A centrifugal fan is a mechanical device for moving air or other gases. The term "blower" and "squirrel cage fan", (because it looks like a hamster wheel), is often used as a synonym. This fan increases the speed and volume of airflow with a rotating impeller.
Centrifugal fans use the kinetic energy of the impeller to increase the volume of airflow, which in turn removes them from obstructions caused by ducts, dampers and other components. Centrifugal fans move the air radially, changing the direction (usually 90 Â °) of airflow. They are sturdy, quiet, reliable, and able to operate in various conditions.
Centrifugal fans are constant displacement devices or constant volume devices, meaning that, at constant fan speeds, the centrifugal fan moves a relatively constant volume of air rather than a constant mass. This means that the air velocity in the system remains even though the mass flow rate through the fan is not.
The centrifugal fan is not a positive displacement device and the centrifugal fan has certain advantages and disadvantages when compared to the positive displacement blower: the centrifugal fan is more efficient, while the blower with positive displacement may have lower capital costs.
The centrifugal fan is one of the most used fans. Centrifugal fans are by far the most common fan type used in the current HVAC industry. They are often cheaper than axial fans and are simpler in construction. They are used in transporting gas or materials and ventilation systems for buildings and vehicles. They are also very suitable for industrial process and air pollution control system.
The centrifugal fan is a drum shape consisting of a number of fan blades mounted around the hub. As shown in the animated image, the hub ignites the driveshaft mounted on the bearings in the fan housing. Gas coming from the side of the fan wheel, turns 90 degrees and accelerates due to centrifugal force as it flows over the fan blades and out of the fan house.
Video Centrifugal fan
History
The earliest mention of centrifugal fans in 1556 by Georg Pawer (Latin: Georgius Agricola) in his book De Re Metallica , where he shows how such fans are used for mine ventilation. After that, the centrifugal fan gradually becomes unused. It was not until the early decades of the nineteenth century that attracted centrifugal fans revived. In 1815 the Marquis de Chabannes advocated the use of centrifugal fans and took British patents in the same year. In 1827, Edwin A. Stevens of Bordentown, New Jersey, installed a fan to blow air into a North American steamer's boiler . Similarly, in 1832, the Swedish-American John Ericsson engineer used a centrifugal fan as a blower on a Corsair steam vessel. A centrifugal fan was invented by Russian military engineer, Alexander Sablukov in 1832, and was used both in the Russian light industry (such as sugar manufacture) and abroad.
One of the most important developments for the mining industry is the fan of Guibal, which was patented in Belgium in 1862 by the French engineer ThÃÆ' Â © ophile Guibal. Guibal fans have a spiral box that surrounds the fan blades, as well as flexible shutter to control the runaway speed, which makes it far superior to the previous open fan design and causes the possibility of mining at very deep depths. Such fans are widely used for mine ventilation throughout the UK.
Maps Centrifugal fan
Construction
The main parts of the centrifugal fan are:
- Fan housing
- Impeller
- Incoming and outbound channels
- Drive shaft
- Drive mechanism
Other components used may include bearings, couplings, impeller lock devices, fan disposal casing, shaft seal plates etc.
Drive mechanism
The fan drive determines the speed of the fan wheel (impeller) and the extent to which this speed can vary. There are three basic types of fan drives.
Live
The fan wheel can be connected directly to the electric motor shaft. This means the speed of the fan wheel is identical to the speed of the motor rotation. With this type of fan drive mechanism, fan speed can not be varied unless motor speed is adjusted. Air conditioning automatically delivers faster speeds because the cooler air is denser.
Some electronics manufacturers have made a centrifugal fan with an external rotor motor (the stator is in the rotor), and the rotor is directly mounted on the impeller fan.
Belt
A set of bundles is mounted on the motor shaft and fan axle, and the belt transmits mechanical energy from the motor to the fan.
The speed of the fan wheel depends on the ratio of the motor sheave diameter to the sheave fan wheel diameter and can be obtained from this equation:
Fan-driven fan speed in belt-driven fan except belt slip. Skid belt can reduce the speed of the fan wheel by several hundred revolutions per minute (rpm).
