Sizing computer room air conditioners for data center energy efficiency

Sizing a data center air conditioner is not like choosing a refrigerator. Bigger is not necessarily better! Correct sizing is even more critical to effective operation and energy efficiency than right-sizing the uninterruptible power supply (UPS). But with so many factors that determine capacity, it can be a bit tricky.

When someone plays with the thermostat at home (not you of course!), the temperature is never right. It gets too hot, then too cold. It's worse with computer room air conditioners (CRACs). The unit that's the wrong size can mess up cooling. Wrong settings or improper location will make it even worse.

Under-sizing can't cool effectively -- that's obvious. But over-sizing won't either. Thankfully, many CRACs will adjust to a range of loads, but there are many that won't. They all need to be sized realistically, but over-sizing will always result in cooling going on and off too often. It's called "short cycling," which is hard on the machine and does a lousy job of maintaining room temperature and humidity. Yes, temperature swings do hurt computing hardware!

Computer room air conditioners with refrigeration compressors -- the true CRACs -- are available in "multi-step" designs. A 20-ton, four-step unit may activate 5 tons of cooling before enabling the next step, as heat increases to 10, 15 and 20 tons of load. Chilled-water units (more properly called computer room air handlers, or CRAHs) have internal valves that adjust water flow to match the load. They usually work effectively down to about 20% of capacity. But what is capacity, and what the heck is a "ton" of air conditioning?

It's actually pretty simple, but comes from old practices (as do most crazy American measurements). Early air conditioning simply blew air across blocks of ice into the room. Melting 2,000 pounds of ice in 24 hours was defined as a ton of cooling. It happens to take 12,000 British Thermal Units (BTU) per hour (another nutty unit) to do that, so 1 ton of air conditioning = 12,000 BTU per hour.

Today we are starting to rate cooling in kilowatts (kW). A ton of air conditioning can cool about 3.5 kW of heat, so a 20-ton CRAC should cool around 70 kW. If we know our data center power loads, we can choose a unit with the right capacity: no more than 20% over-sized for a fixed-capacity unit, and, if we need growth capacity, maybe as much as 50% larger for a chilled-water or multi-step. But hold on -- there are too many different "capacities" on the data sheets. Which one do we use? We still need to know a couple of tidbits.

Air conditioners have to deal with two kinds of heat. Sensible heat -- the kind we can feel -- is what our computers give off. Latent heat is what evaporates moisture. Simplistically, dealing with moisture or humidity requires more latent capacity from our air conditioners, which steals from sensible capacity. There's not much reason to keep a data center above 45% relative humidity (RH), but if you over-cool you'll pull moisture out of the air (latent cooling) and have to use more energy to re-humidify. The problem is that relative humidity is "relative" to temperature. Warmer air has a lower relative humidity for the same moisture content because it can hold more vapor than cool air. Temperatures in a data center vary widely, so RH depends on where it's measured, which is why we're trying to get away from using it. However, RH is still the most common way to determine humidity.

Most air conditioners are controlled by return air temperature. Believe it or not, hotter return air enables the CRAC to provide more actual cooling capacity. So if you dial down the temperature in a heavily loaded room, you'll get less heat removal, and the place may actually get warmer -- and you'll waste a lot more power doing it! The following chart shows how humidity level and return air temperature can affect performance from the same nominal 22-ton chilled-water air conditioner. Note that at high temperatures, RH must be lower to keep moisture content below maximums.

If you use ducts or the ceiling plenum to channel warm air back to the CRACs, and keep the return air at 80 degrees Fahrenheit or higher by preventing it from mixing with cold, you can get more actual cooling capacity from the same machine. And keeping humidity lower makes it even better at any return air temperature.

But there's more. Air conditioning takes both cooling capacity and air flow. Opening the refrigerator door won't cool the room. Air movement has to carry the heat away from the equipment and back to the CRAC, just like a nice breeze in summer. So more air should be better, right? Not necessarily. If the floor is low, too much air from too big a CRAC means higher velocity, and that means lower pressure. (That nasty physics comes into play again.) Air can actually be sucked down through perforated tiles as far as 8 to 10 feet from the CRAC, which wastes air and energy and also reduces cooling. Too much air can also create under-floor turbulence, like small tornados. That makes under-floor pressure uneven, which further reduces cooling effectiveness. And it's worse if CRACs are placed at right angles to each other. High pressures also make the CRAC fans work harder, wasting more energy.

Thankfully, today we can use variable frequency drives (VFDs) to automatically adjust fan speeds for appropriate air flow, controlled by sensors in the room. These can be retro-fit to most existing CRACs, and can save a lot of energy. (A professional computational fluid dynamics, or CFD, analysis is a good idea before buying any expensive air conditioner.)

So Step 1 is to know your real loads, as covered in a previous article. Step 2 is to see if you can get higher temperature return air back to the CRACs. Step 3 is to decide cold air temperature. The American Society of Heating, Refrigeration & Air Conditioning Engineers (ASHRAE) Technical Committee 9.9 has recently increased the temperature envelope, so there's no need to over-cool the equipment. Step 3 is to set your humidity standard. ASHRAE TC 9.9 now recommends dew point monitoring and control, but existing CRACs may not be able to do that, so you'll still need to control relative humidity. Then, if possible, pick an air conditioner that can adjust to load and choose a sensible capacity that will operate Day 1 in its midrange. That will give the best stability and control.

Let's look at three other important issues before we finish: reheat, humidification, and water temperature. If you have more than three or four CRACs, it should not be necessary to put humidifiers on every unit. Moisture diffuses and stabilizes in the room pretty quickly (another reason for dew point sensing). Putting humidifiers on every air conditioner can be counterproductive if one unit humidifies while another de-humidifies. That's wasted energy for no better result.

Reheat was the norm for years, and it's the biggest energy waster of all. The CRAC over-cools the air and a heater warms it back up to discharge temperature. In many situations it's possible to design without reheat, or to use minimal reheat. But it takes a knowledgeable engineer to make that determination and to provide a proper design.

If you're using chilled-water computer room air handlers, you'll need to have a knowledgeable engineer involved. Published capacity ratings are based on specific entering water temperature and water temperature rise. Chiller plants today may be designed on higher numbers to improve energy efficiency, but that reduces the effective cooling capacity of the CRAHs.

Finally, don't overlook opportunities to use "source-of-heat" cooling. That's beyond the scope of this article, but the more cooling you can get near your highest heat loads, the better your cooling will be, with less energy and less capacity needed from those big CRACs. That's another big opportunity.

Keep cool!

ABOUT THE AUTHOR: Robert McFarlane has spent more than 30 years in communications consulting, with experience in every segment of the industry. McFarlane was a pioneer in the field of building cabling design and a leading expert on trading floor and data center design. He is currently president of the Interport Financial Division of New York-based Shen Milsom & Wilke Inc. and a data center power expert.

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