Aerodynamic characteristics of tangential fan blades. Ushakov, Konstantin Andreevich - Aerodynamics of axial fans and elements of their structures. Example of fan characteristics when equipped with an electric motor
The aerodynamic characteristics of fans show the fan flow depending on the pressure. A certain pressure corresponds to a certain air flow, which is illustrated by the fan curve.
Figure 28. Aerodynamic characteristics of the fan and network
Network characteristics
The resistance of the ventilation system at various flow rates is displayed on the network characteristics graph. The fan operating point is the point of intersection of the network characteristic and the fan curve. It shows the flow characteristics for a given duct network.
Each change in pressure in the ventilation system gives rise to a new network characteristic. If the pressure increases, the network characteristic will be similar to line B. When the pressure decreases, the system line will be similar to line C. (Assuming that the number of revolutions of the impeller remains unchanged).
Figure 29. Pressure changes give rise to new network curves
If the actual network resistance is represented by curve B, the operating point shifts from 1 to 2. This also entails a decrease in air flow. In the same way, the air flow will increase if the network resistance corresponds to line C.
Figure 30: Increasing or decreasing fan speed
To obtain an air flow rate similar to the calculated one, in the first case (where the network characteristic corresponds to B) you can simply increase the fan speed. The operating point (4) will in this case be located at the intersection of the network characteristic B and the fan curve for a higher rotation speed. Likewise, the fan speed can be reduced if the actual network characteristic corresponds to line C.
Figure 31. Pressure difference at different rotation speeds
In both cases there will be some difference in pressure performance from the network characteristics for which calculations were carried out, and this is shown as ΔP1 and ΔP2 in the figure, respectively. This means that the operating point for the design network has been selected to achieve maximum efficiency, and each increase or decrease in fan speed results in a reduction in efficiency.
Network efficiency and characteristics
To make fan selection easier, you can plot several possible network characteristics on a fan graph, and then see between which characteristics a certain fan type operates. If we number the network characteristics from 0 to 10, the fan will blow freely (maximum air flow) on line 10, and choke (zero flow) on line 0. This means that the fan on system line 4 produces 40% of the free flow.
Figure 32. Network characteristics (0-10) on the fan graph
The fan efficiency remains constant along the entire network characteristic.
Fans with backward-curved blades often have higher efficiency than fans with forward-curved blades. But a higher level of efficiency of these fans is achievable only in a limited area, where the network characteristic is represented by a lower flow rate at a given pressure than that of fans with forward-curved blades.
To achieve similar flow rates to forward-curved fans while maintaining a high level of efficiency, you must select a larger backward-curved fan.
Figure 33. Efficiency values for similar sized centrifugal fans with backward-curved and forward-curved blades, respectively
General purpose fans are used to operate in clean air, the temperature of which is less than 80 degrees. Special heat-resistant fans are designed to move hotter air. For work in aggressive and explosive environments, special anti-corrosion and explosion-proof fans are produced. The casing and parts of the anti-corrosion fan are made of materials that do not react chemically with corrosive substances of the transported gas. Explosion-proof design eliminates the possibility of sparking inside the fan housing (casing) and increased heating of its parts during operation. Special dust fans are used to move dusty air. Fan sizes are characterized by a number that indicates the diameter of the fan impeller, expressed in decimeters.
According to the operating principle, fans are divided into centrifugal (radial) and axial. Low pressure centrifugal fans create a total pressure of up to 1000 Pa; medium pressure fans - up to 3000 Pa; and high pressure fans develop pressure from 3000 Pa to 15000 Pa.
Centrifugal fans are manufactured with disk and diskless impellers:
The impeller blades are mounted between two disks. The front disc is in the form of a ring, the rear is solid. The blades of a discless wheel are attached to the hub. The spiral casing of a centrifugal fan is installed on independent supports, or on a frame common with the electric motor.
Axial fans are characterized by high performance but low pressure, therefore they are widely used in general ventilation to move large volumes of air at low pressure. If the impeller of an axial fan consists of symmetrical blades, then the fan is reversible.
