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Fans

General

For the purpose of air-gas mix movement different kinds of gas blowers are used. Provisionally they may be split into two groups based on the pressure ratio of conveyed medium. The first group includes machinery of low pressurization capacity - fans and gas blowers. The second group covers machinery with high pressure boosting capability like compressors.

Let’s review air-gas mix conveying machinery in more detail taking fans as an example.

Fans are machines used to move different air-gas mixes by boosting their pressure up to a maximum of 12-15 kPa. Fans are characterized by a simple single-stage design and low peripheral velocity of shaft rotation. Fans are composed of a housing, impeller with blades installed on a shaft inside the housing and a drive. Fans are driven by electric motors.

Fans are widely used both for household and industrial purposes. Industrial fans have to meet certain requirements in view of more severe operating conditions. Besides being compliant with respective process parameters, industrial fans need to be consistent with high standards of structural reliability and safety.

Fans are used for the conveyance of different air-gas mixes, which may be characterized by critical temperatures, abrasive properties, dust and moisture content. That is why proper material design is a significant criterion in fan manufacture.

Operating principle

In general, operating principle of fans may be described as follows:

In the process of fan operation the working medium is transferred with a certain initial pressure and flow velocity via inlet to the impeller inside the housing. The impeller is fixed on the shaft with a hub. It is actuated by a drive. Impeller rotation generates underpressure facilitating the suction of air-gas mix. The working medium passes through the impeller, which transfers the drive energy thereto, and exits via the outlet. The working media exits at a higher pressure and flow rate due to the energy transmitted by the impeller.

Technical specification. Capacity, pressure and power

Industrial machines designated for moving different kinds of liquids and air-gas mixes are similar in their design. For this reason their key operating conditions are identical.

Depending on the scope of application and operational conditions one can select from a wide range of fans depending on key technical specifications, such as:

1. Capacity (Q) determines the volume of air-gas mix moved per unit of time. Fan capacity may vary from 1 to 1,000,000 m3/s. It is calculated as follows:

Q = V/t  [m3/s]

where:
V – working medium volume [m3];
t – time.

2. Pressure is the quantity of energy transferred to the air-gas mix passing through the fan. Fan pressure is expressed using pressure units. Full pressure created by the fan is composed of static and dynamic elements:

Рf = Рst + Рdyn

where:
Рf – full pressure [Pa];
Рst – static pressure [Pa];
Рdyn – dynamic pressure (Рd = ρω2/2) [Pa];
ω – average velocity of meduim [m/s];
ρ – medium density [kg/m3].

3. Power characterizes the quantity of energy required for the movement of working medium. It is divided into input and useful power. Input power is the energy transferred from drive to the fan, whereas useful power is real energy consumed for working media movement. Input power is higher than useful one, which is explained by different losses for energy transfer.

Fan power is derived from the following equation:

N = (Q·P)/(1000·ŋ) [kW]

where:
Q – fan capacity [m3/s];
Р – pressure generated by the fan [Pa];
ŋ – fan efficiency.

4. Besides the above key process specifications of fans the following secondary indicators are of importance: climatic category, admissible noise in operation, overall dimensions, corrosion resistance, etc. These characteristics are significant for selecting a fan.

Types and classification of industrial fans

Fans are generally classified depending on the flow direction of working medium.  Respectively, there are two most common types of industrial fans:

  • axial;
  • radial (centrifugal).

In axial fans, as the name implies, the flow is moving along the axis or shaft of the fan.

In radial fans the gas flows over the blades from the wheel centre to the edge due to centrifugal force generated by rotation, then exits via spiral casing through the discharge line.

Radial fans






Radial fans are robust, capable of generating relatively high pressures with high efficiency and are suitable for heavy-duty operation.

Radial fans consist of a spiral casing, shaft, impeller with blades and a drive. The fan is installed on a mounting frame (base).






The spiral casing of a fan is usually manufactured from steel sheets welded or riveted together. In case of high-pressure application fan casing is a solid casting. In order to impart rigidity to the spiral casing made of steel sheets  reinforcement with transverse strips and ribs is used. For noise abatement purposes during operation the casing is covered with special noise suppression panels or enclosed.






The main operating element of a radial fan is an impeller, which rotates and moves the working medium. It normally consists of rear and front disks, a hub and blades. Depending on the operating conditions there are several impeller modifications:

  • diskless impellers are used to convey solid-containing media;
  • single disk impellers are mostly used for media with solid impurities;
  • double-disk (drum, ring, with conical front disk) impellers are good for clean working media within a wide pressure range;
  • triple-disk wheels are designed for double-suction fans.

