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Kruger Ventilation Industries certifies that the BDB series shown herein is licensed to bear the AMCA Seal. The ratings shown are based on tests and procedures performed in accordance with AMCA publication 211 and AMCA publication 311 and comply with the requirements of the AMCA Certified Ratings Program. Kruger Blower Selection, free kruger blower selection software downloads.Kruger Blower Selection Software Free Trial
*Products
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Fans are used for moving gases (e.g. air) from one place to another for extraction, air-conditioning, compression, etc. They do this by rotating a series of angled blades (or vanes) that pull the air through an aperture.
There are a number of fan types: impeller, axial, centrifugalA, Sirocco, etc. all of which have individual benefits (volume, pressure, speed, power, efficiency, etc.) but all of them will shift gases at the same rate based upon the input power. Differences such as efficiency or flow rate occur in the type of fan due to particular design advantages that favour one characteristic over another. For example, an impeller fan has a higher efficiency when transporting clean (light air) at high flow rates (high speed), whereas a straight-bladed Sirocco fan is more efficient when propelling heavy gases (vapours and particulates). Multi-stage fans are normally used to increase outlet pressure, but are comparatively expensive.
Airflow through the impeller is generated by rotating profiled blades (Fig 1) in a cowling that cut into the air at their inlet tip pushing the air back along the blade and, in the case of centrifugal fans, also from centrifugal forces generating a partial vacuum on the inlet side of the fan due to the entrained air being thrown outwards according the relationship a = v²/r
Apart from the electrical and mechanical components, the efficiency of a fan is to a large extent dependent upon the shape and orientation of the blades. All fans of a given power rating will rotate at a speed commensurate with the air resistance, i.e. the lower the air resistance, the faster the rotation and the greater the flow.
Multi-stage fans are used where a very high outlet pressure is required. I.e. each fan in the sequence increases pressure over the previous fan until you have achieved the pressure required. One normal axial fan operating at maximum efficiency can achieve a velocity pressure (pᵥ) of up to 0.5psi (≈3,500N/m²). A high-efficiency, multi-stage (series of fans) turbo-blower can achieve pressures more than a hundred times greater.Generic Fans
CalQlata has tried to keep the operation of this calculation option as simple as possible, given that it is recommended for general purpose calculations only and not for actual purchase specifications (see Fan Calculator – Technical Help below).
Fig 2. Fan Air Pressures
Fig 2 shows the pressures through a fan, each of which is described below:
Inlet Pressure; is the static pressure on the inlet side of the fan. This should also include the velocity pressure on the inlet side (if known) that is constant and in-line with the fan. You can include this effect if you wish by using the following formula:
pᵢ = pᵢ ± ½.v².ρᵢ {use ’+’ if the direction of movement is towards the fan and ’-’ if it is moving away from the fan (which is an unlikely event given the suction direction)}
Outlet Pressure; is the static pressure on the outlet side of the fan. This should also include the velocity pressure on the outlet side (if known) that is constant and in line with the fan as well as the velocity pressure (pᵥ) generated by the fan. You can include this effect if you wish by using the following formula:
pₒ = pₒ ± ½.v².ρₒ {use ’+’ if the direction of movement is towards the fan and ’-’ if it is moving away from the fan}
Velocity Pressure; is the pressure generated by the gas moving through the fan
Discharge Pressure; is the sum of the velocity pressure and the difference between the outlet pressure and the inlet pressure (Fig 2)
Static Pressure; is the maximum of the inlet and outlet pressures
Pressure Head; is the head generated by the discharge pressure at the outlet side of the fanFan Blade Design (Axial and Centrifugal)
The shape of your blades and the direction they travel will define the performance characteristics of your fan.
