Like most other rotating machinery, fans experience wear and require periodic maintenance and repairs. Dynamic surfaces in bearings and belt drives degrade over time. Fan blade surfaces may erode from abrasive particles in the airstream, and motors eventually require replacement or rewinding.
Although some degree of wear is unavoidable, operating the system at efficient levels reduces the risk of sudden equipment failure and can lower the cost and frequency of maintenance.
Fan system problems can be grouped into two principal categories: problems that are related to the fan/motor assembly and problems associated with the system. A systems approach is important to help understand the total costs and performance impacts of these problems.
Fan/Motor Assembly Problems
Problems with the fan/motor assemblies can result from improper component selection, poor installation, or poor maintenance.
Belt Drives. Belt drives are frequently the most maintenance-intensive component of a fan/motor assembly. Common problems include belt wear, noise, and rupture. Belt wear can lead to efficiency and performance problems. As belt slippage increases, it can translate directly into lower fan output. Insufficient belt tension can also cause high noise levels through belt slap or slippage. In some cases, belts will develop one or more smooth spots that lead to vibrations during fan operation.
In contrast, belt tension that is too high increases the wear rate, increases load on the bearings, and can create an increased risk of unexpected downtime.
In multiple-belt drive assemblies, uneven loading of the belts causes uneven wear, which can affect the life and reliability of the whole drive unit. Poor belt drive maintenance also promotes costly system operation. Contaminant build-up on the belts often results in increased slippage and noisy operation. The presence of abrasive particles tends to accelerate belt wear.
Belts are not the only item in a belt drive assembly that develop problems. The sheaves themselves are subject to wear and should be periodically inspected. Because sheave diameter has a significant effect on fan speed, the relative wear between the driven and the driving sheave can affect fan performance.
Bearings. As with most rotating machinery, the bearings in a fan/motor assembly wear and, over time, can create operating problems. To prevent such problems from causing unplanned downtime, bearings should be a principal maintenance item. There are two primary bearing types in fan/motor combinations: radial and thrust. In general, radial bearings tend to be less expensive than thrust bearings in terms of material cost and installation requirements. Because of the nature of the airflow, axial fans typically require heavier thrust bearings. These bearings tend to be comparatively expensive, making proper fan operation and effective maintenance important.
Common bearing problems include noise, excessive clearance, and, in severe cases, seizure. Because operating conditions vary widely, the history of other fans in similar applications should be used to schedule bearing replacement. Vibration analysis tools can improve confidence in determining bearing condition and planning bearing work. In oil-lubricated bearings, oil analysis methods can help evaluate bearing condition.
Motors. 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. In motor applications below 50 horsepower, the common repair choice is simply to replace a motor with a new one; however, in larger applications, rewinding an existing motor is often more economically feasible. Although motor rewinds are typically a cost-effective alternative, motors that have been previously rewound can suffer additional efficiency losses during subsequent rewinds.
For motor replacements, energy-efficient motors should be considered. A section of the national Energy Policy Act (EPAct) setting minimum efficiency standards for most common types of industrial motors went into effect in October 1997. EPAct should provide industrial end users with increased selection and availability of energy efficient motors. EPAct-efficient motors can be 3 to 8 percent more efficient than standard motors. In high run-time applications, this efficiency advantage often provides an attractive payback period.
Contaminant Build-Up. Some fan types are susceptible to contaminant build-up. The tendency to suffer build-up is related to the velocity and angle of attack of the airflow with respect to the blades. In many cases, especially with backward-inclined blades, this build-up can significantly affect fan performance. Fan types that have blade shapes that discourage material accumulation (for example, radial and radial-tip types) are usually selected for applications in which the airstreams have high particulate or moisture content. However, even in relatively clean air applications, over time, particulate build-up can be a problem. Consequently, fan cleaning should be a part of the routine maintenance program.
In many heating and cooling system applications, highly efficient fan types, such as backward-inclined fans, are increasingly used to lower system energy consumption. An important component in this trend is the use of filters upstream of the fans to lessen material build-up. While these filters can help maintain efficient fan performance, additional attention to filter cleaning and replacement is required to avoid the pressure drops and energy losses that result from clogged filters.
Fan Degradation. In airstreams that have corrosive gases or abrasive particles, fan blade degradation can present a threat to reliable operation. As fan blades degrade, the airflow over the surfaces becomes disrupted and the fan imparts energy less efficiently to the airstream. Certain blade types are particularly susceptible to erosion because of the angle of attack with the airstream. In applications where higher-than-expected blade degradation has occurred, different fan types or fan materials should be considered. Centrifugal fan manufacturers have developed materials and coatings that solve this problem.
Poor system performance can be caused by several factors, including improper system design and component selection, incorrect installation practices, and inadequate maintenance. Improper system design usually means the system is configured so that it has high system effect factors (SEFs) that result in high operating costs, system leakage, and noisy system operation. Poor component selection includes oversizing fans or using ineffective or wasteful flow control devices.
Improper installation practices include on-site modifications to the duct system that result in high SEFs, improper fan rotational speed selection, and incorrect fan rotation.
Inadequate maintenance often means a lack of bearing lubrication and fan cleaning. Contaminant accumulation on fan blades, duct surfaces, and in filters results in decreased system efficiency and inadequate airflow.
