This article takes an in depth look at Centrifugal Blowers.
You will learn:
A centrifugal blower is a motor or pump that moves air using centrifugal force that is created by the rotation of an impeller that pulls air or fluids into the blower and pushes it out through the blower’s outlet. They are made up of an impeller, housing, and drive mechanism with the impeller being the key element that has a series of blades mounted on a central hub connected to a fan shaft.
The drive design of a centrifugal blower, which can be belt or direct drive, determines the speed at which the impeller rotates. The speed and efficiency of centrifugal blowers makes them adaptable to a wide range of applications including various types of driers to HVAC systems.
The angle of the blades of a centrifugal blower determines its efficiency and effectiveness or how fast it moves air through the system. The three varieties of blade angles are forward curved, backward curved, and radial.
In the discussion of centrifugal blowers, the use of the terms blower and fan are used interchangeably. One of the reasons for emphasizing the use of the term blower is to avoid confusion of centrifugal blowers with axial fans that move air in an axial or direct path.
The most prominent difference between the various types of centrifugal blowers is the angle of their blades, which can be backward curved, forward curved, or radial where the blades stick straight out from the impeller.
Centrifugal blowers are an industrial tool that is widely used for high pressure applications and are found in processing industries as an essential element in air pollution control systems. An important distinction between centrifugal blowers and other types of blowers or fans is their ability to provide air flow efficiently at a constant pressure.
Though there isn’t any general formula for classifying centrifugal blowers, they can be broadly divided into six categories. Each class has its range of uses and limitations.
The curve of forward curved centrifugal blowers face toward the direction of the rotation of the impeller. The length of the blades are shorter as compared to radial and backward curved blades. The force of the air placed on the blades is in the direction of the bend of the blades, which can only be achieved with the shorter blades. The velocity vector VR of forward curved blades is greater for forward curved blades compared to radial and backward blades. Forward curved blades create the highest pressure but are the least efficient.
Forward curved centrifugal fans are also known as squirrel cage blowers because the wheel of the blower is in the shape of a squirrel cage wheel. Its structure includes an impeller, shaft, bearings, and housing. Forward curved centrifugal blowers are ideal for domestic furnaces and air conditioning units.
Backward inclined centrifugal fans have three types of blade designs, which are flat single thickness, curved single thickness, and curved airfoil. The blades of backward inclined blowers are much heavier and larger than forward curved centrifugal blowers and move in opposition to the direction of the wheel rotation. The velocity vector for backward curved blade blowers is smaller than radial and forward blade fans.
A reason that backward inclined centrifugal blowers are so widely used is their ability to perform at high speeds with exceptional efficiency since they have a low horsepower requirement and very rugged construction. This also makes them suitable for high static applications.
This design of the backward inclined centrifugal blower is capable of moving both air and gasses that have small amounts of corrosive materials and high temperatures. For extra protection, curved single thickness centrifugal blowers have liners and hard surfaces.
This backward inclined centrifugal blower is very strong and efficient and is a cost effective alternative to backward curved designs. The flat single thickness design has a slightly lower efficiency, which is compensated for by easily installed liners.
The curved airfoil design of backward inclined centrifugal blowers is used in high volume air flow applications that have low pressure. They are mainly used as a method for cleaning air or moving gasses with small amounts of erosive material.
Radial centrifugal blowers have straight blades without any curvature and are oriented radially. The blades are perpendicular to the direction of the wheel rotation. The radial design of the blades prevents dirt or dust from sticking to them. This feature makes them ideal for harsh conditions and material handling applications. Such environments requires that the blades be heavier and deeper with a simple straightforward design.
Certain designs of the blades of a radial centrifugal blower have side rims. The blades are heavier, deeper, and narrower than the ones found on forward and backward inclined centrifugal blowers. Radial centrifugal blowers are capable of producing medium to high pressure and moving higher masses and volumes of fluids.
Airfoil centrifugal blowers provide high volume airflow with low power consumption. They have a round leading edge with a sharp trailing edge. As the airflow approaches the blower, it splits and moves above and below the blade. Air deflection takes place along the convex curve of the top of the blade and the concave curve of the bottom. As air moves over the sharp trailing edge, it leaves the blade.
The design of airfoil blades makes them capable of handling large volumes of clean air at low to moderate static pressure. They run at greater speed and efficiency than backward inclined blowers.