Fig 3 shows the velocity diagram for the air flowing into the fan (inlet) and out of it (outlet).
v₁ᵢ and v₁ₒ: the inlet and outlet velocities of the air through the blades will be the same for axial fans and different for centrifugal fans
v₂ᵢ and v₂ₒ: the circular speed of the inlet and outlet edges of the blade will be the same for axial fans and different for centrifugal fans
v₃ᵢ and v₃ₒ: the speed of the air over the surface of the blade will vary from inlet to outlet for both axial and centrifugal fans
v₄ᵢ and v₄ₒ: the centrifugal velocity component of the air will be zero for the inlet edge of an axial fan blade and will vary from inlet to outlet for both axial and centrifugal fans
vᵢ and vₒ: the absolute velocity of the air at the inlet and outlet edges of the blade and will vary from inlet to outlet for both axial and centrifugal fans
The following table summarises the characteristics you can expect from your fan dependent upon the shape of its blades (Fig 3).CharacteristicBackward Facing { )→ }Straight { |→ }Forward Facing { (→ }SpeedhighmediumlowNoisemediumhighlowPressurehigh (20’ to 40’ WG)medium (8’ to 15’ WG)Low (3’ to 6’ WG)Volume FlowmediumlowhighParticulatesgoodexcellentpoorEfficiency80% x FF65%70% x FF45%70% x FF40%ConstructionheavymediumlightFF = free flow WG = water gauge
Please bear in mind that the backward-straight-forward relationship refers to the inlet tip of the impeller blade (0° < θᵢ < 180°)
It is inadvisable to significantly orientate the outlet tip of an impeller blade in a forward direction (θₒ > 110°) as it would disrupt airflow and give unreliable results.Efficiency
Fig 4. Axial Fan Efficiency
Whilst a fan’s efficiency is not the only consideration for a designer, performance being his/her primary concern, it should not be ignored. Therefore, having achieved the design requirements, the designer should then proceed to optimise operational efficiency.
A fan’s operational efficiencies are primarily dependent upon two factors; blade tip angles and mechnical/electrical equipment. The fan calculator addresses only the blade angles. Mechanical/electrical efficiency must be dealt with by the designer when selecting suitable materials and drive systems. The head losses generated by the blade tip angles (inlet and outlet) define a fan’s ’air’ efficiencies.
These losses are as follows ..
Shock (Lˢ): The air entering a centrifugal impeller changes direction from v₁ᵢ to vᵢ producing a shock load on the blade. The leading (inlet edge) angle can be set to eliminate this shock resulting in v₄ᵢ=0. This loss does not apply to axial fans; i.e. Lˢ=0
Friction (Lᶠ): The air passing over the surface of the blade (v₃ᵢ to v₃ₒ) will slow down as a result of friction between the air and blade.
Energy (Lᵉ): Air leaving the impeller of a centrifugal fan contains stored energy that is not converted into head or velocity. This loss does not apply to axial fans; i.e. Lᵉ=0
.. which is largely determined by the leading and trailing blade angles.
As long as the cross-sectional area of a fan’s diffuser (outer casing; Ac) is greater than the surface area of the outside diameter of the impeller (A or Ao for axial and centrifugal respectively), the fan will exhaust 100% of volumetric flow with the same pressure variation as generated by the impeller (δp). As the diffuser area is reduced, the flow-rate will fall and outlet pressure will increase.
Axial Fans
Axial fans only operate with inlet and outlet angles between 0° and 90° and the outlet angle must be greater than the inlet angle (Fig 3). Moreover, as can be seen in Fig 4, the inlet angle should be as small as possible and there is little to be gained by providing an outlet angle less than 90°
Efficiency varies slightly with impeller diameters (Øᵢ and Øₒ) and operating speed (N) but not with fan length (ℓ).
Centrifugal FansKruger Blower Selection software, free download
As shown in Fig 5, except for very specific performance requirements, there is little to be gained in designing a centrifugal impeller with blade tip angles greater than 90°.
For general applications, maximum isentropic efficiency will be achieved by selecting small inlet angles and large outlet angles, however, this will be at the expense of head efficiency. Optimum efficiencies (head and isentropic) generally occur when inlet and outlet blade tips are set at angles around 45°.
Efficiency at these (optimum) angles varies with impeller diameters (Øᵢ and Øₒ) but is unaffected by variations in operating speed (N).
Axial vs Centrifugal
A comparison between the efficiency and performance of equivalent Axial and Centrifugal impellers is provided below ..
