There are several intrinsically different fan types and each offers different performance characteristics. Consequently, if an engineer wants to achieve specific air movement patterns, and has certain control requirements, it is important to understand the different fan types and control options available. Angus Low reports
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 fan shape selection. The most common fan types (all of which are illustrated diagrammatically on this page) are as follows:
Axial Flow - this type 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 low power input, and are generally selected for simple extraction or cooling applications with very low system resistance, such as moving air from one large space to another, desk fans and condenser cooling in refrigeration.
Tangential - 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 long length, and are ideal for cabinet cooling or process air coverage. Power required is relatively low. 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 - typical impeller blade construction types includes paddle, radial, backward curved and forward curved. Characteristics are low flow rates and high pressures with flow perpendicular to blower axis. Air enters around 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, 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 include air handling units, process heating and cooling, electronic cooling and boiler combustion air.
Mixed flow fans - these combine the features of axial and centrifugal units 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 that an axial and is more akin to the centrifugal.
Designers must also take into consideration factors such as density of the air to be moved; air temperature; humidity and the pressure required to achieve flow across a filter or the pressure drop due to 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 terms of overall cost, the compactness of the fan and 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. And remember, systems can also benefit from recent advances in variable speed drive technology, whether it be simple step by step or infinitely variable speed control coupled with feedback facilities offering constant flow against variable resistance conditions.
Angus Low is with Air Control Industries