Centrifugal fans or blowers have a wide variety of process air applications.  When selecting a centrifugal fan wheel for a particular application, here are three most common types to choose from:

1. Forward Inclined
2. Backward Inclined
3. Radial

In this week’s blog, I will discuss the characteristics of each type of wheel and the applications for which each is best suited.

Air Flow Characteristics

We will start with the airflow characteristics of each type of wheel since this is what matters for determining its suitability for an application.  The three diagrams below show the direction of airflow coming off the blades.

The diagrams show that the air leaving the blades of the radial and backward blades is in a radial direction whereas the air leaving the forward inclined wheel is in a tangential direction.  The diagram also shows that to produce the same absolute air velocity, the tip speed of the forward inclined wheel is the lowest.

Now let’s look at the relative performance of the three types of wheels given these air flow characteristics:

At medium to high pressures and low flow rates, the backward inclined and radial wheels perform the best.  At medium to high pressure and low flow rates, the forward inclined wheel is actually unstable.  It is best suited for low pressure and high volumes.

Applications

Forward inclined wheels are best suited for applications that need a constant flow of air under low static pressure conditions.  Because forward inclined wheels will overload, they are not recommended for variable flow applications.  One of the best characteristics of a forward inclined wheel is that it will produce the same volume of air at lower at a lower speed compared to backward inclined or radial wheels, therefore it is much quieter.  For these reasons, the forward inclined wheel is widely used in residential and commercial air conditioning.  Other applications include oven and dryer exhaust, and process drying and cooling.

Radial bladed wheels are best suited for medium pressure and medium volume applications with steady flow requirements since they also will overload.  Because the wheels can be ruggedly constructed, they are good for applications where the air flow is corrosive or contains materials.  Applications include conveying powdery or granular material and long stringy paper trimmings or fibers.

Backward inclined wheels are more energy efficient than the other two types of wheels and with curved or airfoil style blades, they can come close to the efficiency of an axial fan.   Backward inclined wheels will not overload or stall and are well suited for variable flow applications.  Common applications include dust collectors, fume exhausters and emission control systems.

Conclusion

Making the right choice in a centrifugal fan means selecting the best wheel for the application.  In our 75 years of creating successful environments, Eldridge has helped many customers make the right choice for unique applications such as wind tunnels, railcar dryers and heavy gas fume extraction systems.

How to Select a Fan or Blower

To select a fan, the required data includes the flow rate (CFM), static pressure (SP), and air/gas density.

You have 3 options to select the right fan or blower for your application:

1

Use our

Use our Quick Selector to narrow your choices and sort by Price or Efficiency.

2

Contact our knowledgeable

Contact our knowledgeable local sales engineer for application assistance.

3

Follow the steps below to select a fan using catalog data.

to select a fan using catalog data.

Flow Rate (CFM)
SCFM stands for Standard Cubic Feet per Minute. It is the CFM at standard density, defined as .075 lb/cubic foot.

ACFM stands for Actual Cubic Feet per Minute. It is the CFM at an identified density other than .075 lb/ft3. It is also the required mass flow rate divided by the density of the gas being handled. Since fans and blowers handle the same volume of air regardless of density, the ACFM value (and corresponding density) is the preferred value to use when selecting a fan or blower. Note that ACFM and SCFM are not interchangeable except at .075 lb/ft3 density.

Airflow is rated in cubic feet per minute (CFM) or the metric equivalent, cubic meters per hour (M3/Hr).

1 CFM = 1.6990 x M3/Hr.

If you will be conveying material, make sure you have enough CFM for the duct, pipe or hose size so the material will maintain the required velocity to carry it completely through the system and not settle in the duct, pipe or hose. See Engineering Data catalog for material conveying velocities.

Static Pressure (SP)
Static Pressure is the resistance to airflow (friction) caused by the air moving through a pipe, duct, hose, filter, hood slots, air control dampers or louvers. Static Pressure is rated in inches water gauge (inWG) or the metric equivalent, millimeters water gauge (mmWG). 1 inWG = 25.4 x mmWG.

Standard air has a density of .075 lb/ft3 and is based on a temperature of 70°F and 29.92" Hg barometric pressure (sea level). Fan performance tables are based on using standard air. Corrections for density changes resulting from temperature and/or barometric pressure variations, such as higher altitudes, must be made to the static pressure before selecting a fan or blower based on standard performance data. The metric equivalent is in kilograms per cubic meter (kg/m3). lb/ft3 = 16.018 x kg/m3.

The temperature of the air going through the fan or blower will affect the density and performance of the fan or blower. Temperature should be shown in degrees Fahrenheit (°F). The metric equivalent is degrees Centigrade (°C).

°F = 1.8 x °C + 32

If the air temperature will vary, what are the minimum and maximum temperatures?

The altitude the fan or blower will be operating at will also affect the density and performance of the fan or blower. The altitude should be given in feet above sea level. The metric equivalent is meters (m). 1 ft = .30480 x m

Air Temperature Altitude Correction

Chart found on Page 5 of Instructions For How to Properly Select a Fan or Blower

Example:

Select a blower for 1500 CFM at 7" WG at 250°F. and at 6500 feet elevation.

• STEP 1. From the table above, the conversion factor for 250° and 6500 ft. altitude is 1.71.
• STEP 2. The corrected static pressure is: 7" WG x 1.71 = 11.97" inWG at standard conditions. Round off to 12" WG.
• STEP 3. Select fan, using fan performance tables, for 1500 CFM at 12" WG.

NOTE: If the suction pressure on the inlet side of a blower will exceed 15" inWG, a correction for suction pressure (called rarefication) should be made. See chart below.

NOTE: If the suction pressure on the inlet side of a blower will exceed 15" inWG, a correction for suction pressure (called rarefication) should be made. See chart below.

Chart found on Page 4 of Instructions For How to Properly Select a Fan or Blower

EXAMPLES:

1. With 45" of suction pressure on the blower inlet and no discharge pressure on the blower discharge, the total static pressure = 50.3" inWG.
2. With 45" of suction pressure on the inlet and 12" of discharge pressure on the discharge, the total static pressure = 50.3"+12" = 62.3" inWG.
3. With 0" of suction pressure on the inlet and 12" of discharge pressure, the total static pressure = 12" inWG. There is no correction required for discharge pressure.

Centrifugal fans or blowers use one of seven types of wheels that are enclosed in a scroll shaped housing. The air enters the fan wheel through the housing inlet, turns 90 degrees and is accelerated radially and exits the fan housing. Centrifugal fans are typically used for lower flows and higher pressures.

Axial fans use a propeller, having two or more blades, to move air in an axial direction through a cylindrical housing or formed orifice panel. Axial fans are typically used for higher flows and lower pressures. Do not use an axial fan to convey material.