FAQs - Cooling Infrastructure Services

A CRAC (computer room air conditioner) is a self-contained unit with its own compressor and refrigerant circuit, which rejects heat to an outdoor condenser or dry cooler. A CRAH (computer room air handler) has no compressor. The fans in a CRAH blow air across a coil fed with chilled water from a central chiller plant. CRACs suit server rooms and sites without chilled water, while CRAHs are common in larger data centres, where a central chiller plant is more efficient and can use free cooling when it is cold outside.

Direct liquid-to-chip cooling is also called direct-to-chip cooling. It is a process whereby the system pumps coolant through cold plates mounted on the hottest components, such as CPUs and GPUs. A coolant distribution unit (CDU) circulates the coolant and passes the heat to the facility water system. Because it removes most heat directly at the source (IT servers), it is used for high-density AI and HPC racks. The remaining heat from other components still needs some air cooling.

Air cooling with good hot-aisle/cold-aisle containment works well for most racks, typically up to around 20–30 kW each. Above that, as with GPU servers for AI workloads, air struggles to remove the heat efficiently, and liquid cooling becomes a more practical option. Liquid cooling includes rear-door heat exchangers, direct-to-chip cold plates and immersion cooling, where servers sit in tanks of non-conductive fluid. Many sites run a hybrid, keeping air cooling for standard racks and adding liquid cooling only where densities demand it.

A liquid-cooled rear door, also known as a rear-door heat exchanger (RDHx), replaces the back door on a server rack or server cabinet with a water-cooled coil. Hot exhaust air from the servers passes through the coil and leaves the cabinet at or near room temperature, removing heat before it reaches the room. Passive doors rely on the servers’ own fans, while active doors add fans to handle higher rack loads. They are a popular way to add higher-density racks to an existing air-cooled room.

CRACs, CRAHs and liquid cooling systems require regular planned preventative maintenance. For CRACs and CRAHs, this includes checking filters, fans, coils, and compressors, plus checks for F-gas leaks, where the refrigerant charge is 5 tonnes CO₂ equivalent or more. CRAHs and chilled water systems also need valve, pump, and water-quality checks. Liquid cooling adds coolant testing and filtration, pump and CDU servicing, and leak detection checks, because poor water quality can block cold plates and coils. We can tailor our cooling maintenance contracts to suit your site, with emergency response times from the next working day to within 4 clock hours.