Don’s Corner: In-row or Perimeter Cooling, is there really a difference, and if there is, which is better?

Posted by on October 4, 2012

Don Melchert, Critical Facility Specialist

I love two-part questions, don’t you?  It shows me that the person asking it probably has some experience with the subject and is asking a loaded question.  Usually, the inquiry stems from an experience that didn’t go all too well, or, it went much better than planned and the question is asked more from a validation standpoint than an argument.  I’d much rather prefer the latter, of course, but in data centers, that’s usually not the case, is it?

With any subject that’s controversial, you’ll never win everyone over.  In the argument over In-row or Perimeter Cooling, the subject is more volatile, especially if the inquisitor’s job is to maintain an antiquated (read dinosaur) perimeter cooling system.  You just know they’re thinking to themselves, “If that system goes away…now what?”  Ever see the latest version of Willie Wonka and The Chocolate Factory?  When the machines took over production, Charlie’s dad was laid off, but in the end, he came back to work as a repair technician on the machine that replaced him.  What made Mr. Wonka decide to replace a human with a machine?  Was it production, quality, or floor space?  If you’re a Co-lo provider, floor space is high-dollar real estate!  So, for this article, let’s use floor space as our filter, and to keep the playing field even, we’ll assume a chilled water system.

After some research, and I’m putting that lightly, I’ve found the Liebert DS to be the most commonly deployed air conditioning system for data center use.  Really, I’ve seen them all over the globe!  Why shouldn’t they be the most common?  They’re easy to train on, so you can get training almost anywhere; they have a long service history, so MTTF rates are pretty accurate; they’re supported by a ton of people, so the MTTR rates are well documented, too.  Of course they have to be shut down to make most repairs, but that topic’s for another discussion.  Even though they’re the most common, the Liebert DS, and perimeter cooling in general, is losing ground to more efficient, close-coupled, In-Row and overhead solutions.

Floor space is the name of the game, so what kind of real estate does the most commonly found perimeter unit gobble up?  Simple answer: 21sq ft.  I said “simple answer” because I made a comment above about having to turn them off for repairs.  Which is why I found that most perimeter cooling believers had more than one Liebert in the room.  Even without a calculator, that’s 42 sq ft, folks, and that’s only at N+1 redundancy.  Within that 21 sq ft area, a Liebert DS can cool up to 42kW of critical load.  Not too shabby, but if you have any kind of redundancy requirement, you’re trying to get the most out of that space.  With a Co-location facility, now you’re looking at how many customers can I not fit into the room because you’re forced to give up 21 sq ft of real estate?  That kind of talk doesn’t fly too well in a boardroom.  So what’s the alternative?  Let’s look at one, the APC In-row, close coupled cooling, or IRRC.

The In-row RC, takes up only about 3.5 sq ft, but it makes up for it’s diminutive nature by removing 18kW of heat load.  With reference to floor space (…and this is where, somewhere out there, my high school geometry teacher is laughing her evil laugh), you’d need a bunch of IRRCs to equal the same amount of real estate consumed by a Liebert DS.  The answer is 6, Ms. Davidson (flashback to 9th grade).  With 6 IRRCs, one could cool up to 108kW of critical load!   Before you ask, no, you won’t need to make them redundant, as the most common failure items, the fans and power supplies, are all hot-swappable.  Here’s something else to chew on: the IRRCs don’t have to be right next to each other to get the job done.  At about half a rack wide, they can be separated around the room as needed, but you’ll need a sawzall and a darn good explanation afterward if you try to do the same thing with a legacy style perimeter unit!  Please, if you do this to a perimeter unit, invite me out and I’ll buy lunch?

Knowing that I’d have to prove my math, I worked the number backward and concluded that with 108kW of critical load, one would need 2.5 Liebert DS units to remove that much heat, or step up to the next size unit.

That’s where it really strikes home for me, personally.  After performing a CFEP, I’m often challenged with explaining why its important to let go of the ancient ways and consider converting from a perimeter cooled space to a modern, close-coupled solution.  If floor space is the driving factor, then the math, for me, drives the point home fairly well.  If a Co-lo data center can increase their customer base from 12 to 108kW, simply by becoming more efficient in their cooling architecture, maybe Charlie Bucket’s dad might not be laid-off after all.  In today’s job market, that means a lot.

Don Melchert

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