Guide application of axial and centrifugal fans

By definition a fan is a technical stream machine (aggregate). For this purpose the fan is either integrated as the driving force inside a ventilation plat, is standing or hanging in the room, or extracts the air directly to the outside of the building without passing it through elaborate ventilation shafts. Various kinds of application call for a multitude of different designs: A pedestal fan standing on the ground, as a ceiling fan or mounted into walls, windows and ventilation systems. The main difference is not the look of the fans though. The difference lies is the design, the inner construction of the fan itself.

The type of construction and cover of motor, blades and blade wheel decides about the built-up air pressure and the way the gas takes after passing through the device. The power costs for the devices should be as low as possible for the required sphere of action, yet the pressure should be stable and powerful. Whether the device can be used for a low or high flow rate of the ambient air also depends on the type of device. Currently, the two most common units on the market are axial-flow and centrifugal-flow fans. In this guide we are going to explore the different applications as well as pros and cons of each fan type.

Contents:

• Axial fans – functionality and application
• Centrifugal fans – functionality and application
• Special and hybrid types
• Heat recovery axial and centrifugal
• Conclusion / What to buy when you are unsure

Axial fans – functionaliy and application

The weakness of this type of construction is that the generated airflow is not powerful enough to service industrial and professional environments, e.g. when connected to fume hoods in commercial kitchens or workshops. Due to their suction method (axial-flow) most of these models are also insufficient when used as a centralised ventilation unit in newly built homes, because the airflow would simply lose momentum and die down when travelling through extensive and long shafts. Another disadvantage is the extreme noise development in the higher speed range which is quite bothersome in bedrooms, offices and other common rooms.

The benefits of axial construction are that these fans are easy to install, low-priced and do not take up much space, whilst still providing quite a good airflow rate. Axial-flow fans can be used for all kinds of purposes that do not require an enormous air pressure. For example, they are ideal for direct exhaust ventilation, e.g. as a window fan or mounted into the walls or ceilings where they transport air directly to the outside of the building. These fans operate best in small to medium-sized spaces where they provide a pleasant indoor temperature. They come in a variety of subtle, unobtrusive designs, some of them do not have plastic front covers but instead feature a protective grille in front of the rotating blades. Axial-flow fans are usually used in residential environments, for example in the kitchen or bathroom at home. The majority of centrifugal fans in our range is far too powerful and overdimensioned for this purpose.

There are also axial fans with an enormous blade diameter of more than two metres, for example the Big Smooth Eco made by German producer CasaFan. Fans with a precise design that have been constructed for this specific purpose can even be used to circulate air in big tunnels etc. as long as the airflow stall point has been determined during the planning phase. However, this is quite an unusual setting even for the world-wide ventilation market. Usually, centrifugal fans come into play when you are looking for large-scale application, intensive ventilation in entire buildings or storage depots as well as factories. The following explains how these models work.

Centrifugal fans – functionality and application

We offer a number of roof-mounted fans that exhaust the air in a horizontal and vertical direction with a semi-centrifugal or centrifugal construction.
Considerably smaller versions of centrifugal fans are used for ventilation inside computers and projectors, however they are usually applied in industrial and commercial areas in a much bigger size. Thanks to their impressive buildup of pressure they are mainly used in environments with extreme odour and grease in the air. Furthermore, centrifugal fans operate in a potentially explosive atmosphere that contains chemical particles. These locations include commercial kitchens in a hotel or cafeteria, service stations, storage rooms for dangerous goods, garages, battery and boiler rooms. Even the drying by draining of new buildings, damp basements and particularly tall buildings and halls are equipped with centrifugal fans of the appropriate dimension. The operational noise on high speed are usually lower than the sound of axial-flow fans, so they do not disturb work routine in workshops, laboratories and restaurants.

There are no massive drawbacks when it comes to this technology, but centrifugal fans are much too powerful for providing a simple, fresh breeze at home and their looks are not primarily designed to be visually appealing. Furthermore, you require a lot of space for the installation because the air intake and air exhaust are not on the same side. The elaborate technical construction and size of the models has a price, meaning that centrifugal fans are generally more expensive than axial-flow models. Cleaning, maintenance and frequent replacement of the filters should be done by a professional due to the quite complicated, convoluted build.

