ASHRAE Journal - January 2009 - (Page 39) where Tx = Temperature at equipment intake x n = Total number of intakes Tmax – rec = Maximum recommended intake temperature (e.g., NEBS or ASHRAE) Tmax – all = Maximum allowable intake temperature (e.g., NEBS or ASHRAE) Table 1 lists RCIHI and RCILO quality ratings based on numerous modeling studies using the Thermal Guidelines’ Class 1 recommended and allowable environments of 20°C − 25°C (68°F − 77°F) and 15°C − 32°C (59°F − 90°F), respectively. Placement Strategies for Point Loads. The research suggests that there are preferred placement strategies for front-to-rear ventilated point loads in network rooms with all equipment facing the same direction to ensure that proper intake temperatures are maintained for adjoining equipment. Given the high temperature of the exhaust air from modern network gear, directing that air away from other equipment intakes is imperative. This is especially important if that equipment has not undergone and passed NEBS testing. There is also the occasional HVAC outage condition that must be considered. Intake temperatures will often rise dramatically, resulting in higher exhaust temperatures. Some key strategies for neutralizing the impact of hot exhaust air include the following four solutions: • Place point loads on the perimeter aisles and direct the exhaust airflow toward perimeter walls where the elevated temperature will not directly be captured by other equipment. The drawback to this solution is that it may not always be workable; space may not be available, or the point loads may need to be placed close to other related gear. • In Figure 2, lineups A, B, and C have 8 kW point loads. Lineup C exhausts air toward the perimeter wall where it can be returned safely to the (central) air handler via highelevation wall returns without significantly impacting other equipment. However, lineups A and B exhaust hot air directly toward the intakes of the opposite lineups. • Provide supplement cooling to neutralize the hot exhaust air prior to entering other equipment. Figure 3 shows the impact of using liquid-cooled rear-door heat exchangers. Note the absence of hot air behind the point loads, which results in an ideal RCI value of 100% (i.e., no intake temperatures above the maximum recommended). Benefits include room-load reduction, a modular/scalable solution, and marginal need for valuable floor space. Drawbacks include protrusion outside standard frame dimensions and water or refrigerant piping. In typical telecom environments with concrete floors, an engineered solution is required. January 2009 Figure 3 (top): With ( RCIHI = 100%) and without ( RCIHI = 95%) rear-door heat exc hangers (plan view). Figure 4 (middle): With (RCIHI = 99%) and without ( RCIHI = 95%) exhaust deflection devices (plan view). Figure 5 (bottom): Org anized fr ont-to-front (RCIHI = 99%) layout (plan view). ASHRAE Journal 39
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