Descriptions of centrifugal fan impellers and their blades with a list of manufacturers
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Centrifugal fans are air flow fans that pull air in and push it out at a 90° angle. The central component of a centrifugal fan is its impeller that is round and has blades projecting from a central hub. The angle and placement of the blades determine the efficiency of a centrifugal fan. The impeller is attached to a shaft connected to a motor that rotates the impeller.. As the impeller spins, at high velocity, it pulls in the surrounding air and discharges it through the fan’s outlet. The amount of air that is pulled in is determined by the turning power of the centrifugal fan's electric motor and the fan’s inlet.
The mechanism for a centrifugal fan is encased in a housing with a shape that is like the shell of an ammonite. The volute form surrounds the impeller and directs air flow. The type of centrifugal fan is determined by the angle of the blades of the impeller, which can be backward curved, forward curved, airfoil, or radial.
Centrifugal fans, also known as squirrel cage fans due to the hamster wheel impeller, are used to move air and gas for industrial and commercial applications. As an air stream enters a centrifugal fan, the air stream’s speed is rapidly increased by the impeller that increases the air stream's kinetic energy and accelerates it radially as it changes direction.
The basic parts of centrifugal fans are supported by bearings, couplings, hoods, guards, flanges, isolators, and various other standard machine parts. The key components play a major role in the efficiency of a centrifugal fan and provide optimal performance. The essential element of a centrifugal fan is its impeller, often referred to as a hamster wheel, which defines the type of centrifugal fan.
The structure of centrifugal fans, due to their use, requires strength, durability, and endurance. In most cases, they are required to operate continuously, a factor that necessitates careful selection of their materials and the planning of their structure. Aside from the fan's impeller, the main components of a centrifugal fan are its housing, drive, ducts, dampers, and louvers.
The key component and defining feature of a centrifugal fan is its impeller, which can take different forms to meet the needs of various applications. The choice of impellers covers nine different categories. As may be assumed, the shape, design, and blades of each category differ and is what distinguishes them. The function of the impeller is engineered to mechanically transform the rotational movement of the motor into kinetic energy that forces air to move in a specific direction at a higher rate of speed.
The overall performance of a centrifugal fan depends on the design, configuration, and materials of the impeller. As the central part of a centrifugal fan, the impeller directs the movement of the pulled in air into the ventilation system. The rotational movement draws in the air as the fan pushes the air outward, perpendicular to the intake direction due to the centrifugal force generated by the impeller.
Shrouded impellers have disk modes at lower frequencies and blade modes at higher frequencies due to the blades being constrained inside the shroud. Although the shroud of shrouded impellers is cumbersome, shrouded impellers are highly efficient, which offsets their large footprint.
The uses for shrouded impellers include pressuring and blow off systems and systems with long duct work or pipe runs. Shrouded impellers, due to their enclosed housing, deliver higher pressure.
Open radial impellers have blades or vanes attached to a central hub without the inclusion of a shroud, housing, or covering. The design of open radial blade impellers limits their use to low pressure, low viscosity applications. They are easy to clean but are inefficient, which is the reason they are used for less demanding applications and are seldom used for air movement.
A paddle wheel impeller has equally spaced blades or vanes that extend perpendicularly from the hub of the impeller. The design is used in high pressure applications and has an appearance that is similar to an open radial wheel impeller. Referred to as steel plate radial blade fans, paddle wheel fans have blades that are narrower, deeper, and heavier than forward curved and backward inclined blades. Due to the nature of the paddle wheel design, paddle wheel impellers tend to be more rugged with a minimum ledge that could accumulate dust or sticky materials.
Backward inclined impellers have a backward incline angle to their blades, which makes them ideal for high temperature and abrasive air streams. The design is to accommodate clean air applications with centrifugal fans that can handle a wide range of air flows that have static pressure and tolerance for dust and moisture. The backward incline of the blades produces lower sound levels and lower energy costs.
In dust collection systems, centrifugal fans with a backward inclined impeller helps remove dust filled air from the environment. Induced draft air from the impeller design pulls air into a dust collector or baghouse. Many backward inclined impeller centrifugal fans are included as a part of air pollution control systems.
Backward curved impeller centrifugal fans have blades that are curved away from the direction of the fan’s rotation. The blades are larger and have more curves to them to increase the efficiency of the airflow and increase pressure. Backward curved impellers are housed in circular or rectangular casings.
