Common maintenance tasks on fan systems include:
- Periodic inspection of all system components
- Bearing lubrication and replacement
- Belt tightening and replacement
- Motor repair or replacement
- Fan cleaning
The most costly consequence of improper maintenance is unscheduled downtime. Causes of this downtime vary according to the demands of the application. Because each system places particular demands on its air-moving equipment, maintenance requirements vary widely.
Maintenance Schedules To minimize the amount of unscheduled downtime, basic fan system maintenance should be performed at reasonable intervals, the length of which should be determined by either hours of operation or calendar periods. The maintenance interval should be based on fan manufacturer recommendations and experience with fans in similar applications.
Factors that should weigh into this schedule include the cost of downtime, the cost and the risk of catastrophic failure, and the availability of back-up equipment. In systems that do not have abnormally severe operating demands, a typical maintenance schedule would include the items on the checklist.
Belt Inspection. In belt-driven fans, belts are usually the most maintenance-intensive part of the fan assembly. As belts wear, they tend to lose tension, reducing their power transmission efficiency. Even new, properly adjusted belts suffer losses of 5 to10 percent. As belt conditions degrade, these losses increase. Because noise is one of the ways in which the energy loss of belts is manifested, poor belt condition can add significantly to the ambient noise level.
Belt inspection is particularly important to the operation of large fans because of the size of the power losses. For example, in a 200-horsepower (hp) fan, a 5 percent decrease in power transmission efficiency results in a 10-hp loss, translating to $3,270 annually for a continuously operating system.
Basic Maintenance Checklist
- Belts. Check belt condition, tightness, and alignment. Also check sheave condition.
- Bearings. Determine bearing condition by listening for noises that indicate excessive wear, measuring bearing operating temperature, or by using a predictive maintenance technique, such as vibration analysis or oil analysis. Lubricate bearings in accordance with fan manufacturer instructions. Replace bearings, if necessary.
- System Cleaning. Fans and system components that are susceptible to contaminant build-up should be cleaned regularly.
- Leaks. Check for ductwork leakage that can lead to energy losses and poor system performance.
- Motor Condition. Check the integrity of motor winding insulation. Generally, these tests measure insulation resistance at a certain voltage or measure the rate at which an applied voltage decays across the insulation. Also, vibration analysis can indicate certain conditions within the motor windings, which can lead to early detection of developing problems.
Although belt inspection and tightening is usually a routine task for any mechanic, increased awareness of the costs associated with poorly adjusted belts can improve the attention devoted to this maintenance effort.
In multiple-belt arrangements, whenever one belt degrades to the point of requiring replacement, all the belts should be replaced at the same time. As belts wear and age, they exhibit different properties; consequently, replacing only one or two belts in a multiple-belt arrangement creates a risk of overloading one or more of the belts. Exposing all the belts to roughly the same operating time minimizes the risk of uneven loading.
Establishing proper belt tightness is essential to minimizing the energy losses associated with belt drives. However, care should be taken to prevent overtightening the belts. This leads to high radial bearing loads, accelerated wear, and shorter bearing replacement intervals.
Fan Cleaning. In many fans, performance decline is largely because of contaminant build-up on fan blades and other system surfaces. Contaminant buildup is often not uniform, resulting in imbalance problems that can result in performance problems and drivetrain wear. Because fans are often used in ventilation applications to remove airborne contaminants, this problem can be particularly acute. Fans that operate in particulate-laden or high-moisture airstreams should be cleaned regularly.
Certain fan types, such as backward-inclined airfoil, are highly susceptible to build-up of particulates or moisture. These build-ups disturb the airflow over the blades, resulting in decreased fan efficiency and higher operating costs.
In high-particulate or moisture-content applications, radial-blade, radial-tip, and forward-curved blade type fans are commonly used because of their resistance to contaminant build-up. If, for some other reason, a different type of fan is used in a high-particulate or high-moisture service, then fan inspection and cleaning should be performed more frequently than normal.
Leakage. System leaks degrade system performance and increase operating costs. Leaks tend to develop in flexible connections and in areas of a system that experience high vibration levels. Leakage decreases the amount of air delivered to the point of service; consequently, one of the first steps in troubleshooting a system that has experienced declining performance is to check the integrity of the ductwork.
