Factors Determining Fan Selection

As we already know, fan selection is a complicated process that starts with a basic knowledge of system operating requirements and conditions such as airflow rates, temperatures, pressures, airstream properties, and system layout. The variability of these factors and other considerations, such as cost, efficiency, operating life, maintenance, speed, material type, space constraints, drive arrangements, temperature, and range of operating conditions, complicate fan selection. However, knowledge of the important factors in the fan selection process can be helpful for the purposes of reducing energy consumption during system retrofits or expansions. Often, a fan type is chosen for nontechnical reasons, such as price, delivery, availability, or designer or operator familiarity with a fan model. If noise levels, energy costs, maintenance requirements, system reliability, or fan performance are worse than expected, then the issue of whether the appropriate fan type was initially selected should be revisited.

Fans are usually selected from a range of models and sizes, rather than designed specifically for a particular application. Fan selection is based on calculating the airflow and pressure requirements of a system, then finding a fan of the right design and materials to meet these requirements. Unfortunately, there is a high level of uncertainty associated with predicting system airflow and pressure requirements. This uncertainty, combined with fouling effects and anticipated capacity expansion, encourages the tendency to increase the specified size of a fan/motor assembly.

Designers tend to protect against being responsible for inadequate system performance by “overspecifying.” However, an oversized fan/motor assembly creates a different set of operating problems, including inefficient fan operation, excess airflow noise, poor reliability, and pipe/duct vibrations. By describing some of the problems and costs associated with poor fan selection, this sourcebook is intended to help designers and operators improve fan system performance through better fan selection and improved operating and maintenance practices.

Noise. In industrial air blower , noise can be a significant concern. High acoustic levels promote worker fatigue. The noise generated by a fan depends on fan type, airflow rate, and pressure. Inefficient fan operation is often indicated by a comparatively high noise level for a particular fan type.

If high fan noise levels are unavoidable, then ways to attenuate the acoustic energy should be considered. Noise reduction can be accomplished by several methods: insulating the duct; mounting the fan on a soft material, such as rubber or suitable spring isolator as required to limit the amount of transmitted vibration energy; or installing sound damping material or baffles to absorb noise energy.

Rotational Speed. Fan rotational speed is typically measured in revolutions per minute (rpm). Fan rotational speed has a significant impact on fan performance, as shown by the following fan laws:

Airflowfinal=Airflowinitial (RPMfinal/RPMinital )

Pressurefinal=Pressureinitial (RPMfinal/RPMinital )2

Powerfinal=Powerfinal (RPMfinal/RPMinitial )3

Rotational speed must be considered concurrently with other issues, such as variation in the fan load, airstream temperature, ambient noise, and mechanical strength of the fan.

Variations and uncertainties in system requirements are critical to fan type and fan rotational speed selection. Fans that generate high airflow at relatively low speeds (for example, forward-curved blade centrifugal fans) require a relatively accurate estimate of the system airflow and pressure demand. If, for some reason, system requirements are uncertain, then an improper guess at fan rotational speed can cause under-performance or excessive airflow and pressure.

Airstream temperature has an important impact on fan-speed limits because of the effect of heat on the mechanical strength of most materials. At high temperatures, all materials exhibit lower yield strengths. Because the forces on shafts, blades, and bearings are proportional to the square of the rotational speed, high-temperature applications are often served by fans that operate at relatively low speeds.

Airstream Characteristics. Moisture and particulate content are important considerations in selecting fan type. Contaminant build-up on fan blades can cause severe performance degradation and fan imbalance. Build-up problems are promoted by a shallow blade angle with surfaces that allow contaminants to collect. Fans with blade shapes that promote low-velocity air across the blades, such as backward inclined fans, are susceptible to contaminant build-up. In contrast, radial tip fans and radial blade fans operate so that airflow across the blade surfaces minimizes contaminant build-up. These fans are used in “dirty” airstreams and in material handling applications.

