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Problem Identification:
- The customer complains about high noise and vibration levels occurring at certain values during fan operation. Support has been requested from the Resonance Institute technical team to resolve this issue.
- Preliminary assessments indicate that the high noise and vibration originate from the heat exchanger located in the fan channel body area.
Diagnosis:
- Measurements taken from the heat exchanger body revealed that surface vibrations reached levels of up to 15 mm/s RMS.
- The dominant frequency was determined to be 70Hz.
- Upon careful examination of the spectrum graphs, the presence of a natural frequency in the 70Hz region is noticeable.
- In light of this information, the fan flow rate was scanned between 0% and 100% to determine whether there was a relationship between the change in fan flow rate and the vibration level and frequency.
- It has been observed that the vibration frequency does not change with fan flow rate, but the amplitude increases dramatically in the 60-90% flow rate range.
- Calculations revealed that the pipes facilitating heat transfer within the exchanger generate a vortex around 70Hz at the specified fan flow rates.
- This frequency value corresponds to the vibration frequency observed in the measurements.
- It is thought that the vortex frequency formed behind the pipe bundle causes resonance by exciting the structural natural frequency of the pipe bundle and/or the acoustic natural frequencies of the fan channel cavities.
- The vibration pattern created by this effect on the heat exchanger surfaces is as shown in the simulation below.
Result:
- It has been stated that in order to eliminate the resonance condition, dissipative panels should be placed inside the fan ducts before the heat exchanger and the number of plates securing 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 have loosened, thereby eliminating the resonance condition.
