Q. We are recommending high plume dilution lab exhaust fans from Strobic Air for a project. What would you recommend for vibration isolation?
A. Strobic Air recommends that the Tri-Stack Fan be mounted on a 1½” wide strip of ¼” neoprene pad. No further vibration isolation is required or recommended.
Tri-Stack fans are balanced to below 0.5 mil at the blade pass area, with measurements on the roof base as low as 0.1 mil in the vertical. At this virtually nonexistent vibration level, at the fan frequency of 1170 RPM, the recommended vibration pad (50 durometer neoprene, 0.14” static deflection, 502 CPM average natural frequency) will have an efficiency well over 90%.
The Tri-Stack fan mixed flow impeller has a non-stall characteristic and is perfectly stable along the entire pressure versus volume curve. Other centrifugal type fan wheels experience a stall region under low flow, high-pressure system conditions. During fan stall one side of the fan blade is doing more work than the other. In this condition, the centrifugal fan vibrates violently, mandating spring vibration isolators. Again, the Tri-Stack Impeller is perfectly stable and can smoothly operate at no flow, full pressure and it will not go into stall.
Vibration is primarily transmitted radially from the direction of rotation. The Tri-Stack mixed flow impeller is oriented so its radial direction is in the horizontal plane and thus transmits minimal vibration downward into the roof. Centrifugal fan arrangements radial component are in the vertical plane, therefore a majority of the vibration is transmitted into the roof.
We recommend that the Tri-Stack fan be mounted on neoprene pad. Spring vibration isolators are not necessary and will detract from system stability. In addition, flexible connections are required when using spring isolators and are widely recognized as point source failures.
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