Technical Manual · CDU & Liquid Cooling
Liquid-Cooling CDU Toolkit — Methodology & Formulas
Every input, equation, constant, output, and reference behind the four-resource CDU Toolkit — selection guide, maintenance checklist, interactive Mini-BMS cockpit, and deep comparison. The sizing math is anchored to the GB300 campus engine js/dcai-engine.js (DCAI_CALC) reading js/dcai-model.js (DCAI_MODEL). No back-solved constants, no random values.
01 Purpose & engineering basis
The CDU Toolkit is a four-resource set for selecting, operating, visualising, and comparing liquid-cooling Coolant Distribution Units in AI/HPC data centers. It answers four design questions: which CDU topology fits a given density and infrastructure constraint, how many CDUs are required and what flow does each carry, what are the live operating parameters and fault signatures, and how do CDU types compare on TCO, field-failure modes, and vendor support.
The four constituent resources are: (1) Selection & Deployment Guide — a reference document with verified vendor specs, sizing math, and installation requirements; (2) Installation, Inspection & Maintenance Checklist — operational parameter bands, water-quality criteria (ASHRAE TC 9.9 W-classes, OCP limits), commissioning procedure, symptom table, and PM cadences; (3) Mini-BMS Cockpit — an interactive cockpit per CDU type with animated P&ID, live simulated parameters, and fault injection; (4) Deep Comparison — source-tagged comparison across field issues, BMS/DCIM integration, vendor after-sales, and TCO.
The governing sizing authority is the adopted GB300 campus basis (data/dcai-parameters.json, spec gb300-500mw-2026-09-06): "CDU_duty = ceil(liquid_heat_kWth / 1,000)", executed by js/dcai-engine.js (DCAI_CALC). Supporting fluid standards are ISO 4413 (hydraulic fluid power safety), ASHRAE 15 (refrigeration-system containment), and ASHRAE TC 9.9 (data-center liquid-cooling best practice). The Mini-BMS cockpit inherits DCAI_CALC constants for Rule-1 consistency — parameter bands, standby logic, and pump-power figures trace to the same adopted model as the datahall dashboard.
02 Inputs
Inputs split into two groups: (a) cockpit parameters used by the Mini-BMS to drive live simulation and fault injection, and (b) selection filters used by the Selection Guide and Deep Comparison. The sizing formulas consume the cockpit parameters; the checklist uses measured values against the same bands.
Cockpit / sizing parameters
| Field | Symbol | Unit | Range / typical | Meaning |
|---|---|---|---|---|
Rack IT load per hall power.rack_it_hall_kwe | IT | kW | 124,960 | Total rack IT load for the data hall (880 racks × 142 kW), per the DCAI_CALC adopted basis. |
Liquid capture ratio heat.liquid_capture_ratio | α | — | 0.85 (adopted) | Fraction of rack IT heat removed by the secondary hydronic loop. |
CDU unit rating equipment.cdu_unit_kwth | R | kW | 1,000 (CoolIT CHx1000) · 121 (per-rack) | Heat-rejection capacity per CDU. Engine default = 1,000 kWth CoolIT CHx1000 row CDU. Vertiv CoolChip 121 kW remains the per-rack class reference. |
TCS deltaT tcsDeltaT_K | ΔT | K | 10 (adopted) | Secondary-loop supply/return temperature difference: 40 °C supply → 50 °C return (warm, dry-only). |
Water density rhoKgPerM3 | ρ | kg/m³ | 1,000 | TCS loop water density (STANDARD constant). Adjust for glycol mixtures. |
Water specific heat cpKjPerKgK | Cp | kJ/(kg·K) | 4.186 | TCS loop specific heat capacity (STANDARD constant). |
CDU pump head pumpHead_Pa | H | Pa | 250 000–500 000 | Total dynamic head across the secondary loop, used in pump-power sizing. |
Pump efficiency pumpEff | η | — | 0.70–0.80 | Pump shaft efficiency. CDU VFD pumps typically 72–78%. |
Selection filters (Selection Guide & Comparison)
| Filter | Options | Drives |
|---|---|---|
| CDU topology | In-rack · In-row · Sidecar · L2L EoR · L2A | Selection guide topology columns; Comparison field-issue rows. |
| Cooling loop type | L2L (liquid-to-liquid) · L2A (liquid-to-air) | Infrastructure tie-in requirements; facility water dependency. |
| Density band | <20 kW · 20–60 kW · 60–200 kW · >200 kW per rack | Topology recommendation matrix in the Selection Guide. |
| OEM | Vertiv · Asetek · Schneider · Liqtech · ZutaCore · Custom | Vendor matrix in the Deep Comparison; after-sales score. |
| Redundancy target | N · N+1 · 2N | Running count upper-bounds; spare CDU pre-positioning. |
03 Calculation methodology
Four sizing steps executed sequentially by DCAI_CALC. Function names map one-to-one to js/dcai-engine.js. The Selection Guide and Checklist are reference/editorial documents — they state spec values and parameter bands but do not call the engine. The Mini-BMS cockpit inherits the same constants for consistency.
