Primary Fluid Network (PFN) Design: The Main Artery of Liquid-Cooled Data Centres
Liquid cooling has changed how data centres move heat. Direct-to-chip cold plates now cool GPU racks that draw 40 to 150 kW. However, that heat does not disappear at the rack. It travels through two loops before it leaves the building.
The second loop, the secondary fluid network, carries coolant from the CDU to the racks. The first loop is the primary fluid network, or PFN. It carries heat from every CDU out to the chillers, dry coolers or cooling towers. In other words, the PFN is the main artery of a liquid-cooled facility.
A well-built primary fluid network keeps every CDU supplied with the right water at the right temperature. A poor one starves CDUs, wastes pump energy and makes maintenance risky. This guide explains how to size, lay out, protect and test a PFN.
What is a primary fluid network?
A primary fluid network is the piping of the facility water system. It links the heat rejection plant to the primary side of each CDU heat exchanger. It includes the main supply and return headers, branch lines to each CDU, valves, strainers, meters and air vents.
The PFN and the SFN never share fluid. Instead, the CDU heat exchanger passes heat from one loop to the other. As a result, engineers can design each loop for its own job.
The SFN is small, clean and tightly controlled, because its coolant flows through cold plates. The PFN is larger and runs across the whole building. It often carries plain treated water, and it must handle much higher flow rates. So the PFN focuses on scale, while the SFN focuses on cleanliness.
Sizing the primary fluid network
Start with the total heat load of the hall. Then add the load you plan to add over the next few years. Undersized headers are very hard to fix once the hall is live.
Next, set the design temperatures. Liquid cooling allows warm facility water, often in the 25 to 40°C range. Warmer water means more hours of free cooling and lower energy costs. ASHRAE classes W17 to W45 describe these supply temperature bands.
Flow follows from the heat load and the temperature rise. For example, a 5 MW hall with a 10 K rise needs about 120 litres of water per second. That is roughly 7,200 litres per minute. A larger temperature rise cuts flow and pipe size, but it also reduces the margin at each CDU.
Pipe size then follows from velocity. Main headers often run at 2 to 3 m/s, while branches run slower. At 2.5 m/s, our 5 MW example needs a header of about DN250. Finally, check that the total pressure drop sits within the pump head, with margin for strainers and future CDUs.
Layout and redundancy
The layout of a PFN decides how well it copes with failures and maintenance. Most critical facilities avoid a simple single header. Instead, they use a ring main or a dual-path design.
In a ring main, water can reach each CDU from two directions. Therefore, you can isolate a section for repair while every CDU stays online. This supports concurrent maintainability, which many operators now require.
Each CDU connection also needs its own isolation valves, a strainer and a flow or energy meter. In addition, a balancing or pressure-independent control valve keeps flow steady as other CDUs change load. Air vents at high points and drains at low points complete the branch.
Plan for growth from day one. Valved and capped tap-off points let you add CDUs later without draining the network. Because AI rack density keeps rising, spare capacity in the PFN is usually cheap insurance.
Materials, water treatment and filtration
The PFN uses different materials from the SFN. Large headers are often carbon steel, because it is strong and cost-effective at big diameters. However, stainless steel is common for branches, CDU connections and areas that need higher cleanliness.
Water treatment is critical in either case. A closed PFN should run with a corrosion inhibitor and a biocide programme. Regular sampling checks pH, conductivity, inhibitor level and bacteria. If the network touches an open cooling tower, the risk of dirt and scale is much higher.
Filtration protects the CDU heat exchangers. Strainers at each CDU catch debris, while a side-stream filter cleans a share of the flow all the time. As a result, plate heat exchangers stay clean and keep their rated approach temperature.
Fabrication quality matters too. Shop-made spools with controlled welding give better joints than site welding alone. Clean, cap and tag each spool, so it arrives on site ready to install.
Testing and commissioning
You must prove a primary fluid network before any CDU depends on it. First, pressure test each section above its design pressure, commonly at 1.5 times. Next, test the whole network again after installation.
Then flush the network with CDUs bypassed, so debris cannot reach their heat exchangers. Flushing continues until strainers stay clean and samples meet the agreed limits. After that, treat the water and vent the system.
Finally, balance flow to each CDU against design. The handover pack should include as-built drawings, test certificates and the water quality baseline.
PFN design checklist
- Total heat load, plus planned growth
- Supply and return temperatures agreed with the CDU supplier
- Flow, pipe size and pressure drop checked
- Ring main or dual path for concurrent maintainability
- Isolation, strainer, meter and balancing valve at every CDU
- Materials matched to pipe size and cleanliness needs
- Water treatment, sampling and side-stream filtration
- Pressure testing, flushing and a documented handover
Conclusion
The primary fluid network rarely gets attention, but every watt of IT heat passes through it. Get the sizing, layout and water quality right, and every CDU performs as designed. Get them wrong, and the whole hall pays the price.
The best results come when one team designs the PFN, the CDUs and the SFN together. That way, temperatures, flows and pressures line up across the whole liquid loop.
At RexEdge, we design and fabricate both primary and secondary fluid networks in-house. So the whole liquid cooling system, from plant room to rack, comes from one accountable partner. Talk to our engineering team about your next liquid-cooled project.