CDU Selection & Deployment Guide

Q: What's the difference between an L2L and an L2A CDU?

An L2L (liquid-to-liquid) CDU rejects chip-side heat to a facility water loop through a plate heat exchanger — highest capacity and efficiency, but needs a chilled/condenser-water plant. An L2A (liquid-to-air) CDU rejects to room air via an internal coil — no facility water, the retrofit enabler, but the room CRAH/CRAC must absorb the heat, capping density.

Q: How do I size the coolant flow to my rack load?

From Q = m·cp·ΔT, for PG25: flow (L/min) ≈ 14.7 × Q(kW) / ΔT(°C). At ΔT ≈ 10 °C that is about 1.5 L/min per kW; OCP's band is 1.25–2.0 L/min/kW for a 7.5–12 °C rise. A 120 kW rack ≈ 180 L/min. Stay within the cold-plate minimum per-chip flow and the CDU dP window.

Q: What coolant and water quality does a cold-plate loop need?

PG25 (25% propylene glycol) is the de-facto direct-to-chip fluid. The secondary TCS loop has OCP acceptance limits: pH 8.0–10.5, conductivity 100 ppm. The facility loop has its own looser limits.

Q: What filtration does the loop need?

Facility/primary 150–500 µm, secondary/TCS main filter 25–50 µm, and a cold-plate side-stream/polishing filter down to < 5–10 µm (some vendors 0.2 µm). Trend filter dP and replace at the vendor threshold.

Q: Why must the supply temperature stay above the room dew point?

Running secondary supply below room dew point condenses moisture on cold plates, pipes and electronics. The CDU holds supply ≥ 2–3 °C above room dew point (re-setting upward if humidity rises). Warmer ASHRAE classes (W40/W45) keep clear of condensation and enable chiller-less free cooling.

Q: What is a quick-disconnect standard like UQD / UQDB?

OCP UQD (Universal Quick Disconnect) and UQDB (blind-mate) standardise coupler dimensions and performance so different makers interoperate on single-phase water/glycol lines. Typical: 100 psi working / 300 psi burst, ≥ 5,000 mating cycles, dripless ~0.02–0.07 mL per connect; UQDB adds ±1 mm blind-mate tolerance. Never mix couplers.

Q: In-row vs in-rack vs facility-scale — which should I pick?

In-rack is fastest and isolates one rack but multiplies units (more PM + leak surface). In-row is the density workhorse — fewer larger units feeding several racks, easier to service, but a central single-point-of-failure needing N+1. Facility-scale L2L gives economy of scale across many rows with the largest redundancy footprint.

Q: Is direct-to-chip liquid cooling safe near electronics? What about leaks?

It is mainstream for AI/HPC. Leaks are the most-reported field issue, so the safety case uses dripless/blind-mate quick-disconnects, drip trays + secondary containment, leak-detection sensors at QDs/manifolds/low points, and an automatic response (alarm → isolate / pump-shutdown or bypass-standby). Two-phase dielectric systems make any leak non-conductive.

01 · Primer

Where the CDU sits in the cooling chain

In a direct-to-chip deployment the heat path is: chip cold-plate → rack manifold → CDU → facility water. The CDU's job is to keep two loops separate. The primary (facility) loop brings comparatively warm, lower-quality chilled or condenser water from the plant. The secondary (technology / TCS) loop is a clean, filtered, temperature- and flow-controlled circuit feeding the cold plates. The CDU's heat exchanger transfers the load between them while the CDU's pumps, filter and controls protect the IT side. Get the CDU wrong and the rack throttles or trips; get it right and a 132 kW NVL72 stays in spec.

How to read the links. Every link below was fetched and checked. VERIFIED = the page/PDF loaded (HTTP 200) and its content matched the model. VENDOR PORTAL = a working product/support page where the datasheet or manual lives, but the exact deep PDF could not be auto-verified (some vendor sites block automated checks) — use the on-page model search. Specs are sourced from the vendor pages cited; where a number is a family/approximate figure it is marked “approx.” Always confirm the exact configuration with the vendor before procurement.

