Hello everyone, we are facing an issue with our furnace that has seven overhung fans attached to the side wall. In an effort to reduce beat frequency, we have set every other fan to operate at different speeds - 1080 and 980 RPM. The problem arises when the fans change direction from clockwise to counterclockwise during operation. This switch in direction results in a significant increase in vibration levels, reaching up to 1.04 ips in the axial direction and 0.8 ips in one of the radial directions. Additionally, there are impacts observed in both axial and radial directions, with a Crest Factor of up to 7.4 Gs. The fan vendor suggests that the issue is structural, possibly due to a resonance problem, based on past experiences. However, no additional testing has been conducted to support this claim. Unfortunately, we are unable to perform further testing until January when the furnace will be taken down. I am puzzled by the vendor's explanation of a resonance problem as the high 1X vibration and Crest Factor trends suggest otherwise. Details about the bearings being Link Belt P363 and belt driven are also worth mentioning. My queries are as follows: - Can resonance cause an impact at 1X running speed in the axial direction? - Why does the vibration spike significantly when only three or four fans change rotational direction? - Is this issue primarily structural or related to balance/mechanical factors? Thank you for your insights and assistance. Have a great day, Charlie
One common question is: What specific role do fans play in changing direction during a cycle? Do they blow air into the boiler (furnace) and then draw air out at the end of the cycle? Are they induced draft fans, forced draft fans, or both depending on the cycle? Do they simply keep air circulating around the boiler (furnace) without actually feeding air to fire it up? And do all 7 fans change direction simultaneously? Thank you and have a wonderful day, Ralph.
Hello Ralph, in this process, we are heating large aluminum ingots to a specific temperature using overhung vane fans that reverse simultaneously. This ensures that the ingots are heated evenly on both ends. Thank you for your inquiry. Have a great day, Charlie.
Hey Charlie, I'm curious if electric fans pull in warmer air when they are operating in a specific direction versus the opposite direction.
After reaching the desired temperature, the fans are consistently circulating hot air. Have a great day, Charlie.
Hello Charlie, I was wondering about the amplitude of the "three or four fans are affected" before they increase with a change in direction. I'm also curious if there is a phase shift when the amplitude increases. Thank you and have a wonderful day! Merry Christmas, Ralph.
Hi Charlie, Based on the information given, it seems like both your concerns are valid. In answer to your first question, resonance can indeed cause an increase in 1X running speed vibration particularly if the chosen operational speed of your fans aligns with the natural frequency of the system. Regarding your second query, the vibration spike when three or four fans change rotational direction might be related to how their counter-oscillations interact with each other, causing a greater exertion on the fan assembly and hence higher vibrations. Finally, while I wouldn't rule out the structural aspects, bearing in mind the vendor's suggestions, the issues you're facing might be a combination of structural, balance and mechanical factors. The change in belt tension during the shift in rotation could be affecting the system balance, thereby adding to the vibration levels. A thorough analysis with the help of vibration specialists once your furnace is down can help you figure out the root cause. Good luck with your problem-solving, Charlie!
Hi Charlie, this is indeed a complex issue you're dealing with. Addressing your questions, yes, resonance can theoretically cause an impact at 1X running speed in the axial direction, but it's generally uncommon. Secondly, the significant spike in vibration when three or four fans change rotational direction could be due to phase changes or an imbalance caused by the fans moving at different speeds in different directions. Lastly, it could be a combination of structural and mechanical issues. The mechanical aspect could be related to imbalance in rotation while the structural issues could be due to the resonance. The specific bearings in use could also contribute to the overall system dynamics. Ultimately, a thorough testing could provide clear answers. Hope this helps until your further testing in January. Good luck!
Hey Charlie, the fact that vibrations spike dramatically when fans change rotational direction suggests that there might be an issue with the fan drive system, rather than a purely structural problem. As you have mentioned Link Belt P363, I'm assuming you're using a belt-driven fan, and this could also contribute significantly to the vibrations. In belt-driven systems, the drive system and its tension can directly impact fan performance, especially when directional rotation changes. Another possibility to consider could be some wear in the fans' bearings, causing them to vibrate more when direction changes. As for your query on resonance, while it's not common, instances of resonance causing an impact at 1X running speed in the axial direction are not unheard-of, particularly if the natural frequency and running speed coincide.
Hi Charlie, Given the variables at play, it's tough to definitively point to a single cause without a thorough inspection. However, from the details you've provided, it does seem plausible that a resonance condition could occur at 1X running speed, particularly if some structural component's natural frequency coincides with this. The fluctuation of vibration with change in rotational direction could potentially be related to how the forces interact with the direction of the fan blades. As for your question about whether this is structural or mechanical, my hunch would lean towards it being a mix of structural, mechanical, and operating issues that interplay to create this situation. Potentially, if there's enough imbalance within the fans themselves, that could exacerbating the issue significantly. Regular maintenance and balancing might help alleviate it. Remember though, these are just theories based on the information provided, and I'd strongly recommend further inspection and testing as soon as possible to confirm the issue. Good luck!
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Answer: - Yes, resonance can cause impacts at 1X running speed in the axial direction. It is important to investigate further to determine if resonance is indeed the cause of the increased vibration levels during directional changes in fan operation.
Answer: - The vibration may spike significantly when only three or four fans change rotational direction due to potential factors such as unbalance, misalignment, or structural issues. Further analysis and testing may be needed to pinpoint the exact cause of the vibration spike.
Answer: - The issue of increased vibration during directional changes in fan operation could be a combination of structural, balance, and mechanical factors. It is essential to conduct thorough testing and analysis to determine the root cause and develop an effective solution.
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