By Matt Warner

1. The Downside of ACU Duty Cycling Some data centers employ a strategy of duty cycling to control the cooling units. This means that the potential cooling available through each floor grille in the room changes over time. These changing patterns of cooling availability throughout the room can hinder effective load growth management . By Matt Warner

2. The Virtual Facility 6SigmaRoom software is used to create a Virtual Facility. The Virtual Facility is a 3D virtual view of a data center, with individual representations of each of the physical components: servers, racks, ACUs, PDUs, chilled water pipes, power and data cables, floor grilles… The Virtual Facility can be used to predict the air flow and temperature in the data center.

3. The Virtual Facility enables the visualisation air flow under the raised floor. In this view, the air flow under the raised floor is visualised and coloured by pressure. • High pressure regions can be seen in the corners of the room and • In this case, the regions of low pressure under the floor are be caused by: • Recirculation • High velocity air flow near ACUs • Low pressure regions that are located under floor grilles will cause low airflow through these grilles. • In the worst case, a negative pressure under a floor grille will draw warm air down into the floor void. Air Flow

4. The resulting air flow through floor grilles is different at each stage of the duty cycle. If a duty cycle strategy is used to control the ACUs, the air flow patterns under the raised floor will change over time: In this example 1 of the 8 ACUs operates in stand-by mode at any one time. The ACU in this mode is changed at regular time intervals. Air Flow Grille Flow

5. The Potential Cooling available from each floor grille can be seen each stage of the duty cycle. The cooling potential of each floor grille is shown below. Looking at one particular floor grille in the general racking area shows how it changes over time. The table shows how many additional HPDL360s could be cooled by this supply of air in a cabinet already containing a Cisco 3825. In 5 of the 8 stages of the cycle, this grille could cool a router and at least 10 servers in a rack, in another cycle is only has the potential to cool the router !!! Potential Cooling

6. A constant-duty strategy means predictable cooling potential from each floor grille. With no duty cycle strategy the floor grilles have a potential cooling that is constant and predictable: With no duty cycling, the grille examined in the previous page would have a constant potential cooling of 2.2 kW which can cool the router and 9 servers in a rack. Duty Cycling Constant Duty

7. Future Facilities Ltd. Recommends operating facilities with all ACUs operating at constant duty. This means the patterns of cooling availability throughout the room can be predicted and used during load growth management.Changing from a strategy of ACU duty cycling to a constant duty strategy is beneficial in the majority of situations, however Future Facilities recommends the use of 6SigmaRoom or 6SigmaManager to confirm this before implementation. Slide 7

8. providing design & management software and services for data centre facilities 1 Salamanca Street Albert Embankment London SE1 7HX http://www.futurefacilities.com Tel: +44 (0) 20 7840 9540 Fax: +44 (0) 20 7091 7171