Built-in fan mounted on a shaft electric machine, must create a pressure sufficient to ensure required consumption cooling medium in the ducts of the machine's ventilation system. Fans are designed taking into account the design features of a specific type of machine.
Below is a simplified method for calculating the built-in fan, based on data from serial machines general purpose. In such machines, centrifugal fans with radial blades are mainly used, Working wheel which changes its flow direction to radial.
The outer diameter of the fan wheel is selected in accordance with the type of ventilation system and machine design. With axial ventilation, the outer diameter of the impeller (Fig. 7.7) is chosen as large as possible.
Rice. 7.7. Fan wheel
Based on the selected outer diameter of the fan, the peripheral speed is determined, m/s:
.
(7.49)
The maximum fan efficiency value approximately corresponds to the mode when the nominal fan pressure 
,Where 
-
the pressure developed by the fan in idle mode, i.e., with the outer diameter holes closed, when the air flow is zero. The nominal flow rate is approximately:
,
Where 
- fan flow rate, m 3 /s, operating in short circuit mode (by analogy with an electrical circuit), i.e. in open space.
From the condition of maximum efficiency it is accepted
.
(7.50)
Section at the fan outlet edge, m2,
,
(7.51)
where 0.42 is the nominal efficiency of the radial fan.
Fan wheel width
,
(7.52)
where 0.92 is a coefficient that takes into account the presence of ventilation blades on the surface of the ventilation grille (surface 
).
Wheel inner diameter 
determined from the condition that the fan operates at the maximum efficiency value, i.e. at 
And 
. Using the equations for the static pressure developed by the fan, Pa, we find the pressure developed by the fan at idling:
,
(7.53)
Where 
=
0.6 for radial blades; 
kg/m 3 - air density.
Knowing the air flow V,
resistance ventilation system
and determining the peripheral speed at the inner edge of the fan:
,
(7.54)
find the inner diameter of the fan wheel, m:
.
(7.55)
In built-in fans the ratio 
lies within 1.2...1.5.
The number of fan blades is:
.
(7.56)
To reduce ventilation noise, it is recommended to select the number of fan blades so that it is an odd number. For exhaust ventilation, numbers depending on the diameter of the fan can also be recommended: when 
mm 
, at 
mm 
, at 
mm 
, at 
mm 
.
For fans of asynchronous motors of the 4A series, it is recommended to select the number of blades according to table. 7.6.
Table 7.6. Number of fan blades
|
Rotation axis height, mm |
Number of blades at |
|
|
|
|
|
The number of fan blades for DC machines is selected approximately:
.
(7.57)
Meaning 
round to the nearest prime number.
After calculating the fan, it is necessary to clarify the results of the ventilation calculation.
To determine the actual air flow 
and pressure 
and build the combined characteristics of the fan and the ventilation tract of the machine. The fan characteristic can be expressed with sufficient accuracy by the equation
Characteristics of the ventilation tract according to (7.50)
.
(7.59)
In Fig. 7.8 shows graphs constructed using equations (7.58) (curve 1 ) and (7.59) (curve 2 ). The coordinate of the intersection point of these characteristics is determined by solving the equations
(7.60)
Rice. 7.8. Fan characteristics
Power consumed by the fan, W,
,
(7.61)
Where 
- energy efficiency of the fan, which can be taken to be approximately
(7.62)
The ventilation calculation of an electric machine during course design is carried out using a simplified method. More detailed calculations of individual types of machine designs are given in Chapter. 9-11.
Comments:
After the air duct network has been designed and calculated, it is time to select the right one for this system. ventilation unit for air supply and treatment. The heart of the ventilation system is the fan, which sets air masses in motion and is designed to provide the necessary flow and pressure in the network. An axial type unit often plays this role. In order for the necessary parameters to be maintained, the axial fan must first be calculated.
An axial fan is used in duct systems to move large masses of air.
General concept of the design of the unit and its purpose
An axial fan is a bladed blower that transfers the mechanical energy of rotation of the impeller blades to the air flow in the form of potential and kinetic energy, and he spends this energy to overcome all resistance in the system. Impeller axis of this type is the axis of the electric motor, it is located in the center of the air flow, and the plane of rotation of the blades is perpendicular to it. The unit moves air along its axis due to blades turned at an angle to the plane of rotation. The impeller and electric motor are mounted on the same shaft and are constantly located inside the air flow. This design has its disadvantages:
- The unit cannot move high-temperature air masses that could damage the electric motor. Recommended maximum temperature is 100°C.
