CRAC / CRAH units

CRAC (computer room air conditioner) and CRAH (computer room air handler) units are the workhorses of data center air cooling. A CRAC contains its own refrigerant compressor and works like a large direct-expansion air conditioner; a CRAH has no compressor and instead uses chilled water from a central chiller plant flowing through cooling coils. CRAC vs CRAH in practice: CRAHs dominate in large facilities because central chillers with economizers are more efficient at scale and let operators run water-side free cooling in cold climates, while CRACs suit smaller sites, edge rooms and buildings without a chilled-water loop, at the cost of a compressor in every unit and a higher partial-load PUE. Units are placed around the perimeter of data halls or in galleries, pushing cold air under raised floors or through containment aisles, typically at supply temperatures of 18–27 °C per ASHRAE guidelines; variable-speed fans and hot/cold aisle containment are the usual efficiency upgrades. Air cooling with CRAC/CRAH units handles rack densities up to roughly 20–40 kW; beyond that, rear-door heat exchangers and then direct-to-chip or immersion liquid cooling become necessary, which is why AI halls are designed around CDUs rather than CRAH capacity.