Everything You Need To Know About Industrial Fans From The Experts

Fans provide solutions to various industrial processes within a vast range of industries including mining, electrical power production, building heating and ventilation (HVAC), metals production and processing, cement production, petrochemical and many more.

Industrial fans are most commonly used to supply air for purposes of combustion processes or cooling and to extract waste gasses or waste heat from a process.

From cooling and exhausting heat, to dust collection, material handling, air conditioning, and chemical processing, fans are the backbone of many industrial processes.

No matter the industry or the process the principal function of an industrial fan or blower is to provide and accommodate a flow of air or gas within industrial operations.

Generally, most industrial fans or blowers can be categorised into one of four fundamental types, which are named in relation to the direction of flow through the impeller:

• Axial flow fans
• Centrifugal Fans
• Mixed Flow Fans
• Crossflow fans

These fan types differ from each other in design and functionality and ultimately are selected based on the amount of airflow, pressure, and efficiency required for the process. This along with specific considerations related to the intended process conditions such as the gas temperatures or erosive / corrosive elements contained in the gas.

In addition to the above classifications, International Standards and the National Legislation Regulations of many countries recognise another special classification fan called a “Jet Fan”. The naming of this class of fan is associated with its function rather than the flow direction through the impeller.
 

Axial Fans In Industrial Industries

An axial flow fan has a flow through the impeller which is predominantly parallel to the axis of the impeller rotation.

In industrial applications, the impeller is often contained in a circular housing which closely encompasses the impeller. The circulating blades draw air into the fan from an industrial process or system and expel the air on the same parallel axis.

Axial fans are generally efficient machines which require low power inputs and produce relatively high flow rates at low to medium pressures.

Uses vary from small applications in HVAC systems to huge processes like tunnel ventilation and supporting heat exchange in cooling towers. Due to the vastness in appliances, axial fans can vary in size and dimension from less than a foot to over 15m in diameter.

 

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The types of axial fans are:

• Variable Pitch Axial Fan
• Adjustable Pitch Axial fan
• Fixed Pitch Axial fan
• Tubeaxial Fan
• Propeller fan

Variable Pitch Axial Fan has an impeller where the pitch angle of the blades can be changed in unison while the impeller is rotating.
 
Adjustable Pitch Axial Fan has an impeller where the blades are attached to the impeller hub in a way that allows them to be changed when the impeller is not rotating. This is generally achieved by removing a clamp or bolts on each individual blade and manually adjusting to set all the blades to the required pitch angle.
 
Fixed Pitch Axial Fan has an impeller where the blades are attached to the impeller hub at a pitch angle which cannot be changed.
 
Vaneaxial Fan is an axial fan with guide vanes at the inlet, discharge, or both that improve efficiency even further by directing and straightening the flow. They have higher pressure capabilities and medium flow capacity. Considered the most efficient axial fan type uses include heat, smoke and fume removal, process drying, comfort and process cooling, and general HVAC ventilation.
 
Tubeaxial Fan is an axial fan without guide vanes, designed to improve airflow efficiency. They are often used in simple industrial applications including drying ovens and fume exhaust systems.
 
Propeller Fan is an axial flow fan with a simple propeller type impeller, which is mounted within an orifice plate or ring. Propeller fans can handle large volumes of air at low pressure or “free delivery” and moderate temperatures. They experience a large change in airflow with small changes in static pressure and are often used indoors as ventilation exhaust fans, or outdoors in cooling towers or air-cooled condensers. Their efficiency is generally low.

 

The Centrifugal Fan In Industrial Industries

A centrifugal fan, sometimes known as a radial fan, is one where the predominant flow is mainly axial where it enters the impeller and then changed to a predominantly radial direction at the impeller exit, due to the action of centrifugal force on the mass of the gas.

Frequently used in highly contaminated airstreams, material handling applications, and in systems that require flow of air or gas at high temperatures.

Therefore centrifugal fans are widely used in several process plants such as steel, chemical, thermal power, cement and more.

In the majority of centrifugal fans the impeller is enclosed inside a housing or volute casing to enable the flow from the fan to be directed or connected into a ducting system.
 
Tubular Centrifugal Fan is one where the centrifugal impeller is mounted inside a cylindrical enclosure which captures the radial flow exiting the impeller and directs it out of the housing in an axial direction.
 
Centrifugal Plug Fan is one without a housing or enclosure surrounding the impeller.

The air or gas enters the side of the impeller in an axial direction, turns 90 degrees and accelerates due to centrifugal force as it flows over the fan blades and exits the fan housing radially.

 
The airflow passes through a duct system often with various additional components such as dampers, bends, and process components which in turn creates a high resistance to flow in the form of a pressure loss.

The capability to generate low to medium flow rates at medium to high pressures makes centrifugal fans ideal for applications with higher system pressure losses such as cooling and heating systems, dust control, transporting materials, boiler and chemical production processes.

Centrifugal fans are rugged machines designed to withstand difficult operating conditions including high temperatures, abrasive materials, acidic fumes, corrosive gases, and so on.
 
