Guide to Industrial Fan Selection

Types of Industrial Fan:

The most common types of Industrial Fan are as follows:

Axial Fan Type

The axial fan has various blade shapes including Aerofoil, Sickle, Paddle, and Variable pitch. Axial fans are used for relatively high flow rates and low pressures with flow parallel to the axis of fan. They also have lower power input.

Axial fans are generally selected for simple extraction or cooling applications with very low system resistance, such as moving air from one large space to another (i.e. from factory to outside), desk fans and condenser cooling in refrigeration.

Tangential Fan Type

These barrel-shaped fans have curved forward blades that are generally many times longer than any given diameter. Tangential fans tend to scoop the air up and deliver it at relatively low pressure over a longer length, ideal for cabinet cooling or process air coverage.

The main advantage of tangential over axial fans is that they provide extremely smooth laminar airflow over the whole frame width. This makes them suitable for applications such as air conditioning systems, air curtains, computer systems, instrument racks, convector fans, ovens, de-humidifiers and ventilators.

Centrifugal Fan Type

Typical impeller blade construction types include Paddle, Radial, Backward Curved and Forward Curved. Characteristics are low flow rates and high pressures with flow perpendicular to blower axis. Air enters around the centre of the fan and exits around the outside.

Centrifugal fans with Paddle impellers are used when debris will pass through the system because this configuration prevents any clogging.

Fans with backward curved blades produce less air volume than Axial units, but generate considerably more pressure and are the least hungry for power in the centrifugal range. They can also be produced as multistage units to give even higher pressures.

By comparison, the Centrifugal Fans with Forward curved blades develop the best airflow and pressure requirement in size by size comparison, although they will require extra motor power.

Typical applications for centrifugal fans include air handling units, process heating and cooling, electronic cooling and boiler combustion air.

Mixed Flow Fan Type

These combine the features of Axial and Centrifugal Fans by employing the air movement techniques of both. Air enters the inlet side of the fans and exhausts in an axial route from the outlet side. The pressure development of the mixed flow is greater than axial and more akin to the Centrifugal.

Factors For Consideration When Selecting a Fan

Knowing if your application requires high volumes of airflow but low pressure or high pressure and low volumes is just one of the questions that need to be answered in order to determine the correct industrial fan selection.

It is important to also take into consideration factors such as:

• Density of the air to be moved.
• Air temperature.
• Humidity.

When determining the pressure required it is important to consider system restrictions. For example, additional pressure may be required to achieve flow across a filter, or to compensate a pressure drop due to length, size and complexity of ducting.

All ducting will have an effect on the air passing through a system and this will nearly always take the form of a restriction to the flow and therefore have an influence on pressure requirements.

At the earliest stage in the design process the ducting should be carefully evaluated and made as efficient as possible. This will reflect beneficially on the final design in the overall cost, the compactness of the fan and the running costs.

In reality, there are many factors that impact upon fan specification and applications that demand particular air movement characteristics for cooling, exhausting, aerating, ventilating and drying should undergo a ‘system resistance’ assessment. Only by evaluating all the impacting factors (flow resistance, power units, size, space envelope) is it possible to ensure maximum efficiency and reliability of both the fan and the equipment involved.

Fan Selection Checklist

To help ensure that all aspects of fan sizing have been considered the following checklist may be useful:

Volume Flow Rate:

  Velocity Required.

  Does this take into account the worst case open area?

  Density of the air being moved (temperature or different gas).

Pressure:

  Pressure required to generate desired flow through open area.

  Pressure drop across filter.

  Pressure drop due to ducting.

Other Parameters:

  Are there any special hazards (e.g. flammable gas, corrosion, agents etc)?

  Noise level restrictions.

  Inlet and discharge configuration.

  Electrical supply – is speed control required?

  What are the physical size constraints?

  Even flow across aperture – if a large area then poor distribution could occur.

  Dwell time – is the velocity across the filter giving desired dwell times.

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