Technology Primer

Definitions

Air Horsepower

The work done by the fan or the power output of the fan. See the Performance Relationships section for more details.

Application Range

The range of operating volumes and pressures, determined by the manufacturer, at which a fan will operate satisfactorily.

Blast Area

The fan outlet area minus the projected area of the cutoff.

Blocked Tight Static Pressure

The operating condition in which the fan outlet is completely closed resulting in no air flow.

Brake Horsepower

The actual horsepower a fan requires or the power input of the fan.

Drive Losses

The power used to overcome belt, pulley, and bearing friction.

Fan Static Pressure

The total pressure minus the fan velocity pressure, also the static pressure at the fan outlet minus the total pressure at the fan inlet.

Fan Total Pressure

The difference between the total pressure at the fan outlet and the total pressure at the fan inlet.

Fan Velocity Pressure

The pressure corresponding to the fan outlet velocity.

Free Delivery

The operating condition of maximum fan delivery at which static pressure across the fan is zero.

Mechanical Efficiency

The ratio of power output to power input, also referred to as total efficiency.

Outlet Area

The inside area of the fan outlet.

Point of Operation

The cfm-pressure requirements for a given application.

Standard Air

Air at a temperature of 70oF dry bulb and a barometric pressure of 29.92 in. of mercury and a barometric pressure of 0.075 lbs/cu. ft.

Static Efficiency

The efficiency based on static air horsepower.  See the Performance Relationships section for more details.

Tip Speed

The circumference of the fan wheel times the RPM of the fan, expressed in ft/min, also referred to as peripheral velocity.

Equipment

Axial Fans

Axial fans can be categorized into three major varieties, distinguished by the applications for which they are designed rather than by blade configurations.  The principle types of axial fans are propeller, tubeaxial and vaneaxial fans.

  • Propeller Fans - These are low pressure, high capacity fans that are seldom applied in applications requiring more than 0.75 in. static pressure.  As compared to centrifugal fans, the horsepower required by a propeller fan is lowest at maximum air volume. Centrifugal fans are the opposite in that they require minimum horsepower at no air delivery.
  • Tubeaxial Fans - These are generally considered to be heavy-duty propeller fans. They are built for pressures in the range of 2.5 to 3 in. of water and are generally limited to industrial duty where noise considerations are unimportant.
  • Vaneaxial Fans - These are basically tube axial fans plus vanes. Behind the fan blades are vanes which straighten the spiral flow of air, thus increasing the static efficiency. Also available on some fans is a provision to allow adjustment of the blade angles permitting adjustment of the air volume delivered by the fan.

Because the discharge opening is inline with its entrance, and axial flow fan offers the advantage of simplified duct arrangement. This become important when space considerations must be taken into account.

Centrigual Fans

Centrifugal fans can be categorized into three major varieties, distinguished by the blade curvature. This curvature of the blades determines the performance characteristics of the fan. The following figure shows the three types of centrifugal fan wheels commonly used, with a vector expression of air leaving the blades.

  • Forward Curved Blade Fan - The forward curved blade fan, as the name implies, has its blade tips sloped forward in the direction of wheel rotation. As can be seen from the vector diagrams, the air velocity leaving the the tip of the fan blade is higher for a forward curved fan blade than a backward curved blade fan. Because the fan pressure produced is a function of leaving air motion at the blade tips, a fan with forward curved blades will operate at lower speed for a given application than a fan with backward curved blades. Forward curved blade fans are generally used in high flow, low pressure applications. They are also especially sensitive to particulates in the flow stream.
  • Backward Curved Blade Fan - The backward curved blade fan has its blade tips sloped backwards, opposite to the direction of wheel rotation. Another adaption of the backward curved blade fan utilizes more complex airfoil shaped blades to produce smooth air flow across the blades which reduces turbulence and noise.  As a result, the airfoil design incorporated in a backward curved fan increases efficiency, produces a wider range of higher efficiencies and operates at a lower noise level.
  • Straight Radial Blade Fan - The straight radial blade fan has its blades extending radially outward with no curve or pitch. These are often used in application with heavy particulate flows because the fan design is less sensitive to solids build up on the fan blades. These fans are also generally characterized by greater noise ouputs and lower efficiencies.

Whether a forward curved or backward curved blade fan is to be used is determined by fan size as well as the particular application. Size for size  backward curved blade fan will generally require less horsepower than a forward curved blade fan for a given application. Despite the higher speed required for a backward curved blade fan they are as quiet or quieter than a forward curved fan, when properly selected. These factors generally make backward curved blade fans a logical selection for large application (generally greater than 24 inch diameter wheels).

Due to the high speed of backward curved fans the belt speed may be excessive in the smaller applications. In these applications, the lower speed characteristics generally make the forward curved blade fan more desirable. In addition, the lower speed is quite adaptable to packaged central station units having more than one fan on a common shaft, due to the required shaft sizing.

Performance Systems

Fan Efficiency Analysis Tools (FEAT)

Common Opportunities

Efficient Fan Blades

Centralize Fan Controls

VFD on Boiler Fan

Additional Information

Walkthrough Checklist

 Are fans or blowers installed that are not sized correctly for the task?

Are fans or blowers being throttled in order to control the flow rate?

Is bypass control being utilized to vary the flow out of the fan or blower?

Does the facility have a cooling tower(s)?

Does the facility utilize high or low pressure blowers to convey material?

Does the facility have more than one baghouse and multiple distribution lines feeding into these baghouses?

Assessment Tools and Data Collection

Analysis Tools and Methodology

Additional Resources