# Coolant distribution unit and separated loops

Canonical: https://semiaifoundry.com/learn/atlas/cdu/
Revision: 2026-10-07
Rights: https://semiaifoundry.com/ai-atlas/licenses.html

Thermal infrastructure · Atlas v1.3.0

# Coolant distribution unit and separated loops

Controls the IT coolant loop and transfers collected heat to another cooling system.

[Explore in Atlas](https://semiaifoundry.com/ai-atlas/#cdu)Rack- or row-scale thermal service module

Learning objectiveTrack a coolant loop as a power boundary.

## The mechanism

A coolant distribution unit sits between the cooling needs of IT equipment and the available facility heat-removal system. In a liquid-to-liquid design, a controlled secondary loop circulates through the equipment while a heat exchanger transfers energy to the primary facility loop. The fluids do not need to mix. Pumps, filters, sensors, and control valves help maintain the required flow, pressure, fluid quality, and temperature. The atlas renders the two loops in separate colors so viewers can trace heat crossing the exchanger while following the actual fluid paths independently. CDU designs vary, including liquid-to-air arrangements. A CDU manages and transfers heat; the final destination still lies elsewhere in the facility cooling chain.

1.  Circulate secondary coolant through the IT equipment.
2.  Transfer its heat across an exchanger to the primary cooling side.
3.  Control the secondary conditions while the facility carries heat onward.

## Science and assumptions

### Sensible heat transport

$$
\dot Q=\dot m c_p(T_{\mathrm{return}}-T_{\mathrm{supply}})
$$

Heat transport equals mass flow times specific heat times the return-to-supply temperature rise.

Symbols and units

Q̇

Heat transported at the selected coolant-loop boundary

W

ṁ

Coolant mass flow

kg/s

c\_p

Coolant specific heat capacity

J/(kg·K)

T\_return

Return-fluid temperature

°C

T\_supply

Supply-fluid temperature

°C

Heat transport increases with mass flow, fluid heat capacity, and the temperature rise across the equipment. This relates device heat load to the thermal circuit that carries it away.

Assumptions Single-phase sensible heat with approximately constant $c_{p}$ and steady flow; real equipment must also satisfy pressure, approach-temperature, and control limits.

## Engineering example

For a known mass flow and temperature rise, Q̇=ṁ$c_{p}$ΔT; the CDU does not make heat disappear, it moves it to a plant boundary.

## Distinctions and tradeoffs

A liquid-to-liquid CDU is not the final heat sink and does not require the two fluid circuits to mix.

Liquid transport supports dense heat loads. Pumps, controls, fluid quality, and facility integration add system requirements.

Accelerators can sustain useful work only while heat is removed. CDU control connects chip cooling to building infrastructure.

## Specifications and interfaces

Illustrated design

Liquid-to-liquid CDU

Other architectures exist.

Separated circuits

Primary and secondary loops

Heat crosses the exchanger without intentional fluid mixing.

Controlled variables

Flow, pressure, temperature

Equipment- and facility-dependent specifications.

### Interfaces

Warm IT coolant Input

Heat collected from equipment

Facility loop Output

Transferred thermal energy for rejection

## Check your understanding

### What crosses between the two separated loops?

1.  Model parameters
2.  Heat
3.  The entire secondary fluid inventory by design

Answer and reasoning

Heat

The heat exchanger transfers thermal energy while maintaining separate fluid paths.

## Evidence and scope

The model excludes pump power, exchanger approach temperature and redundancy.

No current calculator directly models this lesson’s main mechanism. Use the library for related bounds while retaining their stated omissions.

## Sources

-   [Vertiv — Understanding Coolant Distribution Units](https://www.vertiv.com/en-anz/insights/articles/educational-articles/understanding-coolant-distribution-units-cdus-for-liquid-cooling/)
-   [NREL — Data-Center Heat-Balance Analysis](https://www1.eere.energy.gov/buildings/publications/pdfs/rsf/reducing_data_center_loads_large_scale_low_energy_office_building.pdf)

[Published lesson data](https://semiaifoundry.com/ai-atlas/lesson-data/cdu-ba276d9443d0bfb9.json) · [Atlas releases and licensing](https://semiaifoundry.com/ai-atlas/licenses.html)

## Continue exploring

### Foundations

-   [Liquid-cooling coldplate](https://semiaifoundry.com/learn/atlas/coldplate/)

### Related lessons

-   [Integrated accelerator rack: GB200 NVL72 example](https://semiaifoundry.com/learn/atlas/rack/)
-   [GB200 compute tray](https://semiaifoundry.com/learn/atlas/compute-tray/)
-   [Facility cooling and final heat rejection](https://semiaifoundry.com/learn/atlas/cooling-plant/)