Written by: Kyle Geib, Application Engineer Manager at Motivair by Schneider Electric
As data centers deploy liquid cooling systems to support high-density AI workloads, operators need to decide on coolant distribution unit (CDU) placement. The main question is whether to deploy in-rack CDUs or floor-mounted CDUs. Making a choice comes down to the specific requirements of the liquid cooling architecture in each implementation.
The CDU acts as the hub of the cooling infrastructure, circulating liquid from high-density hardware in a closed loop to dissipate heat. CDUs have become critical components in AI deployments as racks start to reach densities of 200+ kW.
In-rack units eliminate external piping runs but take up more rack space that could be used for IT equipment. Conversely, they free up floor space for IT hardware. Floor-mounted CDUs support multiple rack loops, simplifying service and maintenance.
What is the difference between in-rack and floor-mounted CDU?
In-rack CDUs, as the name implies, are mounted directly in a server rack, supporting only the devices on that specific rack. Hoses connect the CDU to an in-rack manifold that feeds lines into the IT servers in a self-contained, rack-based architecture.
In-rack units naturally have smaller pumps and heat exchangers than their floor-mounted counterparts, which are mounted on the floor and occupy about the same space as a single rack. Floor-mounted CDUs are designed to support larger deployments – typically a group of high-density racks – so they require more pipes, hoses, and manifolds for heat dissipation.
Should you use in-rack or floor-mounted CDU?
CDU deployment options boil down to the liquid cooling architecture. In-rack CDUs make sense for standalone high-density racks. Of course, that changes when a group of racks requires liquid cooling, in which case a floor-mounted unit may be a more practical and economical option. As an example, implementations of NVIDIA’s NVL platform for AI infrastructure typically involve four to eight racks, which would mean deploying a floor-mounted CDU.
Designed for singular racks, in-rack CDUs bring cooling directly to the source of heat, enabling precise thermal control and the stability required for ultra-high-density AI and HPC workloads.
This design allows customers to scale efficiently while minimizing distribution losses and aligning investment to demand.
Floor-mount CDUs centralize cooling across multiple racks, delivering controlled performance and easier serviceability in existing environments. While adding longer hose runs, more underfloor manifolds, and external equipment can increase deployment complexity, that complexity becomes much easier to manage with proper planning, experience, and execution. This makes floor-mounted CDUs a flexible path to scale liquid cooling over time.
Another consideration deals with the amount of heat generated by the AI equipment in use. Centralized, floor-mounted CDUs are the better choice for loads exceeding 300 kW, which generate higher heat concentrations. Rack-mounted CDUs typically handle loads of 100 to 300 kW.
What is the best CDU placement for high-density racks?
Some operators prefer to place the units at the end of row for maximum efficiency, but others opt to raise walls between floor-mounted CDUs and the white space. With the latter, CDUs are placed in a mechanical room to save space on the data center floor or keep IT separate from mechanical systems. A third option involves a combination of floor-mounted CDUs deployed in white spaces with units in separate areas for N+1 redundancy.
What is the cost comparison between floor-mounted vs. rack-mounted CDUs?
Whether floor-mounted or in-rack CDUs are more cost-effective depends on the deployment. Floor-mounted units share infrastructure, but add costs to run piping overhead or under the floor to the equipment. In-rack units moderate infrastructure costs but add service complexity – and there are only so many in-rack units you can deploy before the floor-mounted option becomes more economical.
Flexible by design: Choose the CDU that fits your strategy
Motivair by Schneider Electric supports both in-rack and floor-mounted CDU options along with the recently introduced Motivair MCDU-70 CDU series. Designed for reliability and flexibility, CDUs available from Motivair offer multiple models and custom configurations that scale from 105kW up to 2.5MW per unit, seamlessly integrating into floor-mounted and in-rack environments.
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