As AI and HPC (High Performance Computing) workloads push server power envelopes into the hundreds of watts per chip, cold plates (direct-to-chip liquid cooling) are rapidly becoming a core thermal technology in modern data centers.
In a cold plate system, coolant flows directly through channels very close to the CPU/GPU or power device, providing dramatically higher heat removal capacity than traditional air cooling.
In this architecture, the fittings on the cold plate inlets and outlets are critical components. They determine:
- Leak tightness and long-term reliability
- Compatibility with hoses, hard pipes, and manifolds
- Pressure and flow capability
- Maintainability and ease of service
This article provides a practical, engineering-oriented overview of what types of fittings are typically used on cold plates, which standards and sizes are common in the industry, and what trends are emerging.
Why Cold Plate Fittings Matter
Cold plates sit directly on top of high-value components such as CPUs, GPUs, and accelerators. Any leak near these devices can be catastrophic.
At the same time, cold plate loops must handle:
- System pressures commonly in the range of 4–10 bar
- Coolant chemistry (glycol-water, deionized water, dielectric fluids, etc.)
- Thermal cycling and mechanical stress
- Frequent maintenance in some designs (replacement of servers, blades, or GPU modules)
As a result, fittings on cold plates must offer:
- Excellent sealing performance (typically via O-rings or metal-to-metal seals)
- Defined, standardized interfaces (thread or OD dimensions)
- Material compatibility with both coolant and surrounding metals
- Mechanical robustness under vibration, shock, and handling
For system integrators and component suppliers, selecting the right fitting standard and size is a key design decision.
Main Categories of Fittings Used on Cold Plates
Most cold plate inlets/outlets fall into three broad fitting categories:
- Tubing OD (Outside Diameter) based fittings
- Threaded fittings (BSPP, NPT, SAE/JIC)
- Univeral Quick Disconnect Couplings (UQD)
Each category serves different design and integration preferences.
Tubing OD Based Fittings
In many cold plate designs, the port is sized by tubing outside diameter (OD). The tubing itself can be:
- Stainless steel hard pipe
- Polymer hose (EPDM, PVC, silicone, etc.)
- Nylon or other engineered plastics
- Braided stainless or hybrid hoses
Typical OD Sizes
The following OD sizes are commonly seen in server, HPC, and data center liquid cooling:
| Imperial OD | Metric OD (approx.) | Typical Use Case |
|---|---|---|
| 1/4″ | 6.35 mm | Small modules, low-flow or legacy systems |
| 5/16″ | 7.94 mm | Older designs, some OEM internal systems |
| 3/8″ | 9.5 mm | Mainstream CPU/GPU cold plate loops |
| 1/2″ | 12.7 mm | High-power AI/HPC cold plates |
| 5/8″ | 15.88 mm | Rack-level or manifold supply/return lines |
Today, 3/8" and 1/2" OD are the dominant sizes for direct-to-chip liquid cooling in modern servers.
Threaded Adapter Fittings on Cold Plates
Threaded ports are widely used on cold plates because they offer:
- Flexibility to adapt to different tubing sizes
- Robust mechanical retention
- Good compatibility with industrial plumbing standards
The most common thread families in cold plate applications are:
BSPP / G-Thread (Most Common for Cold Plates)
BSPP (British Standard Pipe Parallel), often referred to as G-thread, is arguably the most widely used thread type on cold plates, especially in Europe and Asia.
Typical sizes:
| Thread Size | Major OD (approx.) | Typical Use on Cold Plates |
|---|---|---|
| G 1/4″ | ~13.1 mm | De facto standard for cold plate ports |
| G 3/8″ | ~16.7 mm | Higher-flow cold plates |
| G 1/2″ | ~20.9 mm | High-capacity plates or manifolds |
Key characteristics:
- Parallel thread; sealing is achieved with an O-ring or gasket at the face
- Easy to assemble and disassemble without damaging the threads
- Well-suited for modular systems and field maintenance
In practice, G 1/4" is the dominant port standard on many commercial cold plates.
NPT (National Pipe Taper) Threads
In North America, some liquid cooling products use NPT (tapered) threads:
| Thread Size | Typical Use Case |
|---|---|
| 1/4″ NPT | Cold plate and water block ports in some designs |
| 3/8″ NPT | Higher flow circuits or manifolds |
Key characteristics:
- Tapered thread provides a wedge-type mechanical seal
- Often requires thread sealant (e.g., PTFE tape or paste)
- Not ideal for frequent disassembly because repeated tightening can damage threads or sealing surfaces
NPT remains common in industrial and legacy systems, but many newer data center designs prefer parallel threads with O-ring sealing (e.g., G-thread).
SAE / JIC 37° Flare Fittings
SAE/JIC 37° flare fittings are widely used for metal tubing in higher pressure or more “industrial” liquid cooling systems.