The features that differentiate backward curved impellers are their high efficiency and better airflow that reduces energy loss. The backward curve enables backward curved impellers to handle higher pressure, increased resistance, and pressure drops, which enables them to endure applications with high static pressure. In many cases, since backward curved impellers do not accumulate dust or dirt, they are the first choice for clean air circulation in cleanrooms.
Airfoil impeller blades are a form of backward inclined centrifugal fan blade that uses airfoil shaped blades that save on energy and operate quietly. The name of airfoil impeller’s blades is due to their appearance, which is similar to that of airplane wings. The design assists in creating more air pressure and turbulence, enabling them to save on energy and operate quietly. Impellers with airfoil blades move a lot of air with little noise and low power.
The use of airfoil impellers is in locations where noise interferes with the ambiance but high volumes of air are required. In hospitals, areas have to be kept clean but have quiet air flow to avoid irritating patience. Theaters and auditoriums require continuous and quiet air flow to avoid interrupting presentations and entertainment.
Forward curved multi-vane impellers operate at lower speeds, reducing energy while providing exceptional performance. The design is efficient, reliable, and minimizes noise, like airfoil impellers. Forward curved multi-vane impellers are commonly used for low pressure and high air flow applications where high efficiency is required. The design of the impeller is compact allowing the centrifugal fan to fit in tight limited spaces. Forward curved multi-vane impellers are capable of functioning in a wide range of applications including exhaust, ventilation, cooling, and material handling ones.
High volume air flow forward curved centrifugal fans are space saving and efficient. In many instances, they outperform other types of centrifugal fan impellers but require regular maintenance to ensure peak performance.
The term forward curved multi-vane impeller refers to the impeller’s structure that includes several shallow blades curved in the direction of the fan’s rotation to move high volumes of air at low pressure. As has been used to describe other forms of impellers, forward curved multi-vane impellers have the hamster or squirrel cage style that is typical of other centrifugal fans. The popularity of forward curved multi-vane impellers is due to their effectiveness and low cost.
Backward curved radial impellers are a version of the open radial blade design. They are capable of handling high volumes of air with high efficiency and are known for being non-overloading because changes in static pressure do not overload their motor. The curved blade design and larger blades allows for better air flow control that reduces energy losses.
In contrast to forward curved blades, backward curved radial blades are capable of handling higher pressure with increased resistance and pressure drops while maintaining air flow capacity. The downside of this aspect of backward curved radial blade impellers is their higher level of noise due to the increased turbulence the larger blades create. Aside from the noise level, the backward curved design prevents the accumulation of dust, dirt, and debris on the blades. This contaminate free characteristic enables backward curved impellers to be used in environments where clean air is a necessity.
Radial tip impellers have straight blades with a forward curved tip. The blades of a radial tip impeller are self-cleaning due to the curve. They are highly proficient at handling high volume air flow with dirt, dust, and particulate matter. Unlike open radial blade impellers, radial tip impellers are capable of handling high pressure applications. The straight feature of the radial tip blades is resistant to particle buildup, which makes them suitable for handling industrial processes where dust and dirt are present.
The leading edge of radial tip impellers cuts through the air flow and aligns with the hub of the impeller. As can be concluded, radial tip impellers are more efficient and resist particulate buildup. The positive energy of the blade face expels debris, preventing its accumulation. Radial tip blades are capable of handling 650°C (1202°F) air flows at up to 700,000 m³/h, and pressures as high as 12,000 Pa, enabling them to handle dirty gases with high volumes of dirt and debris.
The blade angle of an impeller is crucial to the performance of an impeller. The three basic angels are backward, forward, and radial. The correct angle of impeller blades enhances impeller efficiency and energy transfer. The design of an impeller blade determines the rate at which air will be pulled into a centrifugal fan with backward curved blades being the most efficient.
Backward Curved Impeller Blades – Backward curved impeller blades are curved away from the direction of the rotation of the fan’s impeller for smooth air flow and efficiency in high static pressure application. Due to the ability of backward curved impellers to handle high pressure applications, the blades are made of highly durable materials, such as aluminum sheet metal, which ensures the blades can withstand demanding environments while delivering consistent high performance. In addition, the backward design provides better control of air flow.
