Mar 072013

A fan is a device used for forced relocation of gases in premises of industrial and domestic nature. А fan is a rotor to which there are fixedly fastened vanes (blades), which upon rotation of the rotor exert pressure upon the air (gas), swirl it and repel it in a certain direction.


The configuration of the blades determines the direction in which the air is cast away. Usually fans are used as key elements in air-conditioning systems, ventilation and heating as well as for blowing cooling radiators of different devices. Fans are used to supply air, to maintain combustion processes, they are also used for ventilation of factory halls, machine rooms, dyeing premises and public buildings, as well as for air curtains – hot or cold. There are fans with left and right rotation. Fans are used for the transportation of flour, sawdust, metal dust, wood fibre, etc. The use of fans began in the late XIX century and in the beginning they were used for the ventilation of mines, in ships holds, for speeding up the drying process, etc.
The main parameters of fans are:

• Flow rate – the volume of air which passes through the fan per unit time. The flow rate is measured in (m3/s).

• full pressure – the increase of air energy which passes through the fan. Full pressure is measured in (Pa).

• static pressure – it is a resultant magnitude formed by the difference between the full and the dynamic pressure and determines the kinetic energy of the air after the fan. It is also measured in (Pa).
• Power – this is the power of the electric motor of the fan and is measured in watts (W).

The flow rate is the main parameter when choosing a fan. The flow rate is calculated by Bernoulli’s formula and it is the volume of a fluid which flows through a defined area per unit time. When choosing a fan we have to consider:
– The type of premises (industrial, commercial, residential) .
– The type of ventilation system that will be used (plenum, exhaust or mixed).
– The necessary flow rate (it is determined based on how many times the air in the room needs to be changed per unit of time).
– The type of the power supply (single-phase or three-phase).
– The noise regulations in and around the site.


Recently more and more households put new aluminum or PVC joinery, thus perfectly insulating their homes, but then there is a problem with moisture and mould, which is a source of toxins. The air quality in residential and work areas is essential. Many of our health problems nowadays are caused by the air in our own homes or workplaces. It has been proved that air quality in well sealed and perfectly-insulated homes is not healthy. In spite of professionals’ advices to regularly ventilate the rooms most Bulgarians don’t do this because of the high costs of heating and electricity. In the home of a family of three, 6-14 kg of water are given off daily through breathing (30-100 g/h from one person), cooking, bathing, laundry, as well as from plants and aquariums. In order to let 10 kg of water out we need to move 3000 m3 of air. This means that an average of 7 times per day there should be an exchange of air in the housing to remove excess moisture. This however leads to heat waste and high bills for heating. As we all know the combination of temperature, humidity and air flow in the room determine whether a person in this room would feel comfortable. The sufficient oxygen in the room and the excess of carbon dioxide is also an important factor.

We all know that when you gather too many people in a small room, after a while the stuffy air reduces their working capacity and causes rapid fatigue. The concentration of radon, other harmful gases, odors and dust increases in the absence of air circulation. So, good ventilation is the permanent solution. All the above problems can be solved when a certain amount of fresh air comes from the outside and takes the stale and polluted air out of the room, and this can be done through properly calculated and situated fans – plenum and exhaust ones. This will significantly improve air quality without losing heat (in winter).

When calculating the flow rate of the fan we need to consider the type of premises and the types of activities in them. Below we have shown the approximate recommended standards of how many times we need to change the air in the room per hour:

   Commercial and public places:                                              Domestic premises:

Bakeries                               20-30 times                           Kitchen                              6 times
Banks                                   3-4  times                             Bathroom                           5 times
Cafes  and bars                   10-15 times                           Laundry room                    10 times
Canteens                              5-10 times                            Basement                          4 times
Cinemas and theatres          5-8 times                              Garage                              4 times
Conference halls                  8-12 times                             WC                                   10 times
Lobbies                                3-5 times
Garages                               6-8 times
Sports halls                          6-12 times
Hairdresser’s                       10-15 times
Laboratories                         8-12 times
WC                                       8-15 times
Offices                                 4-8 times
Libraries                               3-5 times
Recording studios                10-12 times
Restaurants                          6-10 times
Classrooms                          2-4 times

Another indicator is what is the normal amount of air for a person:

During normal activity – 20-25 m3 / h
During normal activity and smoking – 30-35 m3 / h
During light physical work – 45 m3 / h


An example for calculating the flow rate of a fan:

The flow rate of a fan is calculated by the formula:
D = V.p.k (m3 / h)
V is the volume LxWxH, in m3
p – rate frequency of the air exchange, how many times the air in the room needs to be changed per hour.
k – ratio for filling up the room. How much furniture there is in the room. The value of the ratio is chosen in the range between 0.7 to 0.95. The more furniture there is the less is the value of this ratio.