Axial:
ε = 100%; H = 15.5m; P = 268W; δp = 202Pa
Centrifugal:
ε = 74.4%; H = 14.3m; P = 322W; δp = 181Pa
.. making the axial fan more efficient, primarily due to the negligible losses from shock and outlet energy that are always present and need to be optimised in centrifugal fans.Aspect Ratio
CalQlata defines the aspect ratio (ф) of an impeller thus: ф = ID/OD
The radial depth of a high aspect ratio (0.75<ф<1.0) impeller is relatively shallow compared with its OD
High aspect ratio impellers are used for high-pressures and low flow rates (small impeller volume). However, the flow rate in wide high aspect ratio impellers can be improved by matching the shape of the input orifice to that of the impeller’s cross-section
The radial depth of a medium aspect ratio (0.5<ф<0.75) impeller is relatively high compared with its OD. Such impellers provide greater flow rates but reduced pressure potentialKruger Blower Selection Software Free Edition
Centrifugal fans are normally fitted with impeller aspect ratios greater than 0.5
Axial fans are normally fitted with impeller aspect ratios less than 0.5 (where flow is of greater importance than pressure)
Irrespective of design criteria, an impeller’s aspect ratio should ensure that its airflow is not compromised. With particular regard to centrifugal fans; the impeller inlet area should be no less than the inlet area of the blades; π.Øᵢ²/4 ≥ π.Øᵢ.w.Impeller ID
It is important to ensure that the inlet diameter of your centrifugal impeller is sufficient given the available inlet pressure (ambient or artificial) for the desired outlet mass or volumetric flow rate.
For example; an impeller of 0.5m diameter with an ID of 0.1m will never achieve the flow rate for which the impeller OD is capable unless the inlet pressure/flow-rate is artificially increased.How Many Blades?
The number of blades (in your impeller) does not affect Fans’ calculation results.
I.e. it is entirely up to you as to how many blades you use in your impeller.
Fans’ calculations are based upon all the entrained air passing through the impeller with each rotation, which is normal practice for optimum blade configurations.
However:
Too few blades; the air trailing each blade will be turbulent, reducing operational efficiency. I.e. your fan will not actually achieve the desired/calculated flow-rate and/or pressure.
Too many blades will also reduce fan efficiency through increased skin friction and impeller mass (i.e. greater operational power).
A few rules:
1 Blade: Airflow will occur according to our calculations for about 1/3rd of the impeller volume, the rest of the air within the impeller will be turbulent making your fan extremely inefficient. Such a configuration is also difficult to balance.
2 Blades: Significantly improved airflow characteristics than one blade designs but still generates significant turbulence (behind each blade). Blade balancing is easier to achieve than one blade designs
3 Blades: Excellent for impellers with small aspect ratio (e.g. axial fans) and much simpler to balance than 1 and 2-Blade designs
4 Blades: Better airflow than the 3-Blade configuration but 33% greater skin friction. Airflow improvement more than offsets losses from skin friction
5 Blades: Best configuration for all medium aspect ratio impellers
6 Blades: Losses from increased skin friction and mass begin to exceed airflow gains
>6 Blades: A general rule for large aspect ratio impellers (ф > 0.75) is to set the straight-line distance between the internal tips (toes) of adjacent blades approximately equal to the depth (radial height) of each blade.
Skin friction has a greater effect on flow-rate than pressure in fast fans.
I.e. it is advisable to minimise the number of blades in high flow-rate fans.
However many blades you decide to install, you should ensure that they should not overlap
If you are considering a forward facing blade configration for a centrifugal fan, you will need to increase the number of blades significantly over the above rules in order to ensure sufficient inlet velocity. Fans will not generate a result for forward facing configurations with insufficient blades.Casing
A fan casing may be any shape or size as long as its inlet and outlet diffusers do not impede airflow beyond that intended by the designer.
For example; Fans does not consider the manufacturing quality of the impeller casing, nor does it consider internal bends or deformations affecting the flow-path.Inlet Diffuser
For the purposes of this description; the inlet area of a diffuser is the orifice nearest (adjacent) to the impeller.
