7947 Equipment Engineering: Vibration Analysis & Solutions for Fans

This report addresses the problem of excessive vibration in a centrifugal fan used for blowing hot air over malt, as part of the 7947 Equipment Engineering and Design course. It identifies potential causes of the vibration, categorizing them into simple issues like unbalanced mass and complicated issues like misaligned shafts, cracks, and loose connections. The report further discusses these causes in detail, including misalignment, resonance, cracks in the rotor and shaft, loose connections, and unbalanced mass. It then provides recommendations for preventing recurrence, focusing on addressing resonance, unbalanced rotor mass, and misalignment, suggesting solutions such as monitoring operating speeds, lagging internal surfaces, and ensuring proper shaft alignment. The report also includes a list of references.

VIBRATION IN CENTRIFUGAL FANS
Name of Student
Institution Affiliation

Vibration in centrifugal fans.
Introduction.
In every society today, fans are a very crucial equipment in houses, offices, industries,
and halls. This is because they help in regulating temperatures by creating an effect of cooling on
the environments they are operating on. In contradiction to this, Centrifugal fan is this case is
used to blow hot air over a malt. They have gained popularity in industries because of their
ability to blow a large volume of air with a good efficiency. However, despite their ability to
perform the assigned job like any other machine, they have a challenge which if not taken care of
could cause a deleterious problem. The challenge is the vibration problems. Mechanical or
electromechanical machines usually produce vibrations up to a normal level which is acceptable
for a proper functioning of the machine. However, if the vibration threshold is surpassed then
problems associated with excessive vibrations may erupt and could ground the fan forever.
As elaborated above, centrifugal fans also experience the problem of excessive vibration.
Excessive vibration is caused by a number of factors as explained below. The factors are further
grouped into two domains. They domains include simple issues and complicated issues. Under
simple issues, there are unbalanced problems which may occur due to mass differences on the
shaft of the motor and the rotor of the motor alike. Secondly, talking about the complicated ones,
there are such issues like having loose connections in the mechanical parts of the fan, having
cracks in the rotor and in the shaft and having misalignments in the shaft attached to the rotor.
The causes of excessive vibration as mentioned above have been further expounded as below.

Causes of excessive vibration in the centrifugal fan.
1. Misaligned shafts.
Alignment of shaft refers to the way which the shaft is attached to the motor drive. This is
important because if were carelessly done, the problem of excessive vibration could arise and
cause more problems. In addition misalignment of the shaft can be caused by incorrect
placement of bearings, rugged or bent shafts. With reference to the pictured diagram of fan the
excessive vibrations could have been caused by these factors. Misalignment of the shaft also
causes excessive axial vibration. This can only be sensed by an axial vibration probe. It's,
therefore, possible that the centrifugal fan pictured above could be suffering from the excessive
vibration due to misalignment of the shaft.
2. Resonance.
To understand resonance in this context, it’s important to know that two factors must be put
into consideration. These factors are structural resonance and critical speeds. Critical speed refers
to the speed beyond which if the fan operates could result in excessive vibration. Generally, fans
are predesigned to work with less speed as compared to the manufacturer's critical speed. On the
contrary, structural resonance may be hard to tell. This is because all existing structures have a
natural frequency to resonate on. Evidently, from our case diagram, resonance could have caused
the problem of damaged surfaces where bearings were attached.

3. Cracks in the rotor and shaft.
Cracks are very harmful to a fan and any other mechanical machine. Even the slightest of
cracks could cause disastrous failure to the fan. The centrifugal fan in this case reportedly had
excessively high vibration levels despite impeller balancing. This observation could be
connected to the presence of cracks in its rotating parts that is, the shaft and rotor.
4. Loose connections in the mechanical parts.
Primarily, when diagnosing the centrifugal fan shown above of what could possibly cause the
excessive vibration then looseness in connections would be the first factor to consider. Looseness
can be caused by unwinding of nuts, bolts, and screws that hold the parts together due to the
normal vibrations of the rotor when operating. Loose connections are characterized by
subharmonic degrees of vibration and harmonic degrees of vibration. Having loose shaft and
rotor connection in a fan is normally unpredictable and difficult to tell, though easy to handle
when detected due to its nature of inducing excessively high unbalanced type level of vibration.
5. Unbalanced mass of shaft and rotor.
Unbalance in the rotor and shaft of the fan is the most common cause excessive motor vibration.
In this centrifugal fan, due to its function of blowing hot air, it’s in order to say that the high
temperatures could have caused the rotor body to expand unevenly thus causing some nature of
imbalance. Secondly buying fan parts in bits from different brands could also cause the
differences in the mass. It’s therefore advisable to buy a fully assembled fan and replace broken
parts with identical new parts from the same brand.

Recommendation.
According to the stated problem of excessive vibration in the centrifugal fan, by
identifying what could be the possible causes of the problem then it becomes simpler to mitigate
them so that a similar occurrence may not be seen in future. To begin with, when dealing with
resonance issues, just as briefed earlier in this article, one needs to understand that resonance in
vibration is subdivided into two, that is, critical speed and structural resonance. Critical speed
mapping is always done during the design stage of a fan so that in its operation it doesn’t surpass
the critical speed. Likewise, I would recommend that fan operator to monitor the operating
speeds of their fans so that they don’t exceed the critical speed. Structural resonance on the other
hand always tend to vary with operating speed, but can easily be detected by vibration
measurement test.
Coming to the problem of unbalanced rotor mass, since it’s the major cause of excessive
vibrations I would suggest that the potential factors that could cause the imbalance be looked
into one by one then acted upon. Factor number one would be how to deal with the heat from the
hot air and the best solution to it is ensuring that the fan internal surfaces are lagged so as to
prevent the heat from causing unbalanced heating of the fan.
Misalignment as another potential cause of the vibrations may be as a result of the
carelessness of the installers. To avoid this in future care needs to be observed to ensure proper
aligning of the shafts and the rotor. To do this, an addition of an axial vibration probe would
sense vibrations and facilitate proper alignment of the shaft and rotor.

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