Facilities Design   1 comment

For those who are looking to add a rack or two to your facility.

Computers are monstrously inefficient when it comes to shedding waste heat. Operationally, most of the wattage sucked by your computer is going to be vented into the local environment as waste heat.

If you use Dell machines, they have a handy widget that will let you populate a virtual rack, select a production use (say, “compute node”), and tell you how many watts your cluster is going to chew through. If you dig around on your vendor’s site, you can usually find out maximum wattage for whatever sorts of devices you need to stick into your machine room. Total up the wattage and multiply it by 3.413, and you get the BTU/hour thermal output of your devices. Divide that number by 12,000, and you get the total tons of coolant needed to keep those devices from overheating. (note: for those interested in where “tons of coolant” came from, this dates back to the days when refrigeration guys were primarily concerned with turning liquid water into ice, and thus a “ton of coolant” was the amount of refrigeration required to produce one ton of ice in a day). From Wikipedia:

Domestic and commercial refrigerators may be rated in kJ/s, or Btu/h of cooling. Commercial refrigerators in the US are mostly rated in tons of refrigeration, but elsewhere in kW. One ton of refrigeration capacity can freeze one short ton of water at 0 °C (32 °F) in 24 hours. Based on that:

Latent heat of ice (i.e., heat of fusion) = 333.55 kJ/kg ≈ 144 Btu/lb
One short ton = 2000 lb
Heat extracted = (2000)(144)/24 hr = 288000 Btu/24 hr = 12000 Btu/hr = 200 Btu/min
1 ton refrigeration = 200 Btu/min = 3.517 kJ/s = 3.517 kW[11]

A much less common definition is: 1 tonne of refrigeration is the rate of heat removal required to freeze a metric ton (i.e., 1000 kg) of water at 0 °C in 24 hours. Based on the heat of fusion being 333.55 kJ/kg, 1 tonne of refrigeration = 13,898 kJ/h = 3.861 kW. As can be seen, 1 tonne of refrigeration is 10% larger than 1 ton of refrigeration.

Most residential air conditioning units range in capacity from about 1 to 5 tons of refrigeration.

Now, data centers more or less doubled in power consumption between 2000 and 2005. According to the EPA’s analysis of hardware trends, this is expected to go up another 40-70% between 2005 and 2010. So if you’re planning on being in a facility for the next ten years or so, it’s probably best for you to spend some time figuring out exactly what your average power consumption is per rack (for a 42U high density compute rack, I’m working with about 22kW per rack as a baseline for 2008), and assuming between now and 2020 that number is going to go up by somewhere between 100-120%. That’s somewhere in the range of 42-45kW per 42U rack.

To put this in perspective, a 45kW rack would require just shy of 13 tons of coolant in refrigeration capacity. Ten racks loaded with this sort of equipment means you’re requiring not one, but two 65 ton industrial strength chillers to keep that room at a constant temperature. The energy costs here are staggering. Smart corporations should be looking to move their datacenter operations to someplace where land is cheap, solar panels can be constructed easily, and the average temperature is as cool as possible. Me personally, I’d be looking to stimulate the economy and cutting a good deal somewhere in Montana.

True, not everyone is building high capacity compute nodes. And there are a number of other alternatives in industry gaining traction, such as switching from AC power supplies to DC power supplies, which are much more efficient at idle and consumer less power even under load. With the rise of multi-core processing systems, the idle-to low activity power consumption per machine will probably go down even more… but those high-power activities are still going to burn energy like crazy, and that’s going to produce lots and lots of heat. Expect this curve to keep on going up.

Keep cool, people.


Posted October 30, 2007 by padraic2112 in facilities, hardware, management, tech, work

One response to “Facilities Design

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  1. Pingback: Facilities Design « Pat’s Daily Grind

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