HVAC air handlers, especially those used in office environments, often feature small horsepower motors (typically less than 10 HP). They may be belt-driven or direct drive fan wall units, and balancing these fans properly is essential to maintaining smooth, efficient operation. In office environments, they may also be sitting on isolator springs to minimize the chance of transmitting vibration or noise into the building. With modern variable frequency controls, they can operate at practically any speed.
What to Look for When Diagnosing Small Fan Challenges
I recently inspected a fan just like this for a customer.
From a quick visual inspection, several things were pointed out to the maintenance staff:
1. Dirt/Rust Build-Up
The fan, as well as the inside of the unit, had quite a bit of dust inside the machine. There was also a light glaze of rust on the fan wheel itself. This build-up can lead to imbalances and decreased efficiency, potentially causing vibration and premature wear if not addressed.
Solution: Schedule regular cleaning and maintenance of the fan and housing. Remove dust and debris using a vacuum or compressed air. For rust, gently wire-brush the affected area and apply a corrosion inhibitor or rust-resistant paint to prevent future buildup.
For more info on the topic of fan cleanliness as it relates to balancing, check out this blog post that I co-authored with Brian Rimmer:
A Clean Fan = A Clean Balance
2. Lack of Structural Stiffness
In most cases, these fans are made mostly of thin-gauge steel. While this lightens the weight of the machine and keeps costs down, it makes these units susceptible to resonance. Resonance refers to a phenomenon where the vibrations produced by a rotating machine are significantly amplified when the operating frequency of the machine coincides with its natural frequency, causing excessive vibration that can potentially damage the machinery if not addressed. It is difficult, if not impossible, to balance a rotor in resonance.
Consider: Reinforce the fan housing or frame with stiffening brackets or gussets to increase rigidity and reduce susceptibility to resonance. Alternatively, adjust operating speed to avoid the natural frequency range of the unit or consult an engineer for potential dynamic balancing or vibration-damping solutions.
3. Motor Sheave Larger than Fan Sheave
While this machine may have been designed so that the fan rotates faster than the motor, it is unusual. I recommended that the maintenance staff confirm that the sheaves weren’t mounted to the wrong shafts. It is also a red flag to be mindful of resonance when this configuration is present. Incorrect sheave installation or speed ratios can lead to performance issues, including increased vibration and premature wear.
Solution: Verify that the sheaves are correctly installed on the appropriate shafts according to the manufacturer’s specifications.
4. Isolators
The spring isolators located underneath the machine frame should be checked to ensure they are properly adjusted, isolating the machine vibration from the building. Poorly adjusted isolators can transmit excessive vibration to the building structure, potentially causing noise and damage.
Solution: Inspect the spring isolators for wear, correct tension, and proper isolator adjustment. Replace worn isolators if necessary.
5. Shipping Bolts
In many cases, these machines have shipping bolts to minimize movement inside the machine during shipping and installation. Be sure these shipping bolts have been removed before putting the fan into operation. Failing to remove them can lead to misalignment, excessive vibration, and potential damage to the equipment.
Solution: Locate and remove all shipping bolts before operating the fan. Consult the manufacturer’s documentation for bolt locations and removal instructions. This ensures that the fan is free to operate without binding or internal stress.
6. Sheave Alignment and Belt Tension
Confirm proper sheave alignment and belt tension. Misaligned sheaves can cause uneven belt wear, reduce power transmission efficiency, and increase vibration. Incorrect belt tension can lead to slippage or excessive bearing load, shortening the lifespan of belts and related components.
Solution: Use a laser or straightedge (like the Belt Hog) to verify proper sheave alignment. Adjust the alignment to ensure that the belts run smoothly without misalignment or uneven wear. Check belt tension and adjust according to the manufacturer’s specifications to avoid slippage or premature belt failure.
Simple Maintenance Leads to Smoother Balancing
After discussing with the maintenance team, they decided to clean the fan, remove the rust, confirm the isolator adjustment and sheave configuration, and align and tension the belts, which resolved the problem. In many instances, simple actions like these may reduce vibration and lead to smooth, quiet fan operation. If the fan does require balancing, it will be in a much better condition to perform a successful balance job.
By addressing common maintenance issues first, your fan will be in the best possible condition for balancing, ensuring long-lasting, smooth operation. For precise and efficient balancing, consider using the Acoem Wireless Balancer, the industry’s first wireless, two-plane synchronous solution for rotating machinery that simplifies the balancing process with wireless convenience and real-time accuracy.
