This article is a guide for identifying the most common problems encountered in fans and air-handling systems. This is often an "art" and not a "science" because many problems are subtle and hard to diagnose. It often takes a lot of knowledge, inquisitiveness, common sense, mechanical aptitude, and years of experience to diagnose the cause of some very subtle problems. The intent of this article is to point people in the right direction.
SAFETY CONSIDERATIONSFans and air-handling systems come in many sizes, shapes, and complexities. It is critical to realize the limitations of any investigation so that safety of all personnel and the safe operation of installed equipment is ensured. Operating a piece of equipment or system when there are obvious mechanical, electrical, or aerodynamic instabilities requires extremely good judgment, and investigations should be conducted by only qualified personnel. Catastrophic failure resulting in death or serious physical damage can occur when rotating equipment is involved.
Physical inspections should be made only when the fan and system are shut down and locked out both electrically and mechanically so that windmilling cannot occur. It is strongly recommended that AMCA Publication 202, "Troubleshooting," and AMCA Publication 410, "Recommended Safety Practices for Users and Installers of Industrial and Commercial Fans," be thoroughly read prior to any investigation. Reference should also be made to the manufacturer's installation and maintenance literature. OSHA requirements for guarding should also be reviewed. Proper attire, such as safety shoes, hard hat, safety glasses, no ties or loose fitting clothing, safety harnesses, etc., must be worn. In no case should the troubleshooter become part of the problem due to a lapse in safety.
REQUIRED INFORMATIONEquipment Identification- It is vitally important that the equipment in question be properly identified along with other related information. This allows the equipment manufacturer's records to be accessed allowing a starting point for an initial evaluation of the equipment at the time it left the factory. The following information is necessary:
Detailed Description of the Problem - It is essential for those involved to properly describe the problem in as detailed and clear a manner as possible. Information is often transferred through several different people and "word of mouth" is normally not sufficiently accurate. Complete descriptions including noises are encouraged as they can be very helpful in identifying a problem. As many details as possible should be included. As an example, if a system is low in airflow, related rating parameters such as pressure, power, speed, elevation, and temperature should also be provided. A statement of the actual airflow compared to the wanted airflow should be included. Test measurements and their locations are very useful since the measurements themselves are often a clue to the problem.
The results of this initial inspection should be kept on record for future reference. If a problem does occur later on, it will serve as a beginning point of any evaluation.
Contact Person - If there is a problem, it is necessary that an individual be designated as a contact person who will have continuing intimate knowledge of the fan and system status, knows the problem, and what has already been tried to solve it. This person is to serve as the contact person and liaison for others who may be required to visit the job site. This person's name, title, address, and phone number should be readily available and kept abreast of all actions which may be contemplated.
Problem Priority -There must be some determination as to the seriousness and timeliness of the problem resolution. Many misunderstandings occur when a priority has not been established. It is necessary that all of those involved in the resolution recognize that a problem exists and that an amiable plan of action and solution is usually in the best interest of everyone.
PROBLEM CATEGORIESAs previously stated, this article is intended to point the troubleshooting process in the right direction. Required information has been outlined. It is now necessary to identify the nature or category of the specific problem. The following four categories narrow the focus of attention and speed up the evaluation process.
Aerodynamic Performance - This applies to any of the five rating parameters of flow, pressure, speed, power, and density and how they compare to their respective design quantities.
Noise - This applies to any problem in which the ears are the main sensor. Noise and vibration are similar in that they both have amplitude and frequency, but noise is a much lower amplitude and energy content and is measured in dB referenced to Watts. Generally speaking, noise has a much wider frequency range and a higher upper limit than vibration (63 Hz to 10 KHz).
Vibration - This applies to any problem in which the hands or touching are the main sensor. Amplitude is large when there is a problem. It has a much greater energy content with a smaller frequency range (3 Hz to perhaps 500 Hz).
Premature Failure - Premature failure applies to anything whose life does not meet that which was expected. The term "failure" does not necessarily mean a catastrophic failure such as when something "blows up," but a length of time considered as being less than the useful life of the component.
SUMMARYSatisfactory applications occur when all aspects of the installation are in harmony with each other. Proper operation, constant monitoring, and maintenance are also part of the equation. Troubleshooting must be employed when one of these areas becomes a problem.
Aerodynamic Performance Troubleshooting Symptoms
Noise Troubleshooting General ConsiderationsNoise is generally considered low quality, unwanted sound. The ears sense noise whereas vibration is sensed by feel or touch. Sources of noise can usually be identified by some form of characteristic sound to which we can relate. Words such as tone, rattle, pitch, steady or unsteady, and intermittent are examples. These characteristic words help to define whether the source of the noise is aerodynamic, mechanical, or electrical.
Aerodynamic generated noise is characterized by a continuous broadband frequency spectrum with a superimposed tone. The tone is typically objectionable when it becomes 4-6 dB louder than the rest of the spectrum. The tone can be the blade frequency, which is a function of the fan type. It can become very objectionable when system effects and various controls cause it to rise higher than normal. Additional causes include turbulence, high velocities, and instabilities due to pulsation and surge.
Mechanically generated noise has a different sound quality and characteristic. It has a metallic sound caused by metal-to-metal contact. This contact may be constant or intermittent.
Electrically generated noise is a function of motors, relays, controls, or unbalanced line voltages into the motor. Sometimes improperly matched VFD and motors can cause a substantial increase in the motor noise due to imperfect sine wave simulation.
Vibration Troubleshooting IndexThere are many different sources of vibration. One of the most difficult tasks in troubleshooting fans and systems is the systematic identification of vibration characteristics (amplitude, frequency, location, direction, units of measurement) as a function of operating point location on the fan curve and control settings. Identification of the source can be extremely difficult.
Vibration in housings and ductwork is most often aerodynamically generated. This is a forced vibration in which the energy and characteristics of the airstream are large enough to cause sympathetic vibration in the housing and ductwork. Turbulence, pulsation, and the blade frequency tone are examples of forced vibration due to aerodynamics. Vibration can also be the result of a resonance. This occurs when the natural frequency of a duct or housing panel coincides with a specific aerodynamic excitation such as rotating stall, vortex shedding, or the blade frequency if it is strong enough.
Mechanically generated vibrations occur from unbalance, resonance, looseness, and rubbing. Electrically generated vibrations result from torsional fluctuations, eddy current induced fields, and improper wiring.
Premature Failure Troubleshooting SymptomsPremature failure general considerations: It is obvious that a component that physically fails and flies apart upon start-up is a premature failure. However, premature failures also occur when fans and components do not satisfy their expected life. This is hard to quantify because very few records are kept and once a fan is installed it is easily forgotten. The best prevention against premature failure is a good conscientious preventative maintenance program that includes inspections and the recording of vibration levels. Repairs should take place at the first sign of a problem, and not after damage has occurred to other parts.
In general, the equipment life should be consistent with the application. As an example, HVAC equipment may be expected to last 10 years, industrial equipment about 15 years, and power plant equipment about 30 years. This means that the equipment itself, with proper maintenance, should still be around after these time frames.
Reprinted with permission from Greenheck's Engineering Update, Spring 2005/Winter 2006. For more information, visit www.greenheck.com.
Publication date: 10/09/2006