Updated 2026-10-08
Glossary of liquid cooling terms
Definitions are self-contained — written to be quotable. Terms alphabetized; each term is anchorable (/glossary#term-slug).
| Term | Definition |
|---|---|
| 1.1× rule | The room-cooling sizing rule for liquid-to-air cabinets: room AC capacity ≈ 1.1 × IT load, the extra 10% covering pumps, losses and commissioning margin. |
| Adiabatic assist | Boosting a dry cooler in hot weather by pre-cooling incoming air with water evaporation — recovers capacity at the cost of water consumption and treatment. |
| Approach temperature | The temperature difference between the coolant and the air (or facility water) rejecting its heat. Liquid-to-air capacity scales with this difference — smaller approach means less cooling capacity. |
| ASHRAE W-classes | Coolant temperature classes for liquid-cooled IT: W17 through W45, where the number is the maximum facility supply temperature in °C. Higher classes enable chiller-free or high-efficiency operation if equipment supports them. |
| Blind-mate | A connection design where components engage automatically as they slide together — servers mate to the rack manifold by being pushed in, with no hand-plugged couplings. Specified for racks by OCP’s ORv3 blind-mate manifold specification. |
| CDU | Coolant Distribution Unit — the component that pumps the server coolant loop and transfers its heat to a final sink (facility water or room air). Contains pumps, heat exchanger, filter and controls. Comes in in-rack, in-row and facility sizes. |
| Capture ratio | The share of server heat a cooling architecture removes through liquid: typically 60-80% for direct-to-chip, near 100% for immersion. Drives flow, CDU capacity and the residual air load. |
| Cold plate | A metal block with internal fluid channels bolted onto a CPU or GPU so heat conducts directly into the circulating coolant. The defining component of direct-to-chip cooling. |
| Commissioning | The staged acceptance sequence for a liquid loop: dry pressure test, fill and purge, flow and ΔT verification against design, pressure survey, dew-point check, leak-system test, load ramp, baseline snapshot. |
| Condensation margin | The gap between the coolest wetted surface (coolant supply) and the room dew point. Negative margin means water forms on live hardware — the reason coolant temperature has a floor, not just a ceiling. |
| COP | Coefficient of Performance — cooling delivered divided by electricity consumed by the refrigeration unit. A COP-3 chiller moves 3 kW of heat per 1 kW drawn; the bridge between cooling tons and electrical loads. |
| Cooling tower | A heat rejection device that evaporates water to chill its loop toward the ambient wet-bulb temperature. Efficient in dry climates; output floors at wet-bulb + roughly 5-8 K of tower approach. |
| Derating | The reduction of a cooling system’s capacity as operating conditions worsen — most importantly rising ambient temperature. Capacity-vs-ambient curves state it; very few vendors publish them. |
| Dew point | The temperature at which air becomes saturated and water condenses. In liquid cooling it bounds how cold coolant supply may run without sweating pipes and plates in a humid room. |
| DLC | Direct Liquid Cooling — liquid cooling where coolant touches a plate on the chip (as opposed to immersing whole servers). The mainstream architecture; captures 60-80% of server heat. |
| Dry cooler | A heat rejection device that exchanges heat with ambient air without evaporating water. Output floors at dry-bulb + roughly 8-12 K of approach; no water use, worse performance in hot climates. |
| ΔT (delta-T) | A temperature difference. In loop design, the supply-return coolant difference that sets flow for a given heat load (flow = kW × 60 / (4.186 × ΔT)); in rooms, the air temperature rise across equipment. |
| Fluid analysis | Periodic coolant sampling for pH, inhibitor level, conductivity and biological growth — the early-warning system for corrosion and biofouling in water-glycol loops, typically semi-annual. |
| Floor loading | Structural pressure under equipment: filled mass × 9.81 ÷ footprint, in kN/m². Office floors typically design for 2.5-3.5; computer-room and industrial slabs for 7-12+. Concentrated, not averaged, load governs. |
| Free cooling | Rejecting data center heat without mechanical refrigeration, using ambient air (dry coolers) or evaporation (towers). Viable hours are set by the site’s dry-bulb/wet-bulb statistics versus the required coolant temperature. |
| Friction factor (Darcy) | The dimensionless resistance coefficient in the Darcy–Weisbach pressure-drop equation — about 0.03-0.04 for smooth hose at typical coolant flows. Enters as f × (L/D) × ρv²/2. |
| FWS | Facility Water System — the building’s water loop that ultimately carries heat away. The CDU isolates the clean server loop (TCS) from it. |
| Heat capture split | How a rack’s heat divides between liquid and air: P_liquid = capture × P_IT. The air share (20-40% in DLC) still needs cabinet airflow and room AC. |
| Heat exchanger | A device transferring heat between two fluids without mixing them — the core of every CDU (technology loop ↔ facility side) and of dry coolers. |
