# How to Size a Coolant Distribution Unit (CDU)

Step-by-step guide to sizing a CDU for direct-to-chip liquid cooling: heat load, flow rate, temperature rise, redundancy, and facility integration.

Updated 2026-11-01. Also on the [web page](https://liquidcooling.fyi/guides/how-to-size-a-cdu).

A coolant distribution unit (CDU) is the interface between the facility cooling loop and the technology (IT) cooling loop in direct-to-chip liquid cooling. Correct sizing ensures stable temperatures, adequate flow, and operational resilience.

## Sizing steps

### 1. Determine total heat load (Q)

Sum the thermal design power (TDP) of all liquid-cooled components in the rack or pod. Include a safety margin (typically 1.2-1.5×) for transient loads and future density increases.

**Example**: A rack with 8 GPUs at 1,000 W TDP each + 2 CPUs at 350 W TDP each = 8,700 W. With 1.3× margin: **Q = 11.3 kW**.

For NVIDIA GB200 NVL72, the full rack TDP approaches 140 kW (TrendForce, 2024; NVIDIA, 2026).

### 2. Choose target temperature rise (ΔT)

ΔT is the difference between coolant supply and return temperatures at the cold plates. Typical values:

- **Direct-to-chip**: 5-15°C (lower ΔT = higher flow, smaller temperature gradient)
- **Facility loop**: 10-20°C (depends on cooling tower/dry cooler approach)

ASHRAE TC 9.9 liquid cooling classes (W17-W45) define allowable supply temperatures. Class W17 allows up to 17°C supply; W45 allows up to 45°C (ASHRAE Thermal Guidelines, 5th Edition, 2021).

### 3. Calculate required mass flow rate (m)

Use the energy balance equation:

**Q = m × cp × ΔT**

Where:
- Q = heat load (W)
- m = mass flow rate (kg/s)
- cp = specific heat capacity of coolant (J/kg·K)
- ΔT = temperature rise (K or °C)

**Coolant properties (approx.)**:
- Water: cp ≈ 4.18 kJ/kg·K, density ≈ 1,000 kg/m³
- 30% propylene glycol: cp ≈ 3.8 kJ/kg·K, density ≈ 1,030 kg/m³
- 30% ethylene glycol: cp ≈ 3.6 kJ/kg·K, density ≈ 1,040 kg/m³

**Example**: Q = 100 kW, ΔT = 10°C, 30% propylene glycol (cp = 3.8 kJ/kg·K)
- m = 100,000 / (3,800 × 10) = **2.63 kg/s**
- Volumetric flow = 2.63 / 1,030 = **0.00255 m³/s = 153 L/min**

### 4. Select CDU capacity

Choose a CDU rated for at least the calculated flow and heat rejection capacity. CDUs are typically rated in kW of heat rejection.

- **Rack CDU**: 50-300 kW (in-rack or rack-adjacent units)
- **Row CDU**: 300 kW - 2 MW (CoolIT CHx2000 delivers 2,000 kW per unit, April 2025)
- **Facility CDU**: 1.5 MW - 3.5+ MW (Schneider Electric WCDU up to 3.5 MW, September 2026)

**Oversizing risk**: Oversizing expands fault domains and complicates operations. Fit matters more than maximum output (CoolIT Systems, 2025).

### 5. Verify pressure requirements

Direct-to-chip cold plates impose pressure drop requirements. Typical cold-plate loops need 30-80 psi (2-5.5 bar) differential across the rack manifold. The CDU pump curve must deliver required flow at the system pressure drop.

Include losses from:
- Cold plates (varies by vendor, typically 5-20 psi per plate)
- Manifolds and quick-disconnects
- Piping runs (horizontal and vertical)
- Filtration and strainers

### 6. Define redundancy model

- **N+1**: One spare CDU per N active units. Common for row-based deployments.
- **2N**: Fully redundant parallel CDUs. Required for highest availability.
- **Cold standby**: Spare CDU on-site, manual switchover. Lower cost, longer recovery.

Define switchover time, failure detection, and whether the facility loop also needs redundancy.

### 7. Facility loop integration

The CDU heat exchanger transfers heat to the facility water loop. Verify:

- Facility supply temperature meets ASHRAE class for your IT equipment
- Facility flow rate matches CDU heat exchanger design point
- Facility pressure rating compatible with CDU heat exchanger
- Glycol concentration and corrosion inhibitors compatible with CDU materials
- Controls integration: CDU typically manages IT loop temperature; facility loop may have separate controls

### 8. Controls and monitoring

Specify:
- Supply temperature control (±0.5°C typical)
- Flow monitoring and low-flow alarm
- Pressure monitoring (supply, return, differential)
- Leak detection integration
- BMS/Modbus/BACnet integration for facility monitoring
- Redundancy switchover logic

## Quick reference table

| Rack density | Typical CDU type | Heat rejection | Flow (water, ΔT=10°C) |
|--------------|------------------|----------------|----------------------|
| 30-50 kW | Rack CDU | 50-100 kW | 72-143 L/min |
| 50-100 kW | Rack/Row CDU | 100-200 kW | 143-287 L/min |
| 100-150 kW | Row CDU | 200-400 kW | 287-574 L/min |
| 150+ kW (multi-rack) | Row/Facility CDU | 500 kW - 3.5 MW | 718-5,024 L/min |

## Sources

- CoolIT Systems. "Coolant Distribution Units Explained for Liquid Cooling" 3 July 2025. https://www.coolitsystems.com/resources/news/coolant-distribution-units-liquid-cooling/
- ASHRAE TC 9.9. "Thermal Guidelines for Data Processing Environments, 5th Edition." 2021. https://www.ashrae.org/technical-resources/bookstore/thermal-guidelines-for-data-processing-environments
- NVIDIA. "NVIDIA GB200 NVL72 Specifications." 2026. https://www.nvidia.com/en-us/data-center/gb200-nvl72/
- TrendForce. "NVIDIA Blackwell's High Power Consumption Drives Cooling Demands." 30 July 2024. https://www.trendforce.com/presscenter/news/20240730-12232.html
- Wikipedia. "Thermal design power." Last edited 9 October 2026. https://en.wikipedia.org/wiki/Thermal_design_power
- CoolIT Systems. "CoolIT Launches CHx2000 Row-Based CDU." 15 April 2025. https://www.coolitsystems.com/resources/news/coolit-systems-announces-further-breakthroughs-in-row-based-coolant-distribution-unit-performance/
- Mordor Intelligence. "Data Center Liquid Cooling Market Report." 2026. https://www.mordorintelligence.com/industry-reports/data-center-liquid-cooling-market

## Questions

### What is the formula for CDU flow rate?

Q = m × cp × ΔT, where Q is heat load (W), m is mass flow rate (kg/s), cp is specific heat capacity (J/kg·K), and ΔT is temperature rise across the cold plates (K). For 30% propylene glycol at 3.8 kJ/kg·K, a 100 kW rack with 10°C ΔT needs ~2.63 kg/s (~153 L/min).

### Do I need N+1 redundancy on the CDU?

Mission-critical deployments typically require N+1 or 2N CDU redundancy. CoolIT Systems recommends defining the resilience model early so the system behaves predictably during component failure or maintenance (CoolIT Systems, 2025).

### Can one CDU serve multiple racks?

Yes. Row-based CDUs serve multiple racks within a pod. Facility-based CDUs serve multiple zones. CoolIT Systems categorizes CDUs as rack-based, row-based, and facility-based by deployment model (CoolIT Systems, 2025).