Presentation on theme: "Fan Selection Criteria and Efficiency"— Presentation transcript:
1 Fan Selection Criteria and Efficiency byJohn Magill
2 The Air Movement and Control Association International (AMCA), has met the standards and requirements of the Registered Continuing Education Providers Program. Credit earned on completion of this program will be reported to the RCEPP. A certificate of completion will be issued to each participant. As such, it does not include content that may be deemed or construed to be an approval or endorsement by NCEES or RCEPP.
3 Learning Objectives List available fan types Know fan characteristics that are requiredUnderstand tradeoffs when selecting a fanDefine fan efficiency
4 Outline Fan Types Basic Fan Curve Applications Performance CharacteristicsFan SelectionEfficiency, low noise, size, space and cost considerationsMechanical considerations for a given application including balancing and vibration levels, construction, arrangements, ruggedness, spark resistance, corrosion resistance, high temperature resistance, bearings, motors, drives etc.
7 Centrifugal:Backward Inclined Airfoil-Blade Name is derived from the “airfoil” shape of bladesDeveloped to provide high efficiencyUsed on large HVAC and clean air industrial systems where energy savings are of prime importance
8 Centrifugal:Backward Inclined or Curved Flat-Blade Backward inclined or curved blades are single thickness or “flat”Efficiency is only slightly less than airfoil bladeSimilar characteristics as airfoil bladeSame HVAC applications as airfoil bladeAlso for industrial applications where airfoil blade is not acceptable because of corrosive or erosive environment
9 Backward Inclined or Curved Flat & Airfoil-Blade High volume at moderate pressureNon-overloading power characteristicStable performance characteristicLow noise
10 Centrifugal: Forward Curved Blade Blades are curved forward in the direction of rotationMust be properly applied to avoid unstable operationLess efficient than Airfoil and Backward InclinedRequires the lowest speed of any centrifugal to move a given amount of airUsed for low pressure HVAC systemsClean air and high temperature applicationsTypically smallest size selectionRising power overloading characteristic
11 Centrifugal: Radial Blade The blades are ‘radial’ to the fan shaftGenerally the least efficient of the centrifugal fansFor material handling and moderate to high pressure industrial applications, rugged constructionLow volume at high pressureLarge wheel diameter for a given volume- higher costMaterial handling, self cleaningEasy to maintainRising Power overloading characteristicSuitable for dirty airstream, high pressure, high temperature and corrosive applications
12 Centrifugal: Radial Tip The blades are radial to the fan shaft at the outer extremity of the impeller, but gradually slope towards the direction of wheel rotationMore efficient than the radial blade but less than backward inclinedOffers wear resistance in mildly erosive air streams
13 Axial: Propeller or Panel Fan One of the most basic fan designsFor low pressure, high volume applicationsOften used for ventilation through a wallAvailable in square panel or round ring fanMaximum efficiency is reached near free deliveryReversible blade for reversible flow applications like jet tunnel fansMany axial fans can overload at shutoff
14 Tubeaxial Fan More efficient than the panel fan Cylindrical housing fits closely to outside diameter of blade tipsFor low to medium pressure ducted HVAC systemsAlso used in some low pressure industrial applicationsPerformance curve sometimes includes a dip to the left of peak pressure which should be avoided
15 Vaneaxial Fan Highest efficiency axial fan Cylindrical housing fits closely to outside diameter of blade tipsThe straightening vanes allow for greater efficiency and pressure capabilitiesFor medium to high pressure HVAC systems. More compact than centrifugal fans of same dutyAerodynamic stall causes the performance curve to dip to the left of peak pressure which should be avoided. However anti-stall options available for both unidirectional and reversible axials