High Operating Costs. Many fan systems are designed to support the highest expected operating loads. Because systems are frequently not re-adjusted during periods of low demand, fans often generate higher-than-necessary airflows and incur higher-than-necessary operating costs. Awareness of the costs of inefficient system operation can lead to efforts that reduce these costs and increase system reliability. An important part of evaluating whether operating costs can be significantly reduced is to measure the amount of variability in delivery requirements and determine operating configurations that meet—but do not exceed—these requirements.
Fouling. The accumulation of contaminants in parts of a system can disrupt airflow profiles and create high-pressure drops. Finned heat exchangers and filters are particularly susceptible to contaminant accumulation that can severely impair airflow. In heat exchangers, fouling interferes with heat transfer, which can compound an airflow problem by requiring more airflow to compensate for the reduction in heat exchanger effectiveness. Consequently, fouling can have a compounding impact on energy use.
Another aspect of fouling that can affect fan performance is interference with inlet-guide vane operation or blade-angle adjustment in variable-pitch fans. Inlet-guide vanes are used to change the load on a fan according to system airflow requirements, thus allowing lower energy consumption during periods of low demand. However, because these devices are typically controlled with a mechanical linkage, contaminant build-up on the linkage components can impair proper operation. Similarly, the linkages controlling the position of variable-pitch blades can become fouled with contaminant build-up, limiting blade-angle adjustability.
Where contaminant build-up on mechanical linkages is a problem, it can defeat the energy savings and performance benefits that were intended when the fan system was specified. Consequently, either a greater maintenance effort should be made to keep the linkage action free, or an alternative airflow control solution should be considered. In many dirty air fan applications, adjustable-speed drives are attractive because of the avoided
Airflow Noise. In many systems, airflow noise is a large component of ambient noise levels. Improper fan selection or operating a fan at higher speeds than necessary can create avoidable noise levels that impair worker comfort and productivity.
Insufficient Delivery. Poor system configuration can lead to insufficient delivery. In many systems, designers have improperly calculated the system effect or have attempted to overpower it with additional fan capacity. The system effect stems from poor airflow conditions, and it can cause a fan to operate much less efficiently. This causes a system component to exhibit a higher-than-expected pressure drop. Frequently, a key consequence of the system effect is inadequate airflow.
There are many alternatives to compensate for this problem. A common solution is to increase fan speed, which increases airflow. Although this option is sometimes unavoidable, it results in higher operating costs and increased airflow noise.
Often, a more effective solution to inadequate airflow can be obtained by addressing the fundamental cause of the problem. By configuring the system to improve airflow and by using flow straighteners where appropriate, the performance problems caused by the system effect can be minimized.
Leakage. Some systems are constructed with little attention to joint integrity. In these systems, leakage can have a significant impact on operating cost and system performance. Some system leakage is unavoidable; however, minimizing the amount of airflow and pressure loss can provide key savings.
Over time, system leakage tends to increase. This is particularly true for systems with oversized fans. Higher-than-expected system pressure and high vibration levels cause joint integrity to suffer. As joints loosen, the amount of leakage increases. In systems with extensive ductwork, increases in joint leakage can have a direct impact on airflow delivery and can dramatically increase operating costs.
Unstable Operation.Unstable operation can result from operating certain types of fans at low airflow rates and from the interaction of multiple fans operating in parallel. In single fan configurations, an aerodynamic phenomenon known as “stall” occurs at low airflow rates. The severity of this stall varies according to fan type, but is most severe in axial fans, forward-curved centrifugal fans, and backward-inclined centrifugal fans. The hunting phenomenon associated with fan stall occurs as the fan searches for a stable operating point. Stall occurs when there is insufficient air moving across the fan blades. As the air “separates” from the fan blade, the force on the blade changes, causing the airflow to change as well.
Stall happens largely because of air separation from the fan blades. When this separation starts on one blade, it often initiates an effect that carries over to the next blade, resulting in a cascading effect.
The shape and distance between the fan blades significantly affect how the stall affects fan performance. Some centrifugal fans, such as those with radial blades, show little change in output. This fact is largely because of the way radial-blade fans operate—they do not rely on air slipping across the blade surfaces and tend to have relatively large distances between the blades. As a result, stall problems are not as common in radial-blade fan as they are in other fans.
Axial fans are particularly vulnerable to stall. Because axial fans rely on the lift generated by blade surfaces, stall can create a significant performance problem. In general, axial fans are not recommended for use in systems with widely varying flow requirements, unless a means of keeping airflow rates above the stall point, such as a bleed line or a recirculation path, is available.
A solution to this problem is commercially available. A proprietary design feature, known as an anti-stall device, automatically modifies the flow patterns around the fan blades to provide stable operation at all combinations of flow and pressure. In applications where stall is a risk, this fan design can be considered.
Even in systems in which operating conditions are not expected to create stall problems, fan degradation or a significant increase in system pressure (filter clogging or system fouling) can cause a fan to develop an instability problem. In multiple-fan configurations, fans alternately shifting loads between each other can cause instability. This effect occurs at low-flow rates that are typically to the left of the peak pressure on the combined fan curve. Avoiding this problem requires de-energizing one of the fans or decreasing the system resistance to allow greater airflow.
Imagine a fan selected with great care to provide exactly the performance required in the specifications. Once installed, the air balancer reports that air performance is considerably lower than required. What went wrong?
The term “system resistance” is used when referring to the static pressure. The system resistance is the sum of static pressure losses in the system. The system resistance is a…