- For the same reason, it is not allowed to use this type of unit for moving aggressive media or gases. The transported air must not contain sticky particles or long fibres.
- Due to its design axial fan cannot develop high pressure and is therefore unsuitable for use in ventilation systems of great complexity and length. Maximum pressure, which can provide modern unit axial type, is within 1000 Pa. However, there are special mine fans whose drive design allows pressure to be developed up to 2000 Pa, but then the maximum productivity is reduced to 18,000 m³/h.
The advantages of these machines are as follows:
- the fan can provide high air flow (up to 65,000 m³/h);
- the electric motor, being in the flow, is successfully cooled;
- the machine does not take up much space, is lightweight and can be installed directly in the channel, which reduces installation costs.
All fans are classified according to standard sizes, indicating the diameter of the machine's impeller. This classification can be seen in Table 1.
Table 1
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Description of calculations of blower machine parameters
The calculation of any type of ventilation unit is carried out according to individual aerodynamic characteristics, and an axial fan is no exception. These are the characteristics:
- Volume flow or productivity.
- Coefficient useful action.
- The power required to drive the unit.
- The actual pressure developed by the unit.
The performance was determined earlier when the ventilation system itself was calculated. The fan must provide it, so the air flow value remains unchanged for calculation. If the air temperature is work area differs from the temperature of the air passing through the fan, then the performance should be recalculated using the formula:
L = Ln x (273 + t) / (273 + tr), where:
- Ln — required productivity, m³/h;
- t is the temperature of the air passing through the fan, °C;
- tr is the air temperature in the working area of the room, °C.
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Power determination
Once the required amount of air is finally determined, you need to find out the power required to create the design pressure at this flow rate. The power on the impeller shaft is calculated using the formula:
NB (kW) = (L x p) / 3600 x 102ɳв x ɳп, here:
- L - unit productivity in m³ per 1 second;
- p—required fan pressure, Pa;
- ɳв is the efficiency value, determined by the aerodynamic characteristic;
- ɳp is the efficiency value of the bearings of the unit, assumed to be 0.95-0.98.
The value of the installed power of the electric motor differs from the power on the shaft; the latter takes into account only the load in operating mode. When starting any electric motor, there is a jump in current strength, and therefore power. This starting peak must be taken into account in the calculation, so the installed power of the electric motor will be:
Ny = K NB, where K is the starting torque safety factor.
The values of safety factors for various shaft powers are shown in Table 2.
table 2
If the unit is installed in a room in which the air temperature can reach +40° C for various reasons, then the Ny parameter should be increased by 10%, and at +50° C the installed power should be 25% higher than the calculated one. Finally, this parameter of the electric motor is taken from the manufacturer’s catalog, choosing the closest larger value to the calculated Ny with the calculation of all reserves. As a rule, the blower is installed before the heat exchanger, which heats the air for further supply to the premises. Then the electric motor will start and operate in cold air, which is more economical in terms of energy consumption.
Blower machines of different sizes can be equipped with electric motors of different power depending on the pressure required to be obtained. Each model of the unit has its own aerodynamic characteristics, which the manufacturing plant reflects in its catalog in graphical form. Efficiency is a variable value for various conditions work, it can finally be determined from the graphical characteristics of the fan, based on the values of performance, flow and installed power calculated earlier.
The main task of calculating and selecting a fan is to meet the movement requirements required quantity air, taking into account the resistance of the air duct network, while achieving the maximum efficiency value of the unit.
Snail fans get their name from the shape of the body, which resembles the shell of this mollusk. Today this type of equipment is used both in industry and in residential construction in ventilation systems. Manufacturers today offer several models of snails for ventilation. But they all work on the same principle - the centrifugal force created by the rotation of the blades on the rotor captures air through the snail-shaped inlet and pushes it through a straight outlet located at 90° in a different plane to the inlet.
General information about centrifugal (radial) fans
Coil fans have a dual designation (marking): VR and VC, that is, radial and centrifugal. The first indicates that the blades of the working part of the equipment are located radially relative to their rotor. The second is the designation of the physical principle of operation of the device, that is, the process of taking and moving air masses occurs due to centrifugal force.
It is centrifugal fans in ventilation systems that have proven themselves to be positive side due to high efficiency of air removal.
Operating principle
As already mentioned, fans of this modification operate based on the action of centrifugal force.
- The blades attached to the rotor of the device rotate at high speed, creating turbulence inside the housing.