Generally, centrifugal fans are sub-classified into four basic designations based on the shape of the impeller blades:

• Forward curved blade
• Backward curved blade
• Aerofoil blade
• Radial blade

 

Forward curved centrifugal fans

These generate the highest volume flow rate of all centrifugal fans, however suffer from low efficiency and a requirement for a large number of blades.

Typically they are used in both clean air ventilating and exhaust applications as they are the most sensitive to particle build-up and therefore can only be used for conveying air or gas with low quantities of solid particles entrained in the gas.
 

Backward curved centrifugal fans

These are the most efficient design of centrifugal fan blade and can easily match or exceed the efficiency of axial flow fans when operating at medium pressures and flow rates.

The blade design enables them to operate at high rotational speeds and hence generate high pressures, whilst maintaining a robust construction resistant to high temperatures or abrasive dust particles entrained in the gas.

Aerofoil blades

Designed to produce high volume flows at low pressure and overlap with the output from Axial flow fan products.

It is a common misconception that because the blade has an aerofoil profile it will be the most aerodynamically efficient design. This is not the case and the aerofoil shape of the blade is principally associated with mechanical strength, providing a deeper cross-section more resistant to bending, rather than pure aerodynamic purposes.
 

Radial blade

Also known as paddle blades, they are relatively open construction impellers with fewer blades than other designs. They have the property of being able to generate low to medium flow rates with relatively low pressure.

The most significant feature is the ability to handle significant quantities of highly abrasive dust particles entrained in the gas flow. This is due to the ability to have an extremely robust construction with a self-cleaning characteristic which prevents build-up of solids onto the impeller which could result in it going out of balance. Unfortunately the same features which enable use with highly dirty gases result in a design which has lower efficiency than other centrifugal fan blade types.

They are used in industrial applications where there are high levels of abrasive dust which require a fan solution that will withstand the severe operating conditions and where the lower levels of efficiency can be easily justified by the products ability to provide high levels of availability and product operating life.

Centrifugal fans are inherently efficient, reliable and capable of operating across a wide range of conditions. They have low to medium flow rates at medium to high pressures in contrast to the axial fans, however, there is an area where there is an overlap of potential operating duties. The simple construction and few moving parts mean they require very low levels of maintenance and provide the highest availability of all fan types.

There are centrifugal fans installed in industrial processes which have been operating for 12 years without stopping for maintenance and fans which have been installed for more than 100 years and still in daily use.
 

Mixed Flow Fans In Industrial Industries

The Mixed Flow fan is principally a combination of centrifugal and axial fan fundamental flows within the one impeller.

The main flow is predominantly parallel to the axis of rotation and enters the impeller in an axial direction but is changed by the shape of the blade to induce a radial flow to the vector component of flow direction.

The impeller may be contained within a cylindrical housing or in a volute chamber. In the case of roof-mounted extraction fans, the volute chamber could be the building or room in which the fan is installed.
 

Crossflow Fans In Industrial Industries

The Crossflow Fan is one where the fan impeller scoops or displaces air from one side of the impeller to the other in a similar fashion to a paddle wheel.

These fans are generally very low-efficiency machines and suited to moving relatively small volume flows at very low-pressure rise over the fan. Most commonly this is the type of fan which would have been seen in a small portable fan blown heaters or older type air conditioning units.

The Energy Related Products (ERP) regulations have resulted in the majority of Crossflow fans being removed from the European market due to them not being able to satisfy legislation with regard to minimum levels of fan efficiency.
 

Jet Fans In Industrial Industries

Jet Fan is a special version of an axial or mixed flow fan, where the velocity of the flow jet exiting the fan is utilised to induce a corresponding flow in the surrounding air.

Although academically not considered to be a fundamental fan type in its own right, International standards and legislative regulations place Jet fans in a classification of their own.

Most commonly Jet fans are used for ventilation purposes in Tunnels and Carparks to move exhaust fumes from the general locale.
 

In Conclusion

Axial fans have high airflow rates and low pressures. The flow rate is parallel to the axis of the fan and these fans are often used for cooling and ventilation. The limiting criteria is gasses with high temperature (although Garden City fans may provide a solution if high temperatures are faced) or containing particles of entrained dust. There are a vast number of permutations on size and design detail of axial fans enabling a wide range of applications in handling ambient air or relatively clean and cool gases.
 
Centrifugal fans have medium flow rates at medium to high-pressure capabilities. The airflow changes direction as it passes through the impeller. These fans are ideal for more arduous applications than axial fans such as industrial processes involving transport of hot gas or gas with entrained solid particles, or in applications where the pressure requirements are higher than could be satisfied by an axial fan.

 
Deciding on the correct fan type and design features for any fan application is a complex process. Customers can rely on Howden to help them negotiate their way through the many options.

At Howden, we have the widest range of fans on the market and our application expertise is the reason that we are trusted around the world. Howden can deliver the whole package to meet your needs - achieving maximum efficiency and reducing hassle and cost.
 

 

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