Axial fan diagram:
Roof fans are manufactured axial and radial; are installed on roofs and on the roofless floors of buildings. The impeller of both axial and radial roof fans rotates in a horizontal plane. Schemes of operation of axial and radial (centrifugal) roof fans V:
Axial roof fans are used for general exhaust ventilation without a network of air ducts. Radial roof fans develop higher pressures, so they can operate both without a network and with a network of air ducts connected to them.
Selection of a fan based on aerodynamic characteristics.
For each ventilation system, aspiration or pneumatic transport installation, a fan is selected individually, using graphs of the aerodynamic characteristics of several fans. Based on the pressure and air flow in each graph, the operating point is found, which determines the efficiency and rotation speed of the fan impeller. Comparing the position of the operating point on different characteristics, select the fan that gives the highest efficiency at given values of pressure and air flow.
Example. Calculation of the ventilation unit showed the total pressure loss in the system Hc = 2000 Pa at the required air flowQs=6000 m³/hour. Select a fan that can overcome this network resistance and provide the required performance.
To select a fan, its design pressure is taken with a safety factork=1,1:
Hb= kHc; Нв=1.1·2000=2200 (Pa).
Air consumption is calculated taking into account all unproductive suctions.Q in= Qs=6000 (m³/hour). Let's consider the aerodynamic characteristics of two similar numbers of fans, the range of operating values of which includes the values of the design pressure and air flow of the designed ventilation installation:
Aerodynamic characteristics of fan 1 and fan 2.
At the intersection of values Pv=2200 Pa and Q=6000 m³/hour indicate the operating point. The highest efficiency is determined by fan characteristic 2: efficiency = 0.54; impeller rotation speedn=2280 rpm; wheel edge peripheral speedu~42 m/sec.
Peripheral speed of the 1st fan impeller (u~38 m/sec) is significantly less, which means that the noise and vibration created by this fan will be less, and the operational reliability of the installation will be higher. Sometimes preference is given to a slower fan. But the operating efficiency of the fan must be at least 0.9 of its maximum efficiency. Let's compare two more aerodynamic characteristics that are suitable for choosing a fan for the same ventilation installation:
Aerodynamic characteristics of fan 3 and fan 4.
The efficiency of fan 4 is close to the maximum (0.59). Rotation speed of its impellern=2250 rpm. The efficiency of the 3rd fan is slightly lower (0.575), but the impeller rotation speed is significantly lower:n=1700 rpm. If the efficiency difference is small, the 3rd fan is preferable. If the drive and motor power calculations show similar results for both fans, fan 3 should be selected.
Calculation of the power required to drive the fan.
The power required to drive a fan depends on the pressure it createsHin (Pa), air volume movedQin (m³/sec) and efficiency factor:
N in= H V ·Q V/1000·efficiency (kW); Hb=2200 Pa; Qh=6000/3600=1.67 m³/sec.
Efficiencies of fans 1, 2, 3 and 4 pre-selected according to aerodynamic characteristics, respectively: 0.49; 0.54; 0.575; 0.59.
Substituting the pressure, flow and efficiency values into the calculation formula, we obtain the following power values for each fan drive: 7.48 kW, 6.8 kW, 6.37 kW, 6.22 kW.
Calculation of electric motor power to drive a fan.
The power of the electric motor depends on the type of its transmission from the motor shaft to the fan shaft, and is taken into account in the calculation by the corresponding coefficient (klane). There is no power loss when the fan impeller is directly mounted on the electric motor shaft, i.e., the efficiency of such a transmission is 1. The efficiency of connecting the fan and electric motor shafts using a coupling is 0.98. To achieve the required rotation speed of the fan impeller, we use a V-belt drive, the efficiency of which is 0.95. Losses in bearings are taken into account by the coefficientkn=0.98. According to the formula for calculating the power of an electric motor:
N el= N V / k lane k P
we get the following powers: 8.0 kW; 7.3 kW; 6.8 kW; 6.7 kW.