- The inlet pressure drops, which causes the suction of nearby air, which rushes inward.
- Under the action of the blades, it is thrown to the periphery of the space, where high pressure is created.
- Under its action, the air flow rushes to the outlet pipe.
This is how all centrifugal models work, which are installed not only in ventilation systems, but also in smoke removal systems. About the latter, it must be said that their body is made from aluminum alloy or steel coated with heat-resistant materials, and are equipped with an explosion-proof electric motor.
Design Features
As already mentioned, the main design feature is the snail. It is also necessary to indicate the shape of the blades. Fans of this brand use three types:
- with straight slope,
- with a backward tilt
- in the form of a wing.
The first position is small fans with high power and productivity. That is, they can create conditions in which other models require a large body. At the same time they work with low level noise. The second position is economical option, which consumes 20% less electricity than other positions. Such fans can easily withstand loads.
As for the design that relates to the electric motor, there are also three positions:
- the rotor is fixed directly to the motor shaft through a coupling and bearings;
- through a belt drive using pulleys;
- The impeller is mounted on the electric motor shaft.
And one more feature is the connection points between the fan and the air ducts of the ventilation system. The inlet pipe has rectangular shape holes, exit round.
Kinds
Types of centrifugal fans of snails are three positions, differing from each other in power. This parameter depends on the rotation speed of the electric motor, and therefore the rotor, as well as on the number of blades in the device design. Here are three types:
- Low pressure volute fans, the parameter of which does not exceed 100 kg/cm². Most often they are used in ventilation systems apartment buildings. Install snails on roofs.
- Medium pressure models – 100-300 kg/cm². Installed in ventilation systems of industrial facilities.
- High pressure variety – 300-1200 kg/cm². These are powerful fan units, which are usually included in the air exhaust system of paint shops, in industries where pneumatic transport is installed, in warehouses with fuels and lubricants and other premises.
There is another division of snail fans - according to their purpose. These are primarily general purpose devices. Then there are three more positions: explosion-proof, heat-resistant and corrosion-resistant.
Restrictions on use
- with sticky suspensions with a concentration of more than 10 mg/m³;
- With fibrous materials in the air;
- with explosive inclusions;
- with corrosive particles;
- and in warehouses where explosives are stored.
In all other cases, snails can be used without restrictions. And one more point regulating the conditions of their operation is temperature regime which must not be violated: from -45C to +45C.
Popular models
In principle, there is no model division of snails. There are certain brands that are produced by all manufacturers. And they are divided mainly according to their intended purpose. For example, a VRP fan, where the letter “P” means that this is a dust model, which is used in ventilation and aspiration systems to remove air with a high concentration of dust. That is, this is a specific model that must be used for its intended purpose. Of course, this device can easily handle ordinary air, but it is more expensive than standard VR or VC, because its design uses thick metal to make the body and blades, hence the higher power of the electric motor.
The same applies to VR DU brand fans, that is, for smoke removal. They are made from more quality materials with installation of an explosion-proof motor. Hence their high price. As for other positions, VR is divided into types that have already been mentioned, and each group has its own models with its own technical characteristics.
How to make it yourself
The question posed by the title of this section can be classified as rhetorical. That is, in principle, you can make a snail with your own hands if you have the skills of a tinsmith or welder. Because the device will have to be assembled from sheet metal. And depending on the power and performance of the device, the metal will have different thicknesses.
Plus, making the blades yourself and attaching them to the rotor properly is difficult. Because the rotor will rotate at enormous speed, and if the balancing of the structure is upset, the fan will be torn apart in the first 20 seconds of operation. Yes, and you need to choose the right electric motor, taking into account the power and rotation speed, plus correctly connect it to the fan rotor. So don’t try to do anything with your own hands - it’s dangerous for your own life.
Creating air flow with high density possible in several ways. One of the most effective is a fan. radial type or "snail". It differs from others not only in shape, but also in its operating principle.
Fan device and design
Sometimes an impeller is not enough for air movement and power unit. In conditions of limited space, a special type of exhaust equipment design should be used. It has a spiral-shaped body that acts as an air channel. You can make it yourself or purchase it already finished model.
To form the flow, the design includes a radial impeller. It connects to the power unit. The wheel blades have a curved shape and create a discharged area when moving. Air (or gas) enters it from the inlet pipe. When moving along the spiral body, the speed at the outlet increases.
