Abstract: A data center having multiple rows of computer racks for holding heat-generating computer equipment is cooled by a liquid overfeed air conditioning unit that provides supply air at a temperature of about forty degrees Fahrenheit (40° F. or lower). The air conditioning unit is positioned outside the data center. Ducting from the heat exchanger outlet of the unit is connected to supply registers mounted in aisles that are provided between the rows of computer racks so that very cold supply air enters the space between the racks. Each computer rack includes vertically spaced compartments that house computer components to be cooled. At least one circulation fan is associated with each compartment and causes supply air to flow through the compartment to cool the components. The supply air gradually warms as it flows through successive compartments and is approximately 72° Fahrenheit when it reaches a return register in the ceiling of the data center.