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 DATAHALL_CALC.cduRunningCount() and DATAHALL_CALC.hydronicFlowLPM(), which implement the deterministic engine in js/datahall-calculations.js. 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 21-worked-examples.md Example 7 ("If CDU = 350 kW, running count = ceil(liquid_heat / 350)"), executed by DATAHALL_CALC.cduRunningCount() in js/datahall-calculations.js. 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 DATAHALL_CALC constants for Rule-1 consistency — parameter bands, N+1 logic, and pump-power figures trace to the same locked 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 |
|---|---|---|---|---|
IT load per hall itPerHall_kW | IT | kW | Scenario A 3 564 · B 7 128 | Total IT load for the data hall, per the DATAHALL_CALC locked scenarios. |
Liquid capture ratio liquidCaptureRatio | α | — | 0.85 (locked) | Fraction of IT heat removed by the secondary hydronic loop. 85% from BASELINE-DECISION.md. |
CDU unit rating cdu.ratingKW | R | kW | 350 (EoR) · 121 (per-rack) | Heat-rejection capacity per CDU. Engine default = 350 kW end-of-row CDU (Ex7). Vertiv CoolChip 121 kW is the per-rack reference. |
TCS deltaT tcsDeltaT_K | ΔT | K | 10 (locked) | Secondary-loop supply/return temperature difference: 35 °C supply → 45 °C return. |
Water density rhoKgPerL | ρ | kg/L | 1.0 | TCS loop water density (Ex6 basis). Adjust for glycol mixtures. |
Water specific heat cpKjPerKgK | Cp | kJ/(kg·K) | 4.186 | TCS loop specific heat capacity (Ex6 basis). |
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 DATAHALL_CALC. Function names map one-to-one to js/datahall-calculations.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
DATAHALL_CALC.liquidHeat(itKW, captureRatio). Capture ratio is 0.85 (85%) from BASELINE-DECISION.md. Air-side residual = IT_kW × (1 − α).DATAHALL_CALC · Ex5
Step 2 — CDU running count
DATAHALL_CALC.cduRunningCount(liquidHeatKW, cduRatingKW). Engine default CDU_rating = 350 kW (21-worked-examples.md Ex7 "If CDU = 350 kW"). Ceiling ensures full coverage; fractional CDU is not permissible.DATAHALL_CALC · Ex7
Step 3 — Total hydronic flow and per-CDU apportionment
DATAHALL_CALC.hydronicFlowLPM(qKW, rhoKgL, cpKjKgK, deltaTK). ρ = 1.0 kg/L, Cp = 4.186 kJ/(kg·K), ΔT = 10 K from the locked model. Flow divides equally across running CDUs in a balanced manifold arrangement.DATAHALL_CALC · Ex6 + Ex7
Step 4 — CDU pump power and N+1 redundancy
04 Constants & data sources
Every constant lives in the deep-frozen DATAHALL_MODEL with a // source: tag. CDU-relevant constants:
| Constant | Value | Source | Notes |
|---|---|---|---|
| CDU rating (EoR, default) | 350 kW | 21-worked-examples.md Ex7 | End-of-row CDU sizing basis in the cduRunningCount engine call. |
| Vertiv CoolChip CDU 121 | 121 kW | 00-overview-audit.md Source Sanity | Per-rack class reference; noted as coolChipNoteKW in the model. Not the Ex7 running-count basis. |
| Liquid capture ratio | 0.85 | BASELINE-DECISION.md | 85% of IT heat to the secondary hydronic loop for direct-to-chip halls. |
| TCS supply temperature | 35 °C | BASELINE-DECISION.md | Secondary-loop supply to CDU cold-plate manifold. |
| TCS return temperature | 45 °C | BASELINE-DECISION.md | Return from cold plates; ΔT = 10 K across the loop. |
| Water density (ρ) | 1.0 kg/L | 21-worked-examples.md Ex6 | Pure water at ~35 °C. Glycol mixtures require adjustment. |
| Water Cp | 4.186 kJ/(kg·K) | 21-worked-examples.md Ex6 | Specific heat used in hydronicFlowLPM(). |
| CDU pump power (ref, at 7 128 kW IT) | 120 kW | 21-worked-examples.md Ex9 | CDU pump total at the Scenario B reference IT load — used in PUE bottom-up basis. |
| 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 LPM/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 | 17–45 °C | CDU Mini-BMS · ASHRAE TC 9.9 | Acceptable secondary supply range; dew-point reset ensures ≥dew point + 3 °C to avoid condensation. |
CDU OEM reference models
| OEM / Model | Capacity | Type | Source class |
|---|---|---|---|
| Vertiv CoolChip CDU 121 | 121 kW | In-rack / per-rack L2L | Source Sanity (00-overview-audit.md) — per-rack density class; coolChipNoteKW in DATAHALL_MODEL. |
| Asetek RackCDU D2C | 20–100 kW | In-rack / direct-to-chip | OEM datasheet; representative per-rack class for mid-density HPC. |
| Generic 350 kW EoR CDU | 350 kW | End-of-row L2L | 21-worked-examples.md Ex7 sizing basis. Represents mid-range EoR CDUs from multiple vendors (Schneider EcoBreeze variants, Liqtech, etc.). |
| High-capacity EoR CDU | 500–800 kW | End-of-row L2L | Emerging class for AI/HPC islands >200 kW/rack; not the DATAHALL_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 | IT × 0.85 | kW | Heat rejected to the secondary hydronic loop. Drives all downstream CDU sizing. |
| CDU running count | ceil(liquid_heat / 350) | count | Minimum CDUs needed to cover the liquid heat load at full-capacity rating. |
| N+1 CDU count | CDU_count + 1 | count | Total CDUs provisioned including one full-capacity standby for single-failure tolerance. |
| Total hydronic flow | liquid_heat × 60 / (1.0 × 4.186 × 10) | LPM | Secondary-loop flow required to carry the liquid heat load at 10 K ΔT. |
| Per-CDU flow | total_flow / CDU_count | LPM | Flow each running CDU handles. Used to size manifold headers and check velocity limits. |
| Per-CDU load | liquid_heat / CDU_count | 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
Full-facility peak: 4 halls × Scenario B IT 7 128 kW/hall = 28 512 kW total IT. Verify CDU count, per-unit flow, and per-unit load against DATAHALL_CALC. Scenario B (IT = 7 128 kW/hall) is the reference case for the PUE bottom-up worked example (Ex9).