Depending on the application, the centrifugal fan volute can be general purpose, heat-resistant or corrosion-resistant. It is also necessary to take into account the amount of air flow created:
- low pressure. Application area - production workshops, Appliances. The air temperature should not exceed +80°C. Mandatory absence of aggressive environments;
- average pressure value. It is part of exhaust equipment for removing or transporting small fraction materials, sawdust, grain;
- high pressure. Forms an air flow into the fuel combustion zone. Installed in many types of boilers.
The direction of movement of the blades is determined by the design, and, in particular, by the location of the outlet pipe. If it is located on the left side, the rotor should rotate clockwise. The number of blades and their curvature are also taken into account.
For powerful models you need to do it yourself solid foundation with housing fixation. The industrial installation will vibrate strongly, which may lead to its gradual destruction.
Self-production
First of all, you need to decide functional purpose centrifugal fan. If it is necessary for ventilation of a certain part of the room or equipment, the housing can be made from scrap materials. To complete the boiler, you will need to use heat-resistant steel or make it from stainless steel sheets yourself.
First, the power is calculated and the set of components is determined. The best option The snail will be dismantled from old equipment - an extractor hood or a vacuum cleaner. The advantage of this manufacturing method is the exact match between the power of the power unit and the body parameters. A snail fan can be easily made with your own hands only for some applied purposes in a small home workshop. In other cases, it is recommended to purchase a ready-made industrial-type model or take an old one from a car.
Procedure to make a centrifugal fan with your own hands.
- Calculation overall dimensions. If the device will be installed in a confined space, special damper pads are provided to compensate for vibration.
- Manufacturing of the body. In the absence of finished design You can use plastic sheets, steel or plywood. In the latter case Special attention is given to sealing joints.
- Installation diagram of the power unit. It rotates the blades, so you need to choose the type of drive. For small designs, a shaft is used to connect the motor gearbox to the rotor. IN powerful installations A belt type drive is used.
- Fastening elements. If the fan is installed on the outer casing of, for example, a boiler, mounting U-shaped plates are made. With significant power, it will be necessary to make a reliable and massive base.
This general scheme, according to which you can make a functional exhaust centrifugal unit with your own hands. It may change depending on the availability of components. It is important to comply with the housing sealing requirements, as well as ensure reliable protection power unit from possible clogging with dust and debris.
The fan will make a lot of noise during operation. Reducing this will be problematic, since vibration of the housing during the movement of air flows is almost impossible to compensate with your own hands. This is especially true for models made of metal and plastic. Wood can partially reduce background noise, but at the same time it has a short service life.
In the video you can see the process of manufacturing a case from PVC sheets:
Review and comparison of production ready models
Considering radial fan snail, it is necessary to take into account the material of manufacture: cast aluminum body, sheet or stainless steel. A model is selected based on specific needs; consider an example of serial models in a cast case.
Depending on the size and performance of such units, the operating conditions will also depend: in addition to domestic use, many types of ventilation equipment are widely used in the industrial sector. One example of such equipment is a rounded snail hood.
A radial centrifugal fan of this type is most often installed in production premises and is used to clean the air from dust, sawdust, burning, sand and other industrial waste. A similar air treatment system can be installed in multi-storey building, for example, in a ventilation shaft.
Let's understand the principle of its operation and consider the main stages of constructing a snail hood with your own hands.
Design Features
Scroll hoods differ in structure from standard fans with large blades. Air flows in such equipment move due to centrifugal force resulting from the rotation of a wheel with small specially shaped blades. The speed and power of such hoods may vary depending on the number of blades and motor parameters.
The air purification scheme in radial centrifugal hoods is quite simple: when air gets inside the hood, it begins to be sucked into the rotor, where it begins to rotate and is subject to pressure, gradually moving towards the outlet and being cleared of foreign elements. The general shape of the inlet and outlet channels resembles a snail - hence the name of this hood.
Attention! Designs of this type are useful in that they can both suck in air and ensure its outflow.
The housing of this type of ventilation system is made of durable materials, such as aluminum, brass or steel. Also available for sale plastic structures, but they are less durable and rarely operate at peak efficiency.
Since air treatment can be carried out at high temperatures, the body is processed protective paint, substances resistant to chemicals, and also coated with polymers.
Rotational mechanisms in such a system can be single, or may include two disks with blades required sizes. Both radial and circular blade placement provide high performance device.
Advice: For better cleaning air, purchase fans in which the blades are slightly curved rather than flat.
Despite the uniform shape, such hoods are suitable for many operating conditions, since they differ in orientation to the right or left side, and in overall dimensions. Average the diameter of the main body of such a hood can be from 25 to 150 cm.
