Hot Air Isolation Strategy using Vertical Exhaust Ducts

In addition to the HAC and CAC systems, a third strategy is also available to designers interested in achieving superior performance and efficiency levels. Referred to as the Vertical Exhaust Duct (VED) strategy, it improves overall air isolation and energy performance. This approach requires a gasketed solid rear door to seal off the rear of the cabinet from the rest of the data center and unimpeded flow path for cool air to the server in-takes, either through a high percent open perforated metal front door or opening in a floor tile inside the front boundaries of the cabinet. Computer equipment fans draw that cool air into across the heat producing parts of the equipment and then pass it into the enclosed rear chamber of the cabinet.  It is then drawn up as return air into the vertical duct (sometimes referred to as a chimney) at the top of the cabinet and into the ceiling area plenum or ductwork where it returns back to the CRAC unit. This strategy effectively isolates the equipment and its heat from the room and changes the focus from aisles to individual cabinets that can potentially be arranged more freely.

By isolating cold supply from hot exhaust air, the inefficient mixing of hot and cold air is eliminated, allowing only cold supply air to be directed through equipment. Images courtesy of Chatsworth Products, Inc.

 

The performance of this VED strategy is notable. The working air temperature around the computer equipment drops from the 95 °F to130 °F range down to the 77 °F to 80 °F range. In terms of thermal transfer or ride, this VED approach will vary depending on boundary conditions. If the VEDs are not directly coupled to the enclosed return air path, then this will provide the best thermal ride-through due to the conductive heat absorption of all the extra sheet metal in the data center space, though the lack of coupling obviously allows a path for compromising the complete isolation and thereby diminishing the overall efficiency. When coupled into a fully closed system, ride through improves to the degree that the return air space exceeds the volumetric capacity of the supply space and it diminishes as that ratio is reversed.

As VED air isolation strategies have become integrated into computer rooms and data centers, some misconceptions have arisen that need to be corrected:

• Capacity myth: This misconception is born from a mistaken belief that there is an upper limit to the air cooling capacities of VED systems that are well below today’s potential heat load densities inside a cabinet. This perceived ceiling to air cooling capability is commonly based on how much air can be delivered out of a perforated access floor tile in a raised floor system. A standard value for this is typically placed at around 700 CFM. Hence, assuming that 700 CFM of chilled air is available through a perforated access floor tile located in front of a rack containing common computer equipment, one could expect to cool roughly between 4.5-8 kW of heat. Actual experiments confirm that the chilled air is consumed by the bottom half of the cabinet, thereby cooling approximately half of a potential heat load of over 9 kW. This would lead designers to think that the cabinet would never fully cool and that hot spots would emerge. Therefore, any kind of air cooling solution for high-density heat loads will need to eliminate the dependency on chilled air from a perforated access floor tile in front of the computer equipment. Instead, a fully integrated cabinet with the capability to receive larger amounts of chilled air and properly contain and channel that air is readily available to more than meet the heating capacity of the equipment. In tested conditions, capacities exceeding 30 kW of heat are possible within a single cabinet, allowing a higher density of equipment inside that cabinet with capacity to spare.
• Return air temperature myth: This misunderstanding focuses on a belief that high-density VED air cooling systems create unmanageable high return air temperatures. While in some circumstances that may be true, if an IT manager were to tell his facilities manager that he was experiencing high return air temperatures, the facilities manager would likely respond by telling him to keep up the good work, especially in a chilled water environment. The reason for this unexpected response is quite clear. Chilled water computer room air conditioners (CRAC) improve efficiency; that is, they increase cooling capacity with higher return air temperatures. Of course, there are some caveats to this statement. First, there is not a flexible performance curve for direct exchange (DX) cooling units—this capacity bonus is only available with chilled water units. Secondly, there is a limit to how high this return air can be before the performance curve starts on a path of diminishing returns, primarily by raising the supply air temperature. Hence, the best design decision is to specify chilled water cooling unit solutions that accommodate wider differences in temperature (ΔT) between the supply air and the return air, and which, in fact, deliver superior performance at those higher ΔT; whereas in a DX environment some amount of bypass airflow will be required to control return air temperatures, particularly in blade server applications.. By making the return air plenum large enough (i.e.,  double the supply plenum) this system will be self-regulating for most ranges of airflow fluctuation. In summary, this VED air cooling solution does create high return air temperatures, which is good up to a point and then there are simple site management strategies to allow the data center manager to continue reaping the benefits of high ΔT without driving supply air too high.
• Operating cost myth: The final misperception is a belief that the lower acquisition costs for VED air cooling solutions are overshadowed by significantly higher operating costs, particularly when compared with liquid cooling solutions. The primary basis for this belief is derived from the inherent inefficiency associated with standard hot and cold aisle facilities at higher densities, plus the common over-capacity designs that are typically driven by the need to supply cooling continuously at a worst case level. With the improved efficiency of complete isolation between supply air and return air and the resultant operating economies associated with that separation, close-coupled liquid cooling solutions lose their operating cost advantage. In terms of understanding the difference, particularly in first costs, the cost of construction can be a significant determinant. If a lower density array of cabinets or racks is selected to distribute the heat and avoid hot areas, then more square footage needs to be built to house this equipment, usually at a rate in excess of $200 per sq. ft. Since the VED system has been shown to quite comfortably handle higher densities, the required square footage can be proportionately less as a result. Further, typical high density solutions require some redundancy in cooling equipment in case one unit fails or is inoperable for a time. As for operating costs, with the removal of normal inefficiencies and necessary over-supply through a containment strategy, apples-to-apples comparisons between precision perimeter cooling with containment and close-couple cooling with containment reveal very little differences in energy efficiency ratings (EER, i.e., BTU’s heat removed per kW energy expended to remove the heat). In fact, some types of precision perimeter cooling equipment perform consistently better than popular close-couple systems. However, the benefits don’t stop there. A study conducted by the McKinstry Company of eight different geographic regions around the U.S. reveals a more complete picture of the economic benefits from a complete separation between supply air and return air including:
• 74% reduction in critical refrigeration tonnage
• 35% reduction in peak water use
• 44% reduction in tank that would be required for 24 hours water storage
• 74% reduction in thermal tank volume to allow generators to restart
• 65% reduction in HVAC load on generator, utility distribution
• 78% reduction in three phase component connection requirements
• 89% reduction in floor space requirements for indoor HVAC equipment
• 63% reduction in cost for HVAC water
• 49% reduction in maintenance costs