02 · Taxonomy

CDU types — what each is for

CDUs differ mainly by where they reject heat (liquid-to-liquid vs liquid-to-air) and where they physically sit (in a rack, in a row, beside a rack, or at room scale).

In-rack CDU

~50–200 kW · 4U–8U in the rack

Mounts inside the 19″ rack it serves. Fast to deploy, isolates one rack/pod, no separate floor space. Capacity capped by rack U-height and the facility-water tap available at the cabinet.

In-row / row-based CDU

~120 kW–2.3 MW · floor-standing in the row

Stands in the row and feeds several racks via manifolds. The workhorse for dense AI halls — large heat exchanger, redundant pumps, big integrated filter. Needs facility chilled/condenser water piped to it.

Sidecar CDU

~tens–150 kW · bolts to the rack side

A slim cabinet attached to the side of a rack — between in-rack and in-row. Useful when in-rack U-space is full but a full row CDU is overkill, or for rear-door / single-cabinet retrofits.

Liquid-to-Air (L2A) CDU

~70–240 kW · no facility water needed

Rejects the secondary-loop heat straight to room air via an internal air coil. The key retrofit enabler: lets you run direct-to-chip in a hall that has no chilled-water plumbing — at the cost of lower capacity and adding heat to the room.

Liquid-to-Liquid (L2L) CDU

~120 kW–2.3 MW · needs facility CDW

Rejects to a facility water loop through a plate heat exchanger. Highest capacity and efficiency; the standard for purpose-built AI halls with a chilled- or condenser-water plant.

Facility / room-scale CDU

1 MW–2.3 MW+ · serves many rows

Large L2L units (e.g. 1.35–2.3 MW) that distribute to multiple rows/pods from a central location. Fewer units, central maintenance, but a bigger single-point footprint to design for redundancy (N+1 at the unit level).

03 · Compare

In-row & facility-scale CDUs

Row-based and central L2L/L2A units, ordered by capacity. Specs cite the vendor product page; “approx.” marks family-level or rounded figures to confirm per configuration.

VERIFIED page/PDF fetched, content matched VENDOR PORTAL working product/docs page; deep PDF not auto-verifiable

