectors
• Socket/plug; male/female; body/insert – connector halves fit together by one side inserting into the other. They’re intuitive to use and requires force to connect, which increases as the system pressure increases
• Latched – integrated thumb latches can ease connection/disconnection by allowing one-handed operation; enables hot swapping
• Blind mate – requires a separate retention mechanism, such as a sever blade latch; releasing force disconnects the QD and works best in difficult to see/access locations
• QDs with elbows, swivel joints – integrated swivel joints and elbows eliminate tube kinking and allow easier connection and disconnection in tight spaces

Flow rate, pressure and pressure drop

• Flow rate – flow rates are typically low at the server (o.5 l/min) and much higher at the coolant distribution unit (up to 70 l/min); actual-use flow rates that exceed the connector’s maximum flow rate capacity can lead to seal failure or accelerated part erosion.
• Connector size – specify appropriate connector sizes from server to CDU. They range from 1/8 inch at the server to 1 inch at the CDU. QDs of the same size can deliver significantly different flow performance. Also consider physical space available at the front or back to ensure adequate room for connections, disconnections and ongoing use.
• Pressure – operating break and safety burst pressures should all be assessed. Operating pressure defines the usual and customary pressure ranges during regular system use. Break pressure indicates the point at which a component no longer maintains pressure, which is a higher threshold than safety burst pressure.
• Pressure drop – both flow rate and connector size affect pressure drop

Stop-flow / dripless performance

• Straight-through connectors – neither connector half features a valve necessitating flow stop prior to disconnection
• Single shut-off valve – one side of the QD contains a valve
• Double shut-off valve – both QD halves contain valves; poppet valves trap a small amount of liquid within the coupling body that can drip when disconnected
• Flush-face valves – most dripless/drybreak/non-spill QDs feature flush-face valves that allow no more than a coating of coolant on valve surfaces
• Seal type – many QDs feature O-rings; some connectors feature multilobed seals that offer better shape retention over time, protection against leakage, greater resistance to debris or foreign contaminants, and require less force to connect

Reliability

• Helium vacuum leak test – verifies sealing performance at specific temperatures
• Elevated temperature burst test – demonstrates adequate safety margins above rated operating pressure at higher than ambient temperatures
• Creep rupture test – demonstrates safe use at continuous higher-than-rated pressures and temperatures for an extended period
• Flow rate test – determines CV values
• Drip leak testing and spillage testing – under specific temperature and pressure conditions, measure evidence of drip leaks during simulated use conditions or spillage at disconnection
• Disconnect under flow – quantify resistance of connectors to water hammer and fluid acceleration caused by disconnecting units under flow
• Cycle testing – verifies connector sealing performance after repeated connection/disconnection cycles; some manufacturers conduct 10,000 cycles to validate leak-free performance
• Connect force testing – characterize the force to connect with varying pressures in the disconnected body and insert prior to connection