Unless the purpose of a fan is to generate suction, there is nothing to be gained by restricting inlet airflow. Therefore, the cross-sectional area of the inlet diffuser should be no less than that of the impeller blade inlet.
If the casing inlet includes a diffuser, it is normally considered advisable to taper the diffuser to minimise the effects of surface friction.Outlet Diffuser
For the purposes of this description; the outlet area of a diffuser is the orifice furthest from the impeller.
It is normal practice to design the diffuser outlet to minimise airflow restriction. In this case, the outlet area should be no less than that of the impeller blades.
If the casing outlet includes a diffuser, it is normally considered advisable to taper the diffuser to minimise the effects of surface friction.
Where outlet airflow is to be restricted, this may be achieved by reducing the diffuser outlet area (there is little to be gained by increasing the diffuser outlet area). The relative areas (impeller:diffuser) will define the resultant head, pressure and velocity of the outflowing air; volumetric flow-rate will of course remain unchanged.The Theory (a few tips)
The theory on which this calculator is based is usually credited to Charles Innes. It is now considered to be the industry standard and has stood the test of time since 1916.
It is based upon the velocity of air as it passes over the blade profile (Fig 3). Like all theories it requires you to follow a few basic rules. if you don’t follow the rules, your fan won’t work. This does not mean Innes’ theory doesn’t work, it means that the air will not flow over the fan correctly.
For example the theory assumes a smooth transition from inlet blade tip to outlet blade tip. The two blade tip angles define the profile of your blade. Charles Innes did not create the performance of air over a curved blade, he simply shows us how to calculate it.
If you get it wrong, the results will be meaningless, not just theoretically meaningless but practically also. Your impeller won’t work.
For example:
1) Always try to use a backward facing blade where possible. It generates more head (pressure) and is much more efficient.
2) Paddle blades must be 90° inlet and outlet (not simply close to this value) as they do not drive the air using the blade profile, they drive air out through the impeller using centrifugal force and any other angle will create unnecessary back pressure
3) Always use inlet blade angles considerably less than 90°
4) When setting blade outlet angles greater than 90°, always set the inlet blade angle shallow enough to overcome inward thrust from the outlet tip. The greater the outlet blade angle the shallower must be the inlet tip angle. If you just alter the outlet angle without adjusting the inlet angle you will struggle to find a solution. This is a particularly sensitive calculation as pressure generation is already low; it doesn’t take much to generate a negative pressure.
5) If you are getting negative results, this simply means that your head losses are greater than the head generated.
Outlet blade angles greater than 90° will always give you a bit of a challenge to create a workable solution. The secret here is the ensure that inlet angle is very shallow (e.g. << 45°; i.e. a deep cup-shape blade) to generate the inlet pressure required to overcome the negative pressure at the outlet. Moreover, it is advisable to minimise the number of blades used in such fans.Fan Design Procedure
It is important to remember the following when designing a fan using the Axial and/or Centrifugal calculation options in our fan calculator:
The output results from Fans are for driving the air alone.
Power will increase with material mass & drive mechanism inefficiencies, and the head and flow rates will vary with casing design.
The drive system and casing irregularities are difficult to incorporate in a calculator as the possible variations are infinite.
Hence the the need to follow a suitable procedure when designing your fan (a driven impeller within a casing).
1) List your operating parameters (flow-rate, head, pressure-rise, etc.)
2) Use Fans to size your impeller and set your blade angles. Output co-ordinates can be found in the Data Listing menu. Copy and paste into your spreadsheet for plotting (see Fig 7).
3) The power output (in Watts if you are entering Newtons and Metres) is that needed for movement of the air only.
