The APC Rack Air Removal Unit (ARU) SX is a rack mounted fan system for the NetShelter SX and VX enclosures; designed to draw in exhaust air from rack mounted IT equipment and allow the hot air to be properly managed as part of a total cooling system. The ARU SX consists of a fan box and an adjustable frame; it mounts to the rear of the enclosure, replacing the rear doors, and discharges upwards into a ducting kit. This air removal unit can move a maximum of 2000 CFM of air and is rated up to 23 kW of heat removal. 
APC Rack ARU Airflow Diagram
Rack Compatibility The ARU SX is designed for use with NetShelter SX and VX enclosures. No special adapters or accessories are required to mount on either rack type. The user must determine the width of rack that the ARU will be utilized with, 600 mm or 750 mm and select the appropriate unit - ACF400 or ACF402 respectively. No other selection for ARU SX is required; both models are suitable for mounting on 42U, 47U and 48U racks. The adjustable mounting frame extends as needed and can be adjusted in the field to adapt to the current installation. Ducting Kits The ducting kit for the ARU SX mounts above the unit, attaches to the adjustable mounting frame and allows the fan box to swing open without interfering with the duct kit itself. The kit consists of a ceiling tile adapter and two (2) ducting tubes. The ceiling tile adapter is available in either 24" or 600 mm sizes; the user must select the appropriate kit for their needs - ACF126 or ACF127 respectively. The ducting kit is a ship loose item, purchased separately; however it is required for installations where the ceiling height is less than 12 feet (3.66m) and recommended for all ARU SX installations regardless of ceiling height. Modeling done using computational fluid dynamics (CFD) software shows that the high velocity air jets exiting the ARU will impinge on the ceiling and circulate downward towards the IT equipment rather than forming a layer of hot air at the ceiling; this happens even with relatively tall ceilings. This often makes it easier for the hot exhaust air to be drawn into the IT equipment inlets, the exact opposite of the intended results. Depending on the layout of the space, ceiling height, ARU airflows and other factors in the room, using an ARU without the duct kit could potentially be less beneficial than not using an ARU at all. The images below show the air patterns and temperature profiles of a rack and ARU with and without a ducting kit. 
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| Airflows ducted vs. unducted | Temperature profiles ducted vs. unducted |
Rooms that are extremely tall, well above the 14 foot ceilings studied here, might avoid this situation. With sufficiently tall ceilings the exhaust air will eventually lose momentum and not reflect off of the ceiling. Additional techniques to mitigate the recirculation of hot air may be possible - properly situated return air ducts at the ceiling level to scavenge the hot air, or open grates above the racks to allow the air jets to travel into a drop ceiling plenum for return to a CRAC. These methods draw off the warm air or keep it separated from the cool air in the space without the use of a ducting kit.
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