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Understanding liquid immersion cooling

Image: Adobe Stock / pinkeyes

Chris Carreiro, CTO at Park Place Technologies, explains the specifics of liquid immersion cooling, as well as the challenges – and benefits – of its adoption.

Immersion cooling is a type of liquid cooling used to moderate data centre equipment temperature by submerging it in a cooling fluid. Server immersion cooling helps to dissipate heat and keep components like CPUs performing optimally. Immersion cooling systems prove to be more efficient than traditional data centre cooling methods (like computer room air conditioning, or CRAC) due to the increased thermal conductivity of most liquids compared to air.

Because 1-1.5% of electricity use across the globe is attributable to data centres, companies have been innovating to find a liquid cooling solution that can reduce that energy demand. Dielectric fluid immersion cooling is one solution that could increase CPU density in data centres while consuming less energy. Dielectric liquid cooling depends on the use of a thermally conductive but not electrically conductive fluid that will not disrupt the function of electrical components like servers. Examples of dielectric fluids include mineral oil hydrocarbons, synthetic fluorocarbons, and silicone fluids.

Immersion cooling technology can rely on dielectric fluids purpose-chosen to remain in a liquid state, or fluids intended to cycle through a liquid and gas state within the system. The type of fluid chosen depends on whether a single-phase or two-phase system is being used.

To put it simply, immersion cooling is a subset of several liquid cooling techniques that have been explored. Other types of liquid cooling include direct-to-chip (DTC), rear-door server rack cooling, waterborne data center cooling, and evaporative cooling.

With immersion cooling, whole data centre components are directly submerged into a specially designed tank. In contrast, water-cooled server racks look very similar to traditional rack-mount servers, but they are networked with waterblocks and tubing that circulates fluid to help dissipate heat.

The ‘phases’ in single-phase and two-phase immersion are a reference to states of matter, and not physical stages in the system. The physical footprints of these two immersion cooling tanks are not drastically different, but their cooling cycles and contained fluids set them apart.

In single-phase immersion cooling, heat from the immersed server components is transferred directly to the surrounding fluid. However, the dielectric fluid does not undergo a ‘phase change’ from a liquid to a gas. Instead, the fluid is cycled out of the immersion tank by a coolant pump that runs through a heat exchanger and is returned to the immersion tank at a lower temperature where it continues this heat transfer cycle.

In two-phase immersion cooling, heat from immersed server components causes the special immersion fluid to boil. The resulting steam heats a condenser coil in the top of the sealed chamber. The coolant in the condenser coil is cycled out of the chamber to a heat rejection mechanism (cooling tower, etc.). Then, the coolant is sent back to the sealed chamber at a lower temperature, ready to continue the heat transfer cycle.

Because of the steam from the phase change in two-phase liquid immersion cooling, the chamber must be sealed during operation. This means that performing maintenance requires a cooling and unsealing process that costs valuable operation time (which can cost as much as $5,600 per minute).

The average power usage effectiveness ratio (PUE) within a data centre can be measured by dividing total energy consumed by energy used by computing equipment. This means that as PUE gets closer to 1, efficiency is improving. According to The Register, PUE for a traditional data centre in 2022 was approximately 1.58, while single-phase immersion was able to bring this number down to the 1.05 to 1.10 range.

Not only does immersion cooling improve the energy efficiency of data centers, but it can save valuable space as well. According to a 2023 research article, immersion cooling only requires about one-third of the space to that of an air-cooled configuration. One of the main contributors to this efficiency is the improved rack power density from not having to allow for air flow within servers.

CRAC, one of the main traditional cooling methods for data centres, is reliant on the use of fans. This means that traditional data centers are very loud. Immersion cooling server configurations don’t rely on fans and air flow for cooling. Because of their liquid cooling function, immersion cooling has proven to reduce data center noise.

Launching a liquid cooling solution comes with a list of challenges, like preparing existing hardware for immersion, training maintenance staff (or finding qualified third-party maintainers) on the repair process for immersed gear, and managing vendors for the tanks, dielectric fluid, and more. But the ROI and sustainability benefits make liquid cooling an exciting part of future data center planning.

Picture of Chris Carreiro
Chris Carreiro
CTO at Park Place Technologies

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