Hot aisle and cold aisle containment

Hot aisle and cold aisle containment is a data hall layout that encloses one of the two aisles between rows of racks, to keep supply air apart from return air. Racks are set face to face and back to back: the fronts draw from the cold aisle, the rears exhaust into the hot aisle. The layout is usually credited to Robert Sullivan at IBM in the early 1990s. The enclosure uses end-of-row doors and a roof, or chimneys rising to the return plenum. Without it, hot air recirculates over the racks and the operator compensates by blowing colder and harder. With it, the supply setpoint moves up toward the top of the ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) range and PUE (Power Usage Effectiveness) improves by several hundredths. A risk engineer reads the trade-off on the fire side. An enclosed hot aisle runs at 40 to 45 °C, air velocities dilute smoke, and a spot detector on the room ceiling sees an ignition too late, which is why VESDA (Very Early Smoke Detection Apparatus) sampling points go inside the contained aisle itself. The roof also shields the aisle from sprinklers and from the inert gas discharge. Carriers then expect panels that drop away under heat, or a nozzle network dedicated to the aisle, and a concentration calculation based on the volume actually enclosed. The survey asks for the containment type, the fire rating of the soft curtains and whether the doors are tied to the alarm. Cold aisle containment carries one more drawback: it leaves the rest of the room at return air temperature, which weighs on equipment installed outside the rows.