Because power is calculated thus
https://diarynote.indered.space
Kruger Ventilation Industries certifies that the BDB series shown herein is licensed to bear the AMCA Seal. The ratings shown are based on tests and procedures performed in accordance with AMCA publication 211 and AMCA publication 311 and comply with the requirements of the AMCA Certified Ratings Program. Kruger Blower Selection, free kruger blower selection software downloads.Kruger Blower Selection Software Free Trial
*Products
*FansBy Mounting TypeFan Selector Fan ControllersControllersRun-on timerAncillariesPrefab Modular AssembliesMounting FeetMatching FlangesDuctingAttenuatorsAcoustic LouvresCross Talk AttenuatorsVAV DiffusersElectronic DiffusersThermo-disk DiffusersHealthy indoor environment
*Technical Library
*Adjustable PitchAxial Flow FansMotorsTypes and SpecificationsNoise & AcousticInstallationInst. & Maint. InstructionsDo’s and Don’ts
*Downloads
*Tech TalkLatest IssuesBrochures Catalogue Catalogue DownloadSelection Program Revit Models Revit Models Download
*Contact Us
*Contact AddressesNew ZealandEuropeRequests/enquiriesRequests/enquiries
*Key Projects
*ProjectsDomestic Fire Safety & HazardousIndustrialSports & Leisure
*About Us
*About FantechAbout FantechHistory of Innovation Privacy PolicyPrivacy PolicyTerms of UseTerms of Trading & QuotationTerms of Trading & Quotation
Fans are used for moving gases (e.g. air) from one place to another for extraction, air-conditioning, compression, etc. They do this by rotating a series of angled blades (or vanes) that pull the air through an aperture.
There are a number of fan types: impeller, axial, centrifugalA, Sirocco, etc. all of which have individual benefits (volume, pressure, speed, power, efficiency, etc.) but all of them will shift gases at the same rate based upon the input power. Differences such as efficiency or flow rate occur in the type of fan due to particular design advantages that favour one characteristic over another. For example, an impeller fan has a higher efficiency when transporting clean (light air) at high flow rates (high speed), whereas a straight-bladed Sirocco fan is more efficient when propelling heavy gases (vapours and particulates). Multi-stage fans are normally used to increase outlet pressure, but are comparatively expensive.
Airflow through the impeller is generated by rotating profiled blades (Fig 1) in a cowling that cut into the air at their inlet tip pushing the air back along the blade and, in the case of centrifugal fans, also from centrifugal forces generating a partial vacuum on the inlet side of the fan due to the entrained air being thrown outwards according the relationship a = v²/r
Apart from the electrical and mechanical components, the efficiency of a fan is to a large extent dependent upon the shape and orientation of the blades. All fans of a given power rating will rotate at a speed commensurate with the air resistance, i.e. the lower the air resistance, the faster the rotation and the greater the flow.
Multi-stage fans are used where a very high outlet pressure is required. I.e. each fan in the sequence increases pressure over the previous fan until you have achieved the pressure required. One normal axial fan operating at maximum efficiency can achieve a velocity pressure (pᵥ) of up to 0.5psi (≈3,500N/m²). A high-efficiency, multi-stage (series of fans) turbo-blower can achieve pressures more than a hundred times greater.Generic Fans
CalQlata has tried to keep the operation of this calculation option as simple as possible, given that it is recommended for general purpose calculations only and not for actual purchase specifications (see Fan Calculator – Technical Help below).
Fig 2. Fan Air Pressures
Fig 2 shows the pressures through a fan, each of which is described below:
Inlet Pressure; is the static pressure on the inlet side of the fan. This should also include the velocity pressure on the inlet side (if known) that is constant and in-line with the fan. You can include this effect if you wish by using the following formula:
pᵢ = pᵢ ± ½.v².ρᵢ {use ’+’ if the direction of movement is towards the fan and ’-’ if it is moving away from the fan (which is an unlikely event given the suction direction)}
Outlet Pressure; is the static pressure on the outlet side of the fan. This should also include the velocity pressure on the outlet side (if known) that is constant and in line with the fan as well as the velocity pressure (pᵥ) generated by the fan. You can include this effect if you wish by using the following formula:
pₒ = pₒ ± ½.v².ρₒ {use ’+’ if the direction of movement is towards the fan and ’-’ if it is moving away from the fan}
Velocity Pressure; is the pressure generated by the gas moving through the fan
Discharge Pressure; is the sum of the velocity pressure and the difference between the outlet pressure and the inlet pressure (Fig 2)
Static Pressure; is the maximum of the inlet and outlet pressures
Pressure Head; is the head generated by the discharge pressure at the outlet side of the fanFan Blade Design (Axial and Centrifugal)
The shape of your blades and the direction they travel will define the performance characteristics of your fan.