Radial tip centrifugal blowers have a compact design and are made for high volume air flow where the pressure requirements are high. The tipped blades keep dust and particulate matter moving as a method for handling clean and dirty air flow. The structure of a radial tip centrifugal blower combines the features of a backward inclined blower and radial blower.
A centrifugal radial tip blower can move corrosive and saturated air since it is made of stainless steel and has an epoxy coating with highly durable linings that protect the blower from corrosion.
The most noticeable and discussed feature of inline centrifugal blowers is their rugged construction that includes a backward inclined wheel in an axial tube casing. They are an ideal tool for square ducts as a clean air booster for supply and exhaust systems where other blowers are impractical. Inline centrifugal blowers can be installed vertically or horizontally at any angle.
An inline centrifugal blower has all of the advantages of a centrifugal blower with the size and space consumption of an axial one. The appeal of inline centrifugal blowers is their versatility since they have use in multiple applications.
Plug blowers or fans are known by a wide variety of names. The most obvious and noticeable aspect of plug blowers is they do not have a housing; this gives them the name open type fan or blower. Plug blowers are used in high temperature applications, autoclave vessels, or for dust collection.
Each plug blower is engineered and designed to meet the conditions of a specific high temperature application. Customers are required to provide data on operating temperatures, required circulation, air flow, pressure, and density.
The design of a volute centrifugal blower casing regulates the flow and changes kinetic energy into static pressure. The casings for volute centrifugal blowers are spiral shaped, which makes it possible for them to change the velocity of the flow to force. The spiral design slowly reduces the velocity of the flow as it increases its area. The force rises as the velocity drops.
Volute casings can be single or double volute and are the portion of the blower that receives the flow. They are named after an ancient architecture feature that was placed at the top of a column. With a split volute, the path of the volute is separated leading to two discharge paths.
During the operation of a volute centrifugal blower, the impeller sends the flow to the casing outlet tip where the flow forms a volute that is discharged through the blower's conical tip.
With multistage centrifugal blowers, air is pulled into the first stage through an inlet volute. The different stages are linked through return channels where a discharge volute collects the air from the exit. In the different stages of a multistage centrifugal blower, each stage acts like a single stage centrifugal blower where each stage compresses the flow. As the flow moves from stage to stage, its pressure increases while the flow rate remains the same.
Using multiple impellers to distribute the pressure load, multistage blowers can generate more power and higher pressure using a smaller motor and less energy. As the number of stages increases, the discharge pressure increases, which makes multistage centrifugal blowers capable of producing very high pressure.
The impellers for a multistage centrifugal blower have radial, backward curved, or backward inclined vane designs.
Centrifugal blower performance curves show the operating characteristics of a blower by flow versus pressure and flow versus power. Since air has a variable density, the performance curve of a centrifugal blower shifts with inlet pressure, inlet air temperature, and relative humidity. Other variances are the result of changes in speed, modulating inlet guides, or throttling valves.
The operating flow of a centrifugal blower is determined by plotting its performance curve during unstable flow referred to as surge. The intersection of the curves determines the operating air flow rate from which the power can be ascertained. As the level and static pressure increase, the point of intersection shifts to a lower flow if the blower control system is not modulated.
If a variable level application needs a reduced flow, it is possible to modulate the blower to meet the needs of the reduced flow. Today’s technology has made it possible to improve the efficiency and flexibility of centrifugal blowers where the discharge pressure varies. Control systems are designed to measure a centrifugal blower's flow rate and adjust it when there are variances.
Centrifugal blowers are the most popular type of air movement tool used for industrial and commercial applications. The simple design and structure of centrifugal blowers is the main reason for their popularity as well as their high endurance and reliability.
The design of centrifugal blowers includes a motor, fan wheel, and a housing. The rotating impeller is attached to the fan wheel that catches the air flow and pushes it out the outlet.
For many applications, centrifugal blowers are preferred over axial as a method of rapidly and efficiently moving air.
The electric motor of a centrifugal blower is the most important part of its mechanism. It is the force that rotates the impeller that pulls in the air and blows it out at a right angle. The drive mechanism on a centrifugal blower can be direct or indirect belt driven.
