Fan Performance Characteristics of Centrifugal Fans - Aerovent

Figure 4. Tip Speed/Static Pressure RelationshipRPM — HUNDREDSSTATIC PRESSURE — IN. W.G.454035302520151050.90.80.70.60.50.40.30.20.1Forward Curve FansWHEEL DIA. INCHES2 34F R BThese fans are sometimes known as “volume,” “squirrelcage,” or “sirocco” blowers. The impeller blades aresmall and numerous with a pronounced curvature andshort chord length. The concave blade curvature facesthe direction of rotation. These fans operate at relativelylow speeds and pressures (reference Figure 4)which permits light construction of the impeller, shaft,bearings, and housing.681012162030BLADE INCLINATIONF = FORWARDR = RADIALB = BACKWARD1 2 3 4TIP SPEED — FPM (THOUSANDS)The forward curve fan is used to deliver high air volumesagainst static pressures up to 6" water gauge.However, the majority of applications are for pressures of3" water gauge or less. Note the pronounced dip (stall)in the static pressure curve (Figure 5b). Any selection tothe left of the 40% free delivery point (peak) will resultin an unstable pulsating airflow that will lead to impellerand structural damage. Even though good peak efficienciesare on either side of the peak, selections should belimited to 45% or greater of free delivery.Interestingly, in parallel installations, if selected tooclose to peak, forward curve fans exhibit a tendency notto share the load equally and become unstable. Theseselections should be limited to 55% or greater of freedelivery.The advantage of the forward curve fan is its low speedand quiet operation. The light construction results in a lowcost fan and its relatively high airflow results in a smallfan requiring minimum space, making it ideal for the residentialand commercial heating and cooling market.Disadvantages are that its high horsepower requirementsat or near free delivery (note the rising powercurve in Figure 5b) and its light construction limit itssuitability for most industrial requirements.Highly dependent on the housing for performance, theforward curve impeller is not suitable for plug or plenumfan applications. Without a housing the forward curveimpeller becomes unstable and exhibits a relatively poorperformance.Backward Inclined FansThese are sometimes called “load limiting” or “non-overloading”fans. The impeller blades are larger and heavierthan forward curve blades, usually number from eight totwelve, and are inclined away from the direction of rotation.They are standardly offered in three blade shapes:1. Flat single thickness (Figure 6a)2. Curved single thickness (Figure 6b)3. Curved airfoil (Figure 6c)Figure 6. Backward Inclined FansFigure 5a. Typical Forward Curve Fan6a. Flat Single Thickness, BIFigure 5b. Characteristic Performance of Forward Curve Fans100PERCENT OF NO FLOW STATIC PRESSUREHORSEPOWER AND EFFICIENCY908070605040302010HORSEPOWERSTATIC EFFICIENCYTOTAL PRESSURETOTAL EFFICIENCYSTATIC PRESSURE6b. Curved Single Thickness, BC00 10 20 30 40 50 60 70 80 90 100PERCENT OF FREE DELIVERY6c. Curved Airfoil, BIAPage 2Fan Engineering FE-2400