16 Power Roof Ventilators A variety of backward inclined centrifugal wheels or axial impeller designsAlso available in upblast damper design to discharge air away from the buildingFor low pressure exhaust systems of all building types (roof mounted)
17 Inline Centrifugal Fan Cylindrical housing is similar to a vaneaxial fanWheel is generally an airfoil or backward inclined typeHousing does not fit close to outer diameter of wheelFor low and medium pressure HVAC systems or industrial applications when an inline housing is geometrically more convenient than a centrifugal configuration
18 Mixed Flow Fan Specific Speed between a centrifugal and axial fan Cylindrical housing is similar to a vaneaxial fanHigh volume advantages of axial fansLow sound, high efficiency advantages of tubular centrifugal fans
19 PLENUM / PLUG FAN Housed vs plenum fan This is basically a centrifugal wheel and inlet in a frame without a scroll or housing. The ‘housing’ is the AHU box.Offers tremendous flexibility for inlet and discharge in a AHU applicationMore efficient than a scroll centrifugal for high flows and low SP. All SP rise occurs in the blade passageWall clearance rules must be followed to avoid significant system effect lossesHoused vs plenum fan
20 SO YOU HAVE ALL THESE CHOICES OF FANS TYPES AVAILABLE…WHAT SHOULD YOU DO TO PICK THE RIGHT FAN FOR YOUR APPLICATION?Let’s consider a couple of examples to illustrate the selection process from an efficiency, sound, cost and available space perspectiveAll Air tests based on AMCA std 210, and Sound tests based on AMCA std 300
23 All fans selected at peak SE (Static Efficiency) for Airflow=10,000 cfm, Static Pressure (SP)~2 iwc TypeDia (in)Spd (rpm)BHPSE % (Static Efficiency)LwiA (Inlet Sound Power ‘A’)1Forward Curved- SW (Centrifugal)304765.0961.7892Backward Airfoil – SW (Centrifugal)36.56503.8280.0773Plenum338004.2574.0804Tubular Mixed Flow2710744.4870.2815Tubular Vane Axial2814384.7765.9866Propeller (Axial)19984.9254.4103
24 Narrowing in after main Fan Type Selection.......... LESS EFFICIENTLESS COSTMORE NOISYIn general, for all fan types, as first cost goes down, operating costs (BHP) and noise go up…trade off!
25 Tone at Blade Pass Frequency (Blade Tone) Blade Pass Frequency, bpf= #blades * rpm / 60Sound Power level, Lw, at bpf is a distinct audible tone. This aerodynamic tone can be very annoying and is usually the worst for radial bladed fans, followed by plenums and housed centrifugals.Axial fans have a high pitched tone which is not as annoying.The bpf tone is a spike in Lw over the surrounding broadband noise spectra.Blade Tone Prominence is defined as the dominant energy level of the blade tone integrated over a narrowband region of the sound spectrum surrounding the blade tone.
26 FT-2FT-2Blade Tone prominenceAcoustic Engineers do not like blade tone prominence to exceed 6dB in addition to low Sound Power Levels (Lw)
27 Fan Selection based on Specific Speed Dimensional Specific Speed, is the fan speed required to raise the SP by 1 iwc with 1 cfm airflow.Ns = N * (Q)^0.5/(SP)^0.75Where, N = Speed (rpm)Q = Airflow (cfm)SP = Static pressure (iwc)Density = lbm/cu ft
29 All fans selected at peak SE (Static Efficiency) for Specific Speed, Ns TypeSpecific Speed, NsMax Static Efficiency (SE%)1Forward Curved-SW (Centrifugal)26,300612Backward Airfoil-SW (Centrifugal)40,000803Plenum50,000754Tubular Mixed Flow65,800705Tubular Vane Axial90,000656Propeller (Axial)126,00059
30 SummaryFan selection is not a trivial process for a given application.Example shown applies to one design operating point. The selections will change for other operating points.There is no magic fan that will result in least cost, best efficiency and low noise for a wide range of operating points.Compromises should be well understood upfront.Direct Drive (DD) selection speeds may further limit selections. Varying width options can optimize DD selections.Mechanical design requirements like balancing and vibration levels, spark and high temp resistance, corrosion resistance, arrangements, motors, bearings, drives can further challenge the selection process.