The installed power of the electric motor is taken with a safety factorkz=1.15 for engines with power less than 5 kW; for motors over 5 kWk z=1.1:
N y= k h· N email
Taking into account the safety factorkz=1.1 the final power of the electric motors for the 1st and 2nd fans will be 8.8 kW and 8 kW; for 3rd and 4th 7.5 kW and 7.4 kW. The first two fans would have to be equipped with an 11 kW motor; for any fan from the second pair, the power of an electric motor of 7.5 kW is sufficient. We choose fan 3: as it is less energy-intensive than sizes 1 or 2; and as more low-speed and operationally reliable compared to fan 4.
The fan numbers and graphs of aerodynamic characteristics in the example of fan selection are taken conditionally and do not refer to any specific brand and standard size. (And they could.)
Calculation of the diameters of V-belt fan drive pulleys.
A V-belt drive allows you to select the desired speed of rotation of the impeller by installing pulleys of different diameters on the motor shaft and the fan drive shaft. The gear ratio of the rotational speed of the electric motor shaft to the rotational speed of the fan impeller is determined:nuh/ nV.
V-belt drive pulleys are selected so that the ratio of the diameter of the fan drive pulley to the diameter of the pulley on the electric motor shaft corresponds to the ratio of rotational speeds:
DV/ Duh= nuh/ nV
The ratio of the diameter of the driven pulley to the diameter of the driving pulley is called the belt drive ratio.
Example. Select pulleys for a V-belt drive of a fan with an impeller rotation speed of 1780 rpm, driven by an electric motor with a power of 7.5 kW and a rotation speed of 1440 rpm. Transmission ratio:
nuh/ nV=1440/1780=0,8
The required speed of rotation of the impeller will be ensured by the following equipment: a pulley on a fan with a diameter 180 mm , pulley on an electric motor with a diameter 224 mm.
Schemes of a fan V-belt transmission that increases and decreases the speed of rotation of the impeller:
Figure 7.24. Installation of a TsAGI axial fan type U.
Rice. 7.23. Roof axial fan.
1-safety grill; 2- collector; 3- body; 4- electric motor; 5- impeller; 6- diffuser; 7- valve; 8-umbrella.
Currently, production of this fan in the roof modification has begun (Fig. 7.23). In this case, the fan wheel rotates in a horizontal plane, being installed on the shaft of a vertically located electric motor mounted on three braces in the shell (housing).
The entire installation is located in a short pipeline, equipped with a safety grille on the air inlet side and an umbrella on the outlet side.
The units are produced in venti-five sizes No. 4, 5, 6, 8, 10 and 12. According to the catalogue, the maximum circumferential speed is 45 m/sec. The maximum developed static pressure reaches 10-11 kg/m2 at static efficiency 0.31.
TsAGI axial fans type U (universal) have a more complex design. The fan wheel consists of a large diameter bushing (0.5 D), on which 6 or 12 hollow blades are fixed. Each blade is riveted to a rod, which in turn is screwed into a special glass and secured with nuts in the sleeve. The blades are rotatable and can be installed at an angle from 10 to 25° to the plane of rotation of the wheel (Fig. 7.24). Installation of the blades at the required angle is carried out according to the markings made on the side surface of the bushing.
The ability to change the angles of the blades, i.e., change the geometry of the wheel, gives this fan versatility, since the pressure it develops increases with increasing angle of the blades.
The fan is designed to be driven by an electric motor via a V-belt drive, so the fan wheel is mounted on a shaft. The shaft has two bearings, the housings of which are placed in box-shaped holders. Each holder has four cast rods ending in flat feet with holes for mounting bolts. Holders with rods and feet form two frames on which the wheel is held. The drive pulley is located in a cantilever at the end of the shaft. Currently (mainly for the needs of the textile industry) fans with 12 blades No. 12, 16 and 20 are produced. The wheel of these machines is very durable and allows peripheral speeds of up to 80-85 m/sec..
Considering that the pressure developed by a type Y fan depends on the angle of installation of the blades, a typical fan should be built for each angle separately. Therefore, for type U fans, a special universal characteristic is given, covering the areas of fan operation in various conditions.
