When fan filter units are the right call
01/01/2001
Fan filter units are more energy efficient and less noisy than you might think. And that's good news for space-challenged cleanrooms.
by Sheila Galatowitsch
For the past five years, construction personnel at Cirent Semiconductor's Orlando, FL, wafer fab have installed hundreds of fan filter units (FFUs) in support areas, while choosing distributed air circulation systems for main production areas. But when another 20,000-square-foot production cleanroom was added to the 150,000-square-foot fab recently, designers opted for the FFUs over more conventional approaches—saving several million dollars in the process.
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By implementing FFUs, Cirent Semiconductor—a subsidiary of Lucent Technologies Microelectronics Division—is following the lead of cleanroom owners in Asia and Europe. In geographic regions where cleanroom real estate is precious and multi-level fabs more common, FFUs predominate. But in the United States, the perceived drawbacks of FFUs have limited their use. For example, in some applications, the technology can prove noisy, relatively energy inefficient and costly for 100 percent filter coverage. Some people believe FFU motors have a tendency to burn out, making them unreliable and a nuisance to maintain.
That's why in the U.S. FFUs are more likely to find use in ancillary support areas of a cleanroom instead of the main production areas where distributed air handling units and fan tower systems prevail (See "Air moving sytstems defined," page 18). FFUs work well in auxiliary spaces because those areas typically require only 25 to 30 percent filter coverage, while production areas require 100 percent coverage.
"No other choice"
In other parts of the world, however, cleanroom owners have embraced the smaller, space-saving FFUs for the same reason that Cirent Semiconductor chose the technology. Lack of space was the deciding factor behind the wafer fab's decision to install about 1,200 FFUs in its new ISO Class 3 (Class 1) production cleanroom. Constructed from spaces originally designed for office use, the cleanroom simply wasn't tall enough to accommodate a more traditional approach. "Ceiling space was so limited, we had no other choice," says Paul Olivarez, the fab's HVAC plant engineer.
The company's vendor, Huntair (Tigard, OR), designed the 234 foot single-fan units to fit inside the structure's bar joists, a move that allowed construction of a 10-foot ceiling and one-foot raised floor. The final design saved several million dollars over an earlier plan, which called for tearing out the concrete slab floor to make the room tall enough to accommodate a pressurized plenum system.
That plan would also have destroyed an upper-level office space that would have had to be rebuilt elsewhere. Besides saving on construction costs, the FFU approach was cheaper and easier to purchase and install, says Olivarez. It also helped Lucent meet a fast-track build schedule.
According to Olivarez, reliability of the several thousand FFUs now operating at the fab is not a problem. "They don't break down, and we have had some of these units running 24 hours a day for five years," says Olivarez, who adds that newer models are more energy efficient and less noisy than older technology. After the fab's successful attempt to incorporate FFUs into the 12-foot production cleanroom, technical personnel are open to using the technology in any size room used for any purpose, "even if we planned to build a facility from scratch."
Energy efficiency
The new FFUs installed in the Cirent fab feature an advanced motor technology that makes them more energy efficient than older FFUs in the support areas and even a standard pressurized plenum system, says Olivarez.
The advent of the electronically commutated motor, or EC motor, may cause more U.S. cleanroom owners to implement FFUs in the future, says Richard Grout, a senior mechanical engineer at IDC (Portland, OR), an international engineering and architectural company.
 An electronically commutated (EC) motor from ebm Industries. |
The EC motor replaces the standard induction motor typically used in FFUs. These alternating-current motors, similar to ones used in home appliances, are not noted for efficiency. By contrast, the EC motors—special brushless direct current motors that operate from the AC line—are as much as 60 percent more efficient than induction motors. Used for a decade in such applications as high-efficiency gas furnaces, they incorporate specialty drive and control electronics that make the FFUs in a cleanroom easier to adjust, monitor and tie together in a network.
The technology also allows complete control of airflow speed without stressing the motor components, says Armin Hauer, senior project engineer at ebm Industries Inc. (Farmington, CT), which manufactures an EC motor system that includes a drive, control electronics and motorized impellers. In FFUs, EC motors are "an excellent means to control air velocity, power consumption and noise," says Hauer.
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An EC motor combined with a "nicely balanced" fan filter system and motor controller make for a "very efficient system with effectively less noise," says Grout. In fact, the marketplace is calling these EC motor-based units "super-FFUs," says Paul Christiansen, director of sales and marketing at Envirco Corp. (Albuquerque, NM), an FFU vendor. Christiansen expects sales of the new FFUs to climb once cleanroom users realize the significant energy savings the technology offers for ISO Class 3 and 5 (Class 1 and Class 100) cleanrooms compared to centralized air handling systems.
Computer-controlled management systems are also improving operation of FFUs. These early warning systems allow HVAC personnel to better monitor FFU performance and respond quickly to unit failure—a significant advantage in systems where hundreds of FFUs are tied together.
 Computer-controlled management systems, such as the CleanPak Control System, improve operations of FFUs like the CleanPak FFU pictured here. |
But the computer package comes at a price. A control system would have added so much to the cost of the Cirent implementation that engineers decided against it. "We monitor the FFUs locally with a pilot light indicator/air pressure switch system that instantly alerts if a unit is not running," says Olivarez.
Some control systems are probably "overkill," says Christiansen, especially since the EC motors are expected to last some 12 to 15 years and FFU filters in an ISO Class 5 cleanroom some five to 10 years. But the control features are well received in Asia, he adds, where Envirco has several ongoing projects.
Making the call
Although FFUs with EC motors and control systems are more expensive than other options, IDC's Grout anticipates that owners of 300 mm production cleanrooms will turn to the technology. Larger tools in these advanced cleanrooms will push up ceiling heights and make interstitial space narrower, which means less room to run services and ductwork.
