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Selection criteria for fans in ventilation devices and installations

published 21 April 2023

If you are faced with choosing the right fan for your device or ventilation system, you may wonder where to start. The careful selection of the appropriate fan involves several criteria. In this publication, we will simplify how to choose the right fan from many solutions.

This article will exclude non-standard designs and applications, constituting a small percentage of selections requiring a more in-depth, individual approach, i.e., applications in explosive environments, environments exposed to the corrosive effects of aggressive chemical compounds, or applications in very high temperatures (e.g., smoke extraction).

Basic classification of fans

Fans are flow devices consisting mainly of a motor and a rotor, which, when set in motion, causes forced airflow. Due to their design, we can distinguish many types of fans. Based on the direction of the airflow pushed by the rotor, they are determined as follows:

Centrifugal fans - the rotor axis of rotation is perpendicular to the direction of the airflow pushed by the fan. We distinguish the two most commonly used types of rotors based on the shape of their blades:

Centrifugal fans with forward curved blades

The forward-curved rotor is characterized by a more significant number of shallow blades facing the direction of rotation. It can be made with either a single air inlet or a wider rotor with air inlets on both sides („double inlet”). The forward-curved rotor generates the same pressure as a backward-curved rotor but at significantly lower speeds. As a result, these fans are relatively low-noise. The forward-curved rotor usually provides a more compact design but requires an additional spiral housing to generate static pressure.

Due to their operating characteristics, forward-curved radial fans are used in devices that require high-pressure and quiet fan operation, such as air curtains, heat recovery units and kitchen hoods.

Backward-curved centrifugal fans - with backward-curved blades

The "backward curved" rotors have a smaller number of longer blades directed opposite the rotation direction. Backward curved rotors require higher rotational speeds than forward curved rotors, but they operate more efficiently thanks to the pressure generation mechanism. The advantage of this design is less susceptibility to motor overload.

Backward fans with an integrated funnel and mounting plate are often used to facilitate and speed up assembly (in the AFL Motors Europe offer, marked as Backward Curved Centrifugal Fan+).

Compact plug fans are used wherever high airflow efficiency and installation resistance are needed. They are ideal for air purifiers, roof fans, heat recovery units, and air handling units. They are used in industrial ventilation applications where the ability to overcome large installation resistances is also required in addition to high efficiency.

Axial fans - the impeller’s rotation axis is parallel to the flow direction of the forced air stream. Fans of this type are often offered in a configuration with a protective grill and a mounting plate with a profiled funnel.

They are used where high airflow, low noise, and low power consumption are required, and the device's design does not impose high static resistance, such as air coolers, chillers, dry coolers, condensers, and heat pumps. They are successfully used in exhaust ventilation applications in industrial facilities or water/gas/electric room heaters. We can proceed to their selection by knowing the classification of fans and their basic features. 

STEP 1 - Characteristic and Operating Point

Ventilators are flow devices, and their basic selection parameters are: 1) air flow often expressed in units of [m3/h] 2) static pressure is often expressed in units of [Pa]. These dependencies are illustrated on curves called fan characteristics.

The other curve is the system curve, which describes the static pressure required by the duct system with all connected components to move a specific volume of air. 

At the point of intersection of both curves, we can determine the operating point of the fan and the system.

Generally, the operating point is determined at the maximum load of the device, and a fan is selected whose characteristic covers this operating point and has a small reserve. When selecting a fan, it should be remembered that the nominal operating point should be as close as possible to the point of maximum efficiency of the fan.

However, during operation, the operating points of the equipment may change: the daily demand for fresh air may vary, and the system’s resistance may also change, for example, due to an increase in the pollution of filters. Due to the number of variables, it is also necessary to ensure that the remaining operating points are within the operating range of the fan, avoiding operating at the edge of its characteristic curve.

In the case of variable operating conditions, it is also essential to adapt the operation of the fan to current requirements. In applications with variable operating conditions, it is advisable to use fan motors that allow for smooth adjustment of their characteristics by changing the rotational speed of the rotor. Additionally, due to regulations, such regulation is mandatory for many ventilation devices.

Step 2 - Selection of fan’s motor type

The following types of motors and their control are commonly used in industrial fans:

-Fans with AC motors - fans with alternating current motors have been widely used for many years. Additional automation elements, such as frequency converters or voltage regulators, must be used to allow for their regulation.

-Fans with energy-efficient EC motors are equipped with electronically commutated, brushless motors with built-in electronics, convenient, efficient, and smooth control with a 0-10V/PWM signal. Their use enables compliance with increasingly higher requirements set by European legal regulations concerning energy efficiency (high efficiency, low power consumption) for:

-motors (Regulation 2019/1781 implementing Directive 2009/125/EC),

-fans themselves (Regulation 327/2011 implementing Directive 2009/125/EC)

-complete ventilation units (Regulation 1253/2014 implementing Directive 2009/125/EC, also called the ecodesign directive).

In addition, there may be national regulations, such as in Poland the Regulation of the Minister of Infrastructure and Construction of April 12, 2002, on the technical conditions that buildings and their location should meet, which specify the maximum allowable unit powers of fans used in ventilation and air conditioning systems.

EC motors are characterized by high efficiency, even at reduced rotational speeds, which saves energy costs associated with devices whose operating parameters are not constant. Fans can operate at reduced performance for a long period. 

STEP 3 - Fan installation

Once we know the operating characteristics of the fan, we can consider other criteria, such as the available space for fan installation. The available space in the device or ventilation system may limit us. The current trend of minimizing the size of ventilation devices may negatively affect the fan's performance. To ensure proper operation, we must provide sufficient physical space to install the fan while maintaining minimum distances from partitions and obstacles. Only then can we ensure its efficient operation without disturbances. Often, such guidelines can be found in the materials provided by fan manufacturers.

STEP 4 - Determining the noise level

In many applications, an important parameter is a noise emitted by the fan. Fans with low sound pressure emissions should be used to avoid excessive noise.

Elements that can reduce the level of emitted sound pressure include using a larger impeller with lower rotational speed, using EC brushless motors with optimized rotor shape, and adding secondary elements such as acoustic dampers or additional insulation.

For example, when selecting a fan for a specific operating point and having sufficient space, it is worth considering a larger impeller, e.g., a 250 mm diameter instead of 200 mm. This way, the larger impeller will operate at a reduced rotational speed to achieve the same operating parameters, lowering the noise level.

STEP 5 - Choosing the suitable model

At this stage, having defined the operating point, available space, and required noise level, we can preliminarily decide which type of fan will be the most suitable for us and choose from many models of axial and centrifugal fans.

Of course, the choice is not easy, as each manufacturer offers hundreds of models in their range. Therefore, please get in touch with our European Sales and Project Support Office for AFL fans if you have any questions or doubts. We will gladly help you select the most appropriate fan model for your application.

We offer a wide range of fans with AC and energy-efficient EC motors, so we will find a fan that meets your expectations.