IBM NeXtScale System Planning And Implementation Manual page 62

Cooling zone 1
Figure 3-9 Cooling zones on the chassis
For each of the cooling zones (zone 1 or zone 2), the FPC sets the respective
fans for the corresponding nodes to the appropriate cooling values that are
required to cool those nodes. The FPC varies the speeds of the fans in zone 1
and zone 2 by at most a 20% difference to avoid unbalanced air flow distribution.
Fan removal: To maintain proper system cooling, do not operate the IBM
NeXtScale n1200 Enclosure without a fan module (or fan module filler) in
every fan module bay. Install a fan module within 1 minute of the removal of a
fan module.
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IBM NeXtScale System Planning and Implementation Guide
Cooling zone 2