Step 1 — Liquid heat load
Step 2 — CDU running count
ceilCount(duty, unit). Engine default CDU_rating = 1,000 kWth (CoolIT CHx1000 row CDU). Ceiling ensures full coverage; fractional CDU is not permissible.js/dcai-engine.js ceilCount() · equipment.cdu_unit_kwth
Step 3 — Total hydronic flow and per-CDU apportionment
flowM3h(qKwTh, rhoKgM3, cpKjKgK, deltaTK). ρ = 1,000 kg/m³, Cp = 4.186 kJ/(kg·K), ΔT = 10 K from the adopted model (warm 40/50°C TCS). Flow divides equally across running CDUs in a balanced manifold arrangement.js/dcai-engine.js flowM3h() · design.flows.tcs_m3h
Step 4 — CDU pump power and standby provision
design.electrical.cdw_pumps_kwe (~2,790 kW facility-wide at the design-day operating point). Standby provision is one unit per hall (107 duty + 1 standby = 108 installed/hall). The Mini-BMS P&ID shows P-01A duty / P-01B standby with auto-changeover on pump failure (F11.2).js/dcai-engine.js pumpKwe() · design.electrical.cdw_pumps_kwe
04 Constants & data sources
Every constant lives in the deep-frozen DCAI_MODEL with a // source: tag and an evidence class. CDU-relevant constants:
| Constant | Value | Source | Notes |
|---|---|---|---|
| CDU rating (row CDU, adopted default) | 1,000 kW | data/dcai-parameters.json equipment.cdu_unit_kwth | CoolIT CHx1000 sizing basis for the CDU duty-count engine call. |
| Vertiv CoolChip CDU 121 | 121 kW | OEM datasheet, Source Sanity | Per-rack class reference for smaller in-rack deployments. Not the adopted facility-scale basis. |
| Liquid capture ratio | 0.85 | data/dcai-parameters.json heat.liquid_capture_ratio | 85% of rack IT heat to the secondary hydronic loop for direct-to-chip racks. |
| TCS supply temperature | 40 °C | data/dcai-parameters.json design.planes.p07_tcs_supply_c | Warm secondary-loop supply to CDU cold-plate manifold, dry-only heat rejection. |
| TCS return temperature | 50 °C | data/dcai-parameters.json design.planes.p08_tcs_return_c | Return from cold plates; ΔT = 10 K across the loop. |
| Water density (ρ) | 1,000 kg/m³ | STANDARD physical constant | Pure water at the TCS operating range. Glycol mixtures require adjustment. |
| Water Cp | 4.186 kJ/(kg·K) | STANDARD physical constant | Specific heat used in flowM3h(). |
| CDU pump power (facility, design-day) | ~2,790 kW | design.electrical.cdw_pumps_kwe | Derived from flow × head at the design-day operating point — never a fixed reference figure. |
| Secondary loop ΔP target | 0.5–3.0 bar | CDU Mini-BMS operating bands · ASHRAE TC 9.9 | Differential pressure across the CDU secondary manifold; alarm outside this band. |
| System pressure (secondary) | 2–6 bar | CDU Mini-BMS operating bands | Secondary-loop static pressure; leak alarm if rapid drop detected. |
| Flow rate target | 1.0–1.5 L/min/kW | CDU Mini-BMS operating bands · OCP Thermal WG | Per-kW flow guidance for cold-plate DCW systems; OCP specification for HPC racks. |
| Secondary supply temperature | 17–50 °C | CDU Mini-BMS · ASHRAE TC 9.9 | Acceptable secondary supply range; dew-point reset ensures ≥dew point + 3 °C to avoid condensation. |
js/datahall-model.js and js/datahall-calculations.js, remain on disk byte-frozen and still pass their own 57 worked-example tests (tools/test-datahall-calc.mjs) as the retirement record — they are historical, not the current basis, and are the only place on this page where the retired figures above describe anything other than a retired reference.CDU OEM reference models
| OEM / Model | Capacity | Type | Source class |
|---|---|---|---|
| Vertiv CoolChip CDU 121 | 121 kW | In-rack / per-rack L2L | Source Sanity — per-rack density class; coolChipNoteKW reference, not the adopted facility-scale default. |
| Asetek RackCDU D2C | 20–100 kW | In-rack / direct-to-chip | OEM datasheet; representative per-rack class for mid-density HPC. |
