Forward curved blades

Features:

• 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

Features

• 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)

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.

• Static pressure Δpfa

• Dynamic pressure Δpd = ρ/2 * c²

• Total pressure Δpt = Δpfa + Δpd

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.

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.

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!

Note:

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.