Separation anxiety

Engineered Systems, Oct, 2008 by John Hazucha, Kevin Dickens

In Kevin Dickens' article, "Overcoming Hot Aisle / Cold Aisle Separation Anxiety," published in the May 2008 issue of Engineered Systems, he showed computational flow diagram (CFD) images of data centers with the following:

* No ceiling or hot/cold aisle separation

* With ceiling plenum but no hot/cold aisle separation

* With ceiling plenum AND with hot/cold aisle separation

However, he did not show:

* No ceiling but with hot/cold aisle separation

Was that analysis done, and if not, how do you know that both physical separations were/are important?

John Hazucha, P.E.

Ellerbe Becket

Minneapolis, MN

Dickens responds:

The answer in our particular case is no. The reason is our design required the ceiling because there would be no other path for the return air.

Specifically, If you think of each hot aisle as an island with an enclosure from the cabinets to the deck then you can see that without a ceiling plenum or duct work to each "hot pod," the hot air would have no path back to our mechanical galleries on the perimeter.

So that begs the question, is there a design where both vertical and horizontal separation is not required? The smart money says yes, but I would say there are some limits. If you didn't create hot pods but instead only contiguous hot aisles by eliminating the enclosure doors on the ends of each pod and extending the vertical separation across the perpendicular aisles, then these "hot alleys" could provide an unencumbered path for the hot air back to the gallery with no chance for mixing with the cold air in the cold aisles. But this concept would likely be limited by the width of the data hall.

Based on the runs we did with and without ceilings without vertical separation, I can say our data hall probably wouldn't have worked with this hot alley approach because the heat just built up too much trying to cross 100 ft of open hall.

But all of this is conjecture without a CFD.

I would say that anything you could do to provide separation while minimizing infrastructure is a plus. Ceilings cost money, so if you come up with design that doesn't need one, that's a good thing.

If you see things differently or if you have a design without a ceiling that worked (with similar Watt density of course) let me know. I'm always willing to learn a new trick.