Data center air-conditioning fans blow cost savings your way

It may not be your first thought, but design and speed control of data center air-conditioner fans can generate significant savings.

When people think about energy savings, data center air-conditioning fans don't necessarily leap to mind. But choosing the right fan design can save up to $50,000 per fan over a 15-year lifespan.

Up to 37% of a data center's energy costs can be attributed to cooling infrastructure, according to EYP Mission Critical Facilities. A central part of cooling data centers is moving the air, and a big part of air movement is the cooling unit's fans. Incremental changes in the type of fan, airflow direction and fan speed offer opportunities for energy efficiency have created better options for today's fan designs.

In the U.S., computer room air-conditioner (CRAC) fans used to be forward-curved centrifugal fans inside the unit blowing air directly down into the underfloor at full steam.

Data center CRAC fan design
Traditional fans' centrifugal design calls for up to three fans to be attached to a single shaft. They operate through a belt-and-pulley system. But where there are belts and pulleys, there is also wear and tear.

For this reason, data center hosting provider 365 Main moved away from standard fans. Jean Paul Balajadia, the company's senior vice president of engineering, said the new fans "don't have pulleys and belts to look at regularly. It saves us a lot of time and money in maintenance and efficiency."

"Belts have losses themselves," added John Bean, the director of innovation racks and cooling at American Power Conversion. "They'll slip a little, and so a few percent of the total motor power is consumed by belt friction losses and elasticity of the belt."

Balajadia calls the new fans "direct drive" fans, but they are also known as plug or plenum fans. Instead of operating through belts and pulleys, the fan motor is connected directly to the fan. In addition, the opening through which a plug fan blows air is bigger, thus allowing for a more even distribution of air underneath the raised floor. With the smaller opening of a centrifugal fan, the air velocity is initially much higher, which can lead to unpredictable and inefficient airflow patterns under the raised floor.

In 2004, Germany-based data center air-conditioner supplier Stulz was the first to bring plug fans into U.S. data centers. Plug fans are standard in Stulz's European models. Oliver Stulz, the president of Stulz's North American branch, said the company initially sold "a couple hundred" plug fans in the U.S. Now, the company has some 10,000 out in the field, he says.

"With a plenum fan, the opening of the unit is completely open, and so there is practically a nondirectional airflow," said Jason Koo, an engineer at Stulz. "With that alone you'll be saving quite a bit on motor horsepower, and it is a fundamental reason why we pick plenum fan over a conventional belt-drive fan."

Controlling the CRAC fan speed
A common law regarding fans goes as follows:

Fan 1 power = Fan 2 power X (Fan 2 speed/Fan 1 speed) 3

This cubic relationship means that if you can reduce fan speed by 10%, fan power consumption decreases by 27%. If you reduce fan speed by 20%, fan power drops by 49%.

This is why Digital Realty Trust, a data center colocation company, decided to put variable-speed drive (VSD) drives on its fans.

"Two years ago, we wrote a business case," said Jim Smith, the vice president of engineering. "Every little bit you save gets exponentiated out. It was pretty overwhelming [for VSDs]. Most people design to that mythical 100% load range. But if you walk around, there are very few people who are at that 100%. It's that part-loading scenario where these things come into play."

With centrifugal fans, users can buy an extra VSD, also called a variable-frequency drive (VFD), that attaches to the motor and can throttle power up and down. Meanwhile, electronically commutated (EC) plug fans are inherently variable speed, because users can dictate how much voltage goes into the fan.

Emerson Network Power has found that a centrifugal fan using a VFD, compared with one without a VFD, could save about $35,000 over its 15-year lifespan.

"With a control system, if the temperature sensors see temperatures go up, the system will produce more air," said Ron Spangler, senior product manager for Liebert's precision cooling at Emerson. "If the temperatures go back down, I can ramp my fans down now. A lot of the time of the year, you might be able to run at 50% to 60% of the cooling capacity and air volume."