Variables
Variable drive enthusiasts can use hydraulic or magnetic couplings (between fan wheel shafts and motor shafts) that allow variable speed. Fan speed controls are often integrated into automated systems to maintain the desired fan wheel speed.
An alternative method to change the fan speed is to use an electronic variable speed drive to control the speed of the motor that drives the fan. It offers better overall energy efficiency rather than mechanical coupling, especially at a greatly reduced speed.
Bearing
Bearing is an important part of the fan. Arm ring oil bearings are widely used in fans. Some arm ring bearings may be cooled with water. Waterproof cushioned sleeves are often used when the fan moves hot gas. Heat is done through the shaft and into the oil, which must be cooled to prevent overheating. Low speed fans have pads in places that are hard to reach, so they use oil-filled pads.
Many turbo blowers use an air cushion or magnetic pads.
Fan and propeller dampers
Fan damper is used to control gas flow in and out of centrifugal fans. They can be mounted on the inlet side or on the outlet side of the fan, or both. The damper on the outlet side forces the flow resistance used to control the gas flow. The inlet vanes damper is designed to control the gas flow by changing the amount of gas or air entering the inlet fan.
The inlet damper (inlet vanes) reduces fan energy usage because of its ability to affect the airflow pattern to the fan.
Fan bar
The fan wheel consists of a hub with a number of fan blades attached. The fan blades in the hub can be arranged in three different ways: forward-curved, curved back or radial.
Forward-curve
The blades are bent forward, as in Figure 3 (a), the curve toward the rotation of the fan wheel. It is very sensitive to particulates. The forward curved blade provides a low level of noise and a relatively small airflow with a high increase of static pressure.
Backward-curve
The blades of the backward-curved blades, as in Fig. 3 (b), the curves to the direction of the spinning wheel of the fan. A smaller blower may have a sloping backward-tilt , which is straight, not curved. The more backward/webbed blower has a rear curve that mimics that of the airfoil cross section, but both designs provide good operating efficiency with relatively economical construction techniques. This type of blower is designed to handle gas flow with low to moderate particulate charge. They can be easily fitted with wear protection but certain knife blades can be susceptible to the buildup of solids. The backward-curved wheels are often heavier than the same-forward equivalents of the curve, as they travel at higher speeds and require more robust construction.
The backward curved fan can have a high specific speed range but is most commonly used for certain medium speed applications - high pressure, medium flow applications.
The backward-curved fan is far more energy efficient than radial blade fans and hence, for high power applications may be a suitable alternative to low-cost radial bladed fans.
Radial straight
The radial blower, as in Fig. 3 (c), has wheels whose blades extend straight out of the center of the hub. Radial bladed wheels are often used on particulate gas streams because they are least sensitive to solid formation on the propellers, but they are often characterized by larger sound output. High speed, low volume, and high pressure are common with radial blowers, and are often used in vacuum cleaners, pneumatic material delivery systems, and similar processes.
Principles of operation
The centrifugal fan uses centrifugal power supplied from the rotation of the impeller to increase the kinetic energy of air/gas. When the impeller rotates, the gas particles near the impeller are thrown from the impeller, then move into the fan casing. As a result, the kinetic energy of the gas is measured as pressure due to the system resistance offered by the casing and channel. The gas is then guided to the exit through outlet channels. After the gas is thrown, the gas pressure in the center of the impeller decreases. The gas from the impeller's eye rushes in to normalize this. This cycle is repeated and therefore the gas can be transferred continuously.
Triangle speed
The diagram called the speed triangle helps us in determining the flow geometry at the time of entry and exit of the blade. A minimum amount of data is required to draw a speed triangle at a point on the blade. Some speed components vary at different points on the blade due to changes in flow direction. Therefore the number of unlimited velocity triangles is possible for the given blades. To illustrate the flow using only two speed triangles, we define the average value of velocity and direction. The triangle speed of each turbo engine has three components as shown:
- Blade U Speed ​​
- V r Relative Speed ​​
- V Absolute speed
Kecepatan ini terkait dengan hukum segitiga penambahan vektor:
This relatively simple equation is often used when drawing speed diagrams. The speed diagram for the forward face blades shown is illustrated using this law. Angle? is the angle made by the absolute speed with the axial and angular directions? is the angle made by the blade with respect to the axial direction.
