ASHRAE Journal - December 2010 - 47

(CFD) model indicated that the inlet Temperature °F 95 temperatures would reach unacceptable levels (Figure 2). And a physi85 cal barrier between hot and cold air 75 on the data center floor was created 65 using rack enclosures (Figure 3 and Photo 1, Page 40). 55 The cold and hot aisle containment 90°F 77°F were also considered, but were found to be impractical in this application (due to the existing section of the data center). The rack level containment allowed for flexibility. Using a VFD on CRAC fans became practical once Left: Insufficient volume of 55°F air still allows equipment Right: Insufficient volume of 68°F air shows equipment the hot air was contained. Additioninlet temperatures to be within ASHRAE Class 1 limits. inlet temperatures exceeding ASHRAE Class 1 limits. ally, direct return of hot air from the racks over the plenum eliminates the Figure 2: Supply Air Temperature CFD. Hot spots with conventional hot aisle/cold aisle air common problems of recirculation of distribution. Note: Section taken from racks in the middle of the row. Conditions deteriorate hot air into the uppermost areas of the as you move out toward the end of the rows. racks in the cold aisle. By eliminating the short-circuit bypass, the computer room units are run more efficiently. The return air temperature to the unit will increase since there is no mixing of air. This allows the return air temperature back to the unit at approximately 90 degrees.
Operation and Maintenance

Efficient ducting of return air results in less issues due to hot spots. And the increased capacity of the CRAC units due to warmer return air temperatures and reduced fan speeds provided greater cooling equipment redundancy. As a result of the design, mechanical equipment operates at lower loads, reducing wear on equipment and increasing equipment life. Additionally, the more efficient heat extraction and increased capacity of each CRAC unit resulted in more CRAC units available for redundancy. The observation of large quantities of condensate flow led to further fine-tuning of system operation for energy efficiency, cost savings and environmental impacts including recovery of the condensate for cooling tower makeup, and fine-tuning of outdoor air and economizer settings. The first year of operation saw 1.9 million gallons (7.2 million L) of condensate recovered. The outdoor air settings were adjusted and the economizer was fine-tuned resulting in a reduction of condensate by 1.1 million gallons (4.2 million L) per year. Associated savings in cooling energy also were realized.
Cost Effectiveness

AC

Rack

Figure 3: Hot air containment schematic. • The payback was conservatively projected to be 19 months. In estimating this payback, it was assumed that the fan speed could be reduced up to 15%, just enough to match the redundant cooling capacity of CRAC units. However, the actual fan speed at full capacity is only 72%, providing 63% of fan energy savings. • The fan speed was much lower during the early days after commissioning when the ICT equipment was being added to the space. • Twelve percent of the CRAC fan savings was negated by the incremental energy use by supplemental fans in the racks to overcome additional pressure drop of the enclosure and duct to connect to the plenum, netting 51% total fan energy savings.
ASHRAE Journal 47

• At full-load operations, the energy cost savings is measured at $1.2 million annually. • The incremental cost of energy features (CRAC VFDs, rack containment, supplemental fans, plenum, and controls) was less than $500,000. • Simple payback was less than five months.
December 2010
ASHRAE Journal - December 2010

Table of Contents for the Digital Edition of ASHRAE Journal - December 2010