Hubs are required to mount impeller on the shaft. They are cast or tailored from blanks.

Blades are an integral part of the impeller. They are mounted on the disk and hub. Blade fastening practice depends directly on the required strength and rigidity of structure, as well as commercial viability. The most reliable way is welding, which is efficient when all the wheel components have the same service life. If operating conditions cause blades to wear faster than disks, riveting or pin connections are used. Blade shape determines fan efficiency and performance.

Type of blades installed on impeller:

  • forward-curved;
  • radial;
  • radial forward-curved;
  • straight  backward-inclined;
  • backward-curved;
  • backward-curved airfoil-shaped.





An important factor contributing to a fan’s efficiency is the gap between impeller and inlet pipe. It should not exceed 1% of impeller diameter.

Fan drive may have the following arrangement:

  • direct coupling between impeller and electric motor;
  • flexible coupling;
  • V-belt drive.

For radial fans several composition patterns of impeller fastening and drive coupling are used.

In case of large fans flexible or belt coupling is recommended. The most popular type is outboard coupling of impeller shaft to the drive, i.e. connection of impeller shaft installed into support bearing assembly mounted outside the casing. The advantages of this arrangement are lack of mechanical losses during transmission and compactness, while they are limited in impeller size. Impeller shaft installation between two support bearings is more reliable and provides for stable operation of a fan. The drawback of this configuration is that it is difficult to mount the fan onto the air pipeline due to complicated design. Outboard coupling is not used for double-inlet fans.

Classification of radial fans

Radial fans are mainly classified by the following operational and design features:

Pressure:

  • low-pressure (up to 1,000 Pa);
  • medium pressure (from 1,000 to 3,000 Pa);
  • high pressure (above 3,000 Pa).

Number of suction ends:

  • single inlet;
  • double inlet.

Direction of rotation (viewed from drive side):

  • right rotation – clockwise impeller rotation;
  • left rotation – counter-clockwise impeller rotation;

Outlet position:

The outlet of a general application fan may have seven different positions, each one offset by 45 degrees from the previous one. The position where outlet is to be installed at 225 degrees is not used in view of difficult pipeline connection.

Spatial orientation of outlets of special application fans may be positioned at every 15 degrees in the range of 0 to 345 degrees (for mill fans) and 0 to 255 degrees (for blower fans).

Depending on media properties radial fans are divided as follows according to their application:

  • general;
  • special.

General application fans are used to convey non-aggressive air-gas mixes, without any solids and dust, at not more than 200о С. These include supply and exhaust ventilation (roof) fans.

Besides a lot of special fans are produced for industrial application. They are designated to move different air-gas mixes characterized by high operating temperatures, abrasive and corrosive properties, solids content, high explosive hazard, etc.  These include:

  • corrosion resistant fans;
  • dust fans;
  • explosion-proof fans;
  • blow fans;
  • mine fans;
  • mill fans.

For each type of fans its material design is selected in view of its operating conditions to provide for reliable, fail-safe operation under normal conditions.

Thus, for a flow part of corrosion-resistant fans, stainless steels, titanium and its alloys are used, different kinds of polymeric materials gain popularity.

Because of high content of solids in the conveyed media fan parts and units are characterized by high-reliability. For this reason they are manufactured from abrasion-resistant materials.

Explosion-proof fans are made of soft materials (aluminium and its alloys) to avoid sparking at impact and friction of moving parts.

Blower fans are designated for the movement of air-gas mixes

at high temperatures, that is why different heat-resistant steels are used for their manufacture.

Axial fans

Axial fans are characterized by simple design and small size. They are often used where application of radial fans is impossible due to limited space. Axial fans consist of a cylindrical casing, impeller with blades and a drive.

Axial fan casing has the shape of a cylinder. Internal diameter should be sufficient for free rotation of the impeller. At the same time maximum distance between the casing wall and impeller blades should not exceed 1.5 % of a blade length. To improve aerodynamic properties and reduce hydraulic losses, fan design may be modified to incorporate the following additional elements: inlet manifold, nose and outlet cones on impeller hub and outlet diffuser.






Axial fan impeller consists of a hub and blades. Blade fastening to the hub is identical to that of a radial fan impeller. Number of blades varies from 2 to 16. Welding, casting (molding) or forging processes are applied for the manufacture of an axial fan impeller.






Impeller blades are installed at different angles to the rotation plane, which provides for efficient control of air and gas feeding. Axial fans may change direction of flow by reversing the direction of impeller rotation. This is done by using reversible (bi-directional) impellers with adjustable blade angle or just by turning the non-reversible ones. Due to the axial fan design the installation is simple.






