Traditional air conditioning typically conditions an entire room or space, such as in an office space or classroom. Air is supplied from the ceiling and distributed throughout the space. This is a cost effective method but is not well suited for some applications due to the limitations in efficiency in cooling the entire space including areas above the occupied zone and the inability to directly cool major heat sources such as found in a data center.
One HVAC system utilized to only condition the occupied space is underfloor air distribution (UFAD). UFAD has been successfully applied in offices, auditoriums, and other spaces with high floor to ceiling heights. UFAD is more effective for these applications because air is delivered to the occupied area of the space and does not utilize energy to condition the areas above the occupants that does not required conditioning. UFAD utilizes warmer supply air temperatures to mitigate drafts resulting in higher system airflow to the space. UFAD requires coordination with the building structure to utilize space above the occupied zone of a room as the return air plenum rather than above ceiling space. UFAD can be challenging to retrofit into an existing building due to the requirement to install a supply air plenum above the existing floor.
Data center HVAC systems are one applications where it is beneficial to direct air only to the sources of heat gain (typically server racks). This instance is less about cooling the entire room or the occupants and more about cooling the equipment in the space. For larger data rooms, the use of underfloor plenums allows the air to be directed to the space in front of the data rack.
To assist in delivering conditioned air directly to the equipment, the racks should be arranged with hot and cold aisles so that the conditioned air is distributed to the cold aisles, drawn through the racks and rejected to the hot aisles. In existing installations this is not always possible however modifying the equipment rack to this arrangement reduces energy consumption. Supply air temperatures may be raised, significantly increasing cooling efficiency which also increases duration that HVAC economizers may be utilized for free cooling throughout the year. Further enhancing the efficiency, physical separations to enclose the cold air aisle and separate them from the hot air aisle increases the effectiveness of directing air to the equipment. Again this is easily accomplished in new installations, existing installations can be difficult but are not impossible. Various manufacturer’s make products that can seal the equipment racks into aisles.
Where able to, the goal is to force the cool air to go through the server equipment and not bypass the heat source, decreasing the efficiency. Sealing the penetrations around the equipment in the rack helps. Pre-manufactured blank-off panels can be used to assist in forcing air directly through the servers. When a floor supply plenum exists, sealing the supply plenum helps tremendously, especially around conduits and other utility penetrations.
If you would like more information regarding mechanical design systems, or if you would like assistance with an upcoming project, please contact Douglas C. Barnhart, PE at (717) 845-7654 or dcb@ba-inc.com.