Fig 3 shows the velocity diagram for the air flowing into the fan (inlet) and out of it (outlet).
v₁ᵢ and v₁ₒ: the inlet and outlet velocities of the air through the blades will be the same for axial fans and different for centrifugal fans
v₂ᵢ and v₂ₒ: the circular speed of the inlet and outlet edges of the blade will be the same for axial fans and different for centrifugal fans
v₃ᵢ and v₃ₒ: the speed of the air over the surface of the blade will vary from inlet to outlet for both axial and centrifugal fans
v₄ᵢ and v₄ₒ: the centrifugal velocity component of the air will be zero for the inlet edge of an axial fan blade and will vary from inlet to outlet for both axial and centrifugal fans
vᵢ and vₒ: the absolute velocity of the air at the inlet and outlet edges of the blade and will vary from inlet to outlet for both axial and centrifugal fans
The following table summarises the characteristics you can expect from your fan dependent upon the shape of its blades (Fig 3).CharacteristicBackward Facing { )→ }Straight { |→ }Forward Facing { (→ }SpeedhighmediumlowNoisemediumhighlowPressurehigh (20’ to 40’ WG)medium (8’ to 15’ WG)Low (3’ to 6’ WG)Volume FlowmediumlowhighParticulatesgoodexcellentpoorEfficiency80% x FF65%70% x FF45%70% x FF40%ConstructionheavymediumlightFF = free flow WG = water gauge
Please bear in mind that the backward-straight-forward relationship refers to the inlet tip of the impeller blade (0° < θᵢ < 180°)
It is inadvisable to significantly orientate the outlet tip of an impeller blade in a forward direction (θₒ > 110°) as it would disrupt airflow and give unreliable results.Efficiency
Fig 4. Axial Fan Efficiency
Whilst a fan’s efficiency is not the only consideration for a designer, performance being his/her primary concern, it should not be ignored. Therefore, having achieved the design requirements, the designer should then proceed to optimise operational efficiency.
A fan’s operational efficiencies are primarily dependent upon two factors; blade tip angles and mechnical/electrical equipment. The fan calculator addresses only the blade angles. Mechanical/electrical efficiency must be dealt with by the designer when selecting suitable materials and drive systems. The head losses generated by the blade tip angles (inlet and outlet) define a fan’s ’air’ efficiencies.
These losses are as follows ..
Shock (Lˢ): The air entering a centrifugal impeller changes direction from v₁ᵢ to vᵢ producing a shock load on the blade. The leading (inlet edge) angle can be set to eliminate this shock resulting in v₄ᵢ=0. This loss does not apply to axial fans; i.e. Lˢ=0
Friction (Lᶠ): The air passing over the surface of the blade (v₃ᵢ to v₃ₒ) will slow down as a result of friction between the air and blade.
Energy (Lᵉ): Air leaving the impeller of a centrifugal fan contains stored energy that is not converted into head or velocity. This loss does not apply to axial fans; i.e. Lᵉ=0
.. which is largely determined by the leading and trailing blade angles.
As long as the cross-sectional area of a fan’s diffuser (outer casing; Ac) is greater than the surface area of the outside diameter of the impeller (A or Ao for axial and centrifugal respectively), the fan will exhaust 100% of volumetric flow with the same pressure variation as generated by the impeller (δp). As the diffuser area is reduced, the flow-rate will fall and outlet pressure will increase.
Axial Fans
Axial fans only operate with inlet and outlet angles between 0° and 90° and the outlet angle must be greater than the inlet angle (Fig 3). Moreover, as can be seen in Fig 4, the inlet angle should be as small as possible and there is little to be gained by providing an outlet angle less than 90°
Efficiency varies slightly with impeller diameters (Øᵢ and Øₒ) and operating speed (N) but not with fan length (ℓ).
Centrifugal FansKruger Blower Selection software, free download
As shown in Fig 5, except for very specific performance requirements, there is little to be gained in designing a centrifugal impeller with blade tip angles greater than 90°.
