Fan Arrays for Efficiency, Redundancy, and Fit

Assessing efficiency, redundancy, fit, and cost

All photos: RMF
A 60,000 cfm fan array
module under construction

All photos: RMF
A 60,000 cfm fan array module under construction

There is an industry-wide push right now to use fan arrays based on efficiency, redundancy, and fit. Not too long ago, the only fans in use in air handlers were double-width, double-inlet (DWDI) fans. In recent years, engineers and facility personnel have begun to understand how plenum fans, if applied correctly, could provide a larger set of solutions for a specific job. Fan array systems can offer a simpler maintenance process, lower noise output, and a smaller environmental footprint than traditional air handling units. In addition, fan array systems can make facility personnel’s jobs easier.

Stony Brook University recently hired RMF Engineering to design the replacement of several air handlers ranging from 60,000 cfm to 160,000 cfm. The original design consisted of two plenum fans per air handler at 66% capacity each—a design that ensures there is always some level of fan redundancy in each air handler, the largest of which is a 66-inch wheel. The largest existing air handlers were located in a penthouse, seven stories up, with freight elevator access only. The remaining units were in the basement of the building with standard 6-foot access doors to the mechanical rooms. The basis of the design was field-erected units.

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The engineering team assisted in selecting a contractor to provide an efficient HVAC solution for the air handler project. The project was bid, and the successful contractor promptly recommended a fan array system. RMF had two initial concerns about using a fan array system: the complex controls and the fan efficiencies. However, the benefits of a fan array system were intriguing, and the University gave the go-ahead for the contractor to issue shop drawings.

A 30”x30” single fan module, one of the eighteen fan modules that make up the 160,000 cfm fan array.

A 30”x30” single fan module, one of the eighteen fan modules that make up the 160,000 cfm fan array.

Initial Project Challenges
The first thing RMF noticed when reviewing the equipment schedule was the horsepower differential. RMF’s original design had two 66-inch diameter plenum fans at 759 rpm with a combined 240 BHP, and the fan array consisted of 25 18-inch diameter plenum fans at 3,500 rpm with a full load combined 288 BHP.

Another notable design element was the fan sound power level (LwA). The two 66-inch diameter fans were rated at 95 dBA, while the fan array was rated at 113 dBA. Finally, the electrical wire sizes needed to be modified to accommodate the extra BHP and connected HP loads.

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This efficiency barrier came as no surprise to the design team, as RMF had consulted with a leading fan manufacturer when fan array systems were first introduced, and they had emphatically stated that with all things being equal, larger fans will always be more efficient both in power and in sound.

Despite this, the real advantage—and perhaps energy savings—is the ability of the fan array to load-match the airflow required by selecting which fans need to run, and at what speed. This is comparable to the load-matching characteristics of modular boilers or chillers.

Individual VFDs and disconnects for the 160,000 cfm fan array.

Individual VFDs and disconnects for the 160,000 cfm fan array.

Another concern was the control complexity of the fan array. Each fan was fitted with a micro variable frequency drive, which in turn is controlled by a programmable logic controller (PLC). The system that was installed was a Generation II Fanwall Technology system complete with a stand-alone harmonic filter. However, combining the interface with the existing site DDC controls system was a challenge. Typically, the system should display diagnostics such as fan status, motor frequency, alarms, amps, and watts. It should also have start-and-stop control over individual fans. This was not standard, but needed to be programmed between the PLC controller and the DDC system.

Another challenge that presented itself was how the PLC controller was powered. If a facility manager needed to open the main-unit control panel that houses the drives and disconnects, the PLC power would be disconnected and communication with the DDC control system would be lost. The manufacturer does provide a UPS system to power the PLC momentarily (approximately 10 minutes), however, this was deemed insufficient and a separate power supply was connected to the PLC. To overcome the challenge of value transfer not being automatically transferred through the PLC, a DDC contractor needed to generate a fan matrix with every desired output per fan. This data was in the PLC system, it just needed to be organized. One last setback is that operators could not de-energize specific fans from the BAS. This could only be done at the PLC controller. However, the entire unit could be energized and de-energized from the BAS.

While the airflow for the array is not directly measured, each fan has a piezometer ring with static and velocity taps that are headered together with tubing. The DDC contractor added fan throat areas and used an average velocity static reading to get the total airflow. When one or two fans are de-energized, the fan area is subtracted out of the equation and the new area is used in the airflow calculation. The TAB contractor validated this routine and was within plus or minus 3% accuracy.

Individual fan Variable Frequency Drives (VFDs) and fan disconnects are required to take advantage of the full capacity modulation features offered by the Gen II system. This resolves the bypass issue with single VFDs and allows operators to control up to a certain speed if the drive goes bad. While connecting a fan VFD up to a motor and sizing the wire and breakers is commonplace, wiring and connecting 25 micro drives, the disconnects, the motors, and the PLC controller is typically outside the reach of the design engineer. The manufacturer’s assistance may be needed in these situations.

