Fan Engineering: Types, Classification, Characteristics and Selection of Industrial Fan💨
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Introduction
- A fan is one of the most essential components in ventilation, humidification, and air conditioning systems. It moves air through a system at the desired velocity, volumetric flow rate, or mass flow rate under a specified pressure condition.
- A fan belongs to a class of machinery known as rotodynamic or turbo machines, where energy transfer takes place between the machine and the fluid flowing through it.
- If the energy transfer occurs from the fluid to the machine, the machine is called a turbine, such as a gas turbine, steam turbine, water turbine, or wind turbine.
- If the energy transfer occurs from the machine to the fluid, the machine is known as a pump, fan, or compressor.
- In fans and turbo compressors, air or gas is compressed so that its pressure increases from the inlet to the outlet. Generally, fans are associated with low-pressure applications, typically below 1000 mm WG.
Fan classification
- Fans are mainly classified according to the direction of airflow through the machine. A fan consists of an impeller (rotor) and a casing of different designs.
- The rotor is responsible for transferring energy from the machine to the fluid. This energy transfer is achieved through various impeller designs to obtain maximum efficiency and stable performance characteristics.
- The following are the most commonly used fan types in industrial and commercial applications.
Radial / Centrifugal Fans
- In centrifugal fans, air enters the impeller axially and then moves radially outward through the impeller. During this movement, energy is transferred to the fluid.
- As the energized air leaves the outer periphery of the impeller, the casing collects the airflow and converts part of the kinetic energy into static pressure.
These fans are generally suitable for:
- Low flow rate applications
- High-pressure systems
- Stable operating conditions
- High efficiency can be achieved through proper impeller and casing design.
- The performance characteristics are quite stable over a wide range of operating conditions.
Centrifugal fan impellers may include:
- Aerofoil-shaped blades
- Backward-curved blades
- Radial curved blades
- Forward-curved blades
Backward-curved aerofoil impellers usually provide:
- Higher efficiency
- Lower noise levels
- Better energy performance
These centrifugal fans must be provided with a scroll-type casing of narrow width to convert a part of the kinetic energy of the air leaving the impeller into pressure.
This conversion is achieved through the proper design of the casing. These fans are widely used in industrial applications such as:
- Steel industries,
- Paper industries,
- Sugar industries,
- Cement industries and
- Other process industries.
Cross Flow Fans
- In cross flow fans, air enters through the periphery of the impeller, passes through the interior, and exits again through the periphery.
- The airflow passes across the cylindrical surface of the impeller, which is why it is called a cross-flow fan. In this type of fan, the energy transfer mechanism is entirely different from that of other rotodynamic machines.
- The fan is designed with a specially shaped casing to ensure efficient airflow, performance and uniform air distribution.
Cross flow fans generally:
- Handle large air volumes
- Operate at low pressure rise
- Have lower efficiency compared to other fan types
Due to their lower efficiency and unstable characteristics at off-design conditions, they are not widely used in heavy industrial applications
These fans have not found wide application in industrial environments. However, they are now successfully used for maintaining air curtains in:
- Air-conditioned buildings
- Small air-conditioning equipment and
- Hand-held air dryers
Mixed Flow Fans
- Mixed flow fans are a special class of fans that have been developed in recent years due to advancements in the understanding of turbo machinery fluid mechanics and improved testing facilities.
- Mixed flow fans, ideally fill the cap in design between Centrifugal and axial fans, yielding efficiencies comparable with best designs of centrifugal or axial machines.
- These fans effectively bridge the design gap between centrifugal and axial fans, offering efficiencies comparable to the best designs of both types. In mixed flow fans, the air enters the impeller axially and is discharged diagonally from the rotor.
- The discharged air is collected either by a specially designed scroll casing or by vaned diffusers, which convert a portion of the outlet kinetic energy into pressure before directing the air to the fan outlet.
- Mixed flow fans are widely used in applications where centrifugal or axial fans cannot operate efficiently.
Mixed flow fans provide:
- Good efficiency
- Compact design
- Balanced pressure and airflow performance
In these fans, the airflow enters the impeller axially and is discharged diagonally through the rotor.
Axial Flow Fans
- Axial flow machines are widely used in systems that require a large volume of airflow at comparatively low pressure.
- In these fans, the air enters axially, gains energy from the impeller, and is discharged axially through the rotor. The performance of vane axial fans largely depends on the type of casing provided.
- The addition of a casing around the axial impeller significantly improves fan performance. In many applications, downstream stator vanes or a diffuser casing are also used to recover kinetic energy and increase static pressure.
