Which system in Air Distribution System takes the hot exhaust air from the IT equipment to the CRAC

The air distribution system in a data center is fundamentally composed of a supply side and a return side. The supply system delivers conditioned, cool air from the Computer Room Air Conditioner (CRAC) to the IT equipment inlets. After this air passes through the equipment and absorbs heat, it becomes hot exhaust air. The return system is the designated pathway responsible for capturing this hot exhaust air and directing it back to the CRAC unit to be cooled again. This completes the air-cooling cycle, ensuring efficient thermal management.

B. Supply system: This system performs the opposite function; it delivers cold air from the CRAC to the IT equipment, not hot air from it.
C. Maintenance system: This term refers to the tools, procedures, and personnel involved in the upkeep of data center infrastructure, not a component of the airflow path.
D. Regulation system: This is a control system (e.g., Building Management System) that monitors and adjusts environmental parameters, not the physical pathway for air movement.

1. Geng, H. (2014). Data Center Handbook. John Wiley & Sons. In Chapter 15, "Data Center Cooling," the text describes the basic air-cooling cycle, stating, "The air distribution system consists of a supply part and a return part... The return system takes the hot air from the hot aisle to the CRAC/CRAH unit" (p. 328).

2. ASHRAE TC 9.9. (2021). Thermal Guidelines for Data Processing Environments, 5th Edition. This foundational industry standard details the air management system, consistently referring to the path for heated air from the IT equipment back to the cooling units as the "return air path" or part of the "return system." (Section 3.1 Air Management).

3. Ham, S. W., & Kim, M. H. (2012). Effect of the location of a CRAC on the cooling performance of a data center. International Journal of Refrigeration, 35(5), 1482-1494. The paper's methodology explicitly models the data center airflow, defining the "return air" as the heated air that flows from server rack outlets back to the CRAC inlet. (Section 2.1, Numerical model). https://doi.org/10.1016/j.ijrefrig.2012.03.003