Exhaust Air Heat Exchanger In z14 servers, all water-cooled models have two Exhaust Air Heat Exchanger units that are  installed on the rear of the A and Z frames, as shown in Figure 2-32. These units remove heat  from the internal system water loop and internal air exits the server into the hot air exhaust  aisle.  Figure 2-32 Water-cooled model: Rear view In addition to the PU SCMs cold plates, the internal water loop circulates through these two  heat exchangers. These exchangers are in the path of the exhaust air in the rear of the  frames. These heat exchangers remove approximately 65% of the residual heat from the I/O  drawers, PCIe I/O drawer, the air-cooled logic in the CPC drawer, and the power enclosures.  The goal is for two-thirds of the total heat that is generated to be removed from the room by  the chilled water.  If one client water supply or one WCU fails, the remaining feed maintains PU SMCs cooling.  The WCUs and the associated drive card are concurrently replaceable. In addition, the heat  exchangers can be disconnected and removed from the system concurrently. 84  IBM z14 (3906) Technical Guide