We recently received a call from a customer who makes fiber optic strands. Fiber optic strands are glass tubes that are heated and stretched into long optic fibers that are later incorporated into a data transmission cable. They hired us to help find the cause of a vibration problem in the fan enclosure used in the production of making the fiber optic strands.
The main stranding room has a highly controlled atmosphere that is dust filtered, as well as, having the humidity and temperature control highly monitored. On a floor, just above the production room, sits a fabricated steel box enclosure with two 30 hp direct drive fans. The steel box is decent sized. To give you a good visual idea, it is about one-half the size of an overseas shipping container. There is some bracing in the long spans along with ductwork attached to the outside going in and out. The ducting is about 4’ square in cross section.
Inside the box are two 30 hp direct drive fans. The fan wheels are about 48” in diameter. The motors sit on a chassis suspended on vibration isolators with the required “more than an inch of static deflection.” The fans sit side-by-side. A series of airflow control devices are employed. There is an adjustable damper just outside the enclosure on the inlet duct. Both fans are speed controlled on variable speed AC drives. There are sophisticated inlet dampers built into each fan inlet. The complaint was that vibration was transmitting from the fans into the walls of the enclosure. During full production, the fan enclosure vibration was enough to concern the facility manager. His concern was vibration would begin to break apart the enclosure. Conversely, it seemed unlikely the fans were the culprit of the destructive vibration, as the isolators seemed to be doing what they were intended to do. The vibration seemed to be at much lower frequency than the fan speed. Finding the cause of vibration requires eliminating sources one at a time.