With a direct drive configuration in a centrifugal blower, the drive shaft is connected directly to the motor; this allows for more power but produces increased torque. The higher RPMs, speed, and fan of the blower can be controlled by changing the speed of the motor. The term direct drive indicates that the motor operates without the need for a tensioner, belt, or chain; this decreases the need for maintenance and repairs.
Direct drive centrifugal blowers can be further divided into high and low pressure blowers. High pressure direct drive blowers are used for combustion of solid, liquid, and gas fuels or pneumatic conveying. Low pressure direct drive motors are used for cooling air, dilution, ejection, ventilation, and air circulation.
Indirect or belt driven motors operate using a flexible coupling or V belt. The belt, attached to the motor, has variable speeds, which makes belt driven blowers perfect for low pressure applications. Since power is diverted through the belt, lower torque on the motor assists in lowering maintenance and repair costs.
The design of the drive motor determines the speed at which the fan wheel will rotate. The fan wheel or impeller is the central factor in the operation of a centrifugal blower since it is the element that moves the air flow, creates pressure, and contains the blower’s blades and defines the type of blower.
A centrifugal fan wheel has a cylindrical shape with a series of parallel non-intersecting slits. The individual slits can be straight, angled, or bent to fit the design of the blower. Air leaves the impeller vane tangent to the tip of the vane.
The selection of a centrifugal fan is highly dependent on the choice of fan wheel, which also determines the application for which the blower may be used. Below is a diagram of some of the available fan wheels.
The housing of a centrifugal blower is ducted as a method for directing the flow of the outgoing air. The materials used to produce the housing vary according to the application, with more resilient housings required for harsh and stressful environments.
Housings are available in stainless steel, galvanized steel, and aluminum with stainless steel being the most commonly used.
The discharge angle at which the air flow leaves the centrifugal blower is determined by the design of the housing. The ducting is welded and smooth to prevent any obstructions to the air flow.
The scroll shape of the housing is designed to accelerate the air flow and change its direction twice before the air exits the housing. Housings can be coated or painted as a second method of protection from the working conditions.
At the heart of a centrifugal blower or fan is its air flow. A centrifugal blower pulls air, fluid, gas, or particulate matter into the inlet. The blades of the blower rotate to discharge the air through the outlet. This air flow pattern is very different from that of the traditional fan or blower where the air flow is parallel to the axis of the fan.
The action of the air flow is caused by the rotating blades, which create a low pressure area at the center of the blade and a high pressure area at the edge of the blade that adds kinetic energy to the air.
The low pressure area at the center of the blade produces a vacuum that draws air in. The pulled in air flows from the low pressure area to the high pressure area at the edge of the blower and moves around the impeller and out of the housing or enclosure.
In the diagram below, the blue arrows indicate the air that is being pulled into the low pressure area. The green arrows indicate the air moving from the low pressure area to the high pressure area from which it circulates and is expelled from the blower.
Centrifugal blowers are used where there are requirements for the movement of air. Any fan that has a high pressure ratio is referred to as a blower. The main usage of blowers is in processes that need a constant and steady flow of air such as pneumatic conveying systems.
Of the blower choices, centrifugal blowers have several advantages over the other types due to their wide operating range and high speed. The variations in centrifugal blowers are air flow capacity, blower type, blower dimensions, and operating pressure. Regardless of the variations, centrifugal blowers can be customized and adapted to fit any type of industrial application.
Air movement is a critical part of all industrial operations since it serves as a method for providing comfortable working conditions and the removal of toxic or harmful fumes. Centrifugal blowers that are used for industrial air movement include backward curved, backward inclined, and radial.
Industrial exhaust blowers or fans handle clean dry gases or light particulate saturated gas. This type of venting blower is designed for heavy duty industrial operations where exhaust air has to be replaced with fresh air.
Dust loading centrifugal fans and blowers are part of a dust collection system to assist in the improvement of air quality and to create safe breathing conditions. Centrifugal dust loading blowers capture, collect, and move particulate loaded air through an air scrubbing system.
There are several configurations of this type of centrifugal blower. They may be part of a hood system over a dust contaminated workspace or the initiating piece of an air filtration system such as a baghouse, trap, or a form of ductwork.
The essence of a pneumatic material handling system is the maintaining of constant pressure throughout the system to prevent fallout and clogs. A wide variety of material can be transported using a pneumatic system; this includes sand, pellets, chips, and lumps of coal.
