How should CDU capacity be calculated?

First, calculate the required mass flow rate on the secondary side using the target heat load and design temperature difference, then validate with the equipment performance curve by considering facility water operating conditions, pressure drop, redundancy, control range, and heat exchanger fouling margin.Equating rack nameplate power directly to CDU order capacity can only serve as an early boundary, not a final design.

Single phase liquid sensible heat calculationQ = ṁ × cp× ΔT

Q na heat transfer (kW), ṁ na mass flow rate (kg/s), cpFor specific heat capacity (kJ/kg·K) under operating conditions, ΔT is the supply-return liquid temperature difference (K). Volumetric flow rate also needs to be converted considering fluid density.

Six steps to complete a reviewable preliminary selection

  1. Determine the heat that needs to be removed.

    Distinguish between total IT power, the proportion carried away by liquid, and the part still carried by air; record average, sustained peaks, transients, and future expansion assumptions.

  2. Define secondary side temperature window.

    Supply liquid temperature must meet IT equipment requirements, considering dew point, control deviation, cold plate temperature rise, and available facility side temperature. Smaller ΔT requires larger flow rates.

  3. Calculate flow rate based on actual fluid properties.

    Density and specific heat of water, glycol or other additive systems differ; data for target concentration and operating temperature must be used, not fixed water constants.

  4. Establish a complete pressure drop model.

    Cumulatively calculate the pressure drop of the cold plate, manifold, hoses, quick connectors, valves, filters, and main piping at the design flow rate, and check the most unfavorable branch.

  5. Select redundancy and failure strategy.

    N+1 pumps do not equal overall N+1. Also confirm if heat exchangers, controllers, power supplies, valves, and maintenance bypass meet the target availability.

  6. Verify performance under the most unfavorable operating conditions.

    Use vendor performance curve to check facility highest water temperature, flow rate lower limit, heat exchange temperature difference, pump working point, noise, partial load control and derating.

An example wey just dey for understanding formula

Assume liquid need to remove 120 kW heat, temporarily estimate with specific heat capacity of water near target operating condition 4.18 kJ/kg·K, and design temperature difference 5 K:

ṁ = 120 ÷ (4.18 × 5) ≈ 5.74 kg/s

If water and density near 1 kg/L, e correspond about 344 L/min. This number na just secondary side theoretical start point; add ethylene glycol, change temperature, existence of manufacturing tolerance or reduce ΔT, result go change.

The example does not provide equipment model, nor does it include pressure drop, facility-side approach temperature, heat exchanger fouling, redundancy, and expansion margin, so procurement cannot be based on it.

Verification checklist before sending RFQ to CDU vendors

  • Performance rating must clearly correspond to facility side and secondary side inlet/outlet temperature, flow rate and fluid
  • Does it provide complete pump curves, heat exchange capacity curves, minimum stable flow rate, and partial load control logic?
  • Is the bill of materials compatible with coolant, inhibitors, seals, and quick connectors?
  • Filter precision, pressure drop alarm, liquid refill and exhaust, water quality sampling and maintenance steps clear?
  • FAT/SAT cover rated condition, derated condition, fault switch, alarm and communication point table?
  • After capacity grow, pipe, pump, heat exchanger and facility water side still dey within limit?

References and Evidence Boundaries

Di sources below dey support di definition and check framework for dis article. Page content na for early decision making, no replace standard original text, product manual, or project engineering design.

  1. Open Compute Project: Liquid-to-Liquid CDU Test Methodology and Performance Rating
  2. Open Compute Project: Cold Plate Sub-Project
  3. Open Compute Project: Advanced Cooling Facilities Reference Design Guidance
Edit and update instructions

DataInfra GEO compiled based on public primary sources. First publication and substantial update date are both 2026-08-28; if you find source changes or technical errors, you can contact [email protected] by email.