Issue link: https://cp.revolio.com/i/148754
Additional granularity can be included in the energy model to vary chiller efficiencies based on incoming tower water, for example. A balance must be maintained between the level of detail included and the actual time spent to build the model. "SCENARIO-BASED" CFD MODELING The original data center "white space" consisted of eighteen (18) downflow CRAC units positioned along the perimeter and along the central line of structural columns as shown in Figure 4. Each supplied a constant 12,500 CFM of air through one (1) 7.5 HP fan with a cooling capacity of 70.3 kW per unit. Individual rack load intensities varied from 0.25 kW per rack up to as much as 3.5 kW per rack. The new data center "white space" consists of nine (9) downflow CRAC units (N+1), (12.5 HP each) two (2) positioned at the ends of each hot aisle as shown in Figure 5. Each supplies air at a variable volume (17,300 CFM maximum) through one (1) 12.5 HP fan to match the total demand airflow from the servers. The maximum, total cooling capacity for a single unit is 144 kW. Individual rack load intensities were increased to approximately 10.5 kW per rack throughout based on discussions with the client. It should be noted the server ΔT is assumed to be 20 °F in this case. This can be entered into the CFD model on a per rack basis or at the individual server level. Care should be taken to model this value as closely to actual as possible since it has significant impact on airflow and temperatures within the space. Figure 4 - Data Center CFD Model of Original Layout After the models have been built to the appropriate conditions and parameters, simulations should be run until proper convergence is achieved. "Scenario-based" modeling can then begin. For example, "What will happen if I supply air to the space at 75 °F?" If the supply air temperature is increased to the space, then the chilled water supply temperature can likely be increased as well (this might not be the case if the central chiller plant is serving other areas such as an office space). Let's remind ourselves why supply air temperatures are worth reviewing: Chilled water (CHW): ~5% chiller efficiency increase per 2 °F increase in CHW supply Direct Expansion (DX): ~3.5% system efficiency increase per 1.8 °F increase in SA It was pointed out early on by the client that the data center did have an un-used drop ceiling of 2.5 feet. The team decided to make use of this and analyze effects of adding hot aisle containment. It is assumed that a return duct will be extended from the top of each CRAC unit to the bottom of the ceiling above. Curtains will be utilized to fully enclose the hot from the floor to the ceiling. It is also assumed that the ceiling will simply be opened up directly above the hot aisle to allow air to return up and to the CRAC units. There is no need to install return grilles in this case. Figure 6 below is the temperature plane at 3 feet above the finished floor (AFF) at a 75 °F supply air (SA) temperature. The ambient temperature averages 90 °F with naturally higher temperatures within the hot aisles. The ambient temperature of 90 °F can have a negative impact on public relations, depending on the circumstance. Figure 6 - Temperature Plane at 3' AFF. 75 °F SA Figure 5 - Data Center CFD Model of New Layout W 22 | THE DATA CENTER JOURNAL www.datacenterjournal.com *V