Axial flow fans are widely used in:
- HVAC systems
- Cooling towers
- Ventilation systems
- Industrial exhaust systems
Fan Characteristics
- Fan performance is usually presented in graphical form, where total pressure, static pressure rise, efficiency and power consumption are plotted against the discharge flow rate.
- Dimensionless numbers are also used to represent fan characteristics, as the performance curves of a fan can be applied to all geometrically similar fans operating at different sizes and speeds.
- Fan characteristic curves are essential for selecting the correct fan design for a particular system and for determining the operating range under efficient working conditions.
Fan characteristic curves are essential for:
- Proper fan selection
- System matching
- Determining efficient operating ranges
Fan Efficiency
Fan efficiency varies depending on the fan design and operating conditions. The concept of overall and individual fan efficiencies is commonly illustrated through performance analysis.
To achieve maximum efficiency, the designer must carefully select the:
- Impeller shape
- Blade angle
- Casing dimensions
- Operating speed
The combination of performance requirements such as discharge flow rate, pressure, and rotational speed results in a characteristic number known as the specific speed.
Stages of Energy Loss in a Fan System
- When electrical power is supplied to a fan system, not all the energy becomes useful airflow output. Some amount of energy is lost at different stages of the system. These losses reduce the total efficiency of the fan system.
- The diagram shows the flow of energy from the motor input to the final fan output and identifies where energy losses occur.
- These losses include Copper losses, Iron losses, Heat generation, Mechanical friction.
- Motor losses reduce the motor efficiency before power reaches the fan drive system.
- After the motor, power is transmitted through Belts, Gears, Couplings, Pulleys.
- During transmission, some power is lost because of Belt slippage, Friction, Mechanical wear.
- The rotating shaft of the fan uses bearings and seals.
- Energy losses occur because of Bearing friction, Seal resistance, Lubrication drag.
- These losses reduce the mechanical power delivered to the fan impeller.
- As the fan impeller rotates, friction occurs between Rotating surfaces, Air surrounding the impeller.
- This creates disc friction losses, which consume additional power.
- These are major losses inside the fan caused by Turbulence, Improper airflow, Air separation, Blade design inefficiencies.
- Aerodynamic losses directly affect Airflow performance, Static pressure, Fan efficiency.
- Some air leaks internally or externally through Gaps, Clearances, Poor sealing.
- This reduces useful airflow and decreases overall system performance.
- Motor efficiency indicates how effectively the motor converts electrical energy into mechanical energy.
- This represents how efficiently the fan converts mechanical power into airflow and pressure.
In contrast, axial flow fans are more efficient at high specific speeds, where high discharge and low pressure conditions are required. where specific speed is defined as:
Step-by-step Calculation
- NS = N√Q ÷ H ¾
- NS = Specific Speed
- N = Speed of machine (r.p.m)
- Q = Flow rate or discharge (M³ / s)
- H = Pressure (mm Wg or M of fluid)
📝Example
- Speed N = 1450 RPM
- Flow Q = 0.05 M³ / s
- Head H = 25m
- NS = (1450 × √0.05) ÷ (25¾)
Where,
√0.05 = 0.2236
25¾ = 11.18
NS = (1450 × 0.2236) ÷ (11.18)
NS = (324.22) ÷ (11.18)
NS = 28.99
This indicates a low specific speed pump, typically a radial flow centrifugal pump used for higher head and lower flow applications.
Fan Selection
- The selection of a fan type depends on the required discharge flow rate, pressure, and operating speed, collectively represented by the specific speed.
- Since efficiency is the primary criterion in fan selection, the efficiencies of different classes of fans are evaluated for a given specific speed. Based on this evaluation, either a centrifugal fan or an axial fan is selected.
- After selecting the appropriate fan type, the characteristic curves of the chosen fan are matched with the requirements of the system under consideration to ensure efficient operation.
Conclusion
Fans are essential components in industrial ventilation, HVAC, humidification, and process applications. Proper understanding of fan classification, characteristics, efficiency, and specific speed helps engineers select the most suitable fan for a system.
Centrifugal, axial, mixed flow, and cross flow fans each offer unique advantages depending on airflow and pressure requirements. Correct fan selection improves:
- Energy efficiency
- System performance
- Reliability
- Operating stability
- Equipment life
Understanding fan engineering principles is important for HVAC engineers, mechanical engineers, technicians, and industrial maintenance professionals involved in air movement and ventilation systems.
About the Author – Insight Control System
- Insight Control System provides technical education and practical guidance in the fields of Building Automation Systems (BAS), HVAC controls, PLC programming, sensors, and industrial automation. The content published on this platform is based on field experience, technical documentation analysis, and real-world system applications.
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