Model	Capacity	Type	Secondary flow	dP / head	Approach	Footprint · weight	Fluid · filter · conn · BMS · class	Links
CoolChip CDU 2300 Vertiv	2300 kW	L2L · row	not published	n/p	4 °C	2400×1200×1200 mm ~1793 kg wet	water / PG-25 · 25–50 µm sec, 500 µm pri · 6″ sanitary · Modbus RTU+TCP · W45	Product VERIFIED Datasheet VENDOR PDF
Liebert XDU1350 Vertiv	1368 kW (2912 max @ 8 °C)	L2L · row	1200 L/min (2-pump) 1800 L/min (3-pump)	2.44 bar	4 °C	2069×900×1243 mm ~650 kg	water / glycol · 50 µm sec (triple-redundant), 500 µm pri · 4″ hygienic · Modbus + SNMP + CLI/web · W3 rated	Datasheet VERIFIED
CHx2000 CoolIT Systems	2000 kW	L2L · rack-mt	2125 L/min @ 35 psi	~2.4 bar	5 °C	750×1200 mm ftpt n/p	PG-25 · 25 µm · 4″ tri-clamp · Redfish + SNMP/Modbus/BACnet · N+N hot-swap pumps	Product VERIFIED
CHx1500 CoolIT Systems	1500 kW (1364 @ 4 °C)	L2L · rack-mt	1800 L/min sec 2100 L/min pri	~3.0 bar	5 °C	n/p	PG · 25 µm (50 opt) · 4″ Victaulic · Redfish + SNMP/Modbus/BACnet · N+N hot-swap	Product VERIFIED
ROL4000 “Deschutes” Boyd (→ Eaton)	2000 kW	L2L · 48U row	1890 L/min (500 GPM)	up to 5.5 bar	3 °C	1651×1199×2364 mm ~3134 kg wet	water / PG-25 · 0.2 µm side-stream · OCP UQD (est) · seal-less N+1 · OCP Deschutes	CDU family VERIFIED
MCDU-60 Motivair by Schneider	2350 kW	L2L · floor	1703 L/min (450 GPM) 800 GPM pri	2.2 bar head	n/p	2499×1600×1222 mm n/p	25% PG · filter n/p · 6″ · BACnet/SNMP/Modbus · class n/p · 2 pumps (N+1)	Brochure VERIFIED
GoCool L2L 1500 Delta	1500 kW (1016–1500)	L2L · floor	1500 L/min (1650 max) 1300 L/min pri	n/p	4–6 °C	1200×1200×2300 mm 1900 kg wet	fluid n/p · 50 µm · 4″ sanitary ferrule · SNMP/Modbus/BACnet · N+1 & N modes	Spec VERIFIED
In-Row 2.4 MW (800 VDC) Delta	2400 kW (2025 launch)	L2L · row	~1.5 L/min/kW	n/p	~4 °C	1500×1200×2286 mm n/p	Press-release figures (no datasheet yet) · 800 VDC pumps, N+1	GoCool spec VERIFIED
RackChiller CDU800 nVent	800 kW	L2L · floor	≤1200 L/min/pump 950 L/min (N+N)	2.7 bar sec	4 K / 6 K	800 (1200) ×2200 mm 1135 kg dry	≤20% PG pri / ≤30% PG sec · 250 µm pri, 50 µm (25 opt) sec · 3″ tri-clamp · SNMP v3/Modbus · N+N VSD	Resource library VENDOR PORTAL
CyberCool CDU Stulz	345–1380 kW	L2L · row	n/p	n/p	~4 °C (FWS 32/TCS 36)	600/900×1200×2090 mm n/p	flexible fluid · 50 µm · sanitary quick-release · Modbus/BACnet/SNMP/HTTP · W32–W+	Product VERIFIED
XCRow2000 Envicool	2000 kW	L2L · row	3000 L/min sec	n/p	n/p	1200×2000×2200 mm n/p	SoluKing PG25/EG25 · dual-pump backup	Vendor site VENDOR PORTAL
Waterless EoR CDU ZutaCore 2-PHASE	1200 / 2000 kW	2-phase · EoR	<0.35 L/min/kW	n/p	n/p	n/p	dielectric refrigerant (R-1233zd class) · active-standby hot-swap · waterless / leak-safe in white space	Announcement VENDOR PORTAL
CoolChip CDU 70 Vertiv	70 kW	L2A · row	70 L/min	n/p	15 °C	2300×600×1200 mm 434 kg wet	water / PG-25 · 50/25 µm · 6-fan N+1 · Modbus RTU+TCP · rejects to room air (no FWS)	Datasheet VERIFIED

04 · Compare

In-rack CDUs

Rack-integrated units (typically 3U–4U) for per-rack or small-pod deployments and faster retrofits. Includes single-phase water CDUs and two-phase (dielectric direct-to-chip / waterless) options — the 2-PHASE tag marks the latter.