FFUs already are the air management system used in minienvironments. And since the contamination control focus in 300 mm cleanrooms will be at the tool- and wafer-level, the cleanroom itself will be designed to higher classifications with less than 100 percent filter coverage. "Rather than running ductwork through the plenum area, installing FFUs will be much easier and lower operating costs," says Christiansen.
FFUs are not limited to semiconductor cleanrooms, however. They are suitable for use in industries ranging from pharmaceutical to food processing, particularly for cleanrooms with space and architectural constraints, and retrofit projects involving office buildings and warehouses. Other areas well served by FFUs are corridors, gowning rooms and bay-and-chases.
FFUs may not be the right choice for ballroom-style cleanrooms, suggests Kevin Weist, head of sales for Clean Air Products (Brooklyn Park, MN), an FFU supplier.
But they can offer additional flexibility to pre-fabricated modular cleanrooms of about 2,000 square feet or less, which are designed for quick assembly and easy relocation.
 An inside look at Envirco's Mac10 FFU |
To determine which air management system is the right choice for an application, compare installed cost, operating cost, watts per sq. ft. and requirements for performance and control. FFUs require more personnel hours to install and wire, so installation costs will likely be higher than other systems, says Brian Mazur, senior vice president of sales at Cleanpak International (Clackamas, OR). And because FFUs have more moving parts than other systems, and thus an increased probability of failure, users should think through how a loss of one or more units will impact a cleanroom's cleanliness.
Users should also consider how they will replace filters and components when the need arises. Some FFU models allow filters to be replaced while the entire assembly remains in the ceiling, while other models require the filter module to come down from the ceiling for filter replacement. Likewise, some models allow servicing without taking the unit down from the ceiling.
The convenience of not having to take the unit down adds to the cost, but is essential for some applications. For example, breaking the ceiling barrier in some pharmaceutical cleanrooms would necessitate revalidating the room.
In addition, FFUs are available with a variety of blower configurations, mounts, bafflings for the airflow and sound attenuation. Noise levels will differ from unit to unit; look for units in the 45 to 60 dBa range. Users can eliminate some noise by running the units at the optimum operating points for static pressure and airflow, says Mazur.
Older FFU technologies may respond poorly to high static pressures, which could impact performance and cause the units to emit a whine or tone in the room. If total static pressure is too high, consider adding return air booster fans or other means to reduce the static pressure, says Christiansen.
Other considerations
Users should also be aware that some FFUs may impact temperature control in a cleanroom. With the motors emitting heat at the point of use and the cooling coils located at the return section, there could be a half or full degree difference of temperature from one end of the room to the other. Also on the checklist of FFU concerns: configuration of power feeds and runs; warehousing of spare parts; maintaining the units in a working cleanroom; control of power spikes and shut-offs; UL, FM and other agency listings for insurance purposes; and the impact of a smoke or fire event on the units.
FFUs with EC motors put to rest some of the drawbacks of older units, but the EC motor, which has more components that induction motors, adds to the cost of the system. ebm's Hauer says that energy savings alone will pay back the extra investment within two to four years.
Where cost is a big concern, FFUs with standard induction motors are still a good option for some applications. Technical Air Products Inc. (Grand Rapids, MI) has installed hundreds of its conventional motorized FFUs into ISO Class 5 production cleanrooms. At prices ranging from $450 to $600 per unit, these FFUs are particularly cost-effective for softwall-type cleanrooms requiring 100 percent coverage, says President Don O'Keefe.
The company also offers FFUs with EC motors.
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While U.S. cleanroom owners may never embrace FFUs like their European and Asian counterparts have, the technology has clearly won converts here. Says Cirent's Olivarez: "If you don't have a lot of room, it's the easiest way to go."
Air moving systems defined Air management systems are the last line of defense in a cleanroom. The following three approaches, defined by Richard Spradling of Huntair, are used in a variety of cleanrooms worldwide.
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Distributed fan system: This approach features recirculation air handlers distributed evenly throughout the cleanroom and sized to cover a specific area of the cleanroom. Common fan sizes are 18,000 cubic feet per minute (cfm) and 26,000 cfm. The air handlers include a pre-filter, cooling coil for sensible load and a direct drive plug fan. The air handling unit sits on a fan deck above the cleanroom over the specific area served. The fan discharge is ducted to an open plenum or ducted supply to direct airflow to the HEPA filter ceiling system. Among the advantages of this approach is the redundancy offered in the event of a fan failure, and ease of construction and maintenance.
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Fan tower systems: This design locates the fan on the perimeter of the cleanroom envelope. The air handler features a large cfm fan system, including pre-filters, cooling coils, an inlet acoustical package, direct drive plug fan or vaneaxial fan, and a discharge acoustical package. The fans in this design generally are in the 70,000 to 100,000 cfm range. The fan tower is discharged into a pressurized plenum-type system or through ductwork to the HEPA filter ceiling system. Redundancy in the cleanroom air supply is a big plus with fan tower systems. However, the size of the fan used in these systems requires additional sound attenuation to meet in-room acoustical levels. The attenuation packages add to the overall system static, causing higher horsepower to serve the clean space.
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Fan filter units: Fan filter units are located directly in the HEPA filter ceiling system and include the HEPA filter, recirculation fan and fan housing. This system is generally a low static pressure system to accommodate the HEPA filter and return air path only. Cooling for the fan motors is incorporated into the design of the cleanroom through remote cooling coils or additional loading on the make-up air handler. The FFUs typically are sized as 234 foot units (720 cfm) or 434 foot units (1440 cfm). A failure in this system creates a dead spot in the cleanroom; however, an FFU computer monitoring system could help offset a potential problem.