A coolant distribution unit, or CDU, is the system that manages and delivers liquid coolant to the hardware being cooled — and it has quietly become one of the most important components in modern liquid cooling. As data centers move to direct-to-chip cooling for AI hardware, the CDU is what separates the clean technology coolant from the facility’s water and delivers it at exactly the right flow, temperature, and pressure. This guide explains what a CDU does, how it works, its types and components, and why it matters.
What is a coolant distribution unit?
A coolant distribution unit (CDU) is the bridge between a facility’s cooling system and its liquid-cooled equipment. It performs two essential jobs: it isolates the clean coolant that flows to the equipment from the facility’s water supply, and it controls and distributes that coolant — regulating its flow, temperature, and pressure. In a liquid-cooled data center, the CDU is what makes it possible to deliver precise, reliable cooling to sensitive hardware such as servers and GPUs.
How does a CDU work?
A CDU links two separate loops through a heat exchanger. The primary loop is the facility side, which carries heat away to chillers or another heat-rejection system. The secondary loop is the technology side, which circulates clean coolant directly to the equipment. Inside the CDU, a heat exchanger transfers heat from the secondary loop into the primary loop, so the two never mix.
Around that heat exchanger, pumps circulate the secondary coolant, while control valves, sensors, and a control system hold the secondary loop at the right temperature, flow, and pressure for the equipment. The result is a controlled, clean closed loop serving the hardware, decoupled from whatever is happening on the facility side.
Types of CDU
Liquid-to-liquid (L2L)
An L2L CDU transfers heat from the technology loop into a facility water loop, which then carries it to chillers or a cooling tower. L2L units suit larger deployments where facility water is available, and scale to high capacities.
Liquid-to-air (L2A)
An L2A CDU rejects the heat directly to room air instead of facility water, making it self-contained. This makes it easier to deploy where no facility water loop exists — for example in an existing air-cooled room — usually at a lower capacity than an L2L unit.
CDUs also vary in form factor, from in-rack units that fit inside a server rack, to larger in-row or standalone units that serve multiple racks.
Main components of a CDU
- Heat exchanger — transfers heat between the primary and secondary loops while keeping them separate.
- Pumps — circulate the secondary coolant, often with redundancy for reliability.
- Control valves and sensors — regulate and monitor flow, temperature, and pressure.
- Control system — manages the unit, holds setpoints, and reports status.
- Reservoir and filtration — hold and clean the coolant, protecting the loop and the equipment.
- Manifolds and connectors — distribute coolant to the equipment.
Why CDUs matter
The CDU earns its description as the heart of liquid cooling because of what it protects and enables. By isolating the technology loop, it keeps the coolant reaching sensitive hardware clean and controlled, regardless of the quality or temperature swings of the facility water. By precisely managing flow, temperature, and pressure — often with redundant pumps for reliability — it gives the equipment exactly the cooling it needs, continuously. And by sitting between the facility and the hardware, it lets the two be designed and operated independently.
The CDU in the data center cooling chain
In a liquid-cooled facility, the CDU sits at the center of the chain. A chiller or facility system produces the primary cooling; the CDU transfers that cooling into a clean secondary loop; and that loop carries coolant to cold plates on the processors or to rear-door heat exchangers. This is the architecture behind modern data center cooling for AI and high-density workloads — and the CDU is the component that ties it all together. Some designs even use warm-water cooling, where a relatively warm secondary loop is still cold enough to cool the chips while improving overall efficiency.
Coolant distribution systems from Eldrotec
Eldrotec designs and manufactures the custom hydronics and coolant-distribution systems that liquid cooling depends on — pumps, heat exchangers, manifolds, piping, and connectors, engineered into integrated assemblies and matched to each application’s heat load and conditions. Combined with Eldrotec’s chillers and closed-loop expertise, and built to AS9100 and MIL-STD standards with thermal modelling, these systems deliver clean, precisely controlled coolant for high-density, edge, and custom liquid-cooling needs. Contact the engineering team to discuss a coolant distribution solution for your application.
Frequently Asked Questions
What is a coolant distribution unit (CDU)?
A coolant distribution unit (CDU) is the system that manages and delivers liquid coolant to cooled equipment, such as servers in a data center. It isolates the clean technology coolant loop from the facility’s water, and controls the coolant’s flow, temperature, and pressure. The CDU is the link between the facility cooling system and the liquid-cooled hardware.
How does a CDU work?
A CDU connects two loops through a heat exchanger. The facility (primary) loop carries heat away to chillers or another rejection system, while the technology (secondary) loop circulates clean coolant to the equipment. Heat passes from the secondary loop to the primary through the heat exchanger, and pumps, valves, and controls keep the secondary loop at the right flow, temperature, and pressure.
What is the difference between liquid-to-liquid and liquid-to-air CDUs?
A liquid-to-liquid (L2L) CDU transfers heat from the technology loop into a facility water loop, suiting larger deployments with facility water available. A liquid-to-air (L2A) CDU rejects the heat to room air instead, making it self-contained and easier to deploy where no facility water loop exists, though typically at lower capacity.
What are the main components of a CDU?
A CDU typically contains a heat exchanger to transfer heat between loops, pumps (often redundant) to circulate the coolant, control valves and sensors to regulate flow, temperature, and pressure, a reservoir, filtration to keep the coolant clean, and a control system that manages and monitors the unit.
Why do AI data centers need CDUs?
AI hardware uses direct-to-chip liquid cooling, which requires clean coolant delivered to each cold plate at a precise temperature, flow, and pressure. A CDU provides exactly this, while isolating the sensitive technology loop from facility water. As AI rack densities rise, CDUs have become essential infrastructure for liquid-cooled data centers.
What is the difference between primary and secondary loops?
The primary loop is the facility side that ultimately rejects heat — for example to chillers or a cooling tower. The secondary loop is the technology side that circulates clean coolant directly to the equipment. The CDU’s heat exchanger links the two while keeping them separate, so the equipment receives controlled, clean coolant.