Good initial planning of data center facilities

When initially tasked with creating a data center environment the most basic constraint, the one that will form the foundation of your solution is to understand the relationship between available space, power and cooling.. While there are different approaches as to deciding which side of the triangle is the most important, there is universal consensus that these are most important design considerations.

Before the deployment of blade server technologies, average power consumption per rack was typically 3-8 kilowatts (14 Compaq Proliant DL380 G1 Servers with two 275W power supplies = 7700 watts). With the advent of blade server implementations, power requirements skyrocketed to up to over 20 kilowatts per rack (6 Compaq C-Class Blade Enclosures can require 3255 watts each = 19530 watts). Couple this with the price of electricity steadily increasing you can see why power is a critical design element for any data center.

In complement with the calculations of kilowatt power per rack in your data center, there must be proper cooling. There are two specific factors to consider when sizing cooling requirements; first the actual amount of BTUs (British Thermal Units) required to dissipate the generated heat, and equally as important is the amount of airflow which is measured in CFM (cubic feet per minute). Unlike determining power requirements, which can be easily figured out as multiplying the number of power supplies with the watts generated from these supplies, calculating the amount of BTUs can be a bit more complex.

A good rule of thumb in determining the amount of heat generated per rack is to take 30% of the total wattage and assign that as the heat cost. So to use the above example of the 6 C-Class Blade Enclosures 19530 watts generates 5859 watts of heat. To convert this into BTU’s, multiply the kilowatts by 3412.14. (5.859 KW * 3412.14 = 19,991 BTU), or this can be expressed in cooling tons by dividing the BTUs by 12,000 (19,991 BTU / 12,000 = 1.66 Tons).

The second step is to determine the CFM of cooling. Legacy chiller solutions cool the entire room and as such are the least efficient ways to go as you are applying energy to cool the room, and not the equipment inside of it. Today’s chillers reside either inside of the rack or off to the side of it, or you will find situations of an enclosed area made up of the racks creating hot aisles and cold aisles. This is a more efficient solution as you are directing the chilled air directly on the equipment and not the ambient air in the room.

To accurately measure how many CFM per minute that you need in your rack, it is a bit more complex, in fact you may wish that you stayed awake in high school physics. First, you need to determine the maximum “hot” temperature – meaning air leaving the rack, for example (30°C/86°F). Next determine the “cold” temperature – meaning the air going into the rack, for example (15°C/59°F). Lastly, you need to calculate the total amount of heat generated in kilowatts (5.859 kW). Calculate the amount of air that is needed to be moved using the specific hear of air (1 kJ / kg K). 5kJ/sec means that you will need 5kg/sec for a 1 degree rise, however in this case, with a 15 degree rise this converts to .33kg /sec. To turn .33kg/sec into CFM you multiply .33kg to 1.3 m3/kg for a total of .43m3/sec. There are 35 cubic feet in a cubic meter, so you need about 15 cubic feet per second, or about 900 CFM.

After two segments of the triangle have been calculated, usable space, and its relationship can be examined. Referring back to the Identification of the components of a data center, there are specific components to plan for in a data center design: The room that the server racks reside in, room for the power and mechanical services, the communications demarcation point, and the operations command room. The dimensions of these areas are dependent on the requirements of the data within the data center but after this initial allocation, the remainder of the physical location is determined to be your “available space”, which is measured in rack spaces or 3’x4’

When combined together, one can quickly understand how the combined factors of power, cooling and space form the most important factors in designing a successful data center.

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