The past decade has seen a big shift to the use of multiple direct-drive plenum fans for HVAC equipment. Along with claims of redundancy, sound, and efficiency, the overriding motivation for this shift is the reduction in unit size. This transformation has also been driven by the integration of high efficiency ECM motors with plenum fans. The vibration isolation guidelines for fans in the ASHRAE Applications Handbook have been used by Mechanical Engineers for over 40 years. These guidelines have served the industry well and have changed little over the years. For air moving equipment these guidelines were developed for large belt-driven fans, typically single fans in built-ups systems. Today nearly all large air moving equipment has multiple direct-drive fans. This paper addresses the basics of why we need fan vibration isolation and extends these principles to modern air moving equipment with multiple direct-drive plenum fans. In short, the vibration isolation rules have changed.
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