The Big Chill

This column is in response to a question from an NDCF reader who wanted to learn the pros and cons of false floor cooling versus room flood chilling, where cool air is pumped around the data center. He was particularly interested in finding out the best cooling procedures for data centers with a mixture of standard Intel two-rack-unit-high servers, blade servers, IBM

S/390 mainframes, storage devices such as the HDS 9980V, and storage area networks (SANs).

Answer:

In room flood chilling, heating, ventilation, and air conditioning (HVAC) units push chilled air throughout the main data center airspace to provide general cooling. This works fine for light to medium loaded equipment racks, but may need to be supplemented by special air handling for heavily loaded racks with newer blade servers, where the total power consumption per rack approaches or exceeds 10 kilowatts.

In false floor (plenum) cooling, HVAC units around the edge of the data center room space are connected to ducting, which directs the cooled air into the false floor void. By careful placement of slotted floor tiles between equipment-rack rows, coupled with floor tile cut-outs underneath equipment racks, general cooling and directed cooling for high power dissipation racks can be applied in a controlled manner. In either case, Liebert Corp. X-Treme Density cooling systems can be used to provide directed air cooling for problem areas.

In this case, care should be taken to ensure floor cut-outs in unused racks are closed. Some equipment-rack manufacturers allow the addition of sliding floor louvers to permit easy control of cooling to each rack by opening or closing the louvers depending on the quantity and type of equipment installed in each rack.Floor voids have been typically set at a height of approximately 600 millimeters, or 2 feet. Airflow in this plenum can be disturbed or diminished, however, by large cable bundles on tray-work, and other sub-floor services, such as fire and water detection. There has therefore been a trend to sometimes increase the floor void height to 1 meter (about 39 inches). However, in order to maintain the same air pressure, twice the volume of cooled air must be shifted, requiring a doubling in the capacity of the air-con handlers. Because this results in higher operating costs, raising the floor void height should therefore be avoided, if possible.

A number of alternatives are available, most based on modular racking systems, which either remove the underfloor cable tray-work and place this into modular, integrated, overhead tray systems, and/or provide integrated air handling ducting between and through the racks. In addition, these systems can provide rackmountable or rack-housed air cooling and UPS (uninterruptible power supply) power systems. Such systems include American Power Conversion Corp.s InfraStruXure racking system and the newly launched Trillenium power and cooling rack systems from the alliance between power supply manufacturer, MGE UPS Systems, and rack manufacturer, Knurr AG.

For large data center spaces, it is also useful to perform an airflow thermal analysis with a 3-dimensional modeling tool, such as Flomerics’ Flovent software.

— Dan Stavola, Director, Enterprise Management, Greenwich Technology Partners