Cooling water couplings for data centres and transformers
Cooling water couplings for data centres and transformers: what requirements does liquid cooling place on your connections?
In data centres and transformer installations, cooling water couplings are not a commodity part. A coupling that leaks in the cooling water circuit of a server rack or power transformer can shut down an entire installation, cause water damage or, in the case of transformers, even create a fire hazard.
The requirements for cooling water couplings are therefore fundamentally different from those in pneumatic applications. Leak-tightness, choice of materials and reliability over years are operational preconditions here. This article explains what requirements liquid cooling places on your cooling water couplings, where data centre cooling and transformer cooling differ, and how to make the right choice.
⏱ approx. 7 minutes reading time
Why cooling water couplings are subjected to heavier loads than pneumatic couplings
Traditional pneumatic couplings are designed for air or gas and are not tested for long-term exposure to liquids. Cooling water couplings are permanently exposed to a liquid medium under pressure, at varying temperatures and with additives that can attack seals.
The rise of AI hardware and high-density GPU racks is driving the heat load per server rack to 50 kW and higher. Air cooling is inadequate for this. Cooling water circuits with direct liquid cooling are becoming the new standard, making cooling water couplings a critical part of data centre infrastructure.
| Application | Heat load | Cooling medium | Critical requirement for the cooling water coupling |
| Standard server rack | 7 to 15 kW per rack | Water or glycol-water | Leak-tightness, fast assembly |
| AI and GPU cluster | 50 to 250 kW per rack | Water or dielectric fluid | Zero leakage on connection and disconnection |
| Power transformer | Depending on power rating | Mineral oil or synthetic ester | Oil-resistant, high pressure, long service life |
| Welding installation cooling water | Up to 50 kW per welding source | Demineralised water | Dezincification-resistant, low metal ion content |
| Laser cutting system | Variable depending on power | Distilled water or glycol mix | No metal ions, high purity |
Three cooling principles and their requirements for cooling water couplings
Direct-to-chip cooling (cold plate on the processor)
In direct-to-chip cooling, coolant is sent directly through a cold plate on the processor. The cooling water couplings in this system are located at two levels: CDU to rack manifold, and manifold to individual server. At server level, push-in connections with double shut-off are required, so that the system does not lose any liquid when a server is replaced.
Requirements for the cooling water coupling: zero leakage on connection and disconnection, resistant to demineralised water and glycol, compact design that fits the rack format, tool-free operation.
Rack-level cooling via a manifold
A rack manifold distributes the coolant over several servers and is permanently under pressure. Cooling water couplings on the manifold are disconnected less frequently but are in continuous contact with the medium. Long-term leak-tightness and resistance to cooling water additives are the critical properties here.
Transformer cooling
In power transformers, mineral oil or a synthetic ester circulates as the cooling medium. Cooling water couplings in this system are permanently exposed to oil under pressure, sometimes at high temperatures and in corrosive outdoor environments. FKM seals are mandatory here; standard EPDM is not suitable for oil.
Material requirements for cooling water couplings: what cooling water does to brass
Standard brass contains zinc, which can be leached out in contact with certain cooling water additives or at low pH. This process, dezincification, leads to porous metal residues in the cooling water coupling and contamination of the cooling water circuit. For cooling water couplings, always specify dezincification-resistant brass (DZR) or stainless steel.
Critical for laser cooling and precision cooling
Laser cutting systems require demineralised water with a specific conductivity value. Cooling water couplings with brass parts can release metal ions that damage the laser optics. Only use stainless steel or polymer components for the interior of the coupling in these applications.
Sealing materials for cooling water couplings
| Sealing material | Suitable for | Not suitable for |
| EPDM | Water, glycol-water, mild cooling water additives | Mineral oil, hydrocarbons |
| FKM (Viton) | Mineral oil, synthetic esters, fuels | Hot water above 120 degrees C |
| FFKM (Kalrez) | Virtually all media, incl. aggressive chemicals | Cost too high for standard water |
| NBR (nitrile) | Mineral oil, hydraulic fluids | Ozone, UV, solvents |
Eisele Liquidline cooling water couplings: specifications
Standard brass contains zinc, which can be leached out in contact with certain cooling water additives or at low pH. This process, dezincification, leads to porous metal residues in the cooling water coupling and contamination of the cooling water circuit. For cooling water couplings, always specify dezincification-resistant brass (DZR) or stainless steel.
| Property | Eisele Liquidline specification |
| Housing material | Dezincification-resistant brass (DZR) or 316 stainless steel |
| Seals | EPDM as standard; FKM and FFKM on request |
| Working pressure | Up to 25 bar (depending on series and hose diameter) |
| Temperature range | -20 to +100 degrees C (EPDM); extended range with FKM |
| Hose connection | Push-in connection with release ring |
| Production | Made in Germany, milled from solid |
| Conformity | Declarations of conformity available; RoHS-compliant |
Data centre cooling: single or double shut-off?
For server rack applications with frequent connection and disconnection, cooling water couplings with double shut-off are required. On disconnection, both sides close automatically, so that no liquid is lost. When requesting a quotation, explicitly ask for the double shut-off coupling type for your rack configuration.
Data centre cooling versus transformer cooling compared
| Criterion | Data centre and server rack | Power transformer |
| Medium | Demineralised water, glycol-water | Mineral oil, synthetic ester |
| Working pressure | Low to moderate (2 to 6 bar) | Moderate to high |
| Temperature | 18 to 45 degrees C | Up to 90 degrees C and higher |
| Coupling frequency | High (server replacement, maintenance) | Low (rarely disconnected) |
| Environment | Indoors, controlled climate | Indoors or outdoors, offshore possible |
| Cooling water coupling seal | EPDM for water and glycol | FKM for oil and synthetic esters |
| Risk in the event of a leak | Water damage to IT, downtime | Fire hazard, environmental damage, grid failure |
Frequently asked questions about cooling water couplings
Can I use standard pneumatic couplings as cooling water couplings?
No. Pneumatic couplings have not been tested for leak-tightness under sustained liquid pressure. They can corrode internally due to contact with water and offer insufficient assurance of the leak-tightness that cooling water circuits require. Always use cooling water couplings that are specifically certified for liquid applications.
Which material is best for cooling water couplings in a data centre?
For most data centre cooling water circuits, dezincification-resistant brass (DZR) or 316 stainless steel is the right choice of housing, combined with EPDM seals for water and glycol-water. With demineralised water in laser or precision cooling, only stainless steel or polymer is acceptable because of the risk of metal ion release.
How long do cooling water couplings last in a data centre?
With correctly specified cooling water couplings and a well-managed cooling water circuit (correct pH, corrosion inhibitors, regular water analysis), a service life of 10 years or more is achievable. Couplings that are frequently disconnected during server replacement wear faster. Check seals annually during scheduled maintenance.
Is Eisele Liquidline also suitable for transformer cooling with mineral oil?
Yes, provided the right seal variant is chosen. Standard EPDM is not suitable for mineral oil. Choose the FKM version for oil and synthetic esters. When requesting a quotation, state the specific cooling medium and the operating temperature so that a responsible material recommendation can be made.
Eisele products
Requesting a quotation for cooling water couplings
Cooling water circuits for data centres and transformers are not standard applications. Please include the following information with your request:
- Medium and composition (water, glycol percentage, mineral oil, additives)
- The working pressure and maximum temperature
- The hose diameter and the connection type on both sides
- The coupling frequency: fixed installation or regular disconnection?
- The environmental requirements: indoor installation, outdoor, offshore or cleanroom
- Any conformity requirements or standards that the installation must meet