| Immersion cooling | Submerging entire servers in a dielectric fluid (oil or fluorinated chemistry). Captures nearly all heat but changes hardware, service and fluid management compared to direct-to-chip. |
| Inhibitors | Chemical additives in water-glycol coolant that suppress corrosion and biological growth. Their depletion — tracked by fluid analysis — is the main driver of fluid replacement intervals. |
| kN/m² | Kilonewtons per square meter — the unit of structural floor load. 1 kN/m² ≈ 102 kg/m². A 1,200 kg rack on a 600 × 1,200 mm footprint prints about 16.4 kN/m². |
| kW vs kVA | Real power vs apparent power in AC systems; they differ by the power factor (< 1). Cooling and IT budgets are kW conversations; UPS, feeds and switchgear are kVA — convert explicitly, early. |
| L2A / L2L | Liquid-to-Air vs Liquid-to-Liquid — how a CDU rejects heat. L2A blows it into the room (self-contained, capacity derates with room temperature); L2L exchanges with facility water (higher capacity, needs building plumbing). |
| Manifold | The rack-level supply/return spine distributing coolant to each server position. Good manifolds are blind-mate so servers connect on insertion. |
| Micro data center | A self-contained compute deployment (one cabinet to a small row) in a room not built as a data center — offices, clinics, wiring closets, light-industrial spaces. |
| N+1 | Redundancy scheme with one spare unit beyond the required count — e.g., two pumps where one suffices. In cooling, apply it to pumps, fans and AC units; the failure question belongs in every RFQ. |
| Nameplate capacity | The rating on the brochure, quoted at standard conditions (typically 25 °C air / 45 °C coolant). Actual capacity at your room temperature is lower — demand the derating curve. |
| OCP | Open Compute Project — the industry body whose specifications (UQD couplings, ORv3 racks and manifolds) make liquid cooling parts interoperable across vendors. |
| ORv3 | Open Rack v3 — OCP’s current rack standard family, including the blind-mate manifold specification (Rev 1.0, 2024) that lets compliant servers and racks interoperate. |
| PFAS | Per- and polyfluoroalkyl substances — the regulated chemical family behind fluorinated coolants. Global tightening and 3M’s 2025 manufacturing exit pushed buyers toward water-glycol and defined the modern fluid decision. |
| PG25 | A coolant of 75% water and 25% propylene glycol — the default fluid for direct-to-chip systems. Non-flammable, non-hazardous to ship, PFAS-free; needs corrosion inhibitors and periodic checks. |
| Pressure drop (ΔP) | The head a loop consumes as fluid moves: friction in hoses (Darcy–Weisbach), fittings and couplings, in kPa. If total ΔP exceeds the CDU’s available head, operating flow — and capacity — fall. |
| PUE | Power Usage Effectiveness — total facility energy ÷ IT energy. Air-cooled sites typically run ~1.5; liquid-cooled data centers reach 1.1-1.2. Assumes facility-scale cooling; a single cabinet in a room depends on that room’s AC. |
| Pump head | The pressure a CDU’s pump can supply, in kPa, as a function of flow. The loop’s resistance curve intersecting the pump curve sets real operating flow — read the intersection, not the peak. |
| RDHx | Rear-Door Heat Exchanger — a water-cooled door replacing a rack’s rear panel, capturing exhaust air heat into a liquid loop without touching the servers inside. An air-to-liquid retrofit technology. |
| Refrigeration ton | A cooling unit: 1 ton = 3.517 kW = 12,000 BTU/h — historically the heat to melt one short ton of ice per day. Building HVAC still quotes in tons; divide kW by 3.517 to speak it. |
| Specific heat | Heat per kilogram per degree — water’s 4.186 kJ/(kg·K) is the constant behind the flow formula. Glycol lowers it ~10%, which is why pure-water math slightly undersizes PG25 loops. |
| SS 726:2026 | Singapore’s national standard for liquid cooling in tropical data centers — the first of its kind. Covers direct-to-chip systems inside data centers; excludes indirect approaches and standalone edge cabinets. |
| TCS | Technology Cooling System — the clean, closed coolant loop circulating through server cold plates, kept separate from facility water by the CDU. |
| TDP | Thermal Design Power — a chip’s worst-case heat output rating, in watts. The unit of rack power budgeting: sum GPUs and CPUs, add ~20% overhead, multiply by node count. |
| UQD | Universal Quick Disconnect — OCP-standardized dripless coolant couplings. Compliance means different vendors’ parts connect and seal; the basis of open, mix-and-match liquid racks. |
| UQD sizes | The UQD family from UQD04 to UQD40, where the number ≈ nominal flow in L/min. Single plates run UQD04-06, accelerator nodes 08-12, rack and CDU headers 32-40. |
| Velocity (coolant) | Flow speed in m/s = flow ÷ pipe area. Enters pressure drop squared; above roughly 3 m/s, erosion and noise make stepping up the diameter cheaper than fighting physics. |
| Wet-bulb temperature | The lowest temperature achievable by evaporative cooling at a site’s humidity. High wet-bulb (tropics) disables free cooling and forces mechanical refrigeration into the chain. |
| Wet-bulb, design | The wet-bulb exceeded only ~1% of hours at a site — the value free-cooling designs must survive. Tropical coastal sites design at 27-30 °C, Central Europe at 20-22 °C. |