There are four viable ways to get heat out of a modern rack, and the right one is decided almost entirely by how many kilowatts that rack draws. This page sets out where each path works, where it stops, and what it costs to operate.
Rear door heat exchange
A water coil replaces the rear door of the cabinet. Server fans push exhaust through it and the air returns to the room at or below ambient. Passive doors carry roughly 45 kW; fan-assisted doors reach 90 kW. Nothing inside the server changes, which makes this the least disruptive option available and usually the fastest to approve.
The limit is airflow. Above 90 kW the volume of air needed to carry the heat to the door exceeds what the chassis can move, and no coil design solves that.
Direct-to-chip cold plates
A microchannel plate mounts directly on the CPU or GPU package and takes that heat straight to water. Because the capture happens at the source, 75 to 95 % of rack heat never enters the air path at all. Racks at 120 to 160 kW are routine.
The remaining 5 to 25 % — memory, drives, VRMs, network — still needs an air system, which is why direct-to-chip reduces the CRAH plant rather than eliminating it.
Single-phase immersion
The whole board is submerged in a dielectric fluid. Heat capture is total, every server fan disappears, and the partial PUE lands near 1.04. It is the highest-density and highest-efficiency option we sell.
It also changes how your team works. Boards come out of the tank wet, some components need fluid-compatibility qualification, and the service procedure is genuinely different. Sites that plan for that succeed with it.
Coolant distribution
Every one of the paths above needs a CDU between it and the building. The CDU isolates the two loops so facility water chemistry never reaches a 0.3 mm channel, holds secondary supply temperature to ±0.5 K, and keeps the technology loop inside the pressure the cold plates are rated for.
Choosing between them
Per-rack power decides it. Below 25 kW a passive door is enough. To 55 kW, an active door. To 120 kW, cold plates. Above that, immersion. The selection wizard on our home page runs these thresholds against our actual catalogue and returns specific model numbers.