Musings on IT, data management, whitewater rafting, and backpacking

Wednesday, December 16, 2009

It's not about square feet

For those of you new to data center design: it's not about square feet, or watts per square foot.

It's about total kilowatts, and cubic feet per minute.


Yes, you need a certain number of square feet to hold your equipment cabinets, but probably less than you think with careful planning and looking at all design elements.

Electric power and air are your ultimate limits, friends, and enemies at the same time.

Servers are getting more power hungry. A cabinet full of blade servers can draw 30 KW or more! A 42U cabinet full of 1U servers can total 42 KW.

Virtually all of that electricity is turned into heat. Imagine 42 electric heaters locked into a 2 x 4 x 7 foot cabinet. Gets hot fast.

That's where CFM comes into play. You get that heat out of there by moving lots of air at the right temperature. Forget about water cooling unless you want lots of headaches and design limits. 99.9% of servers use air now and for the foreseeable future -- we just need lots of it.

About 100 CFM per KW more or less. That's for a 30°F temperature rise through your server.

So for my 60 KW data center, I'll need about 6000 CFM of air moving through the servers.

Add 20% for leakage, and I'll need about 7200 CFM.

Without crowding, I can get twelve 42U cabinets into 500 square feet. With a 60 KW limit, that's 5 KW per cabinet. Now you see why it's not about square feet.

60 KW is about four times as much power as we're using now, and our existing data centers total about 1200 square feet. Already we're saving construction and rental costs.

Now, how do we get kilowatts in, and CFM flowing through, reliably and efficiently? Stay tuned.

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