A pneumatic system moves material using a set of feeders, fans, blowers, ducting, and filtration units. The main feature that is used in the selection of the correct centrifugal blower for a pneumatic system is the density of the material, which is the factor that determines the type of blades, drive, and any necessary accessories.
Saturated air is air that has been moistened by humidity or some form of gas. The air in these conditions is normally heavier due to the particulate content and requires a more robust centrifugal blower. The moisture content of the air necessitates the use of a centrifugal blower made of stainless steel and has some form of coating such as epoxy.
The greatest challenge regarding saturated air is corrosion, which can severely damage the blower. All centrifugal fans and blowers for this type of application are made of corrosion resistant metals and may include some form of protective coating. A precision engineered and well planned system leads to lower repair and maintenance costs as well as an extension of the life of the blower.
Heat treatments, stress relieving, industrial furnaces, and ovens use convection to circulate heated air. Centrifugal blowers and fans for this type of operation are required to be highly resilient, durable, and long lasting to withstand the high temperature conditions.
Regardless of the design and engineering of centrifugal fans and blowers for high temperature applications, they are destined to wear out and fail due to the conditions. With this in mind, it is imperative that centrifugal fans used in high temperature applications be closely monitored and maintained to prevent catastrophic failure.
Specially designed plug blowers and fans are most commonly used in high temperature applications. Plug blowers are designed to move high volumes of air at a low static pressure. They are available in several wheel configurations and materials.
The main function of centrifugal blowers for the dairy industry is to move and recycle air in a processing plant. Centrifugal blowers for the dairy industry are required to meet the same Food and Drug Administration (FDA) regulations that apply to any form of equipment used for food processing. Aside from their use as a means for cleaning and recirculating air, centrifugal blowers for the dairy industry are also used as part of the drying process for cheeses and dairy by-products.
The typical use of incineration is to dispose of waste by burning it. Centrifugal blowers for incinerators are designed to meet the gas volume and pressure requirements of incinerators. A forced draft system sends air into the incinerator through the use of a high pressure centrifugal blower. On the exhaust side of the incinerator, an induced draft blower is used.
In a fluidized bed furnace that incinerates sludge, a multistage system of blowers is used due to the high pressure required to provide the proper air supply. These specially designed blowers are made to stop any negative effects from the sludge or putrefied gasses.
The standards for blowers for this application are very high and getting more stringent as concerns for the environment are ever growing. All manufacturers work diligently to ensure their equipment meets the requirements of the Air Movement and Control Association (AMCA) and air quality control.
Centrifugal blowers and fans are a highly efficient and maintenance free method for moving high volumes of air at high pressure. Every industrial operation needs to improve the ventilation in its workspace for the safety of its workers. In this age of ecological awareness, this has become an even more important part of facility management and planning.
In the majority of cases, planners begin designing buildings, factories, and assembly plants with centrifugal blowers due to their reliability. They can be adapted, engineered, and configured to fit the needs of any design or plan. This flexibility is why so many industrial operations rely on them.
The most notable factor regarding centrifugal blowers is their low maintenance cost. After spending thousands of dollars on equipment repairs and installation, the low cost of a centrifugal blower helps in maintaining and controlling operating costs.
With other methods of air circulation, it is important to frequently check for the build up of dust and dirt. This is not necessary with industrial centrifugal blowers. A small vacuum to clean the motor and vents is all that is needed.
The somewhat simple modular design of centrifugal blowers makes them easy to maintain and service.
With any form of industrial device, efficiency of operation ensures lower cost and improved productivity. The constant and even air flow from a centrifugal blower generates energy that reaches 84% static efficiency, which is an ideal standard for maintaining large air flow systems.
One of the main reasons that centrifugal blowers are so widely used is their ability to be adapted, adjusted, and configured to meet the needs of any industrial operation. Centrifugal blowers are capable of operating in any air flow system and can be adjusted to meet the needs of a changing and growing operation.
Centrifugal fans and blowers can be found in any type of industry from paper mills to automobile plants. In each case, they serve a different purpose at the highest possible efficiency.
Many modern day manufacturing processes require equipment that can withstand hazardous and harsh conditions due to the operating environment. Centrifugal blowers and fans are designed to perform in corrosive, toxic, high temperature, and high humidity conditions. Each type of centrifugal blower is engineered to meet the needs of the manufacturing conditions and offer exceptional performance.
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