New Fan Designs Drive Cooling Savings
January 22nd, 2009 By: Rich Miller
Variable speed fans may not seem sexy to some folks. But saving money on data center energy bills is a beautiful thing, and cooling fan design is an area where small changes can generate big savings. Mark Fontecchio at SearchDataCenter reviews new approaches to fan design for computer room air conditioners (CRACs) and air handlers, which include:
- Fans with no belts and pulleys, known as plug or plenum fans, or sometimes as “direct drive”fans.
- Variable speed drive (VSD) fans that allow data center managers to reduce fan speed.
- Variable frequency drive (VFD) that control motor speed.
Data center operators are also deploying fans in multiple locations to fine-tune air flow throughout the data center equipment. That includes the space under the raised floor, and in chillers and cooling towers. Read more at SearchDataCenter.
William CollierPosted January 24th, 2009
Plug Fans, VSD and VFD have been around for decades; so why haven’t manufacturers been using them in the downflow AHU’s? It really hasn’t made sense for any of the manufacturers to use energy efficient fan technology in their units, the AHU’s are nothing more than a large “spot cooler.” They are limited in their distribution capabilities to specific zone’s, in most instances a unit can’t throw air more than 30′ and the zoned area for a unit is generally about 1200 sq. ft. This is what they are designed to do, based on the cooling coils in the unit. If one were to use a plug fan to get better distribution the zone coverage would become larger, however, the W/sf cooling capacity would be diminished.
We all know that rooms are plagued with hot spots, in part this is because of the AHU’s limitations, however, the distribution means (raised floor) is cluttered with pipes, wires and cabling, and furthermore, leaks a significant amount of air. In fact one manufacturer of AHU’s claims that 25% of the supply air is lost because of leakage. At one time people thought this was acceptable and called it collateral cooling.
Obviously, the driver for manufacturers to install better fan technology is the industry wide need to make the data center green, however, the real question is do you want to claim you’re green, or do you want to be green?
It’s unlikely using VSD’s or VFD technology will save energy, in most instances air doesn’t get where it needs to be now, so how can one assume using less air will solve the problems of air distribution in the data center. They will only save energy if they can be used effectively and as long as the distribution means is a “clogged,” “leaky” floor there’s no money or energy to be saved.
Before investing in such fans for new purchases, or retrofitting existing air handling units it’s far more important and cost effective to concentrate on the air distribution method than the fans.
Leaky floors exist because of cable cutouts and floor joints. Sure there are gasketed devices for the cutouts however, they are limited in size and are burdensome for the technician. Floor joints are often a result of edge trim falling off the panels, and poor installation techniques. However, the bigger problem is opening the floor for reconfigurations. With most single level floors the volume of air that would flow through 3 panels removed from the floor is equivalent to the total amount of air produced by a single AHU. Furthermore, removing more than 3 panels can effect the structural stability of the floor.
If one were really interested in saving energy they would use a two-tier raised floor that isolates the air in a lower dedicated pressurized plenum, separate from the wires and cables in the upper tier. This eliminates all possibilities of air leakage, and allows the user to remove an unlimited number or floor panels for an indefinite period of time to work on wiring. By using such a system energy savings are achievable by not only using better fans, and VSD or VFD technology, it allows them to use much larger air conditioning units such as central station air conditioning which is much less expensive to own and much more energy efficient.
Addressing the issue of fans without looking at the overall problem will only lead to disappointing expectations. Overall, the mechanical system needs to be just that, a combination of energy efficient air conditioning and a properly designed distribution plenum–this is how to save lots of energy and money.