Axial fans are driven via direct connection with motor shaft, a coupling or a belt transmission. They are mostly driven by electric motors. Drive connection configuration depends on operating conditions and air/gas properties. In case of clean non-aggressive media electric motor is installed inside the flow. If the media contain moisture or solids, the drive is taken outside the flow.

Classification of axial fans

There are three major types of axial fans:

  • blade-type;
  • blade-type in a cylindrical casing;
  • with guide vanes.

Blade-type fan is the simplest version of an axial fan. It consists of an impeller installed on electric motor shaft without a casing. These fans usually operate at low rotation frequencies and moderate temperatures. They are characterised by high capacity and low values of generated pressure. Blade fans are often used indoors as induced draught fans. For outdoor applications they are included into air cooling and cooling tower systems. Their efficiency is about 50% or less.

The second type of fans is designed as an impeller installed inside a cylindrical casing. Impeller rotation frequency is higher that that of the blade-type fan, which results in higher exit pressure of 250 - 400 Pa. Their efficiency can be as high as 65%.

Axial fans with guide vanes are similar to the prevous type in their design, but they are equipped additionally with guide vanes at the inlet. This solution improves efficiency due to streamlining and guiding the flow. As a result they can achieve high pressures up to 500 Pa at the exit. This type complies with high energy efficiency standards.

Scope of application

Fans are one of the most popular types of machinery used for industrial and household applications. They are designated for air/gas movement mostly for supply-and-exhaust ventilation. But besides ventilation there are plenty of areas and processes, where they can be used, such as:

  • chemical industry (drying and feeding of process gases);
  • metallurgy;
  • cooling systems;
  • mechanical engineering and shipbuilding (aerodynamic stand testing);
  • agriculture;
  • power;
  • construction;
  • de-dusting systems;
  • pneumatic transport.

Radial vs axial fans

Axial and radial fans are based on different operating principles. In an axial fan the flow moves from inlet to outlet pipe along the shaft axis, while in a radial one the flow starts from inlet along the shaft axis, then, changing direction, moves towards the outlet perpendicular to the axis.

Radial fans are mostly utilized in industrial processes in view of a large variety of modifications and scopes of application. They operate in a wide range of capacities and generated pressures. However radial fans are massive and require a lot of space for installation.

Axial fans are notable for their simple design, small size, cost-efficiency and ability to move gases over short distances. Axial fan drive is often installed inside the casing, which is a limiting factor for some media in terms of dust content and allowable temperature. Axial fans are characterized by higher impeller rotation velocity than radial fans. Thus, their noise generation is higher as well.

Examples of problem solving for the calculation and selection of fans

Problem 1. Fan size calculation

Conditions:

A fan is available, which is used for indoor ventilation and is capable to boost pressure Pmax up to 70 Pa maximum. Room air enters a fixed-diameter pipe for which it is assumed that the pressure increases by 7 Pa every meter. The fan was connected to the suction and discharge lines of unknown lengths, after which the measurements have shown underpressure Ps at the fan inlet equal to 32 Pa and overpressure Pd equal to 24 Pa - at the outlet. Measured air velocity ω inside the line was 3 m/s. For measurement purposes air density ρ is assumed to be 1.2 kg/m3.






Problem:

Calculate maximum possible extension of discharge line.

Solution:

Let’s consider fan pressure calculation formula:

P = (Pd+(ωd2∙ρ)/2) – (Ps+(ωs2∙ρ)/2)

where ωs and ωd are air velocities in suction and discharge lines. Since line diameter is constant, i.e. ωs = ωd, the formula may be represented as follows:

P = Pd - Ps = 24 - (-32) = 56 Pa

This implies that the fan has a pressure reserve of 70-56 = 14 Pa under these operating conditions.

An extension of discharge line will result in higher internal resistance, which will involve higher fan head. Consequently, it is possible to calculate the maximum limit of discharge line resistance, until the fan reaches its head limit:

14/7 = 2 m

So, we can deduce, that the discharge line may be extended by 2 meters maximum.

Problem 2. Calculation of fan capacity and pressure






Conditions:

Air is exhausted from a room with atmospheric pressure of P1 = 0.1 mPa via a uniform diameter pipeline d = 500 mm into the atmosphere P2 = 0.1 mPa. Fan flow rate is Q = 2000 m3/h, power consumption - N = 1.1 kW,while shaft rotation speed n is 1000 rpm. The measurements have shown that pressure drop in the suction line is Ps = 60 Pa, in the discharge line – Pd = 80 Pa. For measurement purposes air density ρ is assumed to be 1.2 kg/m3.

