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When I was looking at a a new personal home being built not too long ago, I was surprised to see how small the air ducts were. I asked the person showing me the house "Why are the air ducts so small? This is Texas! Air conditioning is the only way you can live here." They answered "Oh, those are the new high pressure ducts." as if it explained everything. I guess it did explain everything for them, but just being the new thing was meaningless to me.

I found this article over at toolbase about them. As the house in question was in Houston, the bit about this style of HVAC removing 30% more humidity suddenly made it all come into focus. Houston, Texas was built directly on top of a swamp, and one of its two founders (John Allen, of the The Allen Brothers) died within a year of moving there of Malaria or perhaps Yellow Fever. Houston is a damp place. Having your HVAC be better at removing water? Big win.

When I think about the differences in a data center raised floor, I wondered about similar issues. What are the considerations for how tall the floor should be. In the case of the data center I have in Houston, it has 18 inch raised floor and is set up to dissapate 38 watts per square foot. I have another data center on six inch raised floor, and it dissipates 76 watts a square foot. And as mentioned previously in this series, I have seen brand new data centers on 36 inch raised floors, set up for 250 watts per square foot.

Noise

Thinking about the reasons and design considerations for all of this, I looked at a number of web sites documenting HVAC design, and giving formulas for computing how big a duct needs to be to handle a given flow of air. Intrinsic to that is noise. The math works out mostly that if you want a certain level of air flow, the smaller the duct, the faster the air has to move (and, as it related to the example of the home HVAC, the higher the pressure, thus why the person showing me the tiny ducts called it the 'high pressure' system, when the industry appears to call it the 'high velocity' system). That seems intuitively obvious. If you look at the link above about the high velocity ductwork for homes, there is sound suppressing tubing in the design. This all makes perfect sense.

To move air quickly and with pressure requires tighter (less leaky) ducts, more specially designed fans, and there is more friction in the system, causing losses. Small ducts are therefore not as energy efficient.

In theory then, I could figure out how much air I would need to pump in a 6 inch raised floor to meet any given heat dispersion number, but at some point it is going to get very silly. The high velocity air will be whistling between the seams of the floor tiles, it will be noisy, power inefficient, and who knows? I might get the pressure up high enough to make floor tiles start popping up like geysers. Probably not, but that would be funny. Once.

For raised flooring (assuming the room is tall enough) the major expense is in the tiles and the grid, not in the pipes that extend the grid/tiles away from the slab. Those are little more than metal pipes, and it more or less costs the same in labor to bolt/glue the pipes to a floor no matter if the pipes are 6, 18, 36, or even 72 inches long. The bolt/glue and plate are the same.

So, when building new, why not go tall if you can? The taller the floor, the less you have to worry about figuring out the airflow. The more future proof it is in case you need to increase the airflow. Make it 72 inches, and you can walk around down there. With a tall floor you can deal with having some cables running around under the floor and not impeding airflow significantly.

Preexisting Floor

I am looking at something already in place. a 16,000 square foot room with a 250 PSI concrete subfloor, raised 18 inches and set up for 38 watts per square foot. I can add more air handlers to the space and decrease the available floor space, but I only think I need 10,000 square feet, so that is not an issue. The question becomes: what makes sense? How many CRAH's can I install? How many watts per square foot can I deal with in a room like this?

One other part of the equation is that the room was designed for water cooled mainframes back in 1993. I have all sorts of chill water and a dedicated 450 ton chiller up on the roof.

Cold Math

450 ton of dedicated chiller. 16,000 square feet. That means I can drop about 1.5 megawatts into that space. The most I can do in terms of heat dissipation without adding more chilled water is 100 watts per square foot. Drop that to 10,000 square feet, and that will be 160 watts a square foot.

Not bad, but not 250 watts a square foot. Still, if all I need is 160 watts or less, this seems promising.

Airflow

The room I am looking at has a dropped ceiling. With cowls on the CRAHS, I can fairly easily set up hot / cold aisles, and control my cold air supply and warm air return, keeping them properly separated. I can drop the power and networking in from overhead rails, keeping the underfloor fairly free of air dams. So is 18 inches enough for 160 watts per square foot, without turning the floor into a whistling windy mess?

Easily. I have that data center on six inch floor, at 76 watts per square foot, and it is working fine, and makes no obvious noise beyond the usual white noise of data center fans. 18 inches is three times that height, and we are not looking at three times the heat dissipation. It should not even have too much velocity / pressure in it such that the CRAH's are having to work a great deal harder to pump the air around. Since I have 16,000 square feet,and only need 10,000 of it, I can further put any new CRAH's right next to the cold aisles consuming that air, keeping the airflow paths short.

YMMV

I used my real world data center here by way of example. Looking out across the vast landscape of empty, low watts per square foot data centers, I wonder how many of those are in similiar situations. How many were designed for water cooled mainframes and have fairly tall floors to deal with how big things like 370 channel cables used to be. High PSI floors designed to have mainframe motor-generator units sitting on them. How many data centers sitting empty have good bones?

Next: I am going to try and figure out a cost model. How much should I pay per square foot for a new data center versus getting an old data center cheap and refitting it.