fan article by technical associates

7/22/2019 Fan Article by Technical Associates

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CENTRIFUGAL FANSUSING VIBRATION ANALYSIS TO DETECT PROBLEMS

Copyright 2014 Technical Associates of Charlotte, P.C. Page 1

www.technicalassociates.net

PRINCIPLES OF OPERATION:

Centrifugal fans are one of the most versatile and widely used pieces of rotating mechanicalequipment found today. The fan is an essential part of all heating, air conditioning, air handling andventilation systems. Fans are used in automobiles, homes, offices, industry and many other

applications. The most common use of centrifugal fans is to move air and gases for air handling orventilation systems.

The basic principal of operation for a centrifugal fan is that a shaft mounted rotating wheel(impeller) inside a scroll type housing imparts energy to the air stream or gas being moved by thefan. Centrifugal fans utilize centrifugal force (thus their name) to increase the velocity of the air as itpasses between the fan wheel blades and exits at the tip of the fan wheel. This action convertsmechanical energy (shaft torque) into kinetic energy by acceleration of the air stream to a highervelocity (velocity pressure) and potential energy (static pressure). Fans are velocity generators forthe air being moved.

FAN PERFORMANCE:

Fans follow basic laws of performance. These are useful in determining the effect of changes tothe operating parameters and can help determine the cause of operational problems. These basiclaws apply to any type of fan or fan system.

1. The air volume varies directly as the fan wheel (impeller) rotating speed.2. The pressure (static, velocity and total) varies as the square of the fan wheel (impeller)

speed.3. The power required (horsepower) varies as the cube of the fan wheel (impeller) speed.4. At constant speed and capacity, the pressure and power vary directly as the density

of the air moved (lbs per cubic foot).5. At constant pressure, the speed, capacity and power vary inversely as the square root of

the density of the air.6. For a constant weight of air:

a. The speed, capacity, and pressure vary inversely with the density of the air.b. The horsepower varies inversely with the square of the density.

DETECTING FAN PROBLEMS USING VIBRATION ANALYSIS:

Fans are subjected to operational forces generated by their operating speed, static pressures andsystem arrangement. These operational forces cause forced vibration and may originate from therotating parts themselves. Unbalanced fan wheels and drive sheaves are examples of forces thatcause force and couple unbalance. Axial load on fan bearings is generated primarily by thenegative static pressure at the inlet of the fan wheel. The relationship between a fans operatingconditions (flow quantity, pressure, speed and arrangement) generate forces that cause vibrationand problems that reduce the expected life of the bearings and fan components.


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SUMMARY OF COMMON FAN VIBRATION PROBLEMS:(Detected Using Vibration Analysis)

1. Unbalance of Overhung Fan Rotating Parts (Fan Wheel or Drive Sheave):Overhung Rotor Unbalance will cause high 1X RPM in both the axial and radial directions.

Axial readings tend to be in-phase and steady, whereas radial phase readings might beunsteady. Overhung rotors have both force and couple unbalance, each of which will likelyrequire correction. Thus, correction weights will most always have to be placed in 2 planesto counteract both force and couple unbalance.

2. Unbalance of Center-Hung Fan Wheel:Force Unbalance will be in-phase and steady. Amplitude due to unbalance will increase bythe square of speed (a 3X speed increase = 9X higher vibration). 1X RPM is alwayspresent and normally dominates the spectrum. This can be corrected by placement of onlyone balance correction weight in one plane at the Rotor center of gravity (CG).

Approximately 0 phase difference should exist between outboard and inboard horizontal,

as well as between the outboard and inboard vertical direction.

3. Belt Drive Problems and Drive Misalignment:Sheave misalignment is a very common problem with belt-driven fans. When it is present,the vibration at fan speed is typically the highest vibration peak in spectra taken on themotor. It can be confirmed by measuring phase in the axial direction at fan speed.Misalignment will cause phase differences at or approaching 180 between the motor andfan inboard bearings (closest to the pulleys) with phase comparisons made at fan speed.

4. Fan Stall Condition:This unstable condition can cause the ductwork to vibrate excessively and produce ahowling sound. In many cases subsynchronous frequencies will occur at approximately66%-75% of the fan RPM that will oscillate in amplitude and will produce subsynchronoussidebands around 1X blade pass frequency (where blade pass frequency = #blades XRPM).

5. Fan Surge Condition:Surge is a condition resulting from improper fan selection or operation and is recognized bythe whoosh sound as air surges back through the fan. The frequency is subsynchronoustypically at 33%-50% of fan running speed.

6. Fan Bent Shaft Condition:Bent shaft problems cause high axial vibration with axial phase differences tending towards180 on the same rotor. Dominant vibration normally occurs at 1X RPM if bent near shaftcenter, but at 2X RPM if bent near the coupling. Dial indicators can be used to confirm abent shaft.

7. Misalignment Condition:Fan shaft misalignment is a condition typically found in direct drive overhung fans.Misalignment is a condition where the centerlines of two (2) connected shafts typically atthe coupling do not coincide. Parallel misalignment is a condition where the shaft


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centerlines are parallel, but not aligned. The radial vibration spectra will show 2X RPMhigher than 1X and a 3X higher than normal. Axial 1X and 2X will be low with 180 out ofphase across the coupling in the radial and axial direction. Angular misalignment will showhigh axial 1X and some 2X and 3X RPM 180 out of phase axial across the coupling.

8. Belt Wear:Belt wear can be detected by the presence of frequencies not only at Belt RPM, but also atmultiples of belt RPM (harmonics). Typically, when belt wear is present, the peak at 2X beltRPM will exceed the peak at 1X belt RPM. If belt wear is significant, it will usually producemany harmonics of belt speed. Note that belt speed will always be lower than either themotor or fan speed (RPM).

9. Fan Bearing Problems:Peaks (with harmonics) at nonsynchronous frequencies is a symptom of rolling elementbearing wear. Short bearing life for fans is typically the result of poor bearing selection forthe application, such as excessive loads, poor lubrication or high temperatures. If the model

number and manufacturer of the bearings are known, then the specific fault frequencies forthe outer race, inner race, rolling elements and cage can be determined. These faultfrequencies for such bearings are found within tables of most predictive maintenance(PdM) software today.

10. Air Flow and Air Turbulence Problems:Fans will normally have a peak in the spectrum at blade pass or vane pass frequency.Increased amplitude of the peak indicates the possibility of clearance problems at thedischarge of the scroll or flow restrictions causing high discharge pressure. Severe flowturbulence will generate subsynchronous peaks and a raised noise floor.

This article is intended to be a summary of common fan problems that vibration analysis can detectand identify. Formal vibration analysis training and balancing training from an organization likeTechnical Associates of Charlotte can provide an analyst with valuable information andunderstanding of fan problems detected by vibration analysis. Specialized vibration trainingcourses such as time waveform analysis can also be beneficial. In addition, most training serviceproviders offer vibration analysis certification through a written examination as confirmation of thestudents comprehension of the course material and the basics for detecting common fan problemsusing vibration analysis.

James David KeslerSales/Operations ManagerTechnical Associates of Charlotte1230 West Morehead St., Suite 400Charlotte, NC 28208

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