ASHRAE Journal - January 2009 - (Page 37) (540 W/m2). Introducing high-density cabinets in these environments will not go smoothly without an effective implementation plan. For example, a number of 8 kW cabinets may result in a local density of 400+ W/ft2 (4300+ W/m2). Getting enough cool air to such a density may be a tremendous challenge. However, a clever duct design and some basic rules for placement of point loads go a long way. Although not widely accepted, supplemental liquid-cooled solutions may also provide benefits over conventional ducted overhead air. Poor Separation of Hot and Cold Air. Although alternating hot and cold equipment aisles are the preferred scheme of organizing the equipment, many telecom network rooms are not organized this way. It is not uncommon that the equipment is lined up front to back, meaning that all equipment is facing the same direction. And, network gear often has intakes or exhausts on both sides of the equipment. Consequently, cold air needs to be supplied in all aisles. The lack of Background Hot separation of hot and cold air results Most existing network environments Good? in air mixing that leads to poor energy operate with a relatively uniform and efficiency. This arrangement also low heat density of 50 W/ft2 (540 W/ leads to complications with placing m2) or less, which generally means an the point loads. Specifically, introducaverage cabinet heat dissipation of less ing a point load may affect the intake than 1 kW. However, equipment cabi- Better? temperature to equipment located nets are available with power densities directly behind the point load. of 20 kW or greater. For this article, Since all equipment in network high-density cabinets are defined as facilities is air cooled, cool air needs greater than 5 kW. to enter and hot air needs to leave It is common for high-density equipBest? the cabinet. The equipment-cooling ment to be deployed in existing lowCold (EC) class describes where on the density network environments. This is equipment envelope the air enters the result of two key events: • The exponential increase in Figure 1: Temperature CFD data for three equipment and exits.2 Optimal classes, including power density of network equip- environments (plan view). front-to-rear ventilated equipment, ment; and work in concert with the hot- and cold• Widespread industry network upgrades driven by market aisle arrangement. Such equipment (including most point-load forces. equipment) does not necessarily work well in network facilities. Consequently, another challenge has presented itself; high- However, there are some effective work-arounds. density point loads placed in low-density environments. A point load may be a single cabinet or a cluster of cabinets with Issues a significantly higher heat density than the typical average denIn addition to the physical limitations in existing network sity in the equipment room. It is more common for equipment facilities, there are a number of issues with representing and operators to deploy new high-density equipment in their exist- analyzing the conditions. ing buildings, rather than to build a new high-density network The Concept of “Ambient” Conditions. The thermal equipment center. It may seem surprising that point loads and equipment environment is defined by the temperature of the their impact on the existing network environment have not been air drawn into the air-cooled electronic equipment, which is better studied by the industry. the temperature the electronics depend on for reliable cooling and operation. The exhaust temperature or the temperature in Limitations the middle of the aisle, for example, has little to do with the There are many physical limitations in existing network facili- rack cooling effectiveness. This also holds true for the ambient ties. Two of the major limitations are addressed in this section. temperature specified by “GR-63-CORE, NEBS™ RequireLow Design Heat Densities. Most existing network ments: Physical Protection”3 of 1.50 m (59 in.) above the floor facilities were designed for heat densities below 50 W/ft2 and 0.38 m (15 in.) in front of the equipment. In the historically procedure for determining acceptable overall heat densities of the network equipment through a comprehensive computational method, allowing for high flexibility in maximum equipment heat dissipation. This de-facto standard, however, did little to provide guidance on placing high-density point loads in existing switching centers. Verizon Wireless took the initiative in 2006 to perform research to better understand the point-load issues by using the advanced modeling services of ANCIS Incorporated, building on the methods developed in GR-3028-CORE. The objective was to develop a strategy for deploying point loads in typical network rooms. The scope of this article is limited to providing an overview of several limitations, issues, and solutions discovered in the research. Although telecom-centric, the computational methods and point-load strategies are also applicable to many traditional data centers. January 2009 ASHRAE Journal 37
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