Typical sizes:
| SAE Size | Typical Tube OD |
|---|---|
| SAE 4 | 1/4″ OD |
| SAE 5 | 5/16″ OD |
| SAE 6 | 3/8″ OD |
| SAE 8 | 1/2″ OD |
Key characteristics:
- Metal-to-metal seal via a 37° flare surface
- High mechanical strength and pressure capability
- Suitable for stainless steel hard lines and demanding HPC installations
These fittings are sometimes used at the CDU, manifolds, or main loop, and can also be found at cold plate connections in more industrial-style systems.
Univeral Quick Disconnect Couplings (UQD)
To support fast service and minimal downtime, many liquid-cooled systems use Quick Disconnect Couplings (QDCs) near cold plates or at node boundaries.
Typical UQD Sizes
Common UQD size and corresponding tubing:
| QDC Size (Nominal) | Typical Tube OD | Use Case |
|---|---|---|
| ~1/4″ (6.4 mm) | 1/4″ OD | Small modules, low-flow lines |
| 3/8″ (~9.5 mm) | 3/8″ OD | Mainstream CPU/GPU cold plates |
| 1/2″ (~12.7 mm) | 1/2″ OD | High-power AI/HPC loops |
| ≥ 20 mm | Large manifolds | Rack or row-level liquid loops |
Well-known UQD suppliers in this space include:
- Swagelok
- Parker
- Staubli
- CPC / Colder Products
- QC Hydraulics
- Other specialized data center liquid cooling vendors
Modern data center solutions increasingly favor:
- Dry-break (no-drip) QDCs, to avoid any coolant loss during service
- Push-to-connect designs, to reduce maintenance time and complexity
Interface Standardization: ODF and Beyond
As liquid cooling scales, the industry is actively working toward standardized interfaces to improve interoperability.
ODF (Open Direct Fluid) Concept
ODF (Open Direct Fluid)–type initiatives focus on:
- Standardizing tube OD sizes
- Defining flow/pressure classes
- Aligning on sealing approaches and dimensional envelopes
In practice, the most commonly referenced ODs in such efforts are:
- 9.5 mm (3/8" OD)
- 12.7 mm (1/2" OD)
This aligns well with the sizes already dominant in CPU/GPU cold plate designs.
Typical Pressure and Performance Requirements
Cold plate fittings must satisfy the overall performance envelope of the cooling loop, not just the cold plate component.
Typical system-level requirements include:
| Parameter | Typical Value / Range |
|---|---|
| Working pressure | 1–4 bar (cold plate side) |
| System pressure | 6–10 bar (CDU discharge) |
| Burst pressure target | ≥ 20 bar |
| Fitting proof pressure | ≥ 10 bar (application-specific) |
| Temperature range | –20 °C to +85 °C (typical) |
These values can vary by vendor and application, but they indicate why high-quality stainless steel and properly designed fittings are preferred in mission-critical liquid-cooled environments.
Common Materials for Cold Plate Fittings
Material choice must account for:
- Corrosion resistance
- Galvanic compatibility with cold plate and other metals
- Coolant chemistry
- Mechanical and thermal cycling
Typical materials include:
-
Stainless Steel (304 / 316)
- Excellent corrosion resistance
- High mechanical strength
- Increasingly the preferred option for data center liquid cooling
-
Nickel-Plated Copper or Brass
- Good conductivity and reasonable corrosion resistance
- Widely used in mid-range systems
-
Engineering Plastics (PPSU, PEEK, etc.)
- Often used within QDC internals and manifolds
- Less common as the primary threaded port material on cold plates themselves
For long-life, high-reliability HPC and AI data centers, 316 stainless steel fittings are seeing strong adoption, especially on the facility side and in higher pressure segments of the loop.
Practical Summary for Designers and Suppliers
If you are designing a cold plate or supplying fittings into the liquid cooling ecosystem, the following guidelines can serve as a quick reference:
Mainstream Port and Fitting Choices
- Port Thread Standard
- Use G 1/4" BSPP as the primary cold plate port wherever possible
- Tubing OD
- 3/8" OD (9.5 mm) for mainstream CPU/GPU cold plates
- 1/2" OD (12.7 mm) for high-power AI/HPC applications
When to Use Other Standards
- Consider NPT threads mainly for North American or legacy compatibility.
- Use SAE/JIC flare fittings and stainless steel hard lines in more industrial or high-pressure sections (e.g., CDU, distribution manifolds).
- Employ dry-break QDCs at service boundaries where frequent connect/disconnect is required.
Material Strategy
- Favor 316 stainless steel for fittings and adapters where corrosion resistance, strength, and longevity are critical.
- Ensure compatibility with cold plate body materials (often copper or nickel-plated copper) to mitigate galvanic corrosion risks.
Conclusion
Cold plates are at the heart of modern direct-to-chip liquid cooling systems, but they can only perform reliably when their fittings and interfaces are designed to robust industry standards.
QC Hydraulics is a leading manufacturer and supply of stainless steel fittings and adapters,wich are used in liquid cooling systems. Contact Us for more details.