Corrosive airstreams present a different set of problems. The fan material, as well as the fan type, must be selected to withstand corrosive attack. Also, leakage into ambient spaces may be a concern, requiring the fan to be equipped with a shaft seal. Shaft seals prevent or limit leakage from around the region where the drive shaft penetrates the fan housing. For example, in corrosive environments fans can be constructed with expensive alloys that are strong and corrosion resistant, or they can be less expensively constructed with fiberglass-reinforced plastic or coated with a corrosion-resistant material. Because coatings are often less expensive than superalloy metals, fan types that work well with coatings (for example, radial fan blades because of their simple shape) are widely used in corrosive applications; however, wear will reduce the reliability of coatings. Alternately, materials such as reinforced fiberglass plastics have been developed for centrifugal fan applications and function effectively in many corrosive environments. However, there may be size and speed limitations for composite materials and plastic materials.

Airstreams with high particulate content levels can also be problematic for the fan drive train. In direct drive axial fans, the motor is exposed to the airstream. Sealed motors can be used in these applications but tend to be more expensive and, in the event of lost seal integrity, they are susceptible to expensive damage. In axial fans, belt drives offer an advantage by removing the motor from the airstream. In centrifugal fans, the particulate content is less of a factor because the motor or sheave can be located outside of the fan enclosure and connected to the impeller through a shaft seal. Gear drives are occasionally used in applications where speed reduction is required but the use of belt drives is unfeasible because of access or maintenance requirements.

In flammable environments, fans are usually constructed of nonferrous alloys to minimize the risk of sparks caused by metal-to-metal contact. In some applications, certain components of the fan can be fabricated out of spark-resistant materials. Fans that operate in flammable environments should be properly grounded, including rotating components, to minimize sparking because of static discharge.

Temperature Range. To a large degree, temperature range determines fan type and material selection. In high-temperature environments, many materials lose mechanical strength. The stresses on rotating components increase as the fan’s operating speed increases. Consequently, for high-temperature applications, the fan type that requires the lowest operating speed for a particular service is often recommended. Radial blade fans can be ruggedly constructed and are frequently used in high-temperature environments. Component materials also significantly influence a fan’s ability to serve in high-temperature applications, and different alloys can be selected to provide the necessary mechanical properties at elevated temperatures.

Variations in Operating Conditions. Applications that have widely fluctuating operating requirements should not be served by fans that have unstable operating regions near any of the expected operating conditions. Because axial, backward-inclined airfoil, and forward-curved fans tend to have unstable regions, these fans are not recommended for this type of service unless there is a means of avoiding operation in the unstable region, such as a recirculation line, a bleed feature, or some type of anti-stall device.

Space Constraints. Space and structural constraints can have a significant impact on fan selection. In addition to dimensional constraints on the space available for the fan itself, issues such as maintenance access, foundation and structural support requirements, and ductwork must be considered. Maintenance access addresses the need to inspect, repair, or replace fan components. Because downtime is often costly, quick access to a fan can provide future cost savings. Foundation and structural requirements depend on the size and weight of a fan. Selecting a compact fan can free up valuable floorspace. Fan weight, speed, and size usually determine the foundation requirements, which, in turn, affect installation cost.

If the available space requires a fan to be located in a difficult configuration (for example, with an elbow just upstream or downstream of a fan), then some version of a flow straightener should be considered to improve the operating efficiency. Because non-uniform airflow can increase the pressure drop across a duct fitting and will degrade fan performance, straightening the airflow will lower operating costs.

An important tradeoff regarding space and fan systems is that the cost of floor space often motivates designers and architects to configure a fan system within a tight space envelope. One way to accomplish this is to use small-radius elbows, small ducts, and very compact fan assemblies. Although this design practice may free up floor space, the effect on fan system performance can be severe in terms of maintenance costs. The use of multiple elbows close to a fan inlet or outlet can create a costly system effect, and the added pressure drops caused by small duct size or a cramped duct configuration can significantly increase fan operating costs. System designers should include fan system operating costs as a consideration in configuring fan assemblies and ductwork.

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