The performance of the three sizes of fans ranges from 1-6000 to 100,000 m 3 /h. Developed pressures range from 11 kg/m2(with blades installed at an angle of 10°) up to 35-40 kg/m2(when installing the blades at an angle.
The electric motor that drives the fan wheel is usually located on the floor near the wall of the room, in the hole in which the fan is mounted.
The maximum fan efficiency (at blade angles of 20°) reaches 0.62. At smaller and larger installation angles, the efficiency decreases slightly (to 0.5 at 10° and to 0.58 at 25°).
The aerodynamic design of a fan means a set of basic structural elements arranged in a certain sequence and characterizing the flow part of the machine through which air passes. The VOD11P fan implements the aerodynamic design shown in Fig. 7.25 (RK1 + NA + RK2 + SA), i.e. air is sucked into the fan from channel 5 through the collector 6 under the action of aerodynamic forces arising from the rotation of the blades 8 of the impeller RK 1.
Fig.7.25 Aerodynamic design of the VOD11P fan
When leaving the wheel, the swirling air flow hits the blades 9 of the guide vane NA1, which spins it and directs it to the blades 10 of the second-stage impeller RK2. At the same time, a slight twist of the flow is carried out in the NA before entering the RK2 in the direction opposite to the rotation of the rotor, which helps to increase traction on the second wheel. After RK2, the flow enters the straightening apparatus SA. With the help of blades 11, the SA spins the flow and directs it into a diffuser, made in the form of an expanding cone 14 and a shell 13. In the diffuser, along the flow, the open cross-sectional area increases, therefore, the velocity pressure decreases, and the pressure increases. At the same time, the static pressure also increases.
The impellers RK1 and RK2 are rigidly mounted on shaft 4, mounted in bearings 3 and 12 and receiving rotation from engine 1 through coupling 2. Fairing 7 serves to equalize the flow of air drawn into the fan.
In Fig. 7.26. The fan VOD11P is shown in section, which is intended for ventilation of mine workings in mining areas and individual chambers, and is also used when sinking mine shafts, in heating installations, at large enterprises, etc.
The fan consists of a rotor - shaft 2 with two impellers 4 and 10, rigidly fixed to the shaft using keys 3 and locking rings. The impellers of the first stage RK1 and the second stage RK2 have an identical design, consisting of 4 bushings on which 12 blades made of polymer material are placed. Blades 8 and 11 are installed in special sockets, secured using spacer spring rings 6 and pressed by springs 5 to the wheel hub. This fastening of the blades allows you to rotate them manually through special windows in the housing when the fan is stopped within the installation angles of 15 - 45 0 to regulate the flow and pressure. The fan housing consists of two detachable parts, upper 7 and lower 15, made of cast steel in the form of a split cylinder.
Fans are devices designed to create air (in general, gas) flow. The main task that is solved with the use of these devices in ventilation, air conditioning and air treatment equipment is the creation of conditions in the air duct system for the movement of air masses from intake points to emission points or consumers.
For efficient operation of the equipment, the air flow created by the fan must overcome the resistance of the air duct system caused by turns of the lines, changes in their cross-section, the appearance of turbulence and other factors.
As a result, there is a pressure drop, which is one of the most important characteristic indicators influencing the choice of a fan (besides it, performance, power, noise level, etc. play a major role). These characteristics depend, first of all, on the design of the devices and the operating principles used.
All the many fan designs are divided into several main types:
- Radial (centrifugal);
- Axial (axial);
- Diametral (tangential);
- Diagonal;
- Compact (coolers)
Centrifugal (radial) fans
In devices of this type, air is sucked in along the axis of the impeller and released under the influence of centrifugal forces developed in the area of its blades in the radial direction. The use of centrifugal forces will allow the use of such devices in cases where high pressure is required.
The performance of radial fans depends largely on the design of the impeller and the shape of the blades (blades).
Based on this feature, radial fan impellers are divided into devices with blades:
- curved back;
- direct, including rejected;
- bent forward.