For general applications, maximum isentropic efficiency will be achieved by selecting small inlet angles and large outlet angles, however, this will be at the expense of head efficiency. Optimum efficiencies (head and isentropic) generally occur when inlet and outlet blade tips are set at angles around 45°.
Efficiency at these (optimum) angles varies with impeller diameters (Øᵢ and Øₒ) but is unaffected by variations in operating speed (N).
Axial vs Centrifugal
A comparison between the efficiency and performance of equivalent Axial and Centrifugal impellers is provided below ..
Axial:
ε = 100%; H = 15.5m; P = 268W; δp = 202Pa
Centrifugal:
ε = 74.4%; H = 14.3m; P = 322W; δp = 181Pa
.. making the axial fan more efficient, primarily due to the negligible losses from shock and outlet energy that are always present and need to be optimised in centrifugal fans.Aspect Ratio
CalQlata defines the aspect ratio (ф) of an impeller thus: ф = ID/OD
The radial depth of a high aspect ratio (0.75<ф<1.0) impeller is relatively shallow compared with its OD
High aspect ratio impellers are used for high-pressures and low flow rates (small impeller volume). However, the flow rate in wide high aspect ratio impellers can be improved by matching the shape of the input orifice to that of the impeller’s cross-section
The radial depth of a medium aspect ratio (0.5<ф<0.75) impeller is relatively high compared with its OD. Such impellers provide greater flow rates but reduced pressure potentialKruger Blower Selection Software Free Edition
Centrifugal fans are normally fitted with impeller aspect ratios greater than 0.5
Axial fans are normally fitted with impeller aspect ratios less than 0.5 (where flow is of greater importance than pressure)
Irrespective of design criteria, an impeller’s aspect ratio should ensure that its airflow is not compromised. With particular regard to centrifugal fans; the impeller inlet area should be no less than the inlet area of the blades; π.Øᵢ²/4 ≥ π.Øᵢ.w.Impeller ID
It is important to ensure that the inlet diameter of your centrifugal impeller is sufficient given the available inlet pressure (ambient or artificial) for the desired outlet mass or volumetric flow rate.
For example; an impeller of 0.5m diameter with an ID of 0.1m will never achieve the flow rate for which the impeller OD is capable unless the inlet pressure/flow-rate is artificially increased.How Many Blades?
The number of blades (in your impeller) does not affect Fans’ calculation results.
I.e. it is entirely up to you as to how many blades you use in your impeller.
Fans’ calculations are based upon all the entrained air passing through the impeller with each rotation, which is normal practice for optimum blade configurations.
However:
Too few blades; the air trailing each blade will be turbulent, reducing operational efficiency. I.e. your fan will not actually achieve the desired/calculated flow-rate and/or pressure.
Too many blades will also reduce fan efficiency through increased skin friction and impeller mass (i.e. greater operational power).
A few rules:
1 Blade: Airflow will occur according to our calculations for about 1/3rd of the impeller volume, the rest of the air within the impeller will be turbulent making your fan extremely inefficient. Such a configuration is also difficult to balance.
2 Blades: Significantly improved airflow characteristics than one blade designs but still generates significant turbulence (behind each blade). Blade balancing is easier to achieve than one blade designs
3 Blades: Excellent for impellers with small aspect ratio (e.g. axial fans) and much simpler to balance than 1 and 2-Blade designs
4 Blades: Better airflow than the 3-Blade configuration but 33% greater skin friction. Airflow improvement more than offsets losses from skin friction
5 Blades: Best configuration for all medium aspect ratio impellers
6 Blades: Losses from increased skin friction and mass begin to exceed airflow gains
>6 Blades: A general rule for large aspect ratio impellers (ф > 0.75) is to set the straight-line distance between the internal tips (toes) of adjacent blades approximately equal to the depth (radial height) of each blade.
Skin friction has a greater effect on flow-rate than pressure in fast fans.
I.e. it is advisable to minimise the number of blades in high flow-rate fans.
