Definition

  •  A mechanical fan is a machine used to create flow within a fluid, typically a gas such as air.
  • The fan consists of a rotating arrangement of vanes or blades which act on the fluid.
  • Most fans are powered by electric motors, but other sources of power may be used, including hydraulic motors and internal combustion engines.
  • Fans produce flows with high volume and low pressure (although higher than ambient pressure), as opposed to compressors which produce high pressures at a comparatively low volume.

Mechanical fans

Function / Working Principle 

  •  A device which uses the power of a motor to spin a propeller or  impeller to blow air.
  • The cooling fan circulates air through a cooling coil, temperature difference result in a decrease of circulating air temperature.
  • This cold air then enters the refrigeration space to produce cooling.
  • With the introduction of fan clutches and electric cooling fans,   fans have become more efficient  by operating only when they need to.
  • Electric cooling fans also improve the operation of the air conditioning system.
  • The use of cooling fans and evaporative cooling are the most advanced and a move from the traditional ways of maintaining good and fresh air around in rooms and different places.
  • Fans are also responsible for removing the latent heat of vaporization from the refrigerant in a condenser to outside atmosphere.

Mechanical fans

Types and classification 

Fans can be classified under three general types :

(a) Centrifugal fans

(b) Axial fans

(c) Cross flow fans

Mechanical fans

Centrifugal Fans

Centrifugal fans increase the speed of an air stream with a rotating impeller. The speed increases as the air reaches the ends of the blades and is then converted to pressure. These fans are able to produce high pressures, which makes them suitable for harsh operating conditions, such as systems with high temperatures, moist or dirty air streams, and material handling.

Centrifugal fans are categorized by their blade shapes as following :

Radial fans, with flat blades 

Mechanical fans

Advantages:
  1. Suitable for high static pressures (up to 1400 mmWC) and high temperatures.
  2. High durability , Efficiencies up to 75%
  3. Can operate at low air flows without vibration problems.
  4. Have large running clearances, which is useful for airborne-solids (dust, wood chips and metal scraps) handling services.
  5. Simple design allows custom build units for special applications.
Disadvantage
  • Only suitable for low-medium airflow rates
Forward curved fans, with forward curved blades

Mechanical fans

Advantages :
  1. Can move large air volumes against relatively low pressure.
  2. Relative small size.
  3. Low noise level (due to low speed) and well suited for residential heating, ventilation, and air conditioning (HVAC) applications.
Disadvantages :
  1. Only suitable for clean service applications but not for high pressure and harsh services.
  2. Fan output is difficult to adjust accurately.
  3. Driver must be selected carefully to avoid motor overload because power curve increases steadily with airflow.
  4. Relatively low energy efficiency (55-65%)
Backward inclined fan, with blades that tilt away from the direction of rotation

Mechanical fans

Advantages:
  1. Can operate with changing static pressure (as this does not overload the motor).
  2. Suitable when system behavior at high air flow is uncertain.
  3. Suitable for forced-draft services.
  4. Flat bladed fans are more robust.
  5. Curved blades fans are more efficient (exceeding 85%).
  6. Thin air-foil blades fans are most efficient.
Disadvantages:
  1. Not suitable for dirty air streams (as fan shape promotes accumulation of dust).
  2. Airfoil blades fans are less stable because of staff as they rely on the lift created by each blade.
  3. Thin airfoil blades fans subject to erosion.
Axial-flow fans

Axial-flow fans have blades that force air to move parallel to the shaft about which the blades rotate. Axial fans blow air along the axis of the fan, linearly, hence their name. This type of fan is used in a wide variety of applications, ranging from small cooling fans for electronics to the giant fans used in wind tunnels. Axial flow fans are applied for air conditioning and industrial process applications. Standard axial flow fans have diameters from 300–400 mm or 1800 to 2000 mm and work under pressures up to 800 Pa.

Mechanical fans

Direct Drive
Advantages:
1. Low Power consumption.
2. Less Maintenance.

Limitation:
1. Air flow cannot be varied.
2. Cannot work in high temperature application.
3. Since the motor is in direct contact with the air stream, it cannot work in very corrosive areas.

Indirect Drive
Advantages
1. Air flow and pressure can be varied.
2. These types of fans can work in high temperature applications.
3. Very suitable for flammable or corrosive air.

Limitations:
1. These fans are more expensive than the direct driven fans.
2. Maintenance cost is high.
3. Power consumption is more.

Cross-flow fan

CFF is used extensively in the HVAC industry. The fan is usually long in relation to the diameter.

The flow within a cross-flow fan may be broken up into three distinct regions: a vortex region near the fan discharge, called an eccentric vortex, the through-flow region, and a paddling region directly opposite. Both the vortex and paddling regions are dissipative, and as a result, only a portion of the impeller imparts usable work on the flow. The cross-flow fan, or transverse fan, is thus a two-stage partial admission machine.

The popularity of the cross flow fan in the HVAC industry comes from its compactness, shape, quiet operation, and ability to provide high pressure coefficient.

Mechanical fans

 

Advantages:

  1. CFF has higher overall wind, air pressure.
  2. Reduce the resonance generated by the impeller rotation and the noise.
  3. Better being able to control the axis and diameter run-out.