Impellers with backward curved blades
Such an impeller (B in the figure) is characterized by a significant dependence of performance on pressure. Accordingly, radial fans of this type are effective when operating on the ascending (left) branch of the characteristic. When used in this mode, an efficiency level of up to 80% is achieved. At the same time, the geometry of the blades makes it possible to achieve a low level of operating noise.
The main disadvantage of such devices is the adhesion of airborne particles to the surface of the blades. Therefore, such fans are not recommended for polluted environments.
Straight blade impellers
In such impellers (shape R in the figure), the danger of surface contamination by impurities contained in the air is eliminated. Such devices demonstrate efficiency up to 55%. When using straight backward-curved blades, the performance approaches that of devices with backward-curved blades (efficiency up to 70% is achieved).
Impellers with forward-curved blades
For fans using this design (F in the figure), the effect of pressure changes on airflow is negligible.
Unlike impellers with backward-curved blades, the greatest efficiency of such impellers is achieved when operating on the right (descending) branch of the characteristic, and its level is up to 60%. Accordingly, all other things being equal, a fan with an F-type impeller outperforms devices equipped with an impeller in terms of impeller dimensions and overall overall dimensions.
Axial (axial) fans
For such devices, both the input and output air flows are directed parallel to the axis of rotation of the fan impeller.
The main disadvantage of such devices is their low efficiency when using the free-rotating installation option.
A significant increase in efficiency is achieved by enclosing the fan in a cylindrical housing. There are other methods to improve performance, such as placing guide vanes directly behind the impeller. Such measures make it possible to achieve an efficiency of axial fans of 75% without the use of guide blades and even 85% when installing them.
Diagonal fans
With axial air flow it is impossible to create a significant level of equivalent pressure. An increase in static pressure can be achieved by using additional forces to create an air flow, for example, centrifugal forces, which act in radial fans.
Diagonal fans are a kind of hybrid of axial and radial devices. In them, air is sucked in the direction coinciding with the axis of rotation. Due to the design and arrangement of the impeller blades, an air flow deflection of 45 degrees is achieved.
Thus, a radial velocity component appears in the movement of air masses. This makes it possible to achieve an increase in pressure due to the action of centrifugal forces. The efficiency of diagonal devices can be up to 80%.
Crossflow fans
In devices of this type, the air flow is always directed tangentially to the impeller.
This allows significant performance to be achieved even with small impeller diameters. Thanks to these features, diametric devices have become widespread in compact installations such as air curtains.
The efficiency of fans using this principle of operation reaches 65%.
Aerodynamic characteristics of the fan
The aerodynamic characteristic reflects the dependence of the fan flow (performance) on pressure.
There is an operating point on it, showing the current flow rate at a certain level of pressure in the system.
Network characteristics
The air duct network at different flow rates has different resistance to air movement. It is this resistance that determines the pressure in the system. This dependence is reflected by the network characteristic.
When constructing the aerodynamic characteristics of the fan and the network characteristics in a single coordinate system, the operating point of the fan is at their intersection.
Calculation of network characteristics
To construct network characteristics, the dependence is used
In this formula:
- dP – fan pressure, Pa;
- q – air flow, cubic m/h or l/min;
- k – constant coefficient.
- The first point corresponding to the fan operating point is plotted on the aerodynamic characteristic. For example, it operates at a pressure of 250 Pa, creating an air flow of 5000 cubic meters per hour. (point 1 in the figure).
- The formula determines the coefficient kk = dP/q2 For the example under consideration, its value will be 0.00001.
- Several pressure deviations are arbitrarily selected, for which the flow rate is recalculated. For example, with a pressure deviation of -100 Pa (resulting value 150 Pa) and +100 Pa (resulting value 350 Pa), the air flow calculated by the formula will be 3162 and 516 cubic meters per hour respectively.
Each duct network resistance value has its own network characteristic. They are built in a similar way.