| CoolIT CHx1000 (row CDU, adopted default) | 1,000 kW | End-of-row / row L2L | data/dcai-parameters.json equipment.cdu_model — the adopted GB300 sizing basis. |
| High-capacity EoR CDU | 500–800 kW | End-of-row L2L | Alternative mid-range class for smaller AI/HPC islands; not the DCAI_CALC default. |
Standards: ISO 4413:2011 — hydraulic fluid-power system safety (pressure testing, containment, relief-valve requirements); ASHRAE 15:2022 — refrigeration-system safety (coolant concentration limits, leak detection for refrigerant-based CDUs); ASHRAE TC 9.9 — liquid-cooling best practice including W-class water quality acceptance criteria (W1–W4, conductivity, pH, dissolved oxygen); OCP Thermal WG — cold-plate connector and flow-rate specifications for hyperscale HPC; DMTF Redfish §Thermal — CDU BMS/DCIM integration schema.
05 Outputs
The sizing engine returns four primary quantities. The Mini-BMS cockpit derives its live-parameter state from these values; the Selection Guide states the same sizing math as reference text; the Checklist uses measured values against the operating bands.
| Output | Formula / source | Unit | Interpretation |
|---|---|---|---|
| Liquid heat load | rack_IT × 0.85 | kW | Heat rejected to the secondary hydronic loop. Drives all downstream CDU sizing. |
| CDU duty count | ceil(liquid_heat / 1,000) | count | Minimum CDUs needed to cover the liquid heat load at full-capacity rating. |
| CDU installed count | CDU_duty + 1 | count | Total CDUs provisioned including one standby unit per hall for single-failure tolerance. |
| Total hydronic flow | liquid_heat × 3600 / (1,000 × 4.186 × 10) | m³/h | Secondary-loop flow required to carry the liquid heat load at 10 K ΔT. |
| Per-CDU flow | total_flow / CDU_duty | m³/h | Flow each running CDU handles. Used to size manifold headers and check velocity limits. |
| Per-CDU load | liquid_heat / CDU_duty | kW | Thermal load per unit. Confirms CDU is operating within rated capacity with margin. |
| CDU pump power (total) | flow_m3s × head / η / 1000 | kW | Electrical power for CDU circulation pumps — included in the PUE basis as a distinct cooling-ancillary line. |
| Cockpit status flags | live simulation | bool / enum | Mini-BMS: N+1 healthy / N (standby lost) / FAULT; leak=0 / leak detected; chemistry in-spec / drift alarm. |
06 Worked example
Adopted GB300 basis: 4 halls × 880 NVL72 racks/hall × 142 kW/rack = 499,840 kW (499.84 MW) rack IT facility-wide. Verify CDU count, per-unit flow, and per-unit load against DCAI_CALC.snapshot. Per-hall sizing is computed first, then scaled by 4 halls — this is the same order the live engine uses, and it is NOT the same as ceiling the facility total in one step (ceiling is not linearly additive).
- Rack IT per hall:
880 × 142 =124,960 kW → facility× 4 =499,840 kW (499.84 MW) - Liquid heat per hall (85% capture):
124,960 × 0.85 =106,216 kW · Air-side residual (15% of rack IT plus fabric/OOB/storage/UPS-loss/dist-loss/aux): 35,509 kW - CDU duty count per hall:
ceil(106,216 / 1,000) =107 CDUs/hall + 1 standby = 108 installed/hall - Facility total:
107 × 4 = 428 running+1 × 4 = 4 standby= 432 installed facility-wide - Total hydronic flow per hall:
106,216 × 3600 / (1,000 × 4.186 × 10) =9,612.3 m³/h → facility× 4 =38,449.2 m³/h - Per-CDU flow:
9,612.3 / 107 =89.8 m³/h/CDU - Per-CDU load:
106,216 / 107 =992.7 kW/CDU (99.3% of 1,000 kW rating — within capacity) - CDU pump power (facility, design-day operating point): ~2,790 kW —
design.electrical.cdw_pumps_kwe, derived from flow × head, not a fixed reference figure. - Sanity check — per-CDU flow vs OCP band:
89.8 m³/h = 1,496.7 L/min; 1,496.7 / 992.7 =1.51 L/min/kW → essentially at the top of the 1.0–1.5 L/min/kW operating band ✓
07 References & standards
- data/dcai-parameters.json (spec gb300-500mw-2026-09-06) — CDU duty-count sizing basis: "CDU_duty = ceil(liquid_heat_kWth / 1,000)". Canonical authority for the engine's CDU count.