The impeller is the rotating aspect of a centrifugal fan. Connected to the motor, the rapid rotation of the impeller pulls in atmospheric air and changes the direction by 90° and expels the air at higher speed. It is the core of a centrifugal fan and is the component that directs air movement. The structure of an impeller includes a central hub upon which blades or vanes are mounted. The angle and direction of the blades determines the efficiency of a centrifugal fan and its capabilities. In many cases, the design and type of impeller define the application for which a centrifugal fan can be used as well as its strength and durability.
Although the impeller is the central figure of a centrifugal fan, the other components are the supporting cast that assist in air movement and play an important role in protecting, stabilizing and supporting the impeller. Bearings, couplings, hoods, guards, flanges, isolators, and other minor pieces are components of the major basic parts of a centrifugal fan. Aside from the impeller and its blades or vanes, other parts of a centrifugal fan are its housing, motor, inlets and outlets, dampers, and louvers.
The nature of the function of centrifugal fans necessitates components that are capable of withstanding the fans constant use. Variations in a centrifugal fan’s construction are in accordance with the application for which the fan is designed. Although steel, aluminum, and other metals are used in centrifugal fan construction, plastics are also used.
Most centrifugal fans have direct drive motors with the impeller mounted on a shaft extension from the motor. For fans with indirect motors, a belt attached to the shaft drives the impeller, which increases rotational speed and improves output but is energy inefficient. The motor of a centrifugal fan determines the amount of air the impeller pulls in. This aspect of the process is based on the turning power of the motor.
There are several types of centrifugal fan motors that include direct drive, indirect drive, variable frequency drive (VFD), and induction. The speed of centrifugal fan motors varies and can be controlled with the addition of a frequency inverter or integral electronics.
Direct drive motors are motors used on smaller centrifugal fans, have fewer components, higher efficiency, lower maintenance costs, and lower costs overall. The nature and design of direct drive motors makes them more reliable and sturdier. The fan speed, with direct drive motors, is proportional to the speed of the motor due to the direct connection.
Belt driven motors offer more flexibility for matching air flow requirements when equipped with a variable pitch motor pulley, which is common for larger centrifugal fans. The design of belt driven motors includes a wheel shaft and motor shaft that are connected by a belt and mounted on the sheave with a fixed wheel speed.
With variable speed motors, the drive mechanism uses couplings to make the connection between the motor shaft and wheel shaft. Some systems have electronic variable speed drives that automatically control fan speed at the necessary level. Direct drive centrifugal fans can use a variable frequency drive for external control of the speed of the motor. In belt driven centrifugal fan motors, VFDs can be used but are not necessary since the operating speed of a belt driven centrifugal fan motor can be changed by modifying the sheave ratio of the belt.
A special form of drive for centrifugal fans is a coupling drive that is used when the fan and motor are too large and bearings cannot support direct mounting of the shaft to the impeller. The connection between the motor and shaft is through a group of couplings. The connection is flexible, capable of absorbing vibrations and accommodates shaft misalignments. The design allows the motor and fan to have different bearing systems to simplify maintenance and make it possible for removal of components easier.
The motor, direct or indirect, drives the shaft to rotate the impeller using mechanical energy. With direct drive motors, the shaft is connected directly to the impeller from the motor. In the case of indirect drive motors, the shaft is connected to a belt that is connected to the motor. The design of the shaft depends on the weight of the impeller and the velocity of the inlet air. Bearings support the shaft to reduce friction between the rotating elements. This factor allows the impeller to spin smoothly and efficiently.
The shaft must be precision balanced such that it runs smoothly and minimizes vibrations that impact the performance of a centrifugal fan. Most shafts are made of mild steel with alloy steels such as nickel, nickel chromium or chromium vanadium steel being used for applications requiring high strength. Shafts are formed by hot rolling and shaped to size using cold drawing, turning, or grinding.
Air enters a centrifugal fan through the fan’s inlet and exits through the outlet. The inlet and outlet determine the air flow pattern and greatly influence the performance of a centrifugal fan. The inlet controls the amount of air or gas that enters the fan and determines the amount of air the fan can handle. Air exits the fan at the outlet after the air has been accelerated by the impeller.
Of the many aspects of centrifugal fans, the outlet is one of the more interesting. What would seem to be a simple concept of releasing rapidly moving air, the configuration of the outlet can take several forms. The placement of the inlet can take a variety of forms but essentially draws in air like an axial fan. The outlet varies in accordance with the requirements of an application and takes several forms, determining the direction the air will take. In certain centrifugal fans, the exit direction can be adjusted to meet application requirements.
