Centrifugal Fans & Blowers

A centrifugal fan and centrifugal blower both move air or gas using a rotating impeller, but the terms are often used based on the pressure range and application. Centrifugal blowers are generally associated with higher-pressure air movement, while centrifugal fans cover a broader range of airflow and pressure applications. 

An axial fan moves air parallel to the fan shaft, while a centrifugal fan draws air into the impeller axially and discharges it radially through the fan housing. Axial fans are commonly selected for high airflow at relatively low system resistance. Centrifugal fans are generally better suited to systems requiring higher pressure to overcome ductwork, filters, dampers, dust collectors, or other process equipment.

Centrifugal fan sizing requires the system’s required airflow and pressure, followed by evaluation of the fan performance curve. Calculate the total system resistance from ducts, elbows, filters, dampers, hoods, collectors, and other equipment. Also account for operating temperature and air density because they affect fan performance and power requirements. The selected fan should meet the required duty point without operating excessively close to unstable or inefficient regions of its performance curve.

The required CFM depends on the application. For general ventilation, it can be calculated from the required air-change rate:

CFM = Room Volume × Air Changes per Hour ÷ 60

For process exhaust and dust collection, CFM is usually determined from the required capture velocity, hood design, conveying velocity, equipment airflow requirement, or process conditions. The resulting airflow requirement should then be evaluated together with the system pressure to select the fan.

Fan static pressure is determined from the pressure losses throughout the system. These can include losses from ducts, elbows, transitions, filters, dampers, hoods, scrubbers, dust collectors, heat exchangers, and other equipment. The individual pressure losses are added to determine the required system resistance at the design airflow. The fan is then selected to provide the required airflow at that static-pressure condition.

A centrifugal fan performance curve shows how the fan performs at different operating conditions. It typically relates airflow to pressure, and may also provide information about fan efficiency, power consumption, speed, and other operating parameters. The point where the fan’s performance intersects the system resistance curve represents the actual operating point. Performance curves are therefore essential for selecting a fan that will deliver the required airflow and pressure.

First identify the required airflow, usually in CFM, on the horizontal axis and the required pressure on the vertical axis. Find the intersection of these two values to establish the required duty point. Then check whether the selected fan curve passes through or adequately covers this point. Also review the corresponding fan speed, motor power, efficiency, and operating range. The selected operating point should remain within the manufacturer’s recommended performance range.

Static pressure represents the pressure exerted by the air independent of its velocity, while total pressure combines static pressure and velocity pressure.

In simplified terms:

Total Pressure = Static Pressure + Velocity Pressure

Static pressure is particularly important when evaluating the resistance of ductwork and equipment. Total pressure is useful when assessing the overall energy imparted to the air by the fan. The appropriate pressure basis should always match the manufacturer’s fan performance data.

High energy consumption can result from excessive system resistance, operating at an inefficient duty point, incorrect fan selection, excessive fan speed, dirty filters, blocked ductwork, incorrect dampers, oversized or undersized equipment, or poor system design. Operating a fan at a higher pressure than required can also increase power demand. Regular inspection of filters, ducts, dampers, bearings, and fan performance can help identify avoidable energy losses.

The inlet should provide the fan with uniform, stable airflow into the impeller. Sudden transitions, obstructions, elbows placed too close to the inlet, or uneven flow can reduce performance and increase turbulence. The discharge arrangement should also provide adequate space and appropriate duct transitions to minimize unnecessary pressure losses. Following the manufacturer’s recommended inlet and outlet configurations is important for achieving the rated fan performance.

Yes. Duct configuration can significantly affect centrifugal fan performance. Long duct runs, sharp elbows, sudden transitions, undersized ducts, restrictive dampers, and poor inlet conditions can increase system resistance and reduce the airflow delivered by the fan. Proper duct sizing, smooth transitions, appropriate elbow arrangements, and adequate straight sections can help reduce pressure losses and allow the fan to operate closer to its intended duty point.

Pneumatic conveying generally requires a centrifugal blower capable of producing the pressure and airflow required to transport the material at the specified conveying velocity. The appropriate blower depends on material characteristics, conveying distance, pipe diameter, solids loading, temperature, and required pressure. Radial-blade or other high-pressure centrifugal designs are often considered for demanding conveying applications, but the blower should ultimately be selected from its performance curve based on the complete conveying-system requirements.

Air density affects both fan performance and power requirements. As air density changes with temperature, pressure, altitude, or gas composition, the pressure generated by a fan and the power required to move the air can change. A fan selected for standard air conditions may therefore perform differently when handling hot, humid, high-altitude, or process gases. Fan selection should use the actual operating air density whenever conditions differ significantly from standard conditions.

The required motor size depends primarily on the airflow, pressure, air density, fan efficiency, and operating point. A simplified relationship for air power is:

Air Power = Q × ΔP

where Q is volumetric airflow and ΔP is the relevant pressure increase. Actual motor power must also account for fan efficiency, drive efficiency, and appropriate design margins. The motor should be selected from the manufacturer’s performance data rather than based solely on fan diameter or airflow.