What is the difference between a CRAC and a CRAH unit?

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.