| 1.1 |  What Is a Rear-Door Heat Exchanger?

A rear-door heat exchanger (RDHx) replaces or supplements the rear door of a standard server rack with an air-to-liquid heat exchanger. Hot exhaust air from the servers passes through the RDHx coil, transferring heat to a chilled water loop before exiting into the data hall. Active RDHx units include fans to drive airflow through the coil, achieving cooling capacities of 30-80 kW per rack.

| 1.2 |  Why RDHx Is Compelling

RDHx solutions retrofit directly onto existing racks without changes to raised floors, ceiling plenums, or room airflow design. This makes them one of the fastest paths to upgrading cooling capacity for high-density deployments. For operators managing mixed-density environments -- legacy servers alongside GPU-dense AI clusters -- RDHx provides targeted cooling exactly where it is needed.

| 2.1 |  The Engineering Challenge

Fitting a high-performance air-to-liquid heat exchanger into a rack door form factor is a significant engineering constraint. The available depth is typically 100-150 mm. Within that envelope, the heat exchanger must maximize air-side surface area for heat transfer while maintaining low enough air-side pressure drop that the integrated fans (or server fans) can drive adequate flow. Weight is also a concern -- the door must remain operable.

| 2.2 |  Performance Depends on the Heat Exchanger

The cooling capacity of an RDHx is determined almost entirely by the heat exchanger inside it. Higher heat transfer surface density means more cooling per unit area. Lower air-side pressure drop means the fans consume less power. Better thermal effectiveness means the unit can handle higher inlet air temperatures without losing capacity. Every improvement at the heat exchanger level translates directly to system-level performance.

| 3.1 |  Phasic's Advantage

Phasic designs air-to-liquid heat exchangers that maximize cooling performance within the tight dimensional constraints of rear-door installations. Our compact, high-surface-area designs deliver superior heat rejection while keeping air-side pressure drop and weight within practical limits -- enabling higher rack densities without expanding the data center footprint.