Fan Types- Centrifugal, Axial, Forward Curve, Airfoil. What does it all mean?

The cfm Distributors Inc Applied Systems Group meets every other Monday at 8am for training. This training covers a plethora of topics, but a portion of this training is on HVAC design and application. Last month we were discussing fan types and we thought it would be fun to create a document that explains the different types of fan systems you might run across in the field and in HVAC systems, along with their benefits or draw backs.

There are two main types of fan systems:
  1. Centrifugal
  2. Axial
A centrifugal fan changes the air direction twice as the air moves through the fan (the air turns into the wheel, then turns again into the direction of the ductwork/plenum). This type of fan system typically has a housing around the wheel and is sometimes referred to as a “blower”. Centrifugal fan systems are designed to handle low to medium static pressures, up to a few inches, and are very cost effective with ducted systems. This is why centrifugal fans are very popular in supply fan sections for small to medium size rooftops, splits systems, and some smaller custom air handlers.

In an axial fan system, by contrast, the air doesn’t change direction. It flows into and out of the fan in the same direction. The benefit of an axial fan is that it can provide high airflow at very low static pressure and at a low power input. This is why you see axial fans in condenser sections in rooftops or split systems, and in free-blow unit heaters. These fans however, are not designed to handle very much static pressure. See Figure 2 below for a picture of this fan type.
Figure 1 Centrifugal Fan
Figure 2 Axial fan
Each fan system has different types of wheels, or impeller types: Centrifugal impeller styles
  • Forward Curve- Sometimes referred to as a “squirrel cage” fan, forward curve fans are used in most residential and light commercial HVAC equipment. These fans are for low to medium static applications. They are the most budget friendly and the least efficient blade design.
  • Backward Inclined- The performance of the backward inclined wheel is similar to the forward curve, however BI fans tend to be non-overloading, meaning their fan curves do not tend to increase hp as static pressure changes, but cfm falls off.
  • Backward Curved- these wheels are extremely efficient, more robustly built, and very quiet. They are more expensive to build than FC and BI fans.
  • Airfoil- these blades are shaped like that of an airplane wing and offer slightly higher efficiencies and lower noise levels than BC fans, however are more expensive to build.
Axial blade styles
  • Tube Axial – These fans consist of propeller shaped blades mounted in a tube. These fans are general built to be installed in low pressure duct work.
  • Vane Axial- These fans are a variation of the tube-axial fans with air straightening vanes added in front of or behind the propeller blades.
  • Propeller- A propeller axial fan consists of a propeller shaped blade mounted on a flat frame. The construction is often built to be installed in a wall or in an open ceiling or floor mount.
Another type of fan configuration that is becoming more popular is a plenum fan, sometimes referred to as a plug fan.

Plenum fans are outfitted with a centrifugal fan wheel but without the full housing that a traditional centrifugal fan system has (it has just a frame, or no housing at all). There are many benefits to a plenum fan: most plenum fans are direct drive which means no belts to service, and plenum fans have a much higher external static capability and are quieter and more efficient to operate.
Figure 3. Plenum fan
Figure 4. Plenum fan inside of a York 50 ton Rooftop unit
That’s all the fun we have for you today! Let us know if you have any questions by commenting below!

Brad Telker
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