Heat Exchanger Vibration Problem | Resonance Institute

Heat Exchanger Flow Induced Vibration Problem

The customer complains of high noise and vibration occurring between certain values of the fan speed. In order to resolve this problem, support was requested from the Resonance Institute technical team.
As a result of the preliminary evaluations, it was understood that the high noise and vibration came from the heat exchanger located in the fan ducts body area.

In the measurements taken from the heat exchanger body, it was observed that the surface vibrations reached up to 15 mm/s RMS levels.
It has been determined that the dominant frequency is 70Hz.
When the spectrum graphs are examined carefully, the existence of a natural frequency in the 70Hz region is striking.
In the light of this information, the fan flow rate was scanned between 0% and 100% in order to determine whether there is a relationship between the variation of the fan flow and the vibration level and frequency.

It was observed that the vibration frequency did not change with the fan flow, but the amplitude increased dramatically in the 60%-90% flow range.
With the calculations made, it has been determined that the pipes providing the heat transfer in the heat exchanger form a vortex of around 70Hz at the specified fan flow rates.
This frequency value matches the vibration frequency seen in the measurements.
It is thought that the vortex frequency formed behind the tube bundle causes the resonance state by driving the tube bundle structural natural frequency and/or the fan ducts cavity acoustic natural frequencies.
The vibration pattern created by this effect on the heat exchanger surfaces is as seen in the simulation below.

In order to eliminate the resonance situation, it was stated that dissipative panels should be placed in the fan channels before the heat exchanger and the number of plates fixing the heat exchanger pipes should be increased.
It has been determined that, due to the high temperature of the gas flowing through the fan, the structures holding the pipes loosen, and therefore the resonance situation disappears.