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Monday, July 2, 2012

Some useful data center design formulas


Ever tried to translate between IT-speak, electrician-speak, and heating-ventilation-air-conditioning-speak?  Adding to the confusion, HVAC is known as “mechanical” in the building trades.

Here are a few useful formulas and rules-of-thumb for translating units between different disciplines, along with related data center design tips.


I was an IT guy embedded in a scientific research organization, so I tried to reduce everything to watts (power) or watt-hours (energy) to simplify my life.


1 watt = 1 amp x 1 volt

If your favorite piece of IT gear draws 3 amps at 120 volts, that's about 360 watts. Except when it's not, since most equipment rating plates show the maximum amps the power supply will draw. IT power supplies are almost always over-speced, so try to find or measure actual power draw and peak power draw.

For IT power supplies, the formula also depends on the power factor. Virtually all IT power supplies made recently have a PF close to 1.0, so amps times volts is pretty close.

Sometimes computing IT power draw requires a bit more effort. Many IT specs show the power supply rating, but not the actual power draw. Sometimes specs will show “heat load” or something similar, in BTU/hour. So converting between BTU/hour and watts is handy.


1 watt at the circuit = 0.80 watts usable at the power outlets

This rule surprised me on many occasions. A typical 20 amp, 120 volt circuit should be able to supply 2400 watts, right? Nope. US electrical and fire codes require 80% derating to 1920 watts. Same thing for a 30 amp, 208 volt circuit, derated from 6240 watts to 4992 watts. In most data centers, you will lose additional power capacity going through UPSes, too. Check your UPS specs for details.


1 watt = 3.4 BTU/hour
1 BTU = 3.4 watt-hours

Confused already? Welcome to the exciting world of mismatched units! BTUs are units of energy, BTU/hour are units of power. Sometimes BTU/hour is abbreviated to BTU, to really confuse things.


1 person = 600 BTU/hour
1 person = 175 watts

Most data centers are rarely occupied, but I was responsible for specifying computer labs, high-tech conference rooms, and similar occupied spaces, too, so this rule of thumb can be handy.


1 Ton of Air Conditioning = 12,000 BTU/hour
1 Ton of AC cools about 3.5 kilowatts of IT load at 70° F
1 Ton of AC cools about 5.4 KW of IT load at 90° F

The last two rules can be useful for translating between HVAC-mechanical-speak and IT-speak. Air conditioner specs are often rated for human-occupied areas, where latent cooling is important due to humidity changes. Data centers are rarely occupied, and IT equipment doesn't add or remove water from the air, so you can ignore latent heating loads, and focus only on sensible heating. An air conditioner rated for 1 Ton of latent cooling at 70° F, should be capable of cooling a lot more than 3.5 KW IT sensible-only, at 90° F. This quickly gets into serious HVAC knowledge; make sure you work with a highly-trained HVAC-mechanical contractor who can make these calculations, and explain to you in plain English.

Why the formula for 90° F? Because you can run your data center up to 90° F, save a lot of money on air conditioning capital and operating costs, and stay within the specs of your IT equipment. Most IT equipment will run just fine up to 90° F, a lot at 95° F, and some even higher. You must design to your lowest spec, unfortunately. Just remind your sys admins to wear lightweight clothing to work on hot days!

You can almost ignore humidity, unless you have extended hot-dry or cold-dry spells, and need added humidity to stay above 10% relative humidity. On hot-humid days, the air conditioning lowers RH for free. On cold-humid days, recirculating IT heat to maintain room temperature above 68° F will lower RH for free. Look at the specs of your IT gear: virtually everything is happy with 10%-90% RH, and a lot of equipment is good for 5%-95% RH.

You don't need heaters. (Maybe if you are at the South Pole). All of the electricity going into your IT equipment is turned into heat, which you then work hard to move outside. If the environment gets too cold for your IT specs, recirculate your IT exhaust heat.


1 horsepower motor requires about 850 watts

HVAC fan motors are often rated in hp. The standard definition of 1 HP is 746 watts of motor output; I included a fudge factor for motor efficiency to compute electrical input, if you can't get input from the fan specs. Premium constant-speed motors running at 60% to 100% load are 85% to 95% efficient.

Why should you care? Your backup generators need to run all of your IT load, and all of your HVAC load, including air conditioning compressors and fans. Startup power requirements for air conditioning compressors can drive generator requirements quite high. That's another good reason to tolerate high data center temperatures, so you can specify smaller air conditioning units, with smaller startup power surges, leading to smaller generators.


100 cubic feet per minute cools 1 KW at 30° F delta-T
155 CFM cools 1 KW at 20° F delta-T

These are the most important, and hardest-to-find formulas listed here. As I've mentioned before, your most important data center design units are kilowatts and cubic feet per minute. If you are running without the AC compressors most of the time for efficiency (“free cooling”), you must move enough air through your IT equipment to keep the insides cool. Major IT equipment manufacturers use 30° F delta-T for servers and storage. Delta-T refers to the temperature drop from the cool air intakes to the hot air exhaust, on your IT equipment. I first found versions of these formulas in papers on Chatsworth's web site; Dell and others use the same formulas.

More CFM are relatively cheap for both capital and operating costs, so it's OK to specify a little high to be conservative. On the other hand, too-high CFM creates other problems.

In an ideal world, you want variable speed fans tied to the data center heat load or temperature. Variable speed fans are also called variable frequency drives or VFDs in HVAC-mechanical-speak. VFDs are more expensive, about 8% less efficient at full load, and hard to find in off-the-shelf systems. But VFDs save lots of energy when running at less than full loads; the added expense can have a 12-18 month payback time. For example, at 63% speed, a VFD uses about 25% of full power.






1 comment:

  1. This was a great read on Data Center Design formulas. It was very informative, you have opened my eyes to some things that I did not know. Thank you for sharing this with us Rocky.

    ReplyDelete