Backward inclined fans are used to deliver mediumto high airflow at static pressures up to 20" watergauge. Pressures up to 40" water gauge are attainablewith special construction.The normal selection range for quiet, efficient performanceis from 40% to 85% of free delivery (Figure 7).While these fans do not exhibit a deep stall range likethe forward curve fan, there is a range of instability leftof peak. The single thickness blades are more sensitiveto the breakaway airflow in this area than the airfoil andshould be selected to the right of peak.Figure 7 shows the characteristic of a flat bladedesign; however, it typifies the characteristics of theentire family of backward inclined blade shapes. Onlysubtle differences exist between their static pressurecurves.Figure 7. Characteristic Performance of Backward InclinedFlat Blade FansRadial Blade Fans“Steel plate” or “paddle wheel” are two of the commonnames for radial blade fans. The impeller blades aregenerally narrower, deeper and heavier than forwardcurve and backward inclined blades. A radial bladeimpeller usually comprises six to twelve equally spacedflat blades extending radially from the center of the hub.These impellers are generally of simple design that lendsitself to rugged construction and offers a minimum ofledges, etc., for the accumulation of dust or sticky materials.There are more variations of the radial blade fansthan the forward curve and backward inclined types.Three of the more common impellers are illustrated inFigure 8.Figure 8. Common Radial Blade Impellers100PERCENT OF NO FLOW STATIC PRESSUREHORSEPOWER AND EFFICIENCY908070605040302010HORSEPOWERTOTAL EFFICIENCYTOTAL PRESSURESTATIC PRESSURESTATIC EFFICIENCY00 10 20 30 40 50 60 70 80 90 100PERCENT OF FREE DELIVERYAn attractive feature of the backward inclined typesis the non-overloading characteristic of their horsepowercurves. As Figure 7 illustrates, the horsepower increasesto a maximum as airflow increases, and then drops offagain toward free delivery. This means that a motorselected to accommodate the peak horsepower will notoverload, despite variations in the system resistance orairflow, as long as the fan speed remains constant.Typically the flat bladed design has efficiencies ofabout 82%, while the curved blade and airfoil designsapproach 86% and 90%, respectively.The backward inclined “family” of fans has the highestoperating speeds of all the centrifugal fans (Figure4). While this is a desirable feature for direct connectionto modern “high speed” motor or turbine drives, itcomes with a price. Their high operating speed requiresheavier construction and precision balancing, makingthem fairly expensive compared to forward curve fans.Also, the close running clearances required to maintainfan performance makes them unsuitable for materialhandling. However, in single thickness blade constructionthey can be used in light dust and corrosive air.These fans are used primarily in the industrial marketfor ventilation, clean side of commercial air cleaningdevices, furnace draft and large commercial heating andcooling units. The air leaving the backward inclinedimpeller has less of its total energy in the form of velocitypressure than does the air leaving a forward curveimpeller. Because more of its energy is in the form ofstatic pressure, a backward inclined impeller loses lessenergy in the process of converting from velocity pressureto static pressure in the housing. Therefore, abackward inclined impeller can operate quite satisfactorilywithout a housing, making it suitable for specialtyfans such as plug fans, plenum fans, and in-line centrifugalfans, whose characteristics are all similar toFigure 7, just slightly less efficient.8a. 8b. 8c.OW/BW AW RTRadial Blade Air Wheel Radial TipThe open wheel (OW), paddle wheel; and the backplatewheel (BW), steel plate, are the most common ofthe radial blade impellers, and their typical performancecharacteristics are shown in Figure 9. These fans aregenerally selected to operate from 35% to 80% of freedelivery. However, it should be noted that these fanscan, and do, operate quite successfully left of peak,down to approximately 20% of free delivery.Figure 9. Characteristic Performance of Radial Blade FansPERCENT OF NO FLOW STATIC PRESSUREHORSEPOWER AND EFFICIENCY100908070605040302010STATIC EFFICIENCYHORSEPOWERTOTAL PRESSURETOTAL EFFICIENCYSTATIC PRESSURE00 10 20 30 40 50 60 70 80 90 100PERCENT OF FREE DELIVERYFrom Figure 4 it can be seen that these are mediumspeed fans and are used to deliver low air volumes atmedium to high pressure. The main advantage of theradial blade fan lies in its simple but rugged construction.They are ideal for high static pressure applicationsand for handling airstreams containing a high level ofparticulate.Some of the disadvantages are that they generatemore noise than forward curve and backward inclinedfans, primarily because of the impeller design and highoperating velocities and they exhibit the same risinghorsepower characteristic as the forward curve fans.Because they are low volume fans, larger sizes aregenerally required, taking up a larger installation space.Fan efficiencies are lower than both the forward curveand backward inclined type, but this is generally offsetby their ability to adapt to harsh environments.Page 3Fan Engineering FE-2400

For higher efficiencies most manufacturers offer somevariation of the air wheel (AW) impeller, Figure 8b, tooperate in the same housing as the straight radial bladeimpellers. For even higher efficiencies and airflow, manufacturersalso offer a radial tip (RT) impeller; however,in most cases the radial tip impeller operates in a housingsimilar to the backward inclined design. The radialtip design fills the gap between the clean air backwardinclined fans and the more rugged radial blade fans.Both the air wheel and the radial tip impellers areideal for contaminated airstreams but neither is intendedfor bulk material handling. Both impellers have pressurecharacteristics similar to the backward inclined impellersand a horsepower characteristic similar to the radialblade and forward curve impellers. Unlike the straightradial blade impellers these two impeller designs doexhibit some instability left of peak and should alwaysbe selected to the right of peak.By and large, the radial blade series of fans are usedexclusively in the industrial market for handling and conveyingvarious process materials and gases. They areused for “high pressure” air systems and for combustionair. Generally speaking, they are furnished in belt drivearrangements due to the high shock loads and harshenvironments to which they are exposed.®Aerovent | www.aerovent.com5959 Trenton Lane N | Minneapolis, MN 55442 | Phone: 763-551-7500 | Fax: 763-551-7501