ASHRAE Journal - February 2009 - 53

• • exist such as 2(N+1), 2N, or N+1. The first two options might be used for centers requiring high reliability. However, the latter is used in data centers where lower reliability is required. Many architectural piping configurations exist. However, the chilled water distribution system should be consistent in its reliability, flexibility, and quality with the rest of the datacom system’s components. Figure 13 illustrates a highly reliable and flexible chilled water piping system with redundant piping routes to the connected loads, and redundant connections to the plant. This system is self balancing, as it provides equal pressure in the loop as well as in the cross branches. Also, this layout allows for future equipment additions or service without disruptions to the system operation. In addition, multiple valves are recommended to allow for maintenance of the valves. The design engineer can choose to sort the chilled water system into several cross branches. Each cross branch serves a similar type of equipment function. This requires working closely with the architect planner and facility managers. For example, the need may arise to identify different zones for application, storage, or database servers. They should be sorted according to mission importance and served by different branches of the cooling system. This helps in providing various water temperatures for different application requirements. Also, it is recommended to consider providing a separate chilled water branch for air-handling units serving office spaces. In case the system is unable to meet the required total cooling demand, the chilled water continues to flow to the most critical zone, while throttling or shutting the branch serving the less critical zones. Plant Load Load Load Load Load Load Load Load Load Load Load Load Figure 1: Double-ended loop with dedicated cross branches.3 required airflow.”5 Also, pumps should use VFDs, so they can ramp up to their full speed if an operating pump fails. Air-Distribution System UPS for Cooling Equipment To keep temperature and humidity levels at their normal operating setpoints if cooling is stopped due to power sags and interruptions, it is recommended to use an uninterruptible power supply (UPS) to serve the fans of the computer room air-handling units and the pumps that supply chilled water to those units. Another strategy the designers can choose for an uninterruptible cooling system is to use computer room cooling unit fans and secondary chilled water pumps on UPS with chilled water thermal storage tank.4 When the designer chooses not to use cooling equipment on UPS, this subjects the computer room to a high risk of losing cooling during power failure or during thermal cycling (temperature changes from operating setpoint) when power is restored. VFDs in Cooling Units and Pumps Also, using variable frequency drives (VFDs) for fan controls, and using multiple fans with each independent motor for the same computer room unit will enhance the equipment redundancy. “In this manner, the fans operate at a speed to match loads. If a fan fails, the other fans ramp up to maintain the February 2009 Providing redundant computer room air-conditioning (CRAC) units will enhance the flexibility and reliability of the air-distribution system. Redundant equipment is necessary to compensate for failures and allow maintenance to be performed without reducing the remaining online capacity below normal.2 A technique that is often recommended is providing redundant computer room units per specific area square feet, or per actual number of units meeting the load. For example, if you have a certain number of units that meet the peak load in a certain zone, then you will use a redundant unit. If the redundant units are not operational all the time (in standby mode), redundant equipment should be frequently exercised to prevent conditions that enhance the growth of mold and mildew in filters, insulated unit enclosures, and outside air pathways, where spores and food sources for microbial growth accumulate.2 • The use of hot aisle and cold aisle in an alternating arrangement adds to the reliability of the cooling system. “This is best accomplished when the layout of the file-server farm [High-power density data center] area is first being planned, and it is exceedingly more difficult to accomplish when the computer room is already populated with operating hardware.”6 The cold aisle is the area where cold air from the raised floor is blown through perforated tiles. However, the hot aisle does not have perforated tiles. This configuration is illustrated in Figure 2. Blanking panels should be installed in all unused rack and cabinet spaces to maximize and improve the functionality • ASHRAE Journal 53

ASHRAE Journal - February 2009

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