For example, for a bathroom with dimensions 2,40 x 1,40 x 2,50 m , the volume of the room is V = 8,40 m3.
Therefore, the required flow rate of the fan calculated by the above formula is equal to D = V.p.k = 8,4.5.0,9 = 37,80 m3 / h.

The location and the number of fans are essential for the good ventilation. In a larger room, for example, it will be more useful to place several small fans at suitable places rather than one large fan. The fans should be placed in such a way that the fresh air can access any part of the room. Moreover, the volume of the exhausted air should be approximately equal to the blown air. The ventilation devices are placed at the upper part of the room and the plenum ones– at places which provide access to fresh, clean air. Preferably they should be in the diametrically opposite parts of the room. Plenum devices are required in rooms with fireplace, because if you use a ventilation system over the stove in the kitchen, for example, it will create a low pressure area, which can lead to the suction of smoke in the room.

Modern solutions allow automating the process of ventilation of the premises. Automatic control allows fan activation under certain conditions: at a certain humidity in the room, when entering or exiting the bathroom (equipped with presence sensor) or over a period of time. Some fans have a timer plus an integrated fan speed control system.

Types of fans

According to the construction type there are:
• axial fans
• radial fans
• tangential fans
• bladeless fans

In axial fans the air flow is blasted by the fan blades along the axis around which the rotor rotates. This is one of the most common types of fans and is used in the cooling of electronics (computers), consumers (desktop, window), sanitary, turbofan aircraft engines, shaft fans, fans for aerodynamic pipes, etc.











Radial fans draw the air through an inlet located on the axis, inside the rotor. Rotor blades are spiral and push the air in a centrifugal manner. This fan is located in a special spiral casing having the shape of a snail. The inlet air is at right angle to the outlet air. The fan’s blades are curved forwards or backwards. When the blades are curved backwards there is 20% decrease in power consumption and higher tolerance to overloads. When the blades are curved forwards, the fan has smaller size, lower rotational speed and correspondingly lower level of noise. This type of fans is widely used in industry. They are used in aggressive media (dyeing, harmful industries), in hazardous areas, and they have special design.


Tangential fans have the shape of an elongate cylinder, hollow in the middle. The blades are situated on the periphery and are twisted along the axis. The air is swirled and pushed along the entire length of the fan – tangentially. A special diffuser directs the air in the desired direction. These fans create a steady air flow throughout the length of the cylinder. They are quiet, relatively large and have low air pressure. Tangential fans are widely used in air conditioners, air curtains, etc.









Bladeless fans are relatively new products. They have a toroidal shape of the active part, section of an airplane wing and relatively broad and solid basis. These fans have a completely steady air flow. The air is drawn into the cylindrical root of the fan using a technology borrowed from aircraft engines. The air then enters a hollow hoop and then goes out through a hole which is only 1.3 mm wide, having accelerated and entrained the ambient air, creating quite strong airflow. So far, this type of fan seems relatively expensive (about 600 BGN) but hopefully, this will change soon.

According to their purpose and the installation location, fans are divided into:
channel fans – they are placed in an air-conductive channel; they may be radial or axial and are often connected with a ventilation system.


A pipe fan

Ceiling fans – they are placed, as their name suggests, on the ceiling, in combination with an illuminant or independently. They may have a propeller rotating in clockwise direction or counterclockwise. This will change the direction of the airflow.



TEMPO II Fan (100012-13) large


– Roof fans – they are placed on the roof, they have high requirements for moisture and abrasion resistance, they are usually made ​​of galvanized metals, they have the shape of a turbine and a safety frame.

– Multizone fans – they are mounted in a special enclosure which differentiates the space or the room from which the air is drawn. They are indispensable for sites that have only one channel for drawing polluted air and many ventilation areas. They optimize the cost of air-conductors.
We can add portable desktop fans to this classification as well as sanitary (window type) fans, designed for wet rooms. They have protector bars, louvers or very soft blades.
There are, of course, a lot of various applications of fans that are not mentioned here such as car fans, hovercraft fans and others.


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