- design.electrical.cdw_pumps_kwe — CDU/CDW pump power derived from flow × head at the design-day operating point, not a fixed reference figure. Chiller COP is derived from a Carnot fraction over the actual lift — never a nameplate value.
- js/dcai-engine.js flowM3h() — Hydronic flow formula: ρ = 1,000 kg/m³, Cp = 4.186 kJ/(kg·K). Water property basis, unchanged physical constants from the prior engine.
- data/dcai-parameters.json — Adopted basis of design: liquid capture ratio 0.85, TCS 40/50 °C (ΔT 10 K, warm dry-only), 880 racks/hall at 142 kW/rack.
- NVIDIA Enterprise Reference Architecture components — governing hardware reference for the 142 kW/rack GB300 NVL72 basis.
- ISO 4413:2011 — Hydraulic fluid-power safety: pressure-test requirements, containment, relief-valve ratings for CDU secondary circuits.
- ASHRAE 15:2022 — Safety Standard for Refrigeration Systems: coolant concentration limits, leak detection requirements for refrigerant-based CDU variants.
- ASHRAE TC 9.9 (2021) — Liquid Cooling Guidelines for Datacom Equipment: W-class water quality (W1–W4 conductivity, pH, dissolved O2), secondary-loop temperature and pressure ranges, commissioning criteria.
- OCP Thermal WG — Open Compute cold-plate and connector specifications; 1.0–1.5 L/min/kW flow-rate guidance for HPC racks.
- DMTF Redfish §Thermal — CDU BMS/DCIM integration schema:
CoolingLoop,CoolantConnector, SNMP/RESTful telemetry. Referenced in the Mini-BMS architecture section. - OEM datasheets — Vertiv, Asetek, CoolIT — CDU heat-rejection capacities, flow/pressure envelopes, maintenance intervals. See cdu-checklist.html §07 for spare-parts references.
- Engine source —
js/dcai-engine.js(DCAI_CALC) ·js/dcai-model.js(DCAI_MODEL, deep-frozen) ·js/dcai-parameters.js(shared registry) · balance-identity gatetools/test-dcai-engine.mjs. - js/datahall-model.js + js/datahall-calculations.js (RETIRED 2026-09-06) — the byte-frozen retired-era pair, kept only as the retirement record; see §04 "Retired basis" note.
08 Assumptions & limitations
The CDU count formula assumes all CDUs are identically rated and the manifold is hydraulically balanced (equal flow share). In practice, diversity factors, partial-load derating, and manifold pressure-drop asymmetry should be modelled; consult the CDU OEM for site-specific sizing confirmation. The 1,000 kW CoolIT CHx1000 rating is the adopted basis — it names a real vendor model, but actual CDU ratings must still be confirmed against the chosen OEM's current datasheet at time of procurement.
The CDU pump-power figure (~2,790 kW facility, design-day) is derived from flow × head at the engine's design-day operating point — it is not a fixed reference value scaled from a single worked example, and it moves with ambient across the weather year the same way the chiller COP does. Site pump power depends on actual loop head, pipe routing, and equipment arrangement. The 1.0–1.5 L/min/kW operating band from OCP applies to cold-plate direct-to-chip systems; in-row or sidecar CDUs with larger heat exchangers may accept lower per-kW flow.
Water-quality criteria (ASHRAE TC 9.9 W-classes) in the Checklist are acceptance thresholds, not ongoing operating targets; continuous monitoring with automatic dosing is recommended for CDU installations above 200 kW/hall. The FMECA fault codes (F11.1–F11.5) referenced by the Mini-BMS are educational mappings to ai-engineering-maintenance.html — they are not a substitute for a site-specific FMECA analysis. This toolkit and its methodology are an engineering education and pre-design aid; final CDU design and commissioning must be carried out by a qualified mechanical/facilities engineer to the authority having jurisdiction and in compliance with the site's cooling-infrastructure specifications.