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Let's add an air conditioner

This column originally appeared on TechTarget's Expert Answer Center as a post in Robert McFarlane's blog. Robert served as the on-demand expert on the Expert Answer Center for two weeks in October to November 2005, during which he was available to quickly answer questions on data center design as well as to write daily blog entries. Keep an eye on the Expert Answer Center for topics that could help your IT shop.


It happens all the time. Chances are, it's happened to you. Equipment is added. It starts getting hot. You call...

Facilities. They bring in another big air conditioner. They put it wherever it will fit. It's disruptive. It costs a lot of money. And the hot equipment is still hot!

In yesterday's blog we gave the first steps to getting the most out of what you already have, perhaps avoiding or at least postponing, the need to bring in more cooling. But if that still doesn't do it, you're going to need another remedy.

In the old mainframe days, we could just pressurize the floor and the air would come up through the openings right under the equipment. There weren't very many holes, so the air pretty much had to go through them. And the hardware was designed so an opening in the right place would push cold air right into the box, exactly where the manufacturer wanted it. Those days are gone. It's much more complicated now. We're trying to push a lot of air through a lot of perforated tiles, and it just doesn't work as nicely as we think it should. Why?

The air coming out of a computer room air conditioner (CRAC unit) moves fast. Basic physics (Bernoulli's law) tells us that the faster it moves, the lower the pressure -- the very pressure needed to push it through the tiles. Most people are surprised to learn that, even 8-12 feet in front of the CRAC, the pressure may actually be negative, pulling air back down through the tiles rather than pushing it up. The Computerized Fluid Dynamics (CFD) illustration below shows that happening (image is from Innovative Research's "TileFlow" CFD program). Look for the red arrows pointing downward close to the CRAC that hasn't been turned off (opposite the one with the red "X"). You definitely do not want your highest heat hardware closest to your CRAC units. That's the place for patch racks and other relatively benign equipment.

There is a company called Technology Connection that advertises that they have developed analytical methods and products that enable them to reduce that velocity, re-channel the air and equalize the pressure. We have discussed this with them, and their reasoning seems scientifically sound, but we have yet to make a first-hand evaluation of their results. We can only mention them without comments, pro or con.

Second, we now know that air discharges from CRAC's in what are known as "plumes." These are like horizontal columns of air that spread out a little as they get farther away, but not really a whole lot -- especially if there is another CRAC blowing the same direction nearby. In that case the two air plumes create a barrier where they come together, keeping the air from one CRAC from mixing with or supplementing the air from the other. The CFD model illustrates this effect. (It also illustrates the pressure increase in the middle when two opposing air streams collide.) So adding a CRAC without carefully considering where it is going and what else is around may actually reduce the air flow to the places you need it most.

Worse yet is putting one CRAC at a 90-degree angle to another. This often occurs at the corner of a room, especially when there isn't room to put a unit anywhere else. You can easily demonstrate this effect with two garden hoses. Just shoot one stream at the other and see what happens where they meet. Air is a "fluid," so it obeys the laws of fluid dynamics. If one CRAC blows the air from another CRAC away from where you need it, it's counterproductive. If there's no other choice, at least space them as far from each other as possible.

So what can you do? First is to make sure you really need more cooling capacity (see my previous blog entry). Look for openings that waste air. Look for blockages under the floor. Measure your total load (your UPS should tell you) and compare it with your air conditioning capacity. As a quick estimate, figure that 80% of CRAC tonnage is available for equipment cooling. (The rest goes for humidification, normal losses and inefficiencies.) 1 kiloWatt (kW) = 0.283 tons.

EXAMPLE: For a 100 kW load, you need 28.3 tons of cooling. Multiply this by 1.25 (the inverse of the 80% derating) to get 35.4 tons. If you have four 15-ton air conditioners -- and assume 25% redundancy -- you should have 3 x 15 or 45 tons of constant cooling capacity, which is about 27% more than you're supposed to need. So if things are getting too hot, you should be looking at less expensive solutions before you just roll in another CRAC. (This assumes, of course, that you don't have miles of windows adding lots of solar load or some hidden problem with outside air infiltration.) In short, your problem shouldn't be capacity. You're just not getting the air where it's needed. It may take expert analysis to solve this, or to determine whether there's a solution at all based on how your room is laid out, but there are several things you can look for first.

  1. Are your high heat loads located where you have the best cooling and the most spare capacity? It's easier to move the cabinets than to install a CRAC.

  2. Are the dampers wide open on all the perforated tiles, including those where there are low heat loads? (This is bad.) Are there any perforated tiles in "hot" aisles? (This is worse.) Try closing down tiles where less air is needed and closing them completely where no air is needed. Get them out of hot aisles!! More air will be available where you really need it.

  3. Are CRAC units pulling cold air back into the returns? If you have enough ceiling height, try putting extension ducts on top of them so they draw mainly hot air from closer to the ceiling. You can even extend duct work all the way down hot aisles to ensure that only hot air gets back to the units. Again, this is a lot cheaper than more air conditioners, and in many cases will end up working better.

  4. If your cabinet manufacturer makes fan-powered doors or in-cabinet blowers that add onto your cabinets, try them. See if they help. They're not all equal. One type may do a better job than another. You can't always believe the marketing claims, but it's worth a try. Just one caution; blowers that suck the air out of the floor may cool the cabinets they're in very well, but may also steal too much air from other cabinets in the process. Don't solve one problem by creating another.
If none of this works, or can't be done, and you really need additional cooing, consider some of the newer, localized or "spot cooling" solutions. There are several approaches. We can't go into the pros and cons of each in this space, but here is a quick summary. The order has nothing to do with how good they may be, either in general or for your particular needs.

  • Water-cooled cabinets may solve individual problems. If you need more than a few, however, you're probably better off with another solution. (See Knü rr, Sanmina-SCI's Ecobay, RIMatrix5, APC and Liebert XDF).

  • "Heat containment rooms" are areas within your data center. They consist of two rows of cabinets, back-to-back, totally closed-in with a ceiling, end walls and their own power and air conditioning. They completely isolate the hot air from the cold, making them very efficient, and they definitely work. They consist of two rows, back-to-back, and can be made pretty much as long as you want, but it's best to decide on the size you will grow to when you put one in. They can be expanded, but it can be problematic. CAUTION: The center "hot aisle" of this design is definitely hot! Be prepared for objections from your technicians when they need to work inside. (See APC's Hot Aisle Containment.)

  • Flexible "spot cooling" enables you to add cold air to the cold aisle at just the cabinets that need it and to easily add or move cooling if your hardware locations change. It requires fundamental infrastructure (piping and power), but it's no more difficult or disruptive to install than adding one CRAC. Once in place you have modular growth as well as significant flexibility for a number of cabinets from a single installation. Note, however, that you don't put this in for one or two cabinets. It's designed for a significant load, like from four to 20 cabinets. (See Liebert's XD line.)
Whatever solution you may choose, it's best to locate these specialized systems in a designated area of the data center. It's more cost effective, easier and less disruptive to install and manage if it's not scattered all over. And if you still think another CRAC is your best answer (and in some cases it may be), be sure a thorough, knowledgeable analysis has been done -- including CFD modeling -- before you spend that money.


Even though Robert's stint on the Expert Answer Center is over, he is always ready to answer your questions on SearchDataCenter.com. Ask him your most pressing data center design question.


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