Plenum fans and centrifugal fans are two common types of fans used in HVAC systems for air movement. While both serve to move air, they have distinct differences in design, operation, performance characteristics and applications.
What Is a Plenum Fan
A plenum fan is an air-moving device specially designed for HVAC applications that require high airflow at low to medium static pressures. It features a unique “plug” or plenum section housing the fan wheel. The plenum acts as an airflow-straightening chamber that helps convert dynamic pressure to static pressure, improving overall efficiency.
Plenum fans typically use airfoil or backward-curved fan blades. These aerodynamic blade designs enable the fan to move large volumes of air quietly and efficiently. The plenum housing allows air to enter axially into the fan and discharge radially in a perpendicular direction to the intake. This non-ducted design makes plenum fans ideal for applications like air handling units and rooftop ventilation systems.
What Is a Centrifugal Fan
A centrifugal fan is a mechanical device for moving air or other gases using the centrifugal force generated by a rotating impeller. The impeller is composed of a central hub with a number of blades radiating outward to the housing. As the impeller spins, it draws air into the center and accelerates it radially outward due to centrifugal force.
Centrifugal fans come in several different blade configurations, including forward-curved, backward-inclined and radial designs. Each blade type offers advantages for specific applications based on the pressure, flow rate and efficiency requirements. Centrifugal fans excel at generating high pressures, making them suitable for applications involving long duct runs, air filtration or material handling.
Difference Between a Plenum Fan and a Centrifugal Fan
Noise Levels
One notable difference between plenum and centrifugal fans is their noise output. Plenum fans are designed for quiet operation, with their aerodynamic blade shape and airflow-straightening plenum helping to minimize turbulence and associated noise. The lack of ductwork connection also reduces noise transmission.
In contrast, centrifugal fans tend to generate more noise, especially at high operating speeds. The impeller design and interaction with the housing can create turbulence and vibration leading to increased noise levels. Centrifugal fans often require additional sound attenuation measures like duct liners, silencers or vibration isolation to meet noise criteria.
Design
The physical design of plenum and centrifugal fans differs significantly. A plenum fan consists of a fan wheel inside an enlarged plenum section that helps improve airflow and efficiency. The plenum often features turning vanes and an outlet damper to control airflow direction and volume. Plenum fans use airfoil or backward-curved blades for optimal aerodynamic performance.
Centrifugal fans have a scroll-shaped housing that guides air from the rotating impeller to the outlet. The impeller is not shrouded like in a plenum fan. Centrifugal fan housings are classified as single-width single-inlet (SWSI), double-width double-inlet (DWDI), or double-width single-inlet (DWSI) based on their inlet and outlet configurations. Impeller blades can be forward-curved, backward-inclined or radial depending on the application.
Applications
The distinct designs of plenum and centrifugal fans make them suitable for different HVAC applications. Plenum fans are commonly used in air handling units, rooftop units, and central station applications where the fan discharges into a plenum space rather than ducted runs. Their high-volume, low-pressure capabilities are ideal for these applications.
Centrifugal fans excel in applications requiring high pressure for overcoming ductwork resistance. They are frequently used for exhaust systems, dust collection, fume hoods, and material conveying. Centrifugal fans can handle demanding airstreams containing particulates, moisture or chemicals. They are available in a wide range of sizes and configurations to suit various industrial processes.
Energy Efficiency
Fan energy efficiency is a critical factor in HVAC system design and operation. Plenum fans offer several efficiency advantages due to their aerodynamic blade design and airflow optimization in the plenum section. Airfoil and backward-curved blades used in plenum fans provide efficient operation over a wide range of volumes and pressures. The plenum housing helps recover static pressure, further boosting efficiency.
Centrifugal fan efficiency varies based on the impeller blade design. Backward-inclined and airfoil blades generally offer higher efficiencies than forward-curved designs. However, the impeller’s interaction with the scroll housing and ductwork can introduce system effect losses that reduce overall efficiency.
Flow Direction
Flow direction is another distinction between plenum and centrifugal fans. In a plenum fan, air enters axially into the fan wheel and is discharged perpendicularly through the outlet in a radial direction. This airflow pattern allows plenum fans to be used in non-ducted applications where space is limited.
Centrifugal fans also feature radial flow, with air entering axially and discharging radially from the scroll housing. However, the discharge direction is determined by the housing design and outlet orientation. Centrifugal fans are commonly available with bottom horizontal, top angular up-blast, or radial tip-up discharges to suit various ducting arrangements.
Pressure
The pressure capabilities of plenum and centrifugal fans differ based on their design and intended applications. Plenum fans are engineered for high-volume, low-pressure applications, typically handling pressures up to 5 inches of water gauge (1.25 kPa). They provide the large airflow volumes needed for air circulation and distribution in open spaces.
Centrifugal fans are capable of generating higher pressures, with designs available for pressures exceeding 30 inches of water (7.5 kPa). The impeller and housing design allow centrifugal fans to develop the high pressures needed for overcoming ductwork resistance, air filtration, and conveying materials.
