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Tech Tips: When it Comes to Affinity Laws, We're Big Fans

July 21, 2011


Judging by the triple-digit temperatures recorded recently out of our Georgetown, Texas plant, you'd be hard-pressed to imagine the official start of summer is still a week or so away. But sure enough, all around the country, things are heating up - and when things heat up, fan use kicks into high gear.

Of course, here in the data center world, we aren't talking about your usual fan box propped up in a brownstone window - we're talking server fans, industrial sized CRACs, CRAHs and every other kind of swirling, whirling apparatus designed to keep our data processing at a comfortable temperature - even as the outside world bakes and broils.

To that end, fan affinity laws can provide a simple yet effective way to help reduce power consumption in the data center when it comes to the operating set points of your cooling equipment. Take this explanation into consideration:

If you have a data center that needs 10,000 CFM of cooling, you might have an air conditioner that provides 10,000 CFM at 100 percent load, and another one just like it turned off for redundancy. The fan affinity law here essentially operates like the cube effect - instead of running one A/C at 100 percent capacity, you could run them both at 50 percent load. This means that you'd still have the necessary 10,000 CFM with each A/C providing 5,000 CFM. Logically, it would seem that running one at 50% would reduce the energy in half, right? That's where the fan affinity law comes further into play. Instead of X .5, it becomes .5 X .5 X .5 = .125 or 1/8. So with each running at 50 percent load, instead of using 50 percent of the energy, we're only using 25 percent of the energy it takes to run one A/C at full capacity! For you math enthusiasts out there, it breaks down like this:

(Q1/Q2) = (N1/N2)3 whereas (P1/P2) = (N1/N2)3 

Get it? Got it. Good!

When you realize that most traditional data centers may be over provisioned for two to three times normal demand, fan affinity laws begin to look like an awfully attractive solution - especially when you couple this idea with CPI's Passive Cooling® Solutions. With passive cooling, you only need to provide what your demand is 1X. So while there's always savings to be had, with fan affinity laws working hand in hand with passive cooling, there is considerably more efficiency taking place - possibly resulting in paying as little as 25% of 1/3 of what your current A/C costs are!

We'd say savings like that go a long way towards making summer extra cool. Jeff Cihocki, eContent Specialist and Fernando Reyna, RCDD & Senior Technical Support Specialist 

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