Direct-to-Chip Cooling Sends Data Center Heat Through a CDU and Two Separate Loops
An Open Compute Project CDU specification calls for 2,000 kW at a 3-degree C approach. ASHRAE's W32 class caps facility water supply at 32 degrees C. OCP's September 2026 PG25 guide suggests at least annual lab analysis of each loop.
The Open Compute Project's Project Deschutes specification, effective Feb. 26, 2026, describes a coolant distribution unit (CDU) that handles 2,000 kW at 500 GPM on each side with a 3-degree C approach. By standard conversions, that load is about 569 tons, and 500 GPM of plain water would rise about 27 degrees F across the facility side. Glycol would change the second figure.
ASHRAE's Thermal Guidelines and Liquid Cooling Guidelines are sold and were not reviewed for this article. The class descriptions come from a free 2021 ASHRAE TC 9.9 white paper and an OCP paper that reprints the class table. Other figures come from OCP documents and Vertiv's CoolChip CDU2300 manual. OCP's PG25 guide calls itself "a baseline framework of recommended practices," so the equipment manufacturer and fluid supplier govern on any project.
Two Loops and the Heat Exchanger Between Them
OCP's CDU test paper says the unit's heat exchanger separates the technology cooling system (TCS) from the facility water system (FWS). The TCS is the row manifold, rack manifolds and CDU secondary side, feeding cold plates whose channels OCP put at about 100 microns in January 2025. Schneider Electric gave about 50 microns for fin spacing this August. On the primary side, Vertiv says its unit can reject to chilled water, a fluid cooler, a cooling tower or a dry air cooler.
The Deschutes unit carries N+1 sealless pumps, an air separator, a flow-through expansion tank, a primary-side control valve, three-zone leak detection and a controller, at up to 6,900 pounds wet. Its default valve logic holds a fixed approach, which OCP defines as TCS supply temperature minus FWS supply temperature. Vertiv's guide specification adds a dew point override that raises TCS supply when condensation is a risk. TC 9.9's paper also describes designs that leave memory on air, with fan power staying high and heat capture to liquid falling, so an air-side load can remain.
W Classes Describe Facility Water Only
The fifth edition of the Thermal Guidelines, released in 2021, put each class's upper limit in its name, with a lower limit of 2 degrees C (35.6 degrees F) for all. Operating in a class "requires full performance of the equipment over the entire environmental range of the specified class, based on nonfailure conditions," ASHRAE's paper says.
| Class | Former name | Maximum facility water supply |
|---|---|---|
| W17 | W1 | 17 C (62.6 F) |
| W27 | W2 | 27 C (80.6 F) |
| W32 | W3 | 32 C (89.6 F) |
| W40 | New | 40 C (104 F) |
| W45 | W4 | 45 C (113 F) |
| W+ | W5 | Above 45 C (113 F) |
The class sets no TCS temperature. With W32 supply at its limit and a 3-degree approach, TCS supply would be about 35 degrees C.
Water and Fluid Limits in the Public Documents
"It is the responsibility of the installer to make sure all fluid in contact with the heat exchanger is filtered to a level of at least 500 micron," Vertiv's manual says. It bars galvanized pipe and automotive antifreeze, and says glycol above 20 percent may require derating. No public document reviewed here sets numeric chemistry limits for facility water: OCP's guide places the FWS out of scope, and Vertiv sends the question to a fluid treatment specialist.
Secondary-side guidance is far more specific. Vertiv and Deschutes both allow PG25 or deionized water, with Vertiv requiring inhibitors and biocides in the water. OCP warns of bio-growth when glycol falls below about 25 percent, of over-dilution from residual flush water and of overly fine filters stripping antifoam additives.
| Item | Figure | Source |
|---|---|---|
| FWS strainer or flush level | 500 microns | Vertiv; Deschutes |
| TCS inline filter | 25 or 50 microns; 25-44 microns | Vertiv; Deschutes |
| Sidestream filtration | 5-15 percent of flow; strongly recommended above 250 gallons | OCP PG25 guide |
| Filtration during fill | 5-10 microns absolute, as an example | OCP PG25 guide |
| Final flush filter | 5 microns; industry range 1-25 | OCP pre-commission paper |
| Minimum flush velocity | 3.84 ft/s in 3-inch pipe to 4.43 ft/s in 12-inch pipe | OCP pre-commission paper |
| Flush water | pH 5-8; chloride, sulfate and hardness each under 25 ppm; conductivity under 25 µS/cm; bacteria under 100 CFU/mL; turbidity under 1 NTU; iron under 2 ppm | OCP PG25 guide, Table 12 |
| In-use PG25 | 24-29 percent glycol; pH 8.0-10.5 conventional, 6.5-9.5 OAT or hybrid | OCP PG25 guide, Table 14 |
Commissioning Order and Service
OCP's January 2025 pre-commission paper runs TCS work from hydrotest to cleaning to flushing to preparation, with a proof test at 1.3 to 1.5 times anticipated working pressure. Deschutes' validation tests hold 120 psig on the secondary side and 150 psig on the primary side for one hour. OCP's PG25 guide prefers fill within 24 hours of the final flush, or a nitrogen purge and seal.
Vertiv's checklist starts on the primary side. A dedicated chiller should be commissioned at least 24 hours ahead, the fluid supply needs automatic makeup so filling does not trip the chiller, and the two-way valves require an external bypass because they can close flow entirely. A gas test on the unit tops out at 30 psig. Vertiv does not recommend flushing external secondary pipework through the CDU, and it calls for a pump rotation check after first fill.
At startup, OCP calls for bleeding air, checking for leaks while circulating at least 30 minutes and pulling a sample within one to three days to retain for the life of the fluid. It then suggests lab analysis of each loop at least annually. Vertiv warns against no-flow conditions, because idle fluid collects sediment, and says standby units should stay in rotation.
Why This Matters to Contractors
A W32 plant priced for today's racks may not stay W32. TC 9.9's paper said in 2021 that most cold plates could "live easily on W32 water" but warned that products now able to run on W45 may give way to W32 and eventually W27, and that a case study pairing a 500-watt CPU with 12-watt memory modules could push facility water to W27. A facility without mechanical cooling "should plan to add it in the future," the paper says, and it counts the piping, heat exchangers and controls needed to trim that water as added capital.
Sources
- Open Compute Project, "Project Deschutes: Data Center Facilities," 1.0.0, effective Feb. 26, 2026. opencompute.org
- Open Compute Project, "Guidelines for Using Propylene Glycol-Based Heat Transfer Fluids in Liquid Cooled Racks," Revision 2.0, Sept. 15, 2026. opencompute.org
- Open Compute Project, "Liquid to Liquid CDU Test Methodology and Performance Rating," Revision 1.0, 2024. opencompute.org
- Open Compute Project, "Guidelines for Pre-Commission Preparation of Technology Cooling System (TCS) Row Manifolds in Liquid Cooled Data Centers," Jan. 27, 2025. opencompute.org
- ASHRAE TC 9.9, "Emergence and Expansion of Liquid Cooling in Mainstream Data Centers," white paper, 2021. ashrae.org
- Vertiv, CoolChip CDU2300 Installation, Commissioning and Maintenance Guide, SL-80095. vertiv.com
- Vertiv, CoolChip CDU 600/1350/2300 Guide Specifications, SL-80239. vertiv.com
- Schneider Electric, "What coolant should you use for data center liquid cooling?" Aug. 14, 2026. se.com