One of our Exhaust Fan has an interesting vibration behavior. The Vibration of this fan changes with inlet gas temperature change! (It change between 1 to 7 in horizontal vibration at point 4) But this changing is not in same direction, sometimes the vibration increases with temperature increase and some times it is decreases. The Velocity spectrum shows 1X is dominant. Another interesting thing is the magnitude of horizontal and vertical vibration: for point 4 the horizontal vibration is 7 but the vertical vibration is 0.2 mm/s RMS!
Please download the attachment file, it contains some vibration graph and technical data about fan.
I will appreciate you, if you can help me to find the problem.