Vali Sorell, P.E., is a lead technical resource for mechanical design in Syska Hennessy’s Critical Facilities Group. Syska Hennessy provides facility management consulting, engineering design, structured cabling/audiovisual/security design, commissioning and construction services.
Designers have long assumed that data center projects should not use outside air economizers (a.k.a. airside economizers) in warm climates. The reasons are usually obvious – when it’s 90°F or higher outdoors for much of the year, there couldn’t possibly be any benefit in bringing in that warm outside air. However, when specifically considering data center applications, there are several reasons why this logic is faulty.
Benefits from Using 'Warm' External Air
An important design consideration when sizing the cooling devices for data centers is that the higher the supply air temperature that goes into a cooled space, the more efficient is the cooling equipment. ASHRAE Technical Committee 9.9 publishes the recommended “Thermal Envelope” for IT equipment - a range defined as 64.4°F to 80.6°F at the INLET to the equipment.
If air flow management strategies are well implemented in the computer space to assure that the supply air temperature and the IT equipment INLET temperatures are nearly the same, then substantial first cost and operating cost savings can be realized by selecting as high a supply air temperature as possible. A good supply air temperature to design for is 75°F. Although it’s not quite as aggressive as selecting for 80.6°F, it’s much more aggressive than the typical supply air temperatures from a few years ago, which often hovered in the 55°F to 60°F range.
Another aspect to consider is that most IT equipment takes in air through the front of the equipment, then adds the rejected heat from the internal components (such as processors, power supplies, disk drives, and memory), and finally rejects the heat at the back. Typically, the air exiting this equipment can be 30°F warmer than the air entering it. Assuming once again that the temperature of the air entering the IT equipment is at 75°F, the typical discharge temperature should hover at 105°F!
Humidity is a Factor in Cooling
So, the big question is this: Which airstream requires less energy to cool to 75°F? Is it the outdoor air at 90°F or the air returning from the back of the equipment at 105°F? The simple answer would be the air at 90°F. However, reality is not quite that simple because the 105°F at the back of the servers is usually extremely dry; on the other hand, the 90°F outdoor air may be very humid. Yet, these humid conditions can be accounted for when considering hours of availability of airside economizers. Even after those humid hours are excluded from our count, there are still many hours of economizer availability in warm climates.
Compare with Cooling Office Space
To put these hours of availability into perspective, consider that of the 8,760 hours in a year, the hours of airside economizer availability in areas such as Dallas, Raleigh, and Phoenix are somewhere in the range of 4,000 to 5,000 hours. Commercial office spaces with normal 8 a.m. to 5 p.m. operation run for approximately 2,500 hours per year. The airside economizer for data centers can enable a data center to run for more hours in a year than ALL the operating hours of a typical office building without ever turning on its cooling plant.
In summary, one should not eliminate the use of an outside air economizer for a data center simply because of a perception that the climate may be too warm. The updated “Thermal Envelope” has allowed for many more hours of economizer use, and there is promise in the not too distant future that the envelope will be expanded even more. Only a complete Total Cost of Ownership analysis can determine whether there is enough availability of free cooling to justify the extra cost of the economizer.
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