Different directions of centrifugal fan outlets:
The fan housing, known as the casing or volute, contains the impeller and directs air flow through its inlets and outlets. The structure of the housing reduces noise and protects the moving parts of the fan. The movement of the air by the impeller takes place inside the housing, which has to be durable enough to endure the spinning action of the impeller and the rapid movement of air.
A key design parameter of the housing is its curved shape that directs the air and helps in increasing the rapid movement of the air. The tight sealed enclosure of the housing prevents air leaks and ensures the stability of the components. The clearance between the housing and the tips of the impeller blades is normally 0.25% of the impeller’s diameter. As the clearance is reduced, the peak pressure in the fan increases. Vanes and scroll sections are used to shape the housing and guide air flow.
Most centrifugal fan housings look like an ammonite shell without the spiral, which is referred to as a volute casing, referring to a spiral characteristic that is used in architecture. As the air funnels to the outlet, the area of the volute increases. The use of the ammonite volute shape helps smooth the air flow and reduces losses, making a centrifugal fan more efficient.
The volute shape is a precision engineered aerodynamic design that channels air flow with minimal turbulence, increases static pressure, reduces energy loss caused by friction and air leaks, and improves the efficiency of ductwork. Housings have a tight clearance between blade tips and housing for pressure optimization and have materials capable of enduring high temperatures and corrosive environments.
Dampers and louvers are connected to centrifugal fans to manage air flow. Although they have the same purpose, they differ in their function. Louvers are used for ventilation and weather protection with angled slats that allow light through as they block rain and debris. Dampers regulate or stop air flow and are normally mounted on the inlet. They have rotating blades that can open or close the outlet of a centrifugal fan.
The term damper covers a range of devices that manage and regulate air flow. Each type of damper is designed for a specific purpose and takes a different form. Although they are used with centrifugal fans, they have many other functions in an air handling system. In some instances, dampers and louvers are combined to provide multi-air flow control.
The initial categorization of centrifugal fans is by their type of impeller with the basic types being backward, forward, radial, and airfoil. Above the description of the impeller, there are other factors that differentiate centrifugal fans such as their number of inlets with single and double inlets being two common types. As may be assumed, the terms single and double inlets refers to the number of inlets a centrifugal fan may have.
Single inlet centrifugal fans have one air inlet and one air outlet, which leads to high pressure. The design of single inlet centrifugal fans is rather simple with a compact design that takes up less space. The size of single inlet centrifugal fans makes it easy to integrate them into other applications, such as air purifiers and scientific laboratories. In addition, there is less complexity in ducting and airflow management, which saves time and effort during installation.
As may be assumed by their size, there are limitations to the use of single inlet centrifugal fans. They have lower air flow capacity, which restricts the amount of air they can move per unit of time. Single inlet centrifugal fans are not ideal for applications that require high volume of air flow. In addition, single inlet centrifugal fans have a lower static pressure and are used in applications where there is minimal resistance.
The benefits of single inlet centrifugal fans include their ability to provide consistent levels of filtration. As air passes through, a filter captures particulate matter with finer grade filters being able to capture small micron pieces.
Double inlet centrifugal fans have inlets on both sides of the fan housing, which allows the fan to draw air simultaneously from both sides. As may be imagined, the two air flows increase the capacity of a centrifugal fan, enabling the fan to draw higher volumes of air. The higher capacity of double inlet centrifugal fans makes them ideal for ventilation of industrial and commercial air handling systems.
The two inlets of double inlet centrifugal fans and their combined air flow creates a stable and higher-pressure air flow at the outlet, which is useful in applications where air has to be pushed through a long series of ducts or when there is high resistance. The dual inlets of double inlet centrifugal fans necessitate the use of more space to accommodate the wider double configuration. The installation process is difficult due to the complexity of the double inlet centrifugal fan’s structure and size.
Double inlet centrifugal fans are more expensive due to their larger size, performance capabilities, and significantly complex design. These factors are in addition to the difficulty and cost of installing a double inlet centrifugal fan.
Situations that require double inlet centrifugal fans include ones where increased air flow is needed with relatively high installation resistance. The fans matching drives and controls provide the best air flow for ventilation, cooling, and heating. They can be used for heat recovery, air purification, hoods, and air curtains. The two impellers are spaced on a common shaft such that the restrictions are the same on all inlets.