Special and hybrid types

Resembling axial type:
Semi-axial flow fans are often applied inside pipes. They directly suck in the air but will blow it out through a pre-determined opening at the side in a curved angle. This construction does not use a centrifugal impeller.

Diagonal-flow fans allow for high pressure buildup even though they are constructed in an axial way due to a conical shape on the side of the outlet. This generates a high volume flow with a steady pressure. The device requires less revolutions per minute to provide the same performance, making it slightly more quiet than conventional axial-flow fans.

Axial-flow fans with a motor on the outside are quite rare and rather expensive. This elaborate design prevents any dead spots where condensation water could accumulated behind the centre of the impeller.

Resembling centrifugal type:
Cylindric tangential or cross-flow fans look quite similar to centrifugal constructions with an elongated impeller. They suck in air through the surface of the impeller and emit it through a small ventilation slot on the other side of the device. Due to their elongated design cross-flow fans are particularly suitable for use inside pedestal tower fans or fan heaters. Usually, the small motor is located on the end of the impeller and does not create high pressure rates. Instead, this construction allow for rather compact, handy appliances.

Inline fans take in and exhaust air in an axial fashion, however before the air is blown out, it is lead through a centrifugal impeller. This allows a high pressure generation despite a straight airflow direction.

Heat recovery with axial and centrifugal appliances

Both axial-flow and centrifugal units are able to lower heating costs and ensure a constant indoor temperature, what makes the difference is the place of installation and the system used for heat recovery.

Axial-flow TDA (Temperature Difference Attenuation) Vortice fans are ceiling fans that were engineered for the purpose of heat recovery. The principle of recovery by means of axial airflows are based on the thermodynamic exchange of cold and warm air by forcing down heat from the ceiling. This recirculates heating energy in the lower levels of living and working environments. TDA models operate only in a counterclockwise direction and are designed for application in industrial environments. Other ceiling fans with a more appealing and homelike appearance are used in private households where they offer heat recovery with a special reversing feature.

Centrifugal fans allow a different kind of heat recovery. Seeing that they compress the warm air intake very powerfully and then redirect it in its concentrated form, they are an ideal choice for use in ventilation plants with integrated heat exchangers (recuperators). Either the air is transported through a cross-flow with the incoming cold outdoor air so that the heated air is passing on its energy to the cold air. Otherwise the air streams are transported through a heat reservoir (regenerators) one after another, where the cold air can warm up before it is lead to the individual room. Both methods require closed ventilation shafts that are laid throughout the building and originate in the centre of the ventilation system. This long distance from the openings in the building envelope to the rooms can only be covered with the enormous drive force of a centrifugal fan. The air pressure created by an axial-flow fan is much lower and insufficient for this purpose.

Careful: Centrifugal fans require an airtight room to generate pressure. Airtight conditions are commonly found in newly-built houses with heat insulation. If the air also infiltrates the building through a leaky building envelope, window joints and small cracks in wooden beams or gaps in tiles as it is often the case in old buildings, the ventilator might not be able to generate the required pressure. That means the whole system will not have the desired effect. A professional will be able to determine the airtightness of the building, for example with a blower door test.
As you can see, the benefits of heat recovery are achieved both with axial-flow and centrifugal fans. An incredible recovery index of about 90% is achieved with central heat recovery and a centrifugal fan. However, this also means you have to put up with a number of structural alterations for ventilation ducts etc.

Conclusion / What to buy when you are unsure

Are you unsure whether a radial or axial model is more suitable for your individual requirements? In industrial plants it is usually the case that only models with a sufficient protection certification, commonly IP54, can be used that comply with the high safety standards. Those fans are mainly produced as radial version.

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We are happy to help you as much as possible, just give us a call or send an email. If you have any questions about the right size of the fan, this guide will give you some more information.