Introduction
Fans are a critical feature in the design of ventilation systems for dust control. They are used to move the air through the ventilation system, whether to create an exhausting or blowing ventilation system. In an exhausting system, the fan is located at the end or discharge of the ventilation system and is used to “pull” air through the entire system. In a blowing system, the fan is located at the inlet of the ventilation system and is used to “push” air through the entire system [Hartman et al. 1997].
Fan Operating Characteristics
The operating characteristics of the fan are provided by the manufacturer. This information can be in the format of fan performance tables and/or fan performance curves. Fan performance tables provide only the minimum information, i.e., static pressure, airflow, and brake horsepower (BHP) required for selecting a fan [Greenheck Fan Corporation 1999]. Static pressure is based upon the amount of pressure required to overcome the friction loss of the entire ventilation system. Airflow is the amount of air required for the ventilation system. BHP is the horsepower required of the motor to operate the fan at the desired static pressure and airflow.
Fan curves, or performance curves, are graphs that also describe the performance characteristics of the fan. The fan performance curve is provided for a particular model of fan at a given revolutions per minute (RPM). There can be a series of fan performance curves to cover the performance of a selected fan, with each graph representing a different RPM. Additionally, BHP curves for the fan can be included on the graph, with the BHP scale on a separate scaled y-axis. Information about mechanical efficiency and noise is sometimes included if the manufacturer provides this information [Hartman et al. 1997].
Types of fans
There are different types of fans used in ventilation systems, with their selection being dependent upon their operating characteristics. Several basics of fan operation need to be understood in order to properly select a fan for a ventilation system.
There are two basic types of fans: axial-flow fans and centrifugal fans. There are also other fan designs that use or combine the concepts of axial or centrifugal flow; these are axial-centrifugal fans and roof ventilators. Again, the selection of the fan type is based upon the requirements of the ventilation system design.
Axial-Flow Fans
The axial-flow fan category includes propeller fans, tube axial fans, vane axial fans, and two-stage axial-flow fans. Axial-flow fans move the air in a direction that is “axial,” or parallel to the axis of rotation of the fan. Propeller fans are the most common type. They are generally mounted in the wall of the building near a heat source to exhaust the hot air into the outside atmosphere (Figure 1). They can have different drive configurations, being either direct drive or belt drive. The wall-mounted fans can also be constructed with shutters that close when the fan is turned off. The fans for this purpose can be either wall-mounted or pedestal-mounted. Propeller fans are designed to move large volumes of air at low static pressures [Bleier 1998].
Tubeaxial fans are used for exhausting air from an inlet duct (Figure 2, left). They consist of a fan with many blades in a cylindrical housing. The blades are generally shaped as an airfoil to help with air movement. The hub diameter can be 30 to 50 percent of the blade outside diameter [Bleier 1998]. The housing is connected to the inlet duct and also contains the motor support. These fans are used in conditions that require moderate static pressures (higher than those required for propeller fans).
Vaneaxial fans are similar to tubeaxial fans. The vaneaxial fan has a housing that contains guide vanes that are oriented parallel to the airflow (Figure 2, right). These vanes are used to recover the tangential airflow velocity from the fan blade to convert it into static pressure. This tangential airflow velocity is not recovered as static pressure in propeller or tubeaxial fans (this component is lost energy). The hub diameter of a vaneaxial fan is much larger, being 50 to 80 percent of the blade outside diameter [Bleier 1998]. The vaneaxial fan is used for conditions involving high static pressure.
Two-stage axial-flow fans are basically two axial-flow fans configured in series. This configuration allows for operation in conditions involving high static pressure, as each fan’s operating pressure is added together when in series. This fan can be designed to have each fan rotate in the same direction with guide vanes located between each of the fans, or the fans may counter-rotate.
Centrifugal fans
The airflow for a centrifugal fan is different from that of axial-flow fans. For a centrifugal fan, the airflow is drawn into a rotating impeller and discharged radially from the fan blade into a housing. The resulting flow of air is perpendicular to the axial rotation, or parallel to blade motion [Hartman et al.1997], and the housing is used to direct the airflow to the desired location (Figure 3).
There are numerous types of centrifugal fans. The flow through the fan is basically the same for all types, the difference being in the configuration of the blades. Each blade type has its advantages for different applications, as detailed below.
- Airfoil blades have the best mechanical efficiency and lowest noise level.
- Backward curved blades have slightly lower efficiencies compared to airfoil blades. These blades are better suited to handle contaminated air because they are single thickness and can be made of heavier material that can resist the effects of contaminated air on the fan blades.
- Backward inclined blades have lower structural strength and efficiencies. They are easier to produce due to the elimination of the blade curvature.
- Radial tip blades are curved at the tips. These types are used mainly in large diameters (30 to 60 inches) under severe conditions of high temperatures with minimal air contamination [Bleier 1998].
- Forward curved blades produce airflow rates higher than other centrifugal fans of the same size and speed. This allows for the fan to be more compact than other types of centrifugal fans. These fans are often used in furnaces, air conditioners, and electronic equipment cooling.
- Radial blades are rugged and self-cleaning but have low efficiencies. They are suited for airflows containing corrosive fumes and abrasive material from grinding operations.
Conclusion
Fans are essential for ventilation systems, ensuring effective air circulation for dust control and safe operations. Their selection is based on static pressure, airflow, and motor power, guided by performance data. Axial-flow fans are ideal for varying pressure and volume needs, while centrifugal fans excel in handling high temperatures or contaminated air. Understanding fan types and performance ensures efficient, durable, and tailored system designs.
Reference
NIOSH Mining Program Report of Investigations, « Dust Control Handbook for Industrial Minerals Mining and Processing» Second Edition.