Model	Capacity	Form	Secondary flow	dP / head	Approach	Dimensions · weight	Fluid · filter · conn · BMS · class	Links
CHx200 CoolIT Systems	200 kW	4U in-rack	n/p	n/p	n/p	4U · n/p	PG warm-water · N+1 pumps + N+1 PSU · 16 sensors / 4.3″ LCD · leak detect · W17–W+	Product VERIFIED
CHx80 CoolIT Systems	80 kW	4U in-rack	n/p	n/p	n/p	4U · n/p	PG warm-water · N+1 pumps + PSU · 4.3″ LCD + leak detect · ~100 servers / rack · W17–W+	Product VERIFIED
RackChiller CHx CDU nVent Schroff	200 kW+ @ 4 °C	4U in-rack	150 L/min	2.76 bar (40 psi) sec	4 °C	430×950×177 mm 35 kg dry / 41 kg filled	25% PG (OAT) sec, water pri · 100 µm · 1.5″ sanitary tri-clamp · dry-break QD · Webserver/Modbus/SNMP · W4 · 2 pumps N+1 (24 h alt)	Liquid cooling VENDOR PORTAL
Neptune RM100 Lenovo	100 kW	4U L2L (bottom-of-rack)	~87 L/min (HMI)	relief 3.5 bar	n/p (pri 4–11 °C)	4U · n/p	sec DI+inhibitor or OAT PG-25, pri water ≤20% glycol · 50 µm sec · Ethernet/RS485/CAN · 2× 48 VDC pumps N+1 (7-day) · SP 18 °C dew-point reset	Product guide VERIFIED O&M manual VERIFIED
In-Rack CDU Motivair (by Schneider)	80–105 kW	4U (top/bottom of 19″)	n/p	n/p	n/p	483×178×813 mm ~38.6 kg	water-glycol or dielectric · filter n/p · Modbus/BACnet/SNMP · redundant pumps (N+1)	Product VENDOR PORTAL
NeuCool IR150 Accelsius 2-PHASE	150 kW	Rack-integrated (200 mm of an 800 mm enclosure, 42U IT)	n/p	n/p	n/p	800 mm wide · 42U IT · n/p	R-1233zd(E) dielectric (R-515B planned) · 20 µm + PRV · iCDU N+1 hot-swap · SNMP/IPMI/Redfish/DCIM · W27 rated / W45 derated	Product VERIFIED Spec (gated) VENDOR PORTAL
HyperCool In-Rack ZutaCore 2-PHASE	20 – 120 kW (6U up to 100)	19″ 3U / 6U	n/p	3 bar refr / 4.5 bar water	n/p	6U 440×1100×266 mm 90 kg dry (Water HRU)	dielectric refrigerant (zero ODP, low GWP) · 8 L buffer · 1″ tri-clamp · RJ45 TCP/IP · W3 · N+1 pumps · 20 kW air variant = waterless	Solutions VERIFIED 6U datasheet VERIFIED

L2L vs L2A — the deciding question. If the hall already has a chilled-/condenser-water plant, an L2L CDU gives the highest capacity and efficiency. If there is no facility water (a retrofit into an air-cooled room), an L2A CDU lets you run direct-to-chip today — it rejects to room air, so the room's existing CRAH/CRAC must absorb that heat, which caps practical density. Many sites stage L2A first, then move to L2L when the water plant is built.

05 · Sizing

Sizing & installation requirements

How to size a CDU to the rack heat load, the pipe it needs, and the physical install envelope. Figures marked OCP/ASHRAE are published; the rest are typical engineering values — confirm pressure drop and exact limits against the vendor data sheet and a qualified mechanical engineer. Full operating bands, water-quality acceptance criteria and the commissioning procedure live on the CDU Checklist.

1 · Flow & ΔT — the core sizing equation

A loop carries heat per the heat-balance Q = ṁ · c_p · ΔT. Rearranged to volumetric flow for a 25% propylene-glycol secondary fluid (ρ ≈ 1.03 kg/L, c_p ≈ 3.95 kJ/kg·K → c_p,vol ≈ 4.07 kJ/L·K):

flow (L/min) ≈ 14.7 × Q(kW) / ΔT(°C) [PG25; use ≈14.3 for pure water]

At the OCP design point (ΔT ≈ 10 °C) this is ≈ 1.5 L/min per kW. OCP's published acceptable band is 1.25–2.0 L/min/kW for a 7.5–12 °C rise. (OCP cold-plate guidance.)
Worked — rack: a 120 kW rack at 1.5 L/min/kW = 180 L/min through the secondary manifold.
Worked — node: a 9 kW accelerator at 40 → 50 °C (ΔT 10) ≈ 13 L/min at the cold plate.
Tighter ΔT → more flow (bigger pumps/pipe); wider ΔT → less flow but warmer return. Stay within the cold-plate's minimum per-chip flow (chip-vendor spec) and the CDU's dP window.

2 · Pipe sizing (secondary)

Size pipe for a velocity of ~1.5–3 m/s (too slow → fouling/air; too fast → erosion + high dP). Nominal diameters below are at ≈ 2 m/s — always re-check the actual pressure drop over the run.

