Immersion cooling: fluid loops, water loops and flanges
In immersion cooling the servers sit in a tank of dielectric fluid. The fluid carries the heat to a heat exchanger, and from there an ordinary water loop takes over. The flanges on each side see very different fluids.
A single-phase immersion system pumps dielectric fluid from the tank through a heat exchanger, usually in a CDU, and back. A two-phase system boils the fluid in the tank and condenses it on a water-cooled coil. Either way the water side is a normal facility water loop with carbon or stainless flanges. On the fluid side, flanges and especially gaskets must be compatible with the dielectric fluid, which rules out EPDM with hydrocarbon oils.
Single-phase and two-phase
| Type | Fluid | How heat leaves the tank | Pipework |
|---|---|---|---|
| Single-phase | Hydrocarbon-based fluids such as synthetic oils and polyalphaolefins, or synthetic esters; the fluid stays liquid | Pumped through a heat exchanger in a CDU or tank-side unit | A fluid loop from tank to heat exchanger, then a water loop to the plant |
| Two-phase | Engineered low-boiling fluids, many of them fluorinated | Fluid boils on the chips, condenses on a water-cooled coil in the tank lid | Mostly a water loop to the condenser coils; little fluid pipework outside the tank |
Two-phase fluid supply changed when 3M, a major maker of fluorinated fluids, said in December 2022 that it would stop making PFAS by the end of 2025. Fluid choice is now a design decision in its own right, and it decides the seal materials.
The fluid side: materials and gaskets
- Gaskets and seals: EPDM, the usual choice on water loops, swells in mineral and synthetic hydrocarbon oils. Use the elastomer or PTFE grade on the fluid supplier's compatibility list; FKM and PTFE are common choices for oils.
- Flanges and pipe: stainless steel is the usual choice. Dielectric fluids are expensive and must stay clean and dry, so the same cleanliness thinking as a technology cooling loop applies: pickled and passivated stainless, clean welds and a flush before filling.
- Filters and pumps: flanged connections on the fluid loop see the fluid at its working temperature; rate the joint for it.
Put the dielectric fluid on your request for quotation. It does not change the flange, but it changes the gasket, and we will flag it on the line items so the joint is specified correctly.
The water side
The water loop from the heat exchanger or condenser to the plant is a facility water system, with the same choices as any other: A105 carbon steel or stainless, ASME B16.5 Class 150 or EN 1092-1 PN16, and EPDM or PTFE gaskets. Immersion systems can return water warm enough for heat reuse, so check the flange and gasket ratings at the design temperature.
Where the flanges are
| Location | Fluid | Usual flange and gasket |
|---|---|---|
| Tank outlet and return nozzles | Dielectric fluid | 316L or 304L, Class 150 or PN16; gasket to the fluid supplier's list |
| Fluid pumps, filters, heat exchanger | Dielectric fluid | As above |
| Heat exchanger or condenser, water side | Facility water | Carbon or stainless steel, Class 150 or PN16; EPDM or PTFE gasket |
| Facility water mains | Facility water | A105 weld neck or slip-on; B16.47 or AWWA C207 at large sizes |
Frequently asked questions
Can the same flanges be used on the fluid side and the water side?
Usually yes. A 316L or 304L stainless flange suits both. What changes is the gasket: EPDM is fine on water but swells in hydrocarbon dielectric oils, so the fluid side needs the elastomer or PTFE grade the fluid supplier approves.
Do immersion systems still need a water loop?
Yes. The dielectric fluid carries the heat out of the tank, but a water loop carries it from the heat exchanger or condenser to the dry coolers, chillers or heat reuse plant. That water loop is a normal facility water system.
Which fluids are used in immersion cooling?
Single-phase systems mostly use hydrocarbon-based fluids such as synthetic oils and polyalphaolefins, and some use synthetic esters. Two-phase systems use engineered fluids, many of them fluorinated, that boil at low temperature. The fluid choice decides the seal and gasket materials.
Sources
- 3M, announcement of exit from PFAS manufacturing by the end of 2025 (December 2022)
- Fluid and seal manufacturers' compatibility data for hydrocarbon oils and elastomers