Sources of leaks can be identified visually by inspecting for poorly fitting joints, and tears or cracks in ductwork and flexible joints. In systems with inaccessible ductwork, the use of temporary pressurization equipment can determine if the integrity of the system is adequate.
Bearing Lubrication. Worn bearings can create unsatisfactory noise levels and risk seizure. Bearings should be monitored frequently. Bearing lubrication should be performed in accordance with the manufacturer’s instructions. For example, for high-speed fans in severe environments, lubrication intervals can be necessary weekly or more often.
- For oil-lubricated bearings, check the oil quality and, if necessary, replace the oil.
- For grease-lubricated bearings, check the grease quality and, if necessary, repack the bearings. Be careful not to over-grease bearings as this interferes with ball or roller motion and may cause overheating.
- Ensure the bearings are adequately protected from contamination.
In axial fans, anti-friction bearings (ball, roller-type) are predominantly used because of the need for a robust thrust bearing to handle the axial thrust load.
Motor Replacement. Even properly maintained motors have a finite life. Over time, winding insulation inevitably breaks down. Motors in which the winding temperatures exceed rated values for long periods tend to suffer accelerated insulation breakdown. When faced with the decision to repair or replace a motor, several factors must be considered, including motor size, motor type, operating hours, and cost of electricity. For example, in a motor application where the cost of electricity is $0.05/kilowatt-hour, the motor operates 4,000 hours each year at 75 percent rated load, and the rebuild cost is 60 percent of the price of a new motor, the calculated breakeven point between repair and replacement is 50 hp.2 Under these circumstances, in applications requiring less than 50 hp, replacement motors meeting Energy Policy Act (EPAct) efficiency requirements should be selected, while larger motors should be rebuilt. Of course, each facility must establish its own repair/replace strategy. There are several resources that provide guidance in developing such a strategy.
For motor rewinds, ensure that the repair facility has a proper quality assurance program, because poor quality motor rewinds can compromise motor efficiency. Although motor rewinds are often cost-effective, motors that have been previously rewound can suffer additional efficiency losses during subsequent rewinds.
For motor replacements, high-efficiency motors should be considered. High-efficiency motors are generally 3 to 8 percent more efficient than standard motors. In high-use applications, this efficiency advantage often provides an attractive payback period. EPAct, which went into effect in October 1997, set minimum efficiency standards for most general-purpose motors from 1 to 200 hp.
Fan Replacement. Under most conditions, fan blades should last the life of the impeller. However, in harsh operating environments, erosion and corrosion can reduce fan-blade thickness, weakening the blades and creating an impeller imbalance. In these cases, either the impeller should be replaced or an entirely new fan should be installed.
In many applications, fan maintenance is reactive rather than proactive. For example, bearing lubrication is performed in response to audible bearing noises. Fan cleaning is performed to correct an indication of poor fan performance or vibration because of dust build-up. Unfortunately, many fan system problems remain unaddressed until they become a nuisance, by which time they may have resulted in significantly higher operating costs.
Vibration analysis equipment is essentially a refined extension of the human ear. By “listening” to the vibrations of a motor or similar piece of machinery, the instrumentation can detect the early symptoms of a bearing problem, motor winding problem, or dynamic imbalance. By identifying problems before they become worse, repairs can be effectively scheduled, reducing the risk of catastrophic failure.
Fortunately, recent improvements in instrumentation and signal analysis software have increased the availability of vibration monitoring and testing equipment. These devices can be permanently installed with a fan and incorporated into an alarm or safety shutdown system. Vibration monitors offer relatively inexpensive insurance for avoiding costly failures and can improve the effectiveness with which fan maintenance is planned.
Portable vibration instruments can also be used as part of a facility’s preventive maintenance system. Vibrations measured during operation can be compared against a baseline set of data, usually taken when the machinery was first commissioned. Vibration signatures taken at different points in a fan’s operating life can be evaluated to determine whether a problem is developing and, if so, how fast.
A written log or record documenting observations and inspection results is a useful supplement to a maintenance schedule. Often a machinery problem will develop over time. A history of the repairs, adjustments, or operator observations regarding the conditions under which the problem becomes noticeable improves the ability to effectively schedule a repair.
A typical fan system consists of a fan, an electric motor, a drive system, ducts or piping, flow control devices, and air conditioning equipment (filters, cooling coils, heat exchangers, etc.). To effectively improve the performance of…