Secondary flow	≈ Heat @ 1.5 LPM/kW	Bore @ ~2 m/s	Nominal
50 L/min	~33 kW	~23 mm	DN25 (1″)
100 L/min	~67 kW	~33 mm	DN32 (1¼″)
180 L/min	~120 kW	~44 mm	DN40–50 (1½–2″)
400 L/min	~270 kW	~65 mm	DN65 (2½″)
1,500 L/min	~1 MW	~126 mm	DN125 (5″)
3,000 L/min	~2 MW	~178 mm	DN200 (8″)

3 · Heat exchanger & supply temperature (L2L)

Approach (ATD) across the plate HX is typically ~3–5 °C — the secondary supply can't be colder than facility supply + ATD. Rising ATD over time = fouling.
Facility supply temperature sets the achievable secondary temp and your free-cooling story — see the ASHRAE W-class table (W17/W27/W32/W40/W45/W+) on the checklist. Warmer classes (W40/W45) enable chiller-less / economiser operation.
Hold secondary supply ≥ 2–3 °C above room dew point (the CDU's dew-point reset) to avoid condensation on cold plates and pipework.

4 · Physical install envelope — what to confirm

Floor loading for the CDU's filled, operating weight + point load vs the raised-floor / slab rating.
Service clearances front/rear/sides per the manual (door swing, tube routing, pump/filter pull-out space).
Facility water (L2L) at the design flow/temp with isolation valves + strainer — or confirm the L2A room-air heat budget (the CRAH/CRAC must absorb the rejected heat).
Dual power feeds (A/B) with correct breaker rating + earthing for the N+1 PSUs.
BMS / leak-detection integration points (Modbus/BACnet/SNMP/Redfish) and leak-rope routing at base, manifolds and QDs.
Redundancy — N+1 pumps + dual feeds sized so one pump/PSU can carry the full load.

Note. These are sizing first-passes for selection, not a hydraulic design. Confirm flow, pressure drop, NPSH, pipe schedule, fluid spec and structural loading with the vendor data sheet and a qualified mechanical engineer before procurement or installation.

06 · Manuals & docs

Installation, operation & maintenance documentation hub

Manufacturer install guides and O&M manuals are usually delivered with the unit or through an authenticated support portal. Below are the publicly reachable vendor documentation entry points we could verify (HTTP 200) — open the portal and search the exact model for its install / operation / service manual. Login-gated PDFs are intentionally not deep-linked.

CoolIT — Resources & datasheets (CHx family) VERIFIED nVent — Data Solutions resource library VENDOR PORTAL Motivair (by Schneider) — 2026 CDU brochure PDF VERIFIED Boyd — Liquid-cooling CDU family & docs VERIFIED Lenovo — ThinkSystem RM100 CDU (L2A) product guide VERIFIED Stulz — CyberCool CDU product & docs VERIFIED Delta — GoCool L2L 1500 kW specification VERIFIED Vertiv — CoolChip CDU catalog (request datasheet/manual) VERIFIED

Operation & maintenance forms. The site-side companion — operational parameter bands, an installation/inspection/PM checklist, a symptom→cause→action troubleshooting table, and a printable service-record form — lives on the CDU Checklist page.

07 · Standards & next

Governing standards & what this guide adds next

CDU selection, fluid spec and water quality follow published guidance. The deployment checklist and maintenance strategy (next ships) build on these.

ASHRAE TC 9.9 — Emergence & Expansion of Liquid Cooling (white paper PDF) VERIFIED Open Compute Project — Cold Plate / Advanced Cooling VENDOR PORTAL

ASHRAE TC 9.9 defines liquid-cooling water-temperature classes (W17 / W27 / W32 / W40 / W45 / W+ — the number is the maximum supply temperature in °C, with a 2 °C lower limit) and water-quality guidance. OCP adds practical cold-plate guidance (PG25 coolant, ~1.5 LPM/kW at a 10 °C rise, filtration and wetted-material compatibility).

→ CDU Installation, Inspection & Maintenance Checklist (with operational parameters + printable forms) → CDU Mini-BMS — datahall layout, P&ID & live parameters per type → CDU Deep Comparison — field issues, controls, after-sales & TCO (all aspects)

Disclaimer. This guide is an independent, educational reference built from publicly available vendor pages and published standards. Capacities and specs are vendor-stated and may change; figures marked “approx.” are family-level. It is not procurement, safety, or engineering advice — always validate the exact model configuration, fluid spec, and site requirements with the vendor and a qualified mechanical engineer before purchase or installation.

