Fan and Blower Selection Guide: Inline Duct, Compact Cooling and Backward Centrifugal Fans
Air Movement and Ventilation Systems: Cross Flow, Centrifugal and Axial Fan TechnologiesUnderstanding these differences can make it easier to select an appropriate air-moving solution.
Some designs prioritize relatively direct airflow, while others are particularly useful where a system must overcome resistance.
Performance curves and manufacturer documentation should therefore be considered when selecting equipment.
Fundamentals of Air-Movement Equipment
Different designs consequently produce different airflow and pressure characteristics.
Ducts, filters, grilles, heat exchangers, bends, dampers and equipment enclosures can all introduce resistance.
Two similarly sized fans can have substantially different performance characteristics.
Cross Flow Fans
Their geometry can make them useful where air needs to be distributed across a wide, narrow opening.
Air generally passes through the rotating wheel rather than simply moving straight along its rotational axis.
They should be evaluated using product-specific performance data.
Applications of Cross Flow Fans
This geometry can be useful for cooling, air circulation or air-curtain-like applications depending on the equipment design.
This can be useful when uniformity across a width is an important objective.
Obstructions around the inlet and outlet can alter performance.
Centrifugal Blowers
Centrifugal Blowers use a rotating wheel to draw air into the inlet and discharge it outward through the blower housing.
Depending on the specific wheel and housing, they can provide pressure characteristics suited to ducts, filters or equipment passages.
A product should therefore be evaluated according to its documented pressure and airflow characteristics rather than terminology alone.
Centrifugal Fans
A surrounding housing or system configuration then manages the discharge airflow.
Centrifugal Fans are available in multiple configurations suited to different airflow systems.
These fans can be considered for HVAC, ventilation, equipment cooling and industrial air-handling applications where their pressure-flow characteristics are appropriate.
Comparing Centrifugal Fans and Blowers
Commercial terminology can vary according to industry and manufacturer.
A product described as a fan can outperform a product marketed as a blower under certain operating conditions.
Selection should therefore focus on required airflow, system resistance, operating point, efficiency and installation conditions.
Backward Centrifugal Fan Design
This blade arrangement creates different aerodynamic behavior from forward-curved or radial centrifugal designs.
The blade direction alone does not establish a specific efficiency level.
Some configurations operate within housings while others may be incorporated differently.
Comparing Centrifugal Blade Designs
This affects where each design is most appropriate.
Backward Centrifugal Fans can be attractive where efficiency across an appropriate operating range is important.
This helps determine whether the selected fan will operate in an appropriate region.
Understanding Axial Fan Technology
This straightforward airflow path makes axial designs common in many ventilation and cooling applications.
An axial fan should therefore not automatically be selected simply because high airflow is required.
Inlet and outlet obstructions can significantly affect installed performance.
Comparing Axial and Centrifugal Airflow
The primary distinction between Axial Flow Fans and Centrifugal Fans is the way air moves through the rotating assembly.
Axial fans can be well suited to applications requiring relatively direct airflow through an opening or short airflow path.
Neither design is inherently better for all applications.
Inline Duct Fans
Inline Duct Fans are designed to operate as part of a ducted airflow path.
An Inline Duct Fan must overcome resistance generated by the duct network and connected components.
The term inline describes installation arrangement rather than one universal impeller design.
Applications of Inline Duct Fans
The fan must be appropriate for the air being handled and the installation environment.
Excessive Backward Centrifugal Fans bends, restrictive grilles or poorly sized ducts can increase system resistance.
Installing equipment where it cannot be safely accessed can make maintenance unnecessarily difficult.
Small Fans for Equipment Cooling
Their purpose is generally to help move heat away from components or through an enclosure.
A compact fan must still move sufficient air through the actual enclosure resistance.
A poorly arranged enclosure can develop hot spots even when a fan is operating.
Compact Cooling Fans for Electronics
Electronic components can generate heat that must be transferred away to maintain appropriate operating conditions.
Inlet and exhaust openings should therefore be considered together.
Fan selection should account for the pressure drop created by filtration.
Choosing a Fan for Equipment Cooling
The enclosure layout should guide selection.
Simply maximizing airflow at one point does not guarantee uniform cooling.
Fan performance should be considered together with total thermal management.
Understanding Fan Performance
Both characteristics are important when selecting fans.
As system resistance changes, the fan's operating point changes along its performance curve.
System designers should estimate or calculate pressure losses where appropriate.
Understanding Fan Curves
A fan performance curve illustrates the relationship between airflow and pressure under defined test conditions.
Actual installed conditions can introduce additional variation.
Comparing headline maximum values without understanding those conditions can be misleading.
Selecting Fans for Energy Performance
Fan efficiency depends on how effectively electrical and mechanical input is converted into useful airflow and pressure at the required operating point.
Appropriate selection is more important than choosing a fan solely by category.
System design can influence energy consumption as much as fan selection.
Fan Speed Control
Reducing or increasing fan speed can change both airflow and pressure characteristics.
Manufacturer documentation should identify permitted control arrangements.
Control strategies should therefore be coordinated with the complete system.
Reducing Fan and Blower Noise
Noise from a fan system can originate from aerodynamic airflow, the motor, bearings, structural vibration or interactions with nearby components.
Restrictive or turbulent airflow paths can increase aerodynamic noise.
Vibration isolation may be appropriate in certain installations.
Installing Fans and Blowers
Obstructions placed too close to a fan can change its airflow characteristics.
The fan and connected ducts should be installed according to applicable instructions.
Qualified personnel should perform installation where required.
Fan Maintenance
Fans accumulate wear and contamination during service, although the rate depends on their environment.
The underlying cause should be identified rather than assumed.
Fan blades and wheels can create mechanical hazards even when a unit appears simple.
Choosing the Right Fan
Ventilation, equipment cooling, duct extraction and industrial air handling can impose very different requirements.
Axial Flow Fans can suit many relatively direct airflow applications, while Centrifugal Fans and Centrifugal Blowers can be useful where different pressure characteristics are needed.
Backward Centrifugal Fans offer another centrifugal configuration for systems where their performance characteristics are appropriate.
Comparing Common Fan Types
Centrifugal Fans redirect air through their rotating wheel and can provide useful pressure characteristics.
Centrifugal Blowers apply related centrifugal principles in blower-style configurations, while Backward Centrifugal Fans use a particular blade orientation within the centrifugal category.
These categories can overlap.
Airflow and Cooling Fan Questions
What are Cross Flow Fans?
Centrifugal Blowers use a rotating wheel to draw in and redirect air through a housing.
Inline Duct Fans are designed for installation within or along a duct airflow path.
Compact Cooling Fans provide forced airflow in applications where installation space is limited.
Axial Flow Fans generally move air in a direction broadly parallel to their rotational axis.
Centrifugal Fans draw air toward the rotating wheel and direct it outward, producing airflow and pressure characteristics useful in many HVAC and industrial systems.
Backward Centrifugal Fans use blades oriented backward relative to the direction of wheel rotation according to the impeller design.
No.
Not necessarily.
Can maximum airflow be used to select a fan?
Building an Effective Airflow System
Understanding these differences helps narrow the selection process.
Axial Flow Fans provide direct axial airflow, and Inline Duct Fans integrate air-moving equipment into duct networks.
However, no fan category automatically guarantees a particular airflow, pressure, efficiency or noise level.
Airflow demand, static pressure, duct resistance, physical space, thermal load, noise, controls and maintenance all influence the final decision.