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For a single-phase direct-liquid-cooling loop, calculate heat transfer as Q̇ = V̇ × ρ × cp × ΔT. To find the required volumetric flow, rearrange it to V̇ = Q̇ ÷ (ρ × cp × ΔT). In plain terms, the required flow depends on the heat load, the coolant’s properties, and the temperature increase from supply to return. The result is a thermal calculation—not a complete pump, pipe, CDU, or facility-plant design.
Use the heat-balance equation
For a steady, single-phase coolant loop, heat removed per unit time equals the coolant’s mass flow multiplied by its specific heat and temperature rise. Since mass flow is volumetric flow multiplied by density, the equation is:
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Q̇ = V̇ × ρ × cp × ΔT
- Q̇ is cooling capacity or heat-transfer rate.
- V̇ is volumetric flow rate.
- ρ is the coolant density at the relevant operating conditions.
- cp is the coolant specific heat at those conditions.
- ΔT is return temperature minus supply temperature for a coolant that warms as it absorbs heat.
To calculate the flow needed for a known heat load, use V̇ = Q̇ ÷ (ρ × cp × ΔT). ASHRAE presents the underlying hydronic relation as heat rate equal to mass flow multiplied by specific heat and temperature difference; the volumetric form follows by applying fluid density. See ASHRAE Handbook resources and the OCP ACF Reference Design Guidance White Paper.
Calculate flow with water approximations
For standard-condition water, ASHRAE gives density of 1,000 kg/m³ and specific heat of 4.18 kJ/(kg·K). Those values yield these practical approximations:
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| Units | Cooling capacity | Flow rate | Applicability |
|---|---|---|---|
| SI | Q̇ (W) ≈ 4,180 × V̇ (L/s) × ΔT (K) | V̇ (L/s) ≈ Q̇ (W) ÷ [4,180 × ΔT (K)] | Standard-water approximation; not a universal coolant constant. |
| Customary | Q̇ (Btu/h) ≈ 500 × V̇ (gpm) × ΔT (°F) | V̇ (gpm) ≈ Q̇ (Btu/h) ÷ [500 × ΔT (°F)] | Standard-water approximation; not valid unchanged for every coolant. |
Example: 100 kW with a 10 K rise
Using the SI standard-water approximation, a 100 kW load and 10 K return-to-supply rise require approximately 100,000 ÷ (4,180 × 10) = 2.39 L/s, or about 143 L/min. This is arithmetic based on the stated assumptions, not a field test or vendor recommendation. It does not include engineering allowances or establish pump and piping selections.
Establish the heat load to be removed
Use realistic heat-release information for the actual equipment configuration and workload. A power-supply nameplate rating is a safety and regulatory maximum; it does not establish the equipment’s actual power draw during use or its heat release. Manufacturer configuration tools and product heat-release information are more appropriate inputs. ASHRAE states that “The goal of a good datacom facility cooling design is to match cooling capacity to actual heat load.” See ASHRAE Handbook, Chapter 20: Data Centers and Telecommunication Facilities.
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Sum the loads that can occur together on the loop. Identify which heat is carried away by liquid and which may still enter room air. At facility scale, a server or rack flow result is not automatically the required capacity of the whole plant: account for coincident loads, the CDU and heat-exchanger boundaries, heat rejection, operating strategy, and the project’s resilience basis. Equipment requirements vary by configuration.
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Choose a temperature rise the equipment can accept
Use the proposed coolant supply and return temperatures to calculate ΔT as return minus supply. A larger allowable rise reduces the calculated flow for the same heat load, but the chosen temperatures must remain within equipment operating limits and the facility design. Temperature rise can also affect chip temperatures, CDU approach temperatures, heat-rejection choices, and chiller operation; there is no universal ΔT for direct liquid cooling.
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OEM operating envelopes can specify coolant temperature and flow limits, including magnitude, duration, and rate of change. A steady-state heat-balance calculation does not show whether controls can handle transients or keep supply temperature stable when load and cooling change at different rates. ASHRAE discusses those liquid-cooling considerations in Emergence and Expansion of Liquid Cooling in Mainstream Data Centers.
Use the actual coolant and identify the loop
The water factors above do not apply unchanged to glycol mixtures, additives, or dielectric immersion fluids. Use the actual fluid’s density and specific heat at its operating conditions. Glycol and additives reduce water’s specific heat; dielectric immersion fluids can have substantially lower specific heat than water, so more volumetric flow may be needed to carry the same heat under otherwise equal conditions. If the fluid changes phase or its properties change substantially, an enthalpy-based analysis may be more appropriate.
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In many data-center direct-to-chip systems, a CDU heat exchanger separates the facility-water loop from the technology-cooling-system (IT) loop. State which side a flow value describes. The two sides can have different fluids and temperatures, so their flow rates need not match. CDU design also involves fluid separation, isolation, pressure control, and temperature control; the OCP guidance treats these as system considerations.
What the calculated flow does—and does not—size
The heat balance gives a thermal flow requirement for the selected load, coolant, and temperature rise. It does not calculate pipe diameter or pump head. Hydraulic selection also depends on flow velocity, pipe diameter, pressure loss through pipework and fittings, valves, elevation, manifold balancing, CDU pressure drop, and OEM minimum and maximum pressure requirements.
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OCP cautions that its pipe-capacity table is for comparison rather than final design: “Validation by a licensed engineer is recommended, Table 1 provides an estimate of values for comparison only.” ASHRAE likewise notes that equipment flow and pressure-drop requirements vary by configuration and are typically supplied by manufacturers. Do not use a conceptual pipe table or a generic flow-per-kW rule as a final prescription.
Move from thermal calculation to design by reconciling the result with OEM limits, pipe velocity and pressure loss, CDU and heat-exchanger performance, controls, redundancy, water quality, and commissioning measurements. The cited sources do not establish a universal oversizing factor; any project allowance should be identified and justified rather than hidden in the equation.
Why there is no universal flow rate per kW
A flow-per-kW figure is incomplete unless it specifies coolant properties and temperature rise. For the same heat load, changing ΔT changes the required flow; changing from water to a fluid with different density or specific heat changes it too. Compare candidate designs using the actual coolant, allowable temperatures, flow, hydraulic consequences, CDU performance, OEM operating envelope, and heat-rejection method. Higher warm-water operation may enable different heat-rejection choices, but that is a system-level decision, not a result of the flow formula alone.
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