Variable frequency drive (VFD) and harmonic filter for the 160,000 cfm fan array.

Variable frequency drive (VFD) and harmonic filter for the 160,000 cfm fan array.

Benefits of Fan Array
The RMF team was able to work through these challenges, in large part, because of the solid flow of communication and great working relationship between the equipment manufacturer and the contractor. The team was amazed by the advantages of the fan wall system. Even though the fan wall system was about 20% higher in cost than the original design documents, the contractor made up the difference with lower rigging and constructability costs. The 18-inch fans stacked together like building blocks. The additional power wiring for each fan, as opposed to wiring for two larger fans, did seem to add additional material and labor to the project by approximately 50%.

“Zero effect” back draft dampers on the fan array.

“Zero effect” back draft dampers on the fan array.

One pleasant surprise was that the fan wall system had a tunnel length 4 feet shorter than the original design, which enabled the owner to significantly simplify how the unit’s filters can be accessed. The team worked together to move the filter access from a common return plenum to a more traditional filter access section. The filter access was built up with separate air-handling casing walls and access doors added on to the original unit.

The actual fan section of each of the fan array units saved approximately 400 square feet in double-wall casing, compared to the dual 66-inch plenum fan sections in the original design. In addition, the fan array face area was actually smaller than the coil and filter face area in the original design. However, the velocity profile across all elements was within plus or minus 20%. This seemed reasonable since the actual fan suction of the fan array was about 100 square feet, while the original design suction area was only about 52 square feet. Typically, there is a 20% face velocity variation in conventional AHU design, but a larger access area to let the velocity profile equalize out after the cooling coil was necessary. Between the tunnel length reduction of four feet, and the reduced footprint of the fan array, there was an actual 500 square feet of double-wall air handler casing cost savings.

Once the system was up and running, the PLC controller responded to purposely failed fans by ramping up the speed of the remaining fans to maintain capacity. The PLC controller also did a nice job of fan tracking the return fan array with the supply fan array.

The “zero-net effect” back draft dampers were most effective in isolating failed fans, and these devices are much better than having to work with motorized isolation dampers. It is crucial to make sure that the frequency limits are set up in the PLC correctly. During one of RMF’s tests, the harmonic filter started smoking because the setup Hz limit was exceeded. The manufacturer made improvements by adding a DC link to ensure the issue would not occur again.

The direct-drive fans were housed in individual acoustical enclosures. Despite the higher LwA rating and standing either downstream or upstream of the fans, there were no objectionable vibration or sound issues.

As the project came to a close, it was easy to see why facility personnel like the concept of fan arrays. Each 2’x2′ fan module was on a sliding rail, which easily facilitated motor or fan removal without heavy equipment. The same could not be said of the 120 HP, 66-inch diameter plenum fans in the original design. Efficiency is also an attractive benefit to the fan array system. Sixteen fans in the fan array system could be lost and it would still have the same capacity if one of the 66-inch fans in the original design were to fail. This type of redundancy is a facility operator’s dream.

Is Fan Array Right For Your Project?
So, is a fan array system the right fan for the job? Here are four points to consider for your next project:

  1. Fan efficiency: Larger fans at lower rpms yield a more energy- and sound-efficient design at peak design airflows; however, when considering the controllability of fan arrays and the turndown ratio of peak design versus average airflow during a year-long simulation, the fan array may indeed be more efficient. Consult your fan manufacturer for the best solution.
  2. Fan redundancy: There are an optimal number of larger fans that provide a more efficient fan system and also provide a pre-determined level of redundancy. The fan array seems to optimize the number of fans around less than 8,000 cfm per fan and no more than 10 HP per fan. The key question to ask is, “What is the largest motor or fan my facility operators can deal with in an efficient manner if a fan goes down?”
  3. Fit: This is where the fan array stands out. Based on a 2’x2′ fan module, the tunnel length at minimum is usually four feet less than any conventional two- or three-fan selection, not to mention the actual width of the footprint at the fan section is reduced by 6 feet. The fan array system certainly benefited the project at Stony Brook University based on the tight existing conditions where the previous air handlers were located.
  4. Cost: The contractor for the Stony Brook University project bought a system that was 20% higher in price knowing that the project would be competitively bid, because it realized the 20% investment would save 30% in constructability issues. In most competitive bid environments, the fan array system is 20% more and you pay for it. When you look at the support structure for the fan array, along with the extra electric wiring and electronics, it is easy to see where the 20% increase comes from. Having said this, the overall cost to own and maintain the fan array is worth much more than the 20% premium you pay upfront.

Stony Brook University is not unlike most institutions in that it is looking for ways to incorporate systems that will save time and money, whether it is energy related, maintenance related, or both. The modular concept of fans, boilers, and chillers, along with their ability to match varying load conditions and accommodate site-specific space needs, is hard to resist.

Armed with foreknowledge of both the challenges and benefits, you may find that a fan array system is the right fan for your next job. Taking advantage of the benefits simply depends upon how the technology is applied. BE_bug_web


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