"U" is a standard unit indicating the height of the computer server. Each "U" is 1.75 in. A 5 U server is 8.75 in. tall, therefore eight 5 Us fill a 42 U rack. Like most electronic devices, the processing power and space efficiency of the servers has increased considerably in the last several years. As the servers have become more compact and efficient, more servers can fit into the rack or cabinet rack. A casual glance at Figure 2 in ASHRAE's latest publication on data centers1 would give the impression that the rate of increase heat load per square foot of data equipment appears to be leveling off in 2005.
On closer scrutiny, the vertical scale is logarithmic, so that a straight line with a positive slope reflects an increasing rate of growth. A 42 U cabinet installed five years ago with three U servers has a total cooling load of 3 kW to 4 kW, about one ton in six sq ft. Today, the same cabinet can be filled with 7 U blade servers having a total power consumption over 20 kW, or more than 5.5 tons of cooling for the same six sq ft.