– Dr. Eldad Levy, CEO, and Sofi Shtern, applications engineer at
Data and communication intensive firms are facing a thermal crisis as their data centers are driven to deliver ever increasing performance. As power consumption per rack has risen dramatically, managers have turned to CFD (computational fluid dynamics),thermal software, to identify optimum cooling solutions.
Israeli consulting and thermal modeling firm, CAS Ltd uses CoolitDC from Daat Research (http://www.daat.com/), a powerful, user-friendly software that handles complex problems accurately and with a minimum of computing resources—only a laptop is required. Models often can be changed with just a few mouse clicks and the answer quickly recalculated. CoolitDC helped boost performance, efficiency and reliability of data centers at France Telecom (brand name Orange), one of the world’s largest telecommunications companies and at IDF (Israel Defense Force) Data Centers.
At the Orange site in Israel, two data centers had been built originally to handle low power density racks. Performance enhancements implemented over the years had pushed temperatures, at some points in the room, to borderline conditions. The inflow temperature of some racks reached 30 deg C, though the recommended maximum temperature was no more than 27 deg. C. As a result, the racks were populated only up to 2/3 their height because the upper area was too hot.
Each data center contained 80 racks cooled by four CRACs (Computer Room Air Conditioners) mounted in pairs at opposite ends of the room and pumping cold air under the floor. Cold air passed through the perforated tiles, entered the racks and then returned to the CRACs.
As with all their analyses, CAS engineers start by constructing a model of the existing data center and then calibrate the model by comparing its predictions against actual measurements. Typically models vary from actual by only a few percent.
To minimize redesign costs, CAS engineers initially investigated various options to optimize cooling using the existing equipment. The thermal simulations quickly showed that the data center was already at maximum cooling capacity and simple solutions, such as reshuffling servers and plugging air leaks, were insufficient to bring the temperatures down. Adding more powerful new equipment without making serious changes was completely out of the question.
The CAS solution was to create a “cold aisle” by enclosing the space between the two hottest center rack-rows with walls, doors and a ceiling. Inside the aisle, six in-row cooling units were added. To the client’s delight, the proposed solution proved very effective and equally important, was easy and inexpensive to implement.
At the IDF sites, the critical and often mobile nature of the military application demanded exceptionally high reliability from the data centers, which had to run cool even under harsh conditions, and made it impossible to incorporate spare capacity for backup.
A typical site consisted of a 16 x 12 m room containing 8 rows of racks cooled by 4 CRACs. This configuration did not provide uniform cooling and the tops of some racks exceeded allowable temperatures.
CAS investigated establishing a different mix of cold and hot aisles, reducing the air leakage and replacing missing blanking panels which were impacting the efficiency of cold aisle due to mixing of hot and cold air. In some instances, cold aisles not well sealed, were resealed to insure that no uncontrolled mixing of hot-cold air took place. Many other potential solutions were evaluated in a brief span of time: concentrating high power dissipating racks in one place, trying different perforated tiles to increase the airflow, checking different tiles arrangements, etc. All this required computing dozens of different cases in order to arrive at the optimal solution. Speed of modeling, both in making model changes and in actual computing of cases was of the essence.
A solution developed for one IDF data center required completely sealed cold aisles which eliminated all hot spots without any additional cooling equipment. In fact, the simulation showed that CRAC supply temperatures could be increased from 12 deg C to 15 deg. C. while still keeping inlet temperatures under 25 deg C. The design change resulted in over 12% energy savings which more than paid for the consulting effort.
In both the military and telecom applications, CFD thermal analysis eliminated guess-work in developing optimum solutions. Multiple “what if” scenarios were developed and modified with a few clicks of the mouse. Best of all, the investments paid for themselves through lower cooling requirements, energy savings, and reduced downtime.
