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. CRAHs dominate in large facilities because central chillers with economizers are more efficient at scale, while CRACs suit smaller sites without chilled-water loops. 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. Air cooling with CRAC/CRAH units handles rack densities up to roughly 20-40 kW; beyond that, liquid cooling becomes necessary.