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Very good question and observation. As each data center layout is a little different, it is recommended with the Hot Aisle/Cold Aisle rack arrangement to put the CRAC air units at both ends of the hot aisle. There are two situations to consider:
a) If the maximum heat load is less than the cooling load of all the CRAC units at the end of the Hot Aisles, then alternate CRAC units will and can act as the redundant cooling unit. Obviously, the airflow for each row may not be consistent. As will be noted in b) below, the use of a common plenum can help eliminate mixing of hot and cold air.
b) If the maximum heat load equals the cooling load of all the CRAC units at the end of the Hot Aisles, then space and location for the redundant units needs to determined. In this case, it would be recommended to add the redundant unit next to a unit at the end of a row and center the two units on the middle of the Hot Aisle (assumes the units are all 100% front access). This will then suggest that you rotate for equal wear the two units paired together so that one is available as a redundant unit. This however presents a problem in that if a unit fails and is not paired with a redundant unit, then your room balance could be off. To improve on this configuration, you could duct back to the CRAC units from either the complete Hot Aisle or from the center of the Hot Aisle using a common plenum (such as the space above a drop ceiling), to prevent the mixing of hot air across a Cold Aisle. With some supplemental cooling systems, the modules can be oriented to provide redundancy. This eliminates airflow across rows of racks and the need to be concerned with redundant CRAC units.
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