Most often roof fans constitute elements of mechanical ventilation systems or support natural ventilation. Their function is to discharge the appropriate quantities of contaminated air from rooms in order to keep its quality at the right level.
The fan is a flow device which serves the purpose of transferring air in a specific direction. The whole energy supplied to the delivered gas is included in its kinetic constituent. In other words, the fan accelerates the transferred medium, giving it a specific direction and turn. The energy needed for this purpose is supplied by the rotor coupled with the electric motor.
The roof fan is one of many types of fans available on the market, used in ventilation systems. The fans are used as elements of the exhaust installation in the mechanical ventilation system, where fresh air is supplied through air inlets or supply air openings. In certain systems, the clean air is also supplied by the fan. The air exhaust takes place through the central exhaust fan and the appropriate network of ducts connected with all the ventilated rooms. Another possible solution is the use of many fans and the individual exhaust of air from each room.
Also, the so called hybrid systems, which are a combination of the gravity and mechanical ventilations, are constructed more and more frequently. The operating principle consists in the start-up of the fan which supports the natural air run in the situations of increased demand for fresh air (e.g an increase in the number of persons present in the room, an increase in the heat load or concentration of contamination as a consequence of starting the machines operating in the room).
The main advantage of the roof fans is that no additional room for their installation is required. The maintenance is simple and cheap; the disassembly of the device itself is often unnecessary, as the manufacturers use tilt casings, which facilitates access to the most important components. The maintenance or replacement process itself is not burdensome for working persons as it takes place on the roof of the building. When using this type of equipment, no air intakes and air exhausts have to be used, which reduces the costs of the whole installation. A well designed roof fan can use the existing weather conditions such as wind and temperature for improvement of the conditions of its own operation.
The disadvantages in relation to the gravity ventilation systems are the additional costs incurred by the building administrator related to the consumption of electricity necessary to drive the fans. However, this is minimised by more and more economical motors used by manufacturers, fan constructions combined with photovoltaic cells, from which the electricity necessary to drive the motor is taken, and by efficient rotary speed control owing to the use of voltage inverters.
When selecting a roof fan for a particular application, attention must be paid to a number of important parameters. The major ones include the capacity and the static pressure, the type of air extraction, the casing material and rotor material, the supply method, the generated noise and maximum temperature of the transferred medium. The respective models differ from one another in terms of the combination of these parameters.
Majority of the roof fans includes axial or centrifugal fans. The axial fan is used to deliver the low pressure air, and the gas flowing through the rotor does not change the direction (it flows along the motor’s axis). The centrifugal fan has specially shaped vanes, which distribute the air to the sides and then the air is directed to the outlet. They are suitable in places where high static pressure and high volumetric capacity are required.
The types of roof fans may be divided in terms of the direction of the exhaust and the type of the transferred medium. The fans with horizontal exhaust are used in places where the exhausted air contains no pollution and there is no risk of mixing of this stream with the fresh air supplied to the building (e.g. Bahcivan: BRF, BRCF, BACF). An exception in this type of fans refers to smoke-exhaust fans whose design is special, and their placement on the roof is appropriately planned (eg. DV-ROF-RHT). When using the fans with vertical exhaust, it is necessary to remember that the air from such a fan is exhausted several meters upwards and may be transferred by the wind in the vicinity of windows or air intakes, therefore attention must be paid to keeping the appropriate distances between the respective elements of the system. The design of this type of fans is resistant to precipitation getting inside the ventilation system. Also here, there are versions intended for smoke or hot air extraction, e.g. from kitchen (e.g. DV-ROF-V, BRDV)
The variety of solutions presently available on the market allows the selection of the roof fan for each design, combining the advantages of the respective types of equipment with the requirements set by design engineers.