FAQ

Causes of imbalance

A turbine leaving production can, for the most diverse causes, present an imbalance. Thus, the turbine is centered on the axis of rotation by its hub; However, there is little chance that the center of the hub and that of the rear disc will strictly coincide.

As a result, the center of gravity of the turbine is certainly outside the axis of rotation. On the other hand, the blades do not all have strictly the same weight and are not all strictly at the same distance from the axis, which is another cause of movement of the center of gravity of the turbine.

The cone and the disc are neither perfectly centered on the axis of rotation, which is a cause of both static unbalance and dynamic unbalance.

Finally, the turbine can be slightly inclined on its axis either due to the hub, or due to the disc, or even due to the mounting tolerances of the hub on the shaft, which is another cause of static unbalance and unbalance. dynamic.

It should also be noted that, even perfectly balanced upon delivery, a turbine may present an imbalance after a certain period of service, either due to irregular wear due to corrosion or abrasion of the turbine. , either due to clogging or jamming which is inherently even irregular.

Effects of imbalance

The centrifugal forces or moments created in the various parts of a turbine rotating at high speed cause damage of various kinds.

The shaking and jolts they cause create an increase in wear and, consequently, a reduction in longevity.

Under the permanent action of shaking, assembly elements can fail. When passing critical resonance areas, the centrifugal forces created cause very strong oscillations. This can result in fatigue failures and even sudden ruptures.

The shocks also act annoyingly in the vicinity of the fans. The permanent action of vibrations and noise has a very unfavorable influence on the human organism, both from a physical and psychological point of view.

As the centrifugal forces increase proportionally to the square of the rotation speed, balancing is more necessary as the rotation speed chosen is higher.

The noise of a fan has several origins which can be grouped into two main categories:

noises of aeraulic origin

noises of mechanical origin

Noises of aeraulic origin

In general, they predominate over those of mechanical origin.

a) Noise due to wheel movement

The wheel rotates at a speed of n revolutions per second. This can therefore result in a musical sound with a fundamental frequency equal to n hertz. In general, this sound is very low and not very important.

The wheel carries blades which produce wakes rotating at the angular speed of the wheel. These wakes also generate a sound which will be especially intense when they encounter a fixed stator obstacle (for example the nozzle of the casing of a centrifugal fan). Finally, if the stator has a part near the wheel made up of identical sectors (for example, the rectifier blades of a helical fan), certain harmonics of the fundamental sound will be reinforced. In summary, we will have a noise presenting a spectrum of lines: this is the “siren noise”.

In general, siren noise is responsible for a large portion of the overall acoustic power emitted by a fan. Experience shows that a wake becomes dampened after a certain distance and is lost in the general turbulence. It is therefore possible to reduce the siren noise by moving the wake generator away from obstacles they may encounter (volute nozzle, rectifier, etc.).

b) noise due to turbulence

We say that there is turbulence when, at a given point, the speed of the fluid varies in a disorderly manner, around an average value. In themselves, these speed fluctuations do not constitute noise because they are linked to pressure fluctuations. But when in contact with obstacles (walls for example) these speed fluctuations cause pressure fluctuations and produce noise. In principle, such noise has a continuous spectrum of frequencies. But turbulence can excite resonances which then reinforce certain frequencies of the spectrum independent of the speed of rotation of the wheel. In general, turbulence has only a small part in the overall noise produced by the fan.

c) Noise due to gyration

The gyration of a flow in a duct (for example downstream of a axial fan) can produce noise. Here too the noise will be a musical sound of a determined fundamental frequency and the acoustic power will be greater if these separations encounter fixed obstacles.

The fan being a receiving machine which aims to transport a quantity of gas from point 1 to point 2, it very often happens that abrasive dust (silica, cement, metals, wood, etc.) passes through it.

Over time, this dust ends up first wearing out the turbine, where the flow speeds are the highest, and then the static part.

In order to increase the lifespan of the fan, we resort to the use of steels with a high content of chromium carbide or tungsten, which are more resistant to abrasion.

But, there is always a compromise to be made. These steels often have lower mechanical limits (stresses, operating temperature, etc.) than the steels usually used.

You must therefore be very careful in calculating these fans, and each case is particular.

The power absorbed at the fan shaft is calculated by the formula:

Paer = flow * total pressure / η

Where      flow = flow rate passing through the fan ( m³/s)

Pressure = total pressure difference between the discharge and the suction of the fan (in Pascals Pa)

Ƞ = fan efficiency (varies between 0.5 to 0.9)

Take 0.75 to 0.8 as a first approximation.

Example:

flow = 3 m³/s and total pressure = 6500 Pa, with an efficiency η = 70% = 0.7, we obtain:

Paer = 3 * 6500 / 0.7 = 27857 W = 27.86 kW

There are several ways to vary the flow rate of a fan.

– Check valve/adjustment valve

Opening or closing the valve creates a variable pressure loss which has the effect of varying the flow rate.

Disadvantage: the pressure loss created corresponds to significant energy consumption for large fans. We therefore generally use this adjustment mode for low powers.

Non-linear adjustment curve.

Advantage: Cheap – low maintenance

Tilter or “Vane Control”

This is a device placed at the suction of the fan which makes it possible to rotate blades in concentric quarters so as to modify the natural curve of the fan

Disadvantage: Complex mechanics that require regular maintenance.

Advantage: Consumes much less energy than a conventional valve.

Good adjustment precision.

Frequency converter

The frequency converter is an electronic system which allows the motor control frequency to be varied. As the classic asynchronous motor rotates at a speed directly proportional to the frequency, the driven fan also varies speed.

However, the flow rate of a fan is also proportional to its rotation speed. It is therefore easy to adjust the flow rate by changing the frequency of the converter.

Disadvantage: High price

Converter settings

Advantage: Great energy savings. For medium to high power fans, payback can be rapid.

Does not require any special maintenance.

You must first know the density of the gas under Normal CN Conditions (1013 mbar, and 0°c)

Ex :

air: 1.293 kg/Nm³

CO2: 1.96 kg/Nm³

The density in the CN is equal to the molar mass of the gas divided by 22.4.

The density ρ is calculated according to the formula:

ρ= ρCN * (273/(273+t)) * ((101325+p)/101325)< >Or:

t = temperature in °c

P = gas pressure in Pa

ρCN = density of the gas at normal conditions in kg/Nm³