08 · FAQ

Frequently asked questions

Short answers to the questions that come up most when choosing and deploying a CDU. Deeper detail and sources are on the checklist and deep comparison.

What's the difference between an L2L and an L2A CDU?

An L2L (liquid-to-liquid) CDU rejects the chip-side heat to a facility water loop through a plate heat exchanger — highest capacity and efficiency, but it needs a chilled- or condenser-water plant. An L2A (liquid-to-air) CDU rejects straight to room air via an internal coil — no facility water, so it's the retrofit enabler, but the room's CRAH/CRAC must absorb that heat, which caps density. Many sites stage L2A first, then move to L2L when the water plant exists.

How do I size the coolant flow to my rack load?

From the heat balance Q = ṁ·c_p·ΔT, for a 25% propylene-glycol fluid: flow (L/min) ≈ 14.7 × Q(kW) / ΔT(°C). At the OCP design point (ΔT ≈ 10 °C) that's about 1.5 L/min per kW; OCP's published band is 1.25–2.0 L/min/kW for a 7.5–12 °C rise. A 120 kW rack ≈ 180 L/min. Always stay within the cold-plate's minimum per-chip flow and the CDU's dP window. See §05 Sizing.

What coolant and water quality does a cold-plate loop need?

The de-facto direct-to-chip fluid is PG25 (25% propylene glycol) — it balances freeze point, viscosity and biostatic protection far better than plain DI water. The secondary (TCS) loop has published acceptance limits (OCP): pH 8.0–10.5, conductivity < 1,500 µS/cm, chloride < 50 ppm (304SS), hardness < 30 ppm, bacteria < 100 CFU/mL operational, corrosion inhibitor > 100 ppm. The facility (FWS) loop has its own, looser limits. Full tables on the checklist §02.

What filtration does the loop need?

Typical practice: facility/primary 150–500 µm, secondary/TCS main filter 25–50 µm (trending to 25 µm as chip microchannels narrow), and a cold-plate side-stream / polishing filter down to < 5–10 µm (some vendors go to 0.2 µm). Trend the filter differential pressure and replace at the vendor threshold — a clogging filter is a leading cause of rising loop dP.

Why must the supply temperature stay above the room dew point?

If the secondary supply runs below the room dew point, moisture condenses on cold plates, pipes and — worst case — electronics. The CDU's dew-point-aware control holds supply ≥ 2–3 °C above the room dew point (re-setting upward if humidity rises). This is also why warmer ASHRAE water classes (W40/W45) are attractive: they keep you clear of condensation and enable chiller-less free cooling.

What is a quick-disconnect (QD) standard like UQD / UQDB?

OCP's UQD (Universal Quick Disconnect) and UQDB (blind-mate) standardise the coupler dimensions and performance so parts from different makers interoperate on single-phase water/glycol lines. Typical figures: 100 psi working / 300 psi burst, ≥ 5,000 mating cycles, dripless ~0.02–0.07 mL per connect; UQDB adds ±1 mm blind-mate tolerance for rack insertion. Match one QD standard across the whole loop — never mix couplers.

In-row vs in-rack vs facility-scale — which should I pick?

In-rack is fastest to deploy and isolates one rack, but you buy many units (more PM + leak surface). In-row is the density workhorse — fewer larger units feeding several racks, easier to service, but a central single-point-of-failure that makes N+1 essential. Facility/room-scale L2L gives economy of scale across many rows with the biggest footprint to design for redundancy. The TCO trade-off matrix on the deep comparison breaks this down by capex / opex / density / water / retrofit / maintenance.

Is direct-to-chip liquid cooling safe near electronics? What about leaks?

It's mainstream for AI/HPC, but leaks are the most-reported field issue, so the safety case is built in: dripless/blind-mate quick-disconnects, drip trays + secondary containment, leak-detection rope/spot sensors at QDs, manifolds and low points, and an automatic response (alarm → isolate / pump-shutdown or bypass-standby). Two-phase dielectric systems add the advantage that any leak is non-conductive. Field issues, root causes and prevention are catalogued on the deep comparison §01.