Disadvantages:

  1. The manufacturing process is relatively complex.
  2. The industry-specific applications have an impact on air flow (air flow with the impeller will tilt to the flow).
  3. The relative motor load increased (need to re-adjust the motor parameters)

Applications of Mechanical Fans

  1. Climate control (HVAC)
  2. Personal thermal comfort (e.g., an electric table or floor fan)
  3. Vehicle and machinery cooling systems
  4. Ventilation
  5. Fume extraction
  6. Removing dust (e.g. in a vacuum cleaner)
  7. winnowing (e.g. separating chaff of cereal grains)
  8. Drying (usually in combination with heat) and to provide draft for a fire
  9. Fans are often used to cool people, they do not actually cool air (if anything, electric fans warm it slightly due to the warming of their motors), but work by evaporative cooling of sweat and increased heat convection into the surrounding air due to the airflow from the fans. Thus, fans may become ineffective at cooling the body if the surrounding air is near body temperature and contains high humidity.

Assessment of Mechanical Fans 

Fan efficiency / performance:
  • Fan efficiency is the ratio between the power transferred to the air stream and the power delivered by the motor to the fan.
  • The power of the airflow is the product of the pressure and the flow, corrected for unit consistency.
  • The fan efficiency depends on the type of fan and impeller. As the flow rate increases, the efficiency increases to certain height (“peak efficiency”) and then decreases with further increasing flow rate.

Mechanical fans

Methodology of fan performance assessment

1.calculate the gas density :

The first step is to calculate the air or gas density using the following equation:

gas density (γ)= (273*1.293)/(273+t ºC)

Where, t ºC = Temperature of air or gas at site condition.

2.measure the air velocity and calculate average air velocity:

The air velocity can be measured with a pitot tube and a manometer. The total pressure is measured using the inner tube of pitot tube and static pressure is measured using the outer tube of pitot tube. When the inner and outer tube ends are connected to a manometer, we get the velocity pressure (i.e. the difference between total pressure and static pressure).

Mechanical fans

Cp = Pitot tube constant, 0.85 (or) as given by the manufacturer

∆p = Average differential pressure measured by pitot tube by taking   measurement at number of points over the entire cross section of   the   duct.

γ= Density of air or gas at test condition

3. calculate the volumetric flow :

  • Take the duct diameter (or the circumference from which the diameter can be estimated).
  • Calculate the volume of air/gas in the duct by following relation.

Mechanical fans

4. measure the power of the drive motor:

The power of the drive motor (kW) can be measured by a load analyzer. This kW multiplied by motor efficiency gives the shaft power to the fan.

5. calculate the fan efficiency :

Now the fan’s mechanical and static efficiencies can be calculated as follows:

Mechanical fans

 

Static efficiency, which is the same except that the outlet velocity pressure is not added to the fan static pressure:

Mechanical fans

Performance charts 

Manufacturers typically publish catalogs containing performance tables for each specific fan size. These tables are printed in a compact format, showing only the minimum information necessary to select a fan to match a desired performance. Performance tables are very easy to use for making an initial selection, and in most cases, only include stable operating points.

Find the required CFM along the left vertical axis (example 14,000 CFM), then move horizontally to the right to the required static pressure column (example 6.00 SP). At this intersection, you can read both the fan RPM and the BHP (example 1277 RPM and 16.8 BHP).

Mechanical fans

This format provides a quick snapshot of the total capabilities of one given fan model and size. Locate the desired flow along the x-axis and the specified pressure on the left y-axis. At the point of intersection, you can determine the approximate Fan RPM required. To find the motor size required, move upward to the closest HP line (dotted line). You can quickly review charts for several different fan sizes to determine the most desirable selection.

Mechanical fans

System resistance

The term “system resistance” is used when referring to the static pressure. The system resistance is the sum of static pressure losses in the system. The system resistance is a function of the configuration of ducts, pickups, elbows and the pressure drops across Equipment.

  • The system resistance varies with the square of the volume of air flowing through the system.
  • To determine what volume the fan will produce, it is therefore necessary to know the system resistance characteristics.
  • Long narrow ducts with many bends and twists will require more energy to pull the air through them. Consequently, for a given fan speed, the fan will be able to pull less air through this system than through a short system with no elbows.

Thus, the system resistance increases substantially as the volume of air flowing through the system increases; square of air flow.

Typically a system resistance curve is generated with for various flow rates on the x-axis and the associated resistance on the y-axis.

Mechanical fans

References

  1. Bureau of Energy Efficiency (BEE), Government of India. Energy Efficiency Guide Book, chapter 5, p 93-112. 2004
  2. Canadian Blower. Industrial Fans and Blowers, www.canadianblower.com/blowers/index.html
  3. Fan Air Company, product presentation. www.fanair.com/products.pdf
  4. Ganasean, Indian Institute of Technology. Fans, Pumps and Compressors
  5. http://www.greenheck.com/library/articles/10
  6. http://www.extension.umn.edu/distribution/cropsystems/DC5716.html
  7. US Department of Energy (US DOE), Energy Efficiency and Renewable Energy, 1989.
  8. Improving Fan System Performance – a source book for industry  www1.eere.energy.gov/industry/bestpractices/pdfs/fan_sourcebook.pdf

 

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