Chillers for High-Density and Liquid-Cooled AI Racks: Supply Temperatures, CDUs and Sizing November 9, 2026 By Paul Johnson, Technology & R&D, G&D Chillers Short answer: Liquid-cooled AI racks need a chiller plant that delivers warmer, tightly controlled facility water, typically 18 °C to 32 °C (64 °F to 90 °F), to coolant distribution units (CDUs) that feed direct-to-chip cold plates. The best chillers for AI data centers run efficiently at those elevated setpoints, follow fast load swings, and maximize free cooling hours. This guide covers chilled water supply temperatures for AI, how chillers connect to CDUs, how to size the plant, and what G&D's data center chillers deliver. Why do AI racks change chiller requirements? AI training racks concentrate far more heat than traditional IT. GPU racks are now commonly designed at 50 kW to more than 130 kW per rack, compared with 5 kW to 15 kW for typical enterprise racks, which is beyond what air cooling can remove on its own. That pushes most AI deployments to direct-to-chip liquid cooling, where the chiller plant cools a water loop rather than room air. Three things change for the chiller: Higher water temperatures. Cold plates work with warm water, so the plant can run far above legacy 7 °C (45 °F) setpoints. Faster load swings. GPU training jobs can ramp power in seconds, so the plant must follow load without tripping or cycling. Mixed loads. A share of the heat, often 10% to 30%, still goes to room air and needs cooler water for fan walls or CRAHs. Our air vs. liquid cooling guide covers the architecture choice in more detail. What chilled water supply temperature do liquid-cooled AI racks need? ASHRAE defines liquid cooling classes by the maximum facility water supply temperature. Most current direct-to-chip GPU platforms are designed around W27 to W32, but the server vendor's specification always governs. ASHRAE liquid cooling class Max facility water supply Typical heat rejection Chiller role W17 17 °C (62.6 °F) Chillers with economizer Full-time mechanical cooling in most climates W27 27 °C (80.6 °F) Chillers with economizer Chiller trims, free cooling most of the year in cool climates W32 32 °C (89.6 °F) Chillers with economizer or dry coolers Chiller mainly for peak days and hot climates W40 40 °C (104 °F) Mostly dry coolers, chiller optional Supplemental W45 45 °C (113 °F) Dry coolers Rarely needed W+ Above 45 °C (113 °F) Dry coolers, heat reuse Not needed Our ASHRAE temperature guide explains the air-side envelope that still applies to the room. "The question we hear most from AI projects is whether they still need chillers at W32. In a hot climate, yes. The chiller covers the design days, and the rest of the year it runs in partial or full free cooling. Designing for both is what keeps the GPUs inside their spec on the hottest afternoon." — Paul Johnson, Technology & R&D, G&D Chillers How does a chiller connect to a CDU? The chiller plant and the CDU form two separate loops joined by a heat exchanger inside the CDU. The chiller never touches the cold plate coolant directly. Facility water system (FWS), the primary loop. The chiller produces facility water and pumps it to the CDU and to fan walls or CRAHs. CDU heat exchanger. Heat moves from the rack loop into the facility water. The CDU approach is typically 2 °C to 5 °C (4 °F to 9 °F). Technology cooling system (TCS), the secondary loop. The CDU pumps treated coolant through the cold plates at tightly controlled temperature, pressure and flow. Return. Warmed facility water returns to the chiller, where free cooling coils or compressors reject the heat. Because of the CDU approach, the chiller supply setpoint must sit a few degrees below the rack coolant target. Read more in What is a CDU? How do you size a chiller plant for liquid-cooled AI racks? Size the plant for total IT heat, then split it between the liquid loop and the air side. Water flow follows from the temperature difference: at a 10 °C (18 °F) rise, water carries about 1 kW per 1.43 L/min (about 2.4 gpm per ton at a 10 °F rise). Worked example: 1 MW AI hall, 80% liquid capture Step Calculation Result Total IT load 10 racks at 100 kW 1,000 kW Liquid loop load 1,000 kW × 80% 800 kW to CDUs Air-side load 1,000 kW × 20%, plus lighting and UPS losses ~200 kW+ to fan walls Liquid loop flow at 10 °C rise 800 kW × 1.43 L/min per kW ~1,150 L/min (~300 gpm) Chiller capacity with N+1 3 × 500 kW units, any two carry the load 1,500 kW installed For the full method, including safety factors and non-IT loads, use our step-by-step cooling requirements guide. Should AI data centers use one chilled water temperature or two? Many AI facilities now run two temperatures: warm water for CDUs and cooler water for the air side. The alternative is a single warm plant with air-side equipment selected to work at the higher temperature. Approach How it works Pros Cons Single warm loop One plant at ~18 °C to 24 °C for CDUs and fan walls Simplest plant, most free cooling Larger air-side coils, limits on room temperature Dual temperature Warm loop for CDUs, cooler loop for air side Each loop optimized More equipment, more controls Separate plants Dedicated chillers for liquid and air loads Independent redundancy Highest first cost and footprint What do G&D chillers deliver for AI racks? G&D's GDX Series is built around Danfoss Turbocor oil-free magnetic bearing compressors, whose variable speed operation follows fast AI load swings without cycling. G&D can also supply the chillers, CDUs and chilled water skids as one integrated stack, which puts a single engineering owner on the interfaces (G&D integrated cooling stack). Requirement GDX Air GDX Free (GDX-Cool) Capacity per unit 250 to 2,000 kW 250 to 2,000 kW High ambient Full load at up to 45 °C (113 °F) Built for cooler climates Free cooling Available Integrated coils Refrigerants R-134a, R-1234ze, R-513A, R-515B R-134a, R-1234ze, R-513A, R-515B Water use No cooling tower No cooling tower Frequently asked questions Do liquid-cooled data centers still need chillers? Usually yes. Even at W32, most climates have hot hours when dry coolers alone cannot hold the supply temperature, and the air-side load still needs cooling. What is the difference between facility water and the technology cooling system? Facility water is the chiller loop that serves CDUs and air-side equipment. The technology cooling system is the separate, treated loop between the CDU and the cold plates. How much heat does direct-to-chip cooling capture? Typically 70% to 90% of rack heat goes to the liquid loop, depending on the server design. The rest goes to room air. What kind of chiller is best for AI racks? A variable-speed chiller that runs efficiently at warm setpoints and offers free cooling. Oil-free magnetic bearing centrifugal chillers are a strong fit because AI plants run at part load for most hours. Can G&D supply CDUs as well as chillers? Yes. G&D integrates chillers, CDUs and chilled water skids into one modular stack for AI-class facilities. Planning an AI deployment? See G&D's data center cooling platforms and talk to an application engineer about your supply temperature and redundancy targets.