ASHRAE Journal - January 2009 - (Page 12) Energy-Efficient Data Centers Editor’s note: This letter was submitted by Jeff Stein, P.E., of Alameda, Calif. Comments from the authors of the article are embedded (italic) in the letter for readability. The full text of this letter is available at www.ashrae.org/ashraejournal. October’s article “Energy-Efficient Data Centers: A CloseCoupled Row Solution” by John Bean and Kevin Dunlap posits that in-row air handlers (IRAHs) are more efficient and better at eliminating hot spots than computer room air handlers (CRAHs) and central air handlers (CAHUs). It is written by two employees of the largest IRAH manufacturer. The article contains several misleading statements and a flawed analysis. It also fails to point out several key disadvantages of IRAHs. Authors: This manufacturer also offers CRAHs in their offering. The article was intended to be a straight energy comparison of common architectures and practices. The article states that “The major obstacle in [CRAH] architectures is the length scale of air delivery… this separation results in hot spots… [IRAH] allows for a much shorter air delivery and heat removal path.” Length is not the issue. The issue is fan volume required to minimize hot spots and the fan power required. There are many CRAH layouts, some are more vulnerable to hot spots than others. The CRAH layout shown in Figure 1 is one of the worst. A better layout is to turn the racks 90° and use the ceiling as a return plenum with ceiling return grilles in the hot aisles. Such a layout will have no more hot spots than an IRAH system. The real advantage of IRAH is when there are no good CRAH layout options, which is typically only the case when the floor-to-floor height is very low. Authors: True, the length is not directly the issue, the additional surfaces and obstructions associated with the longer underfloor air-distribution patterns drive static needs of the air delivery system. The layout was for illustration purposes only and had no real relevance to this study nor was it intended to convey a preferred method of physical arrangement. Perimeter CRAH have significant mixing (given the typical absence of containment schemes) and much lower return air temperatures, combined with practical limit to how cold they can cool the air. This results in greater airflow needs per IT kW on typical installation. The article states that “Eliminating mixing of hot and cold airstreams….” IRAHs do not eliminate recirculation from hot to cold aisles or short circuiting of IRAH supply air to IRAH return. The best way to avoid hot spots is with aisle containment, 12 ASHRAE Journal which is not discussed in the article. IRAHs have no particular advantage over CRAH when hot or cold aisles are contained. Authors: We certainly agree that hot aisle containment has additional advantages. However, the physical placement of IRAH along with functionality of control algorithms does effectively reduce mixing. Higher cooling DTs provided by this architecture require lower cfm per kW of IT load. The article states that “At lower densities (1 to 5 kW/rack) adequate cooling is provided [by CRAHs]…despite the mixing of air throughout the room.” Many data centers with higher densities (e.g., 10 to 15 kW/rack) are adequately cooled with CRAHs. Success depends on many factors, including floor height and room geometry. Authors: We agree, however, higher design loads do require special consideration and often represent a significant engineering effort. The article states that “Since CRAH and CAHU systems use a plenum for supply air delivery…a significant amount of fan horsepower is required to pressurize the plenum and overcome resistances in the air-distribution system.” The floor pressure only needs to be high enough to achieve the design flow through See Data Centers, Page 49 ashrae.org January 2009 http://www.ashrae.org/ashraejournal http://www.ashrae.org
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