Problem:

Calculate fan-generated pressure, as well as fan capacity as a function of shaft rotation increase up to nn = 1200 rpm with respective change in power.

Solution:

Pipe cross-section area is equal to:

F = (π∙d2) / 4 = (3.14∙0.52) / 4 = 0.2 m2

In order to calculate fan pressure, first we need to find air velocity in the line, which will be the same for discharge and suction lines, since their diameters are equal. Air velocity can be determined from flow rate equation:

Q = F∙ω

from which:

ω = Q / F = 2000 / (3600∙0.2) = 2.8 m/s

After the velocity is found, we can determine fan pressure:

P = (P2-P1) + (Ps+Pd) + (ω2∙ρ)/2 = (105-105) + (60+80) + (2.82∙1.2)/2 = 145 Pa

Consumption rate at increased rpm is determined as follows:

Qn/Q = nn/n

from which:

Qn = Q∙nn/n = 2000∙1200/1000 = 2400 m3/h

To determine power at a new rpm value, we can use another equation:

Nn/N = (nn/n)³

from which:

Nn = N∙(nn/n)³ = 1.1∙(1200/1000)³ = 1.9 kW

The result is that fan pressure is 145 Pa, in case of an increase in rotation velocity up to 1200 rpm, the flow rate will go up to 2400 m3/h, power – to 1.9 kW.

Problem 3. Fan efficiency calculation






Conditions:

Air is sucked from the room via suction pipe ds = 200 mm and is exhausted by the fan into the atmosphere via discharge line dd = 240 mm. Only readings from sensors installed directly on the fan are available. Vacuum gage at the fan entry shows Ps = 200 Pa, while pressure gage at the fan exist shows overpressure of Pd = 320 Pa. Flow meter of evacuated air shows Q = 500 m3/h. Power consumed by the fan, N, is 0.08 kW, shaft rotation velocity, n, is equal to 1000 rpm. For measurement purposes air density ρ is assumed to be 1.2 kg/m3.

Problem:

Calculate fan efficiency and generated pressure.

Solution:

First let’s determine air velocity in the suction and discharge lines. Let’s express and find velocity, ω, from volumetric flow rate equation:

Q = f∙ω

where f = (π∙d2)/4 – pipe cross-section area. From here:

ω = Q/f = (Q∙4)/(π∙d2)

ωs = Q/f = (Q∙4)/(π∙ds2) = (500∙4)/(3600∙3.14∙0.22) = 4.4 m/s

ωd = Q/f = (Q∙4)/(π∙dd2) = (500∙4)/(3600∙3.14∙0.242) = 3.1 m/s

Now that we now air velocity in the discharge and suction lines, as well as inlet and outlet pressure of the fan, we can find fan pressure, P, as follows:

P = (Pd+(ωd2∙ρ)/2) – (Ps+(ωs2∙ρ)/2) = (320+(3.12∙1.2)/2) – (-200+(4.42∙1.2)/2) = 514 Pa

Now we can express fan efficiency η from power formula and determine it:

N = (Q∙P)/(1000∙η)

η = (Q∙P)/(1000∙N) = (500∙514)/(3600∙1000∙0.08) = 0.9

Result: fan efficiency 0.9 and pressure 514 Pa.

Problem 4. Fan pressure calculation






Conditions:

There is a tank for nitrogen storage at overpressure P1 of 540 Pa. Gas is fed into the unit at overpressure P2 of 1000 Pa with the fan connected by a suction line with the storage tank and by a discharge line with the unit. Pressure losses in these lines are Ps = 120 Pa and Pd = 270 Pa respectively. In the discharge line gas flow achieves ω = 10 m/s. For measurement purposes nitrogen density ρ is assumed to be 1.17 kg/m3.

Problem:

Calculate the pressure generated by the fan. 

Solution:

Pressure drop at the suction and discharge points ΔP is:

∆P = P2-P1 = 1000-540 = 460 Pa

Total losses Ptotal in the suction and discharge lines are:

Ptotal = Ps+Pd = 120+270 = 390 Pa

Velocity pressure Pv may be determined as follows:

Pv = (ω2∙ρ)/2 = (102∙1.17)/2 = 59 Pa

Based on the above values we can calculate fan-generated pressure P as follows:

P = ∆P + Ptotal + Pv = 460 + 390 + 59 = 909 Pa

Fan pressure is 909 Pa