As the computer chip industry explodes with innovation, data centre power density is increasing. HPC applications and AI are demand cutting-edge computer chips to perform complex calculations faster than ever. To accomplish this, semiconductor manufacturers are packing more transistors onto silicon wafers, resulting in increased computing power and energy consumption. Intel has plans to squeeze one-trillion transistors onto a single chip by 2030. These chips are consuming more electrical power, which translates to greater heat dissipation. Data centre operators are having to reconsider conventional cooling techniques, to effectively manage heat generated by equipment and within data centres.

Within the datacentre, there are three localised areas that will be considered for cooling: the rack, the server and individual server components, i.e CPU/GPU. Each can be cooled by air condensing, evaporation, convection or circulation of a liquid. When analysing liquid cooling options for enterprise-grade IT hardware there are essentially two main categories of liquid cooling – Immersive Liquid Cooling and Direct-to-Chip Liquid Cooling (sometimes called conductive or cold plate liquid cooling).


What Is Direct Liquid Cooling (DLC)?

Direct-to-chip cold plate cooling is the practice of circulating cool liquid through a plate that is in direct contact with major heat components, such as CPUs and GPUs. The liquid removes heat from the components, effectively cooling down components and infrastructure. The electric components are never in direct contact with the coolant.
Direct-to-chip cooling systems require several components to work:

  • A cooling liquid, which is typically a dielectric fluid engineered for direct-to-chip cooling
  • A plate that the liquid can pass through
  • A circulator that moves the liquid
  • A thermal interface material, which helps conduct heat from the source to the cold plate

    There are two methods of Direct-to-Chip Liquid Cooling:

    • Direct-to-Chip Single Phase
    This method requires delivering liquid coolant with a cold plate placed directly on the chip. With this method, fans are still required to provide airflow through the server to remove the residual heat. While the air-cooling infrastructure is greatly reduced, one is still required for the correct operation of this liquid cooling method. Coolants can be either water or dielectric fluids, but water will infer a downtime risk of leakage, however, Leak Prevention Systems (LPS) are available.

    Direct-to-Chip Two-Phase
    In terms of heat-rejection, two-phase systems are better than single-phase systems and have a lower risk of leakage due to the coolant's state-changing nature. They do however require additional controls which will increase maintenance costs over the lifetime of the system. These systems will always use engineered dielectric fluids.

    The two-phase method works like the previous single-phase method. The difference being that the liquid coolant changes states - from a gas to a liquid and vice-versa, as it completes the cooling loop.

    In terms of heat-rejection, two-phase systems are better than single-phase systems and have a lower risk of leakage due to the coolant's state-changing nature. They do however require additional controls which will increase maintenance costs over the lifetime of the system. These systems will always use engineered dielectric fluids.




    What Is Immersion Liquid Cooling (ILC)?

    Immersive Liquid Cooling - Chassis Single-Phase

    This cooling approach uses a single-phase dielectric fluid and is in direct contact with IT components. Servers are fully or partially immersed in this non-conductive liquid within the chassis effectively removing all sources of heat. The cooling can happen either passively via conduction or actively pumped. Both heat exchangers and pumps can be found inside the chassis or in a side arrangement where the heat is transferred from the liquid to a water loop. This approach also involves no fans, so its operation is nearly silent (0 dB).

    Immersion Cooling - Open Tub - Single-Phase

    Sometimes referred to as an "open bath,” this immersive liquid cooling method involves the IT equipment being completely submerged in fluid. Essentially, it is a rack turned on its back, filled with dielectric fluid - instead of mounting servers horizontally, they are now mounted vertically. These systems are usually fitted with centralised power supplies and the natural dielectric fluid is cooled off through a heat exchanger using a pump which can be installed either inside or outside the tub, or by convection.

    Immersion Cooling - Open Tub - Two-Phase

    As with Single-Phase, the equipment is completely submerged in fluid vertically within a tank. However, with this approach, the dielectric fluid must be capable of changing states from liquid to gas as it heats up. In such a system, submerged and exposed parts will create heat, turning the liquid into a gas, which rises to the surface and condenses on a coil, falling naturally back down once it cools off enough by turning back into a liquid state.



    What Is Air to Liquid Cooling (ALC)?


    Data centres have been using air cooling since data centres have existed and continue to be used extensively. Although technologies have evolved over the years, cooling systems have remained relatively efficient and the basic concept has remained the same. The main differences between air cooling systems lie in how they control airflow. The systems are generally categorized into three types: room-, row- and rack-based and then broken down to using direct air flow or liquid heat exchangers to dissipate heat. Maintaining these systems is a straightforward process with plenty of industry experience behind it.

    However, air cooling also presents several challenges: At the top of the list is its inability to meet modern workload demands. Air cooling simply can't keep up with the increased densities and heavy processing loads. Water restrictions and costs present a challenge for air cooling systems that rely on evaporative cooling or external public water sources. In addition, higher computing densities translate to more cooling fans and pumps, making data centres extremely noisy, where air-cooled facilities can reach upwards of 80 dB in the data hall with workers requiring hearing protection for longer exposures.



    What are dielectric fluids?

    There’s a common perception of risk associated with having liquids in the data center, where it’s believed that the presence of liquid in a cabinet could potentially lead to system outages. However, the fluids used in immersion cooling are dielectric, which means they do not conduct electricity and consequently these liquids pose no risk to technology components.

    Dielectric liquids are used as electrical insulators in high voltage applications, e.g. transformers, capacitors, high-voltage cables, and switchgear (namely high voltage switchgear). Their functions are to provide electrical insulation, suppress corona (loss of electric energy) and arcing (circuit current jump), and serve as a coolant. They are generally split into two categories: fluorochemical and hydrocarbons.

    Fluorochemical fluids (or fluorocarbons), generally with a lower boiling point, are predominantly used for two-phase immersion cooling. Hydrocarbons typically are not used for Two-Phase immersion cooling systems, as most hydrocarbons are combustible and/or flammable. Therefore, hydrocarbons are typically only used in Single-Phase applications. Examples would be mineral, synthetic or natural oils.

    Both fluorochemicals and hydrocarbons can be used for Single-Phase immersion cooling. Fluids with a higher boiling point (above the maximum temperature of the system) are necessary to ensure the fluid remains in the liquid phase.

    Considerations when deciding among various fluorochemicals and hydrocarbons include heat transfer performance (stability and reliability over time, etc.), ease of maintenance, fluid hygiene and replacement requirement, material compatibility, electrical properties, flammability or combustibility, environmental impact, safety-related issues and total fluid cost over the lifetime of the tank or data centres.





    Current Adoption

    While far from mainstream, liquid cooling is positioning itself as the cooling solution for high-performance computing. Its mainstream adoption will however depend on advances in technology and chip designs.

    Retrofitting already existing data centres is costly for some forms of liquid cooling, while the weight of immersion tanks makes it impractical for many current raised floor facilities.

    To ensure high service levels, there are technical considerations, such as avoiding equipment labelling entering into the immersion coolant




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