Axial fans and centrifugal fans are the most common and widely used types of fans. They have several variations that differentiates each type with the basic concept of all types being the same. Deciding which of the fans to use for an application depends on certain variables that have to be considered in order to get the right fan for an application.
Centrifugal fans, also known as blowers, pull air in and push air out at a right angle. The discharged air is cooler and, in some cases, cleaner. Centrifugal fans are highly reliable, low maintenance fans that are adaptable to any environment. Their use extends to all walks of life including homes, offices, factories, laboratories, hospitals, and highly secure and tightly sealed locations. The primary benefit of centrifugal fans is their high static efficiency that produces air flow at a consistent and smooth level.
A major factor that differentiates centrifugal fans from axial fans is their impeller, which resembles a hamster wheel. Much like axial fans, the blades of the impeller take different forms for efficiency and air movement with the basic designs being backward, forward, radial, and airfoil. Although the cage variety is the most common, some forms of impellers come without a cage and have special uses.
Axial fans have been around for centuries dating back to the windmill designs of early in the first industrial revolution. They have a low-pressure output and come in a wide variety of sizes. Axial fans are used for cooling, ventilation, cooling equipment, and air movement in rooms. The basic structure of axial fans includes a hub with different types of blades attached. The varieties of axial blades are in regard to the size and angle of the blades as well as their angle in relation to the hub.
As with centrifugal fans, axial fans draw air in and push out. Unlike centrifugal fans, axial fans draw air directly in and release it straight out without changing the direction the air takes. They generate a high flow rate at low pressure with air having linear movement that is parallel to the axis of the fan.
Axial fans can move high volumes of air at low pressure. They push the air straight through creating a linear air flow path. The method allows for high flow rates and rapid movement in open spaces. In general, they are a high-volume, low-pressure tool for air movement that works best in systems where there is low resistance. The simple design allows for smooth and even air flow with minimum energy loss.
Centrifugal fans radically change the direction of the air flow by pulling it to the center of the fan, whipping it around the impeller, and discharging the air at a right angle. The result of the process is high pressure air output with lower air flow volume. The changes in direction creates turbulence and increased noise but is more efficient in high pressure systems. The complex design of centrifugal fans moves low volumes of air at high pressure.
Axial motors spin blades parallel to the direction of the air flow. They run at higher speed with less torque. The motors of centrifugal fans rotate the impeller that creates air flow at a 90o angle and operate at lower speeds with higher torque. Due to the size of the fans, axial motors are smaller, compact, and lightweight. Centrifugal motors are larger, heavier, and more powerful due to the design of the fan. How much energy the fans use depends on the size of the fan and the application for which it is being used.
Most axial fans have AC or DC motors of 5V up to 230V. The motors for centrifugal fans are three phase AC motors that are designed for industrial use. Smaller single inlet centrifugal fans may have single phase or DC motors.
Although the appearance of both fans is distinct and different, they do have some basic similarities.
| Similarities Between Axial and Centrifugal Fans | |
|---|---|
| Simplicity of Design | Speed Selection |
| Reliable and Durable | Used in Numerous Applications |
| Multiple Options | |
A key difference between centrifugal and axial fans is their blade angles. Centrifugal fans have blades at different angles pointed in different directions for air flow and pressure purposes. They produce high-pressure air flow due to the angle of the blades. Blades of axial fans are in line with the axis to provide high volume, low-pressure air flow.
Axial fans are lighter and less cumbersome than centrifugal fans due to their low-pressure output. The backward curved fans of centrifugal fans have higher pressure, which necessitates heavier and larger housings. The lighter, smaller structure of axial fans makes them less expensive and more portable.
The design of axial fans makes them noisier than centrifugal fans. Depending on size and application, some axial fans need silencers to suppress the noise. Regardless of centrifugal fans being larger, they are quieter and have no need for noise suppression.
The most obvious difference between the two fans is their housing where a centrifugal fan has a volute, circular shape while axial fans can have a housing with a rectangular, square, or round, tubular shape. In both cases, their shape does not restrict them from being available in a wide assortment of sizes and configurations.
In addition, axial and centrifugal fans are terms that cover two groups of particular fan types. Under the terms axial and centrifugal is a long list of different fans that perform different functions depending on an application.
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