However many blades you decide to install, you should ensure that they should not overlap
If you are considering a forward facing blade configration for a centrifugal fan, you will need to increase the number of blades significantly over the above rules in order to ensure sufficient inlet velocity. Fans will not generate a result for forward facing configurations with insufficient blades.Casing
A fan casing may be any shape or size as long as its inlet and outlet diffusers do not impede airflow beyond that intended by the designer.
For example; Fans does not consider the manufacturing quality of the impeller casing, nor does it consider internal bends or deformations affecting the flow-path.Inlet Diffuser
For the purposes of this description; the inlet area of a diffuser is the orifice nearest (adjacent) to the impeller.
Unless the purpose of a fan is to generate suction, there is nothing to be gained by restricting inlet airflow. Therefore, the cross-sectional area of the inlet diffuser should be no less than that of the impeller blade inlet.
If the casing inlet includes a diffuser, it is normally considered advisable to taper the diffuser to minimise the effects of surface friction.Outlet Diffuser
For the purposes of this description; the outlet area of a diffuser is the orifice furthest from the impeller.
It is normal practice to design the diffuser outlet to minimise airflow restriction. In this case, the outlet area should be no less than that of the impeller blades.
If the casing outlet includes a diffuser, it is normally considered advisable to taper the diffuser to minimise the effects of surface friction.
Where outlet airflow is to be restricted, this may be achieved by reducing the diffuser outlet area (there is little to be gained by increasing the diffuser outlet area). The relative areas (impeller:diffuser) will define the resultant head, pressure and velocity of the outflowing air; volumetric flow-rate will of course remain unchanged.The Theory (a few tips)
The theory on which this calculator is based is usually credited to Charles Innes. It is now considered to be the industry standard and has stood the test of time since 1916.
It is based upon the velocity of air as it passes over the blade profile (Fig 3). Like all theories it requires you to follow a few basic rules. if you don’t follow the rules, your fan won’t work. This does not mean Innes’ theory doesn’t work, it means that the air will not flow over the fan correctly.
For example the theory assumes a smooth transition from inlet blade tip to outlet blade tip. The two blade tip angles define the profile of your blade. Charles Innes did not create the performance of air over a curved blade, he simply shows us how to calculate it.
If you get it wrong, the results will be meaningless, not just theoretically meaningless but practically also. Your impeller won’t work.
For example:
1) Always try to use a backward facing blade where possible. It generates more head (pressure) and is much more efficient.
2) Paddle blades must be 90° inlet and outlet (not simply close to this value) as they do not drive the air using the blade profile, they drive air out through the impeller using centrifugal force and any other angle will create unnecessary back pressure
3) Always use inlet blade angles considerably less than 90°
4) When setting blade outlet angles greater than 90°, always set the inlet blade angle shallow enough to overcome inward thrust from the outlet tip. The greater the outlet blade angle the shallower must be the inlet tip angle. If you just alter the outlet angle without adjusting the inlet angle you will struggle to find a solution. This is a particularly sensitive calculation as pressure generation is already low; it doesn’t take much to generate a negative pressure.
5) If you are getting negative results, this simply means that your head losses are greater than the head generated.
Outlet blade angles greater than 90° will always give you a bit of a challenge to create a workable solution. The secret here is the ensure that inlet angle is very shallow (e.g. << 45°; i.e. a deep cup-shape blade) to generate the inlet pressure required to overcome the negative pressure at the outlet. Moreover, it is advisable to minimise the number of blades used in such fans.Fan Design Procedure
It is important to remember the following when designing a fan using the Axial and/or Centrifugal calculation options in our fan calculator:
The output results from Fans are for driving the air alone.
Power will increase with material mass & drive mechanism inefficiencies, and the head and flow rates will vary with casing design.
The drive system and casing irregularities are difficult to incorporate in a calculator as the possible variations are infinite.
Hence the the need to follow a suitable procedure when designing your fan (a driven impeller within a casing).
1) List your operating parameters (flow-rate, head, pressure-rise, etc.)
2) Use Fans to size your impeller and set your blade angles. Output co-ordinates can be found in the Data Listing menu. Copy and paste into your spreadsheet for plotting (see Fig 7).
3) The power output (in Watts if you are entering Newtons and Metres) is that needed for movement of the air only.
Because power is calculated thus
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