Not everyone is sold on varying the fan speed in CRAC units, however. Balajadia of 365 Main said that the company fills its CRAC units' capacity so quickly that it's not worth the extra cost to have fans run at a lower speed for such a short time. M. Sam Sheehan, the director of mechanical engineering at CCG Facilities Integration Inc., a data center consultant from Baltimore, said VFDs can be beneficial when a data center is first built and the room is being balanced – meaning it is calibrated so the right amount of air at the right speed leaves the underfloor through the perforated tiles to cool IT equipment. Other than that, their utility is limited, he said.

"VFD fans can be helpful in balancing the room," Sheehan said. "But we often see exaggerated claims of energy savings on an ongoing basis because your loads don't vary that much in your room. You see slow growth."

Putting the CRAC fan in its place
In the case of the EC plug fans, another important consideration is where to put them. Typically they're installed inside the CRAC unit, pointing downward to the underfloor. In this situation, the user normally installs turning vanes, which are like metal scoops that redirect the air so that it blows parallel to the floor.

But some CRAC manufacturers – Liebert and Stulz among them – now offer an option to have the fans mounted underneath the raised floor and facing horizontally so the turning vanes are unnecessary. That's what 365 Main is doing, a move they say is helping the CRACs to be about 30% more efficient.

Others don't think that the change will make much difference. APC's Bean said the difference between blasting down and horizontally is "so subtle." Liebert's numbers reflect that to an extent – it found savings of about $5,000 per fan over the course of 15 years.

How much does VFD save?
Of course, all these changes cost more up front than a standard centrifugal fan. Spangler said a VFD on a centrifugal fan will set Liebert customers back about $4,000 more per fan. Going to an EC plug fan could be as much as $2,000 on top of that.

Price – and availability – is what kept Digital Realty Trust from using EC plug fans. Smith said it was going to cost about $3,000 extra per fan for Digital to install plug fans instead of centrifugal fans with VFDs. Because the company buys its air-conditioning units in bulk (usually about 100 at a time), $3,000 each adds up. Aside from that, Smith wants to make sure that what he wants is in stock when he makes a bulk order. With EC plug fans – which are still in their infancy in U.S. data centers – he wasn't sure that would happen.

Overall, Liebert found that compared with a full-speed centrifugal fan, using an EC fan mounted under the floor could save a data center $50,000 over 15 years if the fan runs at an average of about 80% capacity. An EC fan in the unit could save $45,000, while a centrifugal fan with a VFD could save $35,000.

Spangler said that if electricity costs 10 cents per kilowatt-hour, "it's reasonable to say that within one to one and a half years, you could pay back that initial investment."

Other data center fan options
These aren't the only choices. APC is pushing its InRow cooling units, which are mounted in between IT equipment racks, rather than the standard-perimeter CRAC units. APC's Bean said if the cooling devices are closer to the equipment, you don't have to use as much power blowing the air hard enough to get there, and the air "tends to circulate more predictably."

Meanwhile, companies such as AFCO Systems offer closed-off IT equipment cabinets with VFD fans. These fans communicate electronically with VFD fans in the CRAC units so that there are delivery and receiver systems monitoring and controlling the amount of air that gets to the IT equipment and adjusts their speeds accordingly. AFCO's CTO, John Consoli, said it makes the cooling system that much more precise.

"It's a system that creates a balance where the amount of air that needs to be delivered is exactly what makes it there," he said. "It rebalances automatically."

Finally, 365 Main and Digital have installed VFD fans on data center chillers and cooling towers, which can yield additional savings. In one case, Digital installed VFDs on the chiller plant. It took measurements and plans to publish results within the next couple months.

365 Main contends that some of the biggest cost savings can be found with chillers and cooling towers.

"When you build a facility, you build it for an enormous amount of capacity, and when you open the doors on day one, it doesn't exist," said Miles Kelly, 365 Main's vice president of marketing and strategy. "VSD fans allow us to run the cooling system in a much more efficient manner. It allows us to incrementally add load instead of adding capacity that would be underutilized."

Let us know what you think about the story; email Mark Fontecchio, News Writer. You can also check out our Data Center Facilities Pro blog.

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