- Facility IT load:
4 halls × 7 128 kW =28 512 kW - Liquid heat (85% capture):
28 512 × 0.85 =24 235 kW · Air-side residual:28 512 × 0.15 =4 277 kW - CDU running count:
ceil(24 235 / 350) = ceil(69.24) =70 CDUs (facility-wide) - Per-hall basis (Ex7 reference):
7 128 × 0.85 = 6 059 kWliquid heat per hall →ceil(6 059 / 350) = ceil(17.31) =18 CDUs/hall · N+1 = 19 CDUs/hall - Total hydronic flow per hall:
6 059 × 60 / (1.0 × 4.186 × 10) =8 686 LPM - Per-CDU flow:
8 686 / 18 =483 LPM/CDU - Per-CDU load:
6 059 / 18 =337 kW/CDU (96% of 350 kW rating — within capacity) - Facility-level Scenario B alternative (single-hall perspective for Ex7): liquid heat 6 059 kW → 18 running CDUs · per-CDU 483 LPM / 337 kW ✓
- CDU pump power (total, Ex9 reference at 7 128 kW IT): 120 kW CDU pump ancillary contribution to PUE basis.
- Sanity check — per-CDU flow vs OCP band:
483 LPM / 337 kW =1.43 LPM/kW → within the 1.0–1.5 LPM/kW operating band ✓
ceil(12 118 / 350) = ceil(34.62) = 35 CDUs; total flow 12 118 × 60 / (1.0 × 4.186 × 10) = 17 380 LPM; per-CDU flow 17 380 / 35 = 497 LPM; per-CDU load 12 118 / 35 = 346 kW. These are the canonical facility-Scenario-A numbers cited in the toolkit summary (Scenario A IT 3 564 kW/hall × 4 halls × 85% = 12 118 kW, 35 CDUs, 497 LPM, 346 kW per unit).07 References & standards
- 21-worked-examples.md Ex7 — CDU running count sizing basis: "If CDU = 350 kW, count = ceil(liquid_heat / 350)". Canonical authority for
cduRunningCount(). - 21-worked-examples.md Ex9 — PUE bottom-up basis: CDU pump power = 120 kW at 7 128 kW IT reference load. Nameplate COP 6.8 (Carrier 19DV) — not calibrated.
- 21-worked-examples.md Ex6 — Hydronic flow formula: ρ = 1.0 kg/L, Cp = 4.186 kJ/(kg·K). Water property basis for
hydronicFlowLPM(). - BASELINE-DECISION.md — Locked basis of design: liquid capture ratio 0.85, TCS 35/45 °C (ΔT 10 K), Scenario A/B IT loads.
- 00-overview-audit.md (Source Sanity) — Vertiv CoolChip CDU 121 = 121 kW per-rack reference; Carrier 19DV 1 231–4 044 kW chiller range.
- 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 LPM/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, doc-21 — CDU heat-rejection capacities, flow/pressure envelopes, maintenance intervals. See cdu-checklist.html §07 for spare-parts references.
- Engine source —
js/datahall-calculations.js(DATAHALL_CALC) ·js/datahall-model.js(DATAHALL_MODEL, deep-frozen) · worked examples gatetools/test-datahall-calc.mjs(57/57).
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 350 kW CDU rating is the Ex7 basis — it is representative of end-of-row CDUs but is not a specific vendor model; actual CDU ratings must be confirmed against the chosen OEM's datasheet.
The pump-power figure (120 kW at 7 128 kW IT) is the PUE-basis reference for Scenario B — it is an ancillary assumption in the bottom-up PUE model, not a design output of the CDU sizing step. Site pump power depends on actual loop head, pipe routing, and equipment arrangement. The 1.0–1.5 LPM/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.