Understanding the Characteristics of Varying Centrifugal Blower Designs

Many people speculate about the confusion on what is considered a compressor, a blower, or simply a fan.  In essence, each of these turbo-machines achieve a pressure rise by adding velocity to a continuous flow of fluid.  The distinctions between fans, blowers, and compressors are quite simply defined by one parameter, the specific pressure ratio.  Each machine type, however, utilizes a number of different design techniques specific to lower and higher-pressure applications.  As per the American Society of Mechanical Engineers (ASME), the specific pressure is defined as the ratio of the discharge pressure over the suction pressure (or inlet pressure).  The table shown below defines the range at which fans, blowers, and compressors are categorized.
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Similarities between the design of fans and blowers occur near the lower end of a blower’s range.  As well, many design parallels exist between high-pressure blowers and compressors.  For the article, we will be investigating the different design characteristics of centrifugal blowers. Blower selection depends on a number of factors including operating range, efficiency, space limitations, and material handled.   Figure 1 shows a number of different impeller blade designs that are available for centrifugal blowers.

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Figure 1 – Impeller Blading Arrangements for Centrifugal Blowers

Each of these blading arrangements have unique operating ranges limited by overload and stall conditions.  Forward curve and radial blowers exhibit a performance curve for horsepower needed that increases with the volume flow rate.  This means that a motor can be overloaded if discrepancies occur which bring the system to flow rates higher than at the operating point.  On the other hand, any backward arrangement blading exhibits a horsepower curve that increases to a maximum as airflow increases and then drops off again.  This allows the engineer to specify a motor to accommodate the peak horsepower, which in turn ensures that the system cannot overload and is therefore considered “non-overloading”.  The stable range of a blower is defined as the condition under which enough air flows through the fan wheel to fill the spaces in between the blades.  Below this range, the instability of the machine will cause one or several section of blades to stall.

In general, forward curved blading arrangements (or sirocco blowers) are better suited for high volume with lower pressure applications.  These blowers and fans operate at relatively low speeds and pressures, which permit lightweight and cost-effective construction.  Restricted stability ranges, overloading at higher flow rates, and low static efficiencies all limit the capabilities of the forward curved centrifugal blower.  Radial centrifugal blowers have the same problem with overloading at high flow rates; however, they are quite beneficial for moving air in dirty environments due to their flat geometry that prohibits dust or sticky materials from rapidly accumulating.  Composed of 6 to 12 rugged blades extending radially from the hub, these types of blowers run at medium speeds and deliver low air volumes at medium to high pressure.

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Figure 2 – Centrifugal Blower Design with Airfoil Impeller Arrangement in AxSTREAM™

Perhaps the most important centrifugal fan impeller orientation, the backward orientation, comes in three standard shapes: backward inclined, backward curved, and backward inclined aerofoil (or airfoil). All of these designs exhibit most of the same characteristics, with some discrepancies in their efficiencies.  Generally, the “flat-bladed” back inclined design achieves an efficiency of about 82%, while the backward curved and airfoil designs near 88% and 90%, respectively.  In addition to high efficiencies, backward oriented blade designs allow for the highest operating speeds of all centrifugal blowers and are considered non-overloading.  The only drawbacks to these types of blowers would be the high cost of manufacturing as well as the inability to handle flows with high particulates due to the close running clearances and complex geometries.

If you would like to learn more about the design of centrifugal fans, blowers, and compressors, please click here (http://www.softinway.com/machine-type/centrifugal-compressor/).

References:

http://www.wesellfans.com/pdf/underStanding-Centrifugal-Fans.pdf

http://www.saylor.org/site/wp-content/uploads/2011/09/Chapter-3.5-Fans-Blowers.pdf

http://www.clarage.com/docs/fan-engineering-letters/fan-performance-characteristics-of-centrifugal-fans—fe-2400.pdf?sfvrsn=2

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