The Open Compute Project (OCP) is focused on developing scalable computing for cloud data centers, artificial intelligence, and other large-scale computing applications. Immersion cooling can improve the energy efficiency and sustainability of those facilities. So, the interest of the OCP in immersion cooling is not surprising.
The OCP immersion cooling requirements define materials compatibility needs and electrical/thermal performance expectations that impact the selection and application of connectors. This article briefly overviews immersion cooling technologies, how technology selection affects connector design and OPC requirements.
Technology choice
The starting point is choosing between single-phase and two-phase immersion cooling. Single-phase is the simplest approach. The cooling fluid remains liquid for the entire process, and the heat is removed using an external heat exchanger or radiator.
In two-stage immersion cooling, the liquid is vaporized, and heat is removed through condensation. Both immersion cooling approaches are included in the OPC requirements.
While standard servers can be modified for immersion cooling, performance can be optimized using purpose-built servers. It may be necessary to change cable routing. I/O placements and other design elements to facilitate the flow of the liquid and/or vapor.
In some cases, sealed power connectors or I/O ports can be necessary to prevent fluid ingress and different materials may be needed to make them.
Single-phase systems use dielectric fluids with higher boiling points, which tend to be more stable and allow a wider range of connector materials. The low-boiling-point dielectric fluids in two-phase systems can damage some standard connector materials.
Material possibilities
For single-phase immersion cooling, recommended connector materials include high-performance plastics like polytetrafluoroethylene (PTFE), liquid crystal polymer (LCP), and seals using fluor elastomers (FKM) like Viton. Metal choices include copper alloys and phosphor bronze.
Recommended materials for connectors in two-phase liquid cooling include high-performance polymers like polyphenylsulfone (PPSU) or polytetrafluoroethylene (PTFE) and seals made with ethylene propylene diene monomer (EPDM) or Viton. Metal choices include plated brass, aluminum, and stainless steel.
Connector ratings can change
Connector ratings, such as the current rating of power contacts and the breakdown voltage, can change when using immersion cooling. For example, a connector with a combination of power and signal pins has been designed for use with either air cooling or single-phase immersion cooling. It has an LCP housing, gold-plated copper alloy, and phosphor bronze contacts.
When used in an immersion-cooled system, the current-carrying capacity and breakdown voltages are 161.7 A per contact and 5,000 Vac, respectively. The corresponding ratings in an air-cooled system are 52.3 A and 4,177 Vac (Figure 1).
Backplane cable assemblies
Backplane cable assemblies have been developed to meet the OCP immersion cooling requirements. These high-density interconnects support the compact design used in immersion cooling systems. They can handle 112Gb/s per channel and are designed for internal cables, external I/O cables, cabled midplanes, and cabled backplanes. Materials used in these backplane cable assemblies include (Figure 2):
- Contacts made with a high-performance copper alloy.
- Housings using a high-performance thermoplastic.
- Cables of silver-plated copper conductors with foam insulation covered by conductive foil.
Quick release connectors
The OPC large quick connector has been specified to support spill-free quick coupling for immersion cooling systems in data centers. It’s defined for flow diameters of 3.2, 6.4, 9.5, and 12.7 mm and a working pressure of 10 bar. Designed to support blind-mate applications, it has a tolerance for radial misalignment of ±5 mm and an angular misalignment tolerance of ±2.7 degrees (Figure 3).
Summary
The OPC immersion cooling requirements impact the materials used in connectors and can also affect some connector performance ratings. The use of immersion cooling is expected to improve the sustainability of large data centers and reduce energy consumption. The OPC has defined a large quick connector for liquid handling in addition to setting out requirements for electrical connectors.
References
Design Guidelines for Immersion-Cooled IT Equipment, Open Compute Project
ElectroCool Dielectric Coolants, Engineered Fluids
Immersion Requirements, Rev 2.0, Open Compute Project
Large Quick Connector Specification, Open Compute Project
Material Compatibility in Immersion Cooling, Open Compute Project
Open Compute Project Connectors, Samtec
Solving Cooling Interconnects for Next-Gen Data Centers and AI Applications, Amphenol
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