As a result, while maintaining the fan rotation speed, the operating point shifts along the aerodynamic characteristic. As the resistance increases, the operating point shifts from position 1 to position 2, which causes a decrease in air flow. On the contrary, when the resistance decreases (transition to point 3a of line C), the air flow will increase.
Thus, the deviation of the actual resistance of the air duct system from the calculated one leads to a discrepancy between the air flow and the design values, which can negatively affect the performance of the system as a whole. The main danger of such a deviation lies in the inability of ventilation systems to effectively perform their assigned tasks.
The deviation of the air flow from the calculated one can be compensated by changing the fan rotation speed. In this case, a new operating point is obtained, lying at the intersection of the network characteristic and the aerodynamic characteristic from the family that corresponds to the new rotation speed.
Accordingly, as the resistance increases or decreases, it will be necessary to adjust the rotation speed so that the operating point moves to position 4 or 5, respectively.
In this case, there is a deviation of pressure from the calculated characteristics of the network (the magnitude of the changes is shown in the figure).
In practice, the appearance of such deviations indicates that the fan operating mode differs from the one that was calculated for reasons of maximum efficiency. Those. regulation of the speed either in the direction of increasing or decreasing leads to a loss of efficiency of the fan and the system as a whole.
Dependence of fan efficiency on network characteristics
To simplify the selection of a fan, several network characteristics are based on its aerodynamic characteristics. Most often, 10 lines are used, the numbers of which satisfy the condition
L = (dPd / dP)1/2
- L – network characteristic number;
- dPd – dynamic pressure, Pa;
- dP – total pressure value.
In this case, the fan efficiency, which is determined by the ratio
- dP – total pressure, Pa;
- q – air flow, cubic m/h;
- P – power, W
In this regard, it is of interest to compare the efficiency of radial fans with backward and forward curved impeller blades. For the former, the maximum value of this indicator is often higher than for the latter. However, this relationship is maintained only when operating in the area of network characteristics corresponding to lower flow rates at a given pressure value.
As can be seen from the figure, at high air flow levels, backward curved fans will require a larger impeller diameter to achieve equal efficiency.
Aerodynamic losses in the network and rules for installing fans
The technical characteristics of the fans correspond to those specified by the manufacturer in the technical documentation if the requirements for their installation are met.
The main one is to install the fan on a straight section of the air duct, and its length should be at least one and three times the diameter of the fan on the suction and discharge sides, respectively.
Violation of this rule leads to an increase in dynamic losses, and, as a consequence, to an increase in pressure drop. If this difference increases, the air flow may decrease significantly compared to the calculated values.
Many factors influence the level of dynamic losses, performance and efficiency. Accordingly, other requirements must be met when installing fans.
Suction side:
- the fan is installed at a distance of at least 0.75 diameters from the nearest wall;
- the cross-section of the inlet air duct should not differ from the diameter of the inlet opening by more than +12 and -8%;
- the length of the air duct on the air intake side must be greater than 1.0 times the fan diameter;
- the presence of obstacles to the passage of air flow (dampers, branches, etc.) is unacceptable.
- the change in the cross-section of the air duct should not exceed 15% and 7% in the direction of decrease and increase, respectively;
- the length of the straight section of the pipeline at the outlet must be at least 3 fan diameters;
- To reduce resistance, it is not recommended to use bends at an angle of 90 degrees (if it is necessary to turn the main line, they should be obtained from two bends of 45 degrees each).
Specific fan power requirements
High energy efficiency indicators are one of the main requirements that are applied in European countries to all equipment, including building ventilation systems. Accordingly, the Swedish Institute of Indoor Climate (Svenska Inneklimatinsitutet) has developed an integral efficiency assessment concept for ventilation equipment based on the so-called specific fan power.
This indicator is understood as the ratio of the total energy efficiency of all fans included in the system to the total air flow in the ventilation ducts of the building. The lower the resulting value, the higher the efficiency of the equipment.
This assessment forms the basis for recommendations for the purchase and installation of ventilation systems for various sectors and industries. So for municipal buildings the recommended value should not exceed 1.5 when installing new systems and 2.0 for equipment after renovation.
