Fan Wall Technology Versus Centrifugal Fans l455w

Fan Wall Technology versus Single Centrifugal Fans William Greco 2404 Greensward N. Warrington, Pa. 18976 [email protected] 12-28-2009

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Executive Summary This report compares the characteristics of Fan Wall Technology (multiple plenum fans) versus use of a single conventional Centrifugal Fan in HVAC systems. Introduction No comparison information is available on the net concerning Fan Wall technology versus conventional centrifugal fans. In recent years a resurgence of multiple direct drive (plenum fans) mounted to the bulkhead wall of air handling units has been observed. These systems are being referred to as “fan wall technology”. The two types of systems have certain benefits that can be compared, and identified against one another. Advantages of using a single Centrifugal Fan The purchase of a single centrifugal fan will be less expensive than the cost of multiple small fans. In a 10,000 cfm installation fan wall technology employing multiple small fans on average will cost 30% to 40% more. The total horsepower for a 10,000 cfm fan wall system will be 10% to 15% higher than a single centrifugal fan , this is a continuous operating cost difference that will exist during the entire life of the installation. If the system requires a minimum operating envelope below the capability of the fan wall system, multiple isolation dampers may be required that are closed down to achieve reduced ratings. The isolation dampers must be installed to prevent backflow through the discharge side of the plenum fan openings. If one of the motors on a fan wall system fails an immediate backflow situation will occur through the discharge side of the failed plenum fan opening. Multiple fan arrangements many times are chosen to operate at higher than the 60 hz with VFD installations. Operation at higher than the 60 hz causes the motors to run hotter than design and often causes motor failure.

Fan Wall Technology versus Single Centrifugal Fans William Greco 2404 Greensward N. Warrington, Pa. 18976 [email protected] 12-28-2009

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Advantages of using a single Centrifugal Fan ( Continued from page 1 ) If a variable frequency drive failure occurs, the multiple fan arrangement system will not be able to achieve the required design point in VFD by mode. VFD’s when placed in by mode the fan goes to full power. Fan wall technology requires more controls and more wiring than a single centrifugal fan. The control sequence for fan wall technology is much more complicated than a single centrifugal fan. Multiple fans using fan wall technology require individual circuit protection for each fan. Small electric motors are less efficient than large motors. Fan wall technology does not provide the with the capability to spring isolate each fan on the bulkhead. Fan wall technology employing multiple fans requires that the manufacturer is the only provider of operating fan curves for each system. Single fan operation manufacturers often provide software to the design engineer which can be used to compare different operating conditions during the design process. No fan wall technology software is readily available to the . Fan walls are not AMCA certified, leaving the at the mercy of the fan wall technology manufacturer to provide correct operating specifications and performance information.

Fan Wall Technology versus Single Centrifugal Fans William Greco 2404 Greensward N. Warrington, Pa. 18976 [email protected] 12-28-2009

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Advantages of using a single Centrifugal Fan ( Continued from page 2 ) System effect as a result of multiple plenum fans in proximity to each other can be a problem. Multiple fans increase the odds of one of the fans hitting a natural frequency, which would cause high vibration levels.

Advantages of using multiple plenum fans Fan wall plenums are much shorter and require less HVAC unit length. Multiple fans increase unit reliability. Fan Walls provide a more homogenous flow pattern through the coils. Motor replacement is simpler and quicker, there are no belts to install or couplings to align. Conclusion Use of Fan Wall Technology versus use of a conventional single centrifugal fan depends on the application. Recently Fan Wall Technology has been euphemistically termed as the ultimate in HVAC technology. The design engineer must carefully weigh the advantages of each system prior to final choice between the two types of systems.