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Fans and Blowers - Saylor.org

Fans and Blowers - Saylor.org

Table 5.3 Types of

Table 5.3 Types of Fans, Characteristics, and Typical ApplicationsCentrifugal FansAxial-flow Fans5. Fans and BlowersTypeCharacteristicsTypicalApplicationsTypeCharacteristicsTypicalApplicationsRadialHigh pressure,medium flow,efficiency close totube-axial fans,power increasescontinuouslyVariousindustrialapplications,suitable fordust laden,moistair/gasesPropellerLow pressure, highflow, low efficiency,peak efficiency closeto point of free airdelivery (zero staticpressure)Air-circulation,ventilation,exhaustForwardcurvedbladesMedium pressure,high flow, dip inpressure curve,efficiency higherthan radial fans,power risescontinuouslyLow pressureHVAC,packagedunits, suitablefor clean anddust laden air /gasesTube-axialMedium pressure,high flow, higherefficiency thanpropeller type, dip inpressure-flow curvebefore peak pressurepoint.HVAC, dryingovens, exhaustsystemsBackwardcurvedbladesHigh pressure, highflow, highefficiency, powerreduces as flowincreases beyondpoint of highestefficiencyHVAC,variousindustrialapplications,forced draftfans, etc.Vane-axialHigh pressure,medium flow, dip inpressure-flow curve,use of guide vanesimproves efficiencyHigh pressureapplicationsincludingHVACsystems,exhaustsAirfoiltypeSame as backwardcurved type, highestefficiencySame asbackwardcurved, but forclean airapplicationsincreases, which can be a disadvantage in material conveying systems that depend on asteady air volume. Because of this, they are most often used in applications that are notprone to clogging.Positive-displacement blowers have rotors, which "trap" air and push it through housing.Positive-displacement blowers provide a constant volume of air even if the systempressure varies. They are especially suitable for applications prone to clogging, since theycan produce enough pressure - typically up to 1.25 kg/cm 2 - to blow clogged materialsfree. They turn much slower than centrifugal blowers (e.g. 3,600 rpm), and are often beltdriven to facilitate speed changes.5.3 Fan Performance Evaluation and Efficient System OperationSystem CharacteristicsThe term “system resistance” is used when referring to the static pressure. The systemresistance is the sum of static pressure losses in the system. The system resistance is afunction of the configuration of ducts, pickups, elbows and the pressure drops acrossBureau of Energy Efficiency96

5. Fans and Blowersequipment-for example bagfilter or cyclone. Thesystem resistance varies with the square of the volumeof air flowing through the system. For a given volumeof air, the fan in a system with narrow ducts andmultiple short radius elbows is going to have to workharder to overcome a greater system resistance than itwould in a system with larger ducts and a minimumnumber of long radius turns. Long narrow ducts withmany bends and twists will require more energy topull the air through them. Consequently, for a givenfan speed, the fan will be able to pull less air throughthis system than through a short system with noelbows. Thus, the system resistance increases Figure 5.5 System Characteristicssubstantially as the volume of air flowing through the system increases; square of airflow.Conversely, resistance decreases as flow decreases. To determine what volume the fanwill produce, it is therefore necessary to know the system resistance characteristics.In existing systems, the system resistance can be measured. In systems that have beendesigned, but not built, the system resistance must be calculated. Typically a systemresistance curve (see Figure 5.5) is generated with for various flow rates on the x-axis andthe associated resistance on the y-axis.Fan CharacteristicsFan characteristics can be represented in form of fan curve(s). The fan curve is aperformance curve for the particular fan under a specific set of conditions. The fan curveis a graphical representation of a number of inter-related parameters. Typically a curvewill be developed for a given set of conditions usually including: fan volume, systemstatic pressure, fan speed, and brake horsepower required to drive the fan under the statedconditions. Some fan curves will also include an efficiency curve so that a systemdesigner will know where on that curve the fan will be operating under the chosenconditions (see Figure 5.6). In the many curves shown in the Figure, the curve staticpressure (SP) vs. flow is especially important.The intersection of the system curve and the static pressure curve defines theoperating point. When the system resistance changes, the operating point also changes.Once the operating point is fixed, the power required could be found by following avertical line that passes through the operating point to an intersection with the power(BHP) curve. A horizontal line drawn through the intersection with the power curve willlead to the required power on the right vertical axis. In the depicted curves, the fanefficiency curve is also presented.System Characteristics and Fan CurvesIn any fan system, the resistance to air flow (pressure) increases when the flow of air isincreased. As mentioned before, it varies as the square of the flow. The pressure requiredby a system over a range of flows can be determined and a "system performance curve"can be developed (shown as SC) (see Figure 5.7).This system curve can then be plotted on the fan curve to show the fan's actualoperating point at "A" where the two curves (N 1 and SC 1 ) intersect. This operating pointis at air flow Q 1 delivered against pressure P 1 .Bureau of Energy Efficiency 97

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