Fans are an essential component of HVAC designs that play a very significant role on the overall performance of the system. They are devices that use electrical energy in order to drive an impeller which rotates causing air to move in a certain direction.

Depending on the ventilation needs of the conditioned spaces and the overall geometry of the spaces, the inlet and/or outlet of a fan can be connected to air ducts in order to supply or extract air to/from spaces located away from the fans. There are various types of fans available in the market today, each with unique characteristics and applications.

In general, fans can be categorized with respect to the resulting air pattern they generate, ie. axial, centrifugal or mixed flow, or with respect to their construction type, ie box fans, plate fans, ducted fans etc. Regardless of the type, the general operation philosophy is common for all fans. A rotating impeller transfers kinetic energy, and consequently changes the velocity and the pressure of the air causing it to move.

Axial and centrifugal fans

The two main categories are axial and centrifugal fans. In general, in an axial fan the outlet air follows a parallel path to that of the inlet air while air movement to the radial direction is limited. Their main advantage is that axial fans are able to move relatively large quantities of air at a low energy consumption. However, the downside is that axial fans cannot increase the pressure of the airflow significantly. As a consequence, axial fans cannot be connected to long ducting networks as they are not able to force air to travel at long distances.

On the other hand, in centrifugal fans air is drawn axially but is then discharged in the radial direction thus causing a change in direction of approximately 90 degrees. Centrifugal fans have the ability to significantly increase the air pressure at the expense of an increase in electrical power consumption.

Main fan types

Box (or cabinet) fans are a very popular choice for both residential and industrial applications. Their primary characteristic is the silent operation which is a result from the fan’s construction, which usually consists of double acoustically lined panels. Box fans also offer high energy efficiency, easy installation and can also be connected to long ducting networks. Both direct driven and belt driven models are available in the market that can cover a wide range of airflows and pressures.

Inline fans do not have to be placed at the beginning or end of a network of ducts but can be installed at any point in a duct. In general, the resulting airflow pattern produced by inline fans has characteristics of both axial and centrifugal flow. This type of flow is often termed as “mixed flow” and is capable of forcing air to travel for longer distances at a relatively low power consumption. Furthermore, inline fans have the additional advantage of operating while producing very low noise levels in comparison to other fan types. This allows inline fans to be installed inside false ceilings.

Other popular types of fans include among others plate axial fans, ducted axial fans, roof fans and domestic wall fans.

Fans for special applications

Fire rated fans are used primarily for smoke extraction purposes in order to prevent the extensive spread of smoke inside a building in the case of fire. They are made off fire resistant materials that allow them to transfer air at very high temperatures in comparison to non fire rated fans. Fire rated can be classified according to their fire resistance capabilities. In most cases, smoke extract fans are classified as F300 (ability to operate at a maximum temperature of 300oC) or F400 (ability to operate at a maximum temperature of 400oC) according to the European standard EN 12101-3. Smoke extract fans are available in almost all types, axial or centrifugal fans, ducted fans, roof fans, box fans etc

ATEX fans are installed in spaces with a potentially hazardous or explosive environment, such as spaces with gaseous petrochemicals. The acronym stands for ATmospheres Explosible fans. ATEX fans are manufactured according to the European Standard EN 14986. ATEX fans are available in various fan types such as ducted or plate axial fans and centrifugal box fans. Anti-corrosion fans are also available in various fan types, for environments that may contain corrosive vapours.

Jet fans are a very popular solution especially for parking spaces as they can offer efficient pollution control of a specific space without the need for long ducting networks. As a results the overall installations requires a lower installation cost and offers easier maintenance.

Fan motors

Ideally, a fan should vary its speed depending on the demand an that particular time. This significantly decreases the power consumption of the fan and also makes the overall system more quite. The type of speed controller depends on the fan’s motor type. The two main types of fan motor are AC motors and EC motors.

AC motors are used in the majority of fans in all industrial and domestic applications. This is mainly due to their robustness, reliability and their wide power range. AC motors come in various types, for example asynchronous AC motors that do now have a permanent magnet, or the more energy efficient synchronous AC motors which use a permanent magnet.

Electronically Commutated (EC) motors use a permanent magnet as a rotor that which rotates inside a stator. The speed controlling electronics are integrated in the EC motor and it converts the alternating current from the power supply into direct current in order to control the EC motor. The required fan speed is communicated to the EC motor via a digital signal, i.e. Modbus RTU, or an analogue signal, i.e. 0-10 V. The main advantages of EC motors is greater energy efficiency compared to AC motors, a longer service life and a low generated noise while in operation.