For ease of installation for industrial purposes, many structures of this type are created modular, and fastening bolts are used to connect them. Accordingly, you can change both the angle of inclination and the details of some parts of such a design for greater efficiency: It’s better to first calculate all the parameters with specialists.
Since snails can differ from each other, you should not focus solely on size and power ratings. Familiarize yourself with their varieties - and make a choice, relying on future operating conditions.
Types of equipment
First of all, snail hoods differ in pressure indicators. Ventilation can be carried out under the following conditions:
- low pressure – up to 100 kg/m2;
- medium – from 100 dl 300 kg/m2;
- high pressure - more than 300 kg/m2 (can reach 1200 kg/m2).
The first type of hoods is suitable for use in both industrial and living conditions. As a rule, such equipment is quite compact, so it can be installed without additional help.
Attention! Low-pressure hoods are sufficient to ensure high-quality air ventilation in the shafts of multi-storey buildings.
Medium pressure fans are used for industrial purposes. Such equipment can more easily withstand difficult conditions operation, it is equipped in accordance with the main fire and technical requirements in production.
The third option is used not only in workshops, but also in laboratories, warehouses, areas where painting is carried out, etc. They can be installed for blowing air conditioning systems or working machines, as well as for pumping air in boiler systems.
Depending on the quality and degree of wear of the structure, there are general volute hoods, heat-resistant, corrosion-resistant systems, as well as heavy-duty equipment that can withstand even explosive reactions.
In most cases, snail-shaped air ventilation systems are used to remove pebbles, wood and metal shavings, wood chips and other production residues from the premises. Their installation must be carried out taking into account safety and labor protection requirements.
How to make it yourself
One of the features of such snails is their different price range. The minimum price for a snail hood will be about 3 thousand, but such devices, as a rule, are not very powerful and are very limited in size. The average price of a high-quality unit will exceed 20 thousand rubles.
Therefore, for domestic needs it is more expedient to produce homemade snail for hood. Standard design such a body will consist of two parts: the engine will be located in one zone, and the blowing blades in the other.
The casing for the snail can be purchased at hardware stores. If you are going to make it yourself, purchase the motor and other parts in advance, since the dimensions will have to be adjusted. It is better to make the case from metals (for example, aluminum and steel). Plastic will be less resistant to mechanical damage, and the tree will quickly catch fire in case of malfunctions.
The fan in such a system will operate at high speed. Therefore, incorrect design of the hood can have bad consequences. Check the quality and reliability of not only the base itself and fastening mechanisms, but also the motor, impeller and fan.
The fan dimensions are selected taking into account the area and degree of contamination of the room. Industrial designs are large.
Important! When installing the motor inside the box of such a hood, make sure that the design includes cooling holes. High temperature stress on the system can lead to an explosion.
Pay special attention to the choice internal materials. Fan operation can be affected not only by temperature, but also by the power of air flow, the amount of debris and dust.
When air with large impurities is sucked in, the blades of the rotating wheel may be damaged. And in order to thoroughly clean the air, the unit must operate at high speed and under high pressure- this creates additional stress on the entire internal structure. That's why it is better to choose parts made of durable materials, such as steel or aluminum.
- choose the right engine size and power: take into account the maximum load on the structure, as well as the required operating speed of the hood;
- When installing such a system vertically, carefully check that the fan and wheel are securely fastened: with rapid air currents they can jump off or change their location;
- materials adjacent to such a hood must be fireproof, like all the parts used in its assembly;
- maintain proportions between individual hood zones: in standard models offered in stores, the optimal ratio of length and width of the structure is taken into account;
- If you are not sure that the assembled hood is safe, contact specialists who will check its serviceability.
note that snail hoods are rarely used in living rooms . Firstly, they take up a lot of space, and secondly, in rooms like a kitchen, the flow of polluted air can have different directions, so it is best to install such a hood in a ventilation shaft, where all the air coming from the apartment is concentrated.
The design of such structures will also play an important role in living rooms, but it is not diverse and does not always harmonize with the interior.
Advice: when placing such a hood in open conditions(outside) make sure weather will not affect its functionality.
Ventilation hoods can be used not only for air purification. In domestic conditions they are excellent cope with heating the room, and also affect the humidity in the room.
The cost of equipment intended for domestic and industrial needs will differ significantly, but, in any case, such units have sufficient power for full operation.
For an example of designing a snail hood, see the attached video.