The following main parameters need to be considered:
- The flow rate required
- The pressure corresponding to the resistance of the circuit, or pressure drop.
- The type of gas to be carried (composition, density)
- The operating temperature,
- The purpose of the fan (draft, blowing, air for combustion, dilution, circulation, etc.),
- The risks (explosion, temperature, corrosion, abrasion, clogging, etc.),
The pressure drop, usually written ΔP, in an aeraulic circuit corresponds to its resistance. It depends on its length, accidents along the way (elbows, narrowing/widening, connections, etc.).
It varies according to the square of the speed of the flow and is proportional to the density of the flow.
We can write
- ΔP in Pa
- K is the pressure drop factor
- ρ is the density, in kg/m³
- V is the speed of the flow at the place of the accident along the way or in the ducts, in m/s
First of all, we have to know the density of the gas in Normal Conditions NC (1013 mbar, and 0°C)
air : 1.293 kg/Nm³
CO2 : 1.96 kg/Nm³
The density in NC is equal to the molar mass of the gas divided by 22.4.
To calculate the density ρ the following formula is used:
ρ= ρCN * (273/(273+t)) * ((101325+p)/101325)
t = temperature in °C
P = pressure of the gas in Pa
ρCN = density of the gas in normal conditions in kg/Nm³
There are several ways to adjust a fan’s flow rate.
– Valve/Control valve
Opening and closing the valve creates a variable drop in pressure which causes a variation in the flow rate.
Disadvantage : the pressure drop created corresponds to quite a high energy consumption for large fans. Therefore, this adjustment mode is usually used for fans with small power ratings.
Non-linear adjustment curve.
Advantage : Inexpensive – little maintenance required
Splitter or Vane Control
This is a device placed at the extraction side of the fan which swivels the blades into concentric quarters in such a way as to modify the fan’s curve
Disadvantage : Complex mechanics that require regular maintenance.
Advantage : Consumes much less energy than a standard valve.
Can be adjusted precisely.
The frequency converter is an electronic device which allows the frequency of the motor control to be adjusted. As the standard asynchronous motor runs at a speed that is directly proportional to the frequency, the fan that is driven also runs at a varying speed.
A fan’s flow rate is also proportional to its rotational speed. Therefore, it is easy to adjust the flow rate by modifying the frequency of the converter.
Disadvantage : Expensive
Configuration of converter
Advantage : Significant energy savings. With medium and large-sized fans the pay-back can be achieved fast.
Does not require specific maintenance.
Formula used to calculate the power absorbed at the shaft of a fan:
Paer = flow rate * total pressure η
In which flow rate = flow rate passing through fan (m³/s)
Pressure = difference of total pressure between the return and extraction of fan ( in Pascals Pa)
Ƞ = fan performance (varies from between 0.5 and 0.9)
Take 0.75 to 0.8 as first approximation.
flow rate = 3 m³/s and total pressure = 6500 Pa , avec un rendement η = 70 % =0.7, on obtient :
Paer = 3 * 6500 / 0.7 = 27857 W = 27.86 kW
As fans are recipient devices designed to convey a quantity of gas from one place to another, abrasive dust (silica, cement, metals, wood, etc.) very often cross through them.
Over the course of time, this dust first wears out the turbine, where the flow velocity is the highest, and then the static components.
To increase a fan’s life, we use steels with a high chromium carbide content or a high tungsten content which are more resistant to abrasion.
But we always have to compromise. The mechanical properties of these steels are often inferior (constraints, operating temperatures, etc.) to those of steels normally used.
Therefore, fan calculations have to be carried out very carefully and each case is unique.
Causes of imbalance
A recently manufactured turbine may, for various reasons, present an imbalance. The turbine is centred on the rotational axis via its hub. However, there is little chance that the centre of the hub and the centre of the rear disc coincide exactly.
This results in the turbine’s centre of gravity moving away from the rotational axis. Moreover, not all the blades are exactly the same weight and they are not all at exactly the same distance from the axis so this is another reason for the shift in the turbine’s centre of gravity.
Neither the cone nor the disk are perfectly centred on the rotational axis and this can cause both static imbalance and dynamic imbalance.
Moreover, the turbine may be slightly tilted on its axis due to the hub, the disk or even due to the tolerances used to mount the hub on the shaft. This can also result in static imbalance and dynamic imbalance.
It should also be noted that even if the turbine is perfectly balanced when delivered, it may present an imbalance after a certain service time, either because of irregular wear and tear caused by corrosion or abrasion of the turbine, or because of clogging or blockages, even if they are irregular.
Effects of imbalance.
The forces or centrifugal moments created in the various parts of a turbine rotating at high speed cause various types of damage.
The shaking and shocks that they cause lead to increased wear and tear and consequently to a reduced lifespan.
If subjected to constant shaking, the components may break down. At certain critical areas of resonance, the centrifugal forces created cause very strong oscillations. This may result in fatigue fractures and even sudden breaks.
In addition, shaking can have unpleasant consequences in the vicinity of fans. Constant vibrations and noises have a very harmful effect on the human organism, affecting people both physically and mentally.
As centrifugal forces increase in proportion to the square of the speed of rotation, it is very important to have a fan balanced, especially when a high rotational speed is chosen.
There are several causes of noise from fans and they can be grouped into two categories:
- noise caused by aeraulic factors
- noise caused by mechanical factors
Noise caused by aeraulic factors
Generally speaking, this type of noise takes prominence over mechanical noise.
a) Noise caused by the wheel movement
The wheel turns at a rotational speed of n revolutions per second. Therefore, this can result in a musical sound with a fundamental frequency equal to n hertz. Usually, this is a very low-pitched insignificant sound.
The wheel has blades that produce wakes rotating at angular velocity to the wheel. These wakes also generate a sound that is particularly intense when it meets a stationary stator object (the lip of the casing of a centrifugal fan for example).
If, near the wheel, the stator has a part formed from identical sectors (blades of outlet guide vane on a propeller fan for example), some of the harmonics of the fundamental sound will be reinforced.
To sum up, a noise presenting a line spectrum is produced, i.e. the “siren noise”.
Generally speaking, the siren noise accounts for a large proportion of the overall acoustic power emitted by the fan. Experience shows that a wake calms down after a certain course and is lost in the general turbulence. Therefore, the siren noise can be reduced by keeping the generator of wakes at a distance from obstacles that the wakes may encounter (volute tongues, outlet guide vanes, etc.).
b) noise caused by turbulence
We say that turbulence occurs when, at a given point, flow velocity varies in a disorderly manner, around an average value. The actual fluctuations of velocity do not constitute a noise because it is related to pressure fluctuations. But when these velocity fluctuations come in contact with obstacles (walls for example) they generate pressure fluctuations and produce a noise. In principle, such a noise has a continuous spectrum of frequencies.
But turbulence can excite resonances which then reinforce some frequencies in the spectrum, regardless of the rotational speed of the wheel.
Generally speaking, turbulence only accounts for a small proportion of the overall noise produced by a fan.
c) Noise caused by gyration
The gyration of flow in a pipe (downstream from a propeller fan for example) can cause noise.
Once again, the noise will be a musical sound with a specified fundamental frequency and the acoustic power will be more pronounced if these surges encounter fixed obstacles.
d) Noise caused by unstable airflow
In general, unstable flows do not produce much noise.
Unstable flows can lead to resonances linked to the size of the pipe. This resonance can be compared to the sounds produced by some wind instruments (flutes, reeds, etc.). This is the case with the noise caused by a fan’s “surge”.