Difference between fan and blower
The property that distinguishes the centrifugal fan from the blower is the achievable pressure ratio. In general, blowers can produce higher pressure ratios. According to the American Society of Mechanical Engineers (ASME), the specific ratio - the ratio of discharge pressure to suction pressure - is used to define fans and blowers (Table 1).
Ratings
Rankings found in the performance tables of centrifugal fans and curves are based on standard air SCFM. The fan manufacturer defines standard air as clean and dry air with a density of 0.075 pounds mass per cubic foot (1.2 kg/mÃ,³), with barometer pressure at sea level 29.92 inches of mercury (101,325 kPa) and a temperature of 70Ã,  ° Ã,  ° F (21Ã,  ° C). Choosing a centrifugal fan to operate in conditions other than standard air requires adjustment for static pressure and force.
At elevations higher than standard (sea level) and temperatures higher than standard, air density is lower than standard density. Air density correction shall be calculated for centrifugal fans specified for continuous operation at higher temperatures. The centrifugal fan moves a constant volume of air within a given system regardless of the air density.
When the centrifugal fan is specified for the given CFM and static pressure under conditions other than the standard, the air density correction factor should be applied to select the right size fan to meet the new conditions. Since the air 200 ° C (93 ° C) weighs only 80% of the 70 ° F (21 ° C) air, the centrifugal fan creates less pressure and requires less power. To obtain the actual pressure required at 200 ° F (93 ° C), the designer must multiply the pressure under standard conditions with an air density correction factor of 1.25 (ie 1.0/0.8) to get the system to operate with correct. To obtain actual power at 200 ° F (93 ° C), the designer must divide power under standard conditions by air density correction factor.
The Air Movement and Control Association (AMCA)
The performance table of the centrifugal fan provides fan RPM and power requirements for supplied CFM and static pressure at standard air density. When centrifugal fan performance is not in standard conditions, performance must be converted to standard conditions before entering the performance table. Centrifugal fans rated by the Air Movement and Associated Controls (AMCA) are tested in the laboratory with a test setting that simulates a typical installation for that type of fan. Usually they are tested and rated as one of four standard installation types as defined in AMCA Standard 210.
The AMCA Standard 210 defines a uniform method for conducting laboratory tests on fans placed to determine the rate of airflow, pressure, power and efficiency, at a particular rotation speed. The purpose of the AMCA Standard 210 is to establish the appropriate procedures and conditions of the fan testing so that the ratings given by different manufacturers are on the same and comparable basis. For this reason, fans must be rated in a standard SCFM.
Losses
Centrifugal fans experience efficiency losses in both stationary and moving parts, increasing the energy input required for certain airflow performance levels.
Impeller entry
The flow on the intake and the back from axial to radial causes a loss in the intake. Friction and flow separation causes impeller blade losses because there are changes in the coming angle. Impeller blade damage is also included in the category.
Leakage
Multiple air leaks and disturbances in the main stream area are caused due to the permission given between the rotation edges of the impeller and the casing at the entrance.
Impeller
Friction and flow separation causes impeller losses that depend on relative speed, diffusion rate, and blade geometry. Balancing dynamic impeller is usually done on a precision balancer, since all the vibrational energy imbalances are lost (for example, it can easily reach 50% of airflow losses in a poorly maintained air conditioning unit).
Diffuser and volute
Friction and flow separation also causes losses to the diffuser. Further losses due to incidents occur if the device works out of its design conditions. The flow of the impeller or diffuser expands in the volute, which has a larger cross-section which leads to eddy formation, which in turn reduces the head pressure. Friction and loss of flow separation also occur due to volute parts.
Disc friction
The viscous absorption on the rear surface of the impeller disc causes the loss of friction.
See also
- Fan channeling
- Mechanical fans
- Standard temperature and pressure
- Loss of three dimensions and correlation in turbo engine
- Fan fan
- Wind turbine
References
Source of the article : Wikipedia
0 Comments