Karl Klein Ventilatorenbau GmbH - Industrieventilatoren

Fan wheels and the design of fans

Forward curved blades


  • High power density (frame size about half as large as for wheels with backward curved blades and the same power)
  • Impulse wheel → wheel mainly generates kinetic energy, which is converted into static pressure by the case → use only with case
  • Total efficiency of 60-65%
  • Also called drum impellers → TLR/TS
  • Good acoustics

Backward curved blades


  • High efficiency (up to total of 80%) 
  • Can also be used without case (at low pressures and high volume flows)
  • Noise level around 3 dB higher than comparable TLR wheel
  • Adjustment of pressure increase via diameter ratio (intake diameter to outer diameter)

Blade shapes

Spiral case

Spiral cases convert dynamic pressure to static pressure. The opening angle of spiral cases should be max. 7° because flow separation can take place if the opening angles are larger. The greater the pressure increase of a fan, the smaller the spiral angle of the case must be.

Important parameters

The operating point is calculated from:

  • Volume flow
  • Static pressure increase Δpfa
  • Total pressure increase Δpt
  • Air performance (pressure increase x volume flow)

Differentiation of pressure values

  • Static pressure Δpfa
  • Dynamic pressure Δpd = ρ/2 * c²
  • Total pressure Δpt = Δpfa + Δpd

Design of fans

Fan characteristic curve:

  • The characteristic curve for pressure in relation to volume flow specifies the volume flow at which a certain pressure increase is achieved.


Plant characteristic curve:

  • As a result of the mechanical friction of the plant air, pressure losses occur. As a rule, these increase quadratically with the flow speed → if the volume flow doubles, the pressure loss quadruples. This ratio between the pressure and volume flow gives rise to a parabolic characteristic curve.

Operating point

The operating point of a fan is always the intersection between the fan characteristic curve and the plant characteristic curve.


Ideally, the operating point should be at 50-70% of the maximum volume flow, since in this range, the greatest efficiencies are achieved. If possible, a fan should not work in the leading part of the characteristic curve, which is normally almost horizontal. This is because the volume flow can dive significantly after just a small change in the pressure increase.

Laws of proportionality

Speed change:

Note: The volume flow changes proportionally to the speed change, the pressure changes proportionally to the square of the speed change, and the power consumption changes proportionally to the cube of the speed change.

Change in wheel size

Note: The laws of proportionality only apply in the case of similar geometries. As the wheel size gets larger, the efficiency generally increases slightly; this change is not recorded here.

Change in air density or temperature

Note: The volume flow remains constant regardless of the density; the pressure change applies to the static, dynamic, and total pressure. Temperature in K!


You require a high volume flow but low pressure → large wheel diameter, low speed

You require a low volume flow but high pressure → small wheel diameter, high speed

You have a low medium density (e.g. due to a large height, high temperature, or steam) → the wheel must be designed for a greater pressure increase

If the speed increases by 20% (60 Hz), the power consumption increases by around 75%.

Density change

The change in the density of the conveyed medium must be taken into account when designing fans if:

  • Gases with densities other than that of air are conveyed
  • Large quantities of steam are in the air
  • The medium temperature changes significantly (the density of gases is proportional to the temperature in kelvin)
  • The static pressure increase significantly exceeds 3000 Pa (compressibility of air)
  • The flow speed exceeds 100 m/s


General equation of state of gases:

Pressure increase significantly above 3000 Pa

Note: This applies only if the temperature change can be disregarded.

Inlet nozzles

u around 1% of D, s around 0.6% of D

Wheels for dust conveyance


On the blade suction side, dust accumulates and is pressed against the blades by centrifugal force. In addition, in this area, the flow speed is lower, so the particles are not blown away. Imbalance can occur.


Wheels with straight blades and a low chord angle - so steeper blades - are used. As a result, particles can be better flung out of the wheel by centrifugal force.

Wheel strength

The reliable top speed is determined as follows for impellers:

  • The wheel is spun at increasing speed and measured until a lasting deformation of 0.5 mm is achieved. This speed is divided by a safety factor to give the reliable top speed.
  • Alternatively: FEM calculation 

Open radial wheels

Wheels without a cover disk can be used to convey extremely long-stranded media. However, note that such wheels are not suitable for large pressure increases, so they are also called conveyor wheels. In addition, the missing cover disk reduces the wheel strength. Typical applications include air circulators in drying plants.