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Sizing computer room air conditioners for data center energy efficiency

SearchDatacenter.com recently had a great article by Bob McFarlane with several handy tips on sizing computer room air conditioners and save on energy. Here are some excerpts:

 

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!

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.

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. 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.

For the full version of the original article, visit  http://searchdatacenter.techtarget.com/tip/0,289483,sid80_gci1371079,00.html?track=sy185# . Free Registration might be required on the site to view content . To know more about Green Rack Systems and the suite of services we offer in addition to sustainable cooling practices for Data centers, contact us at sales@GreenRackSystems.com .

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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Lavenya Dilip was responsible for Marketing at Green Rack Systems.