A centrifugal Blower and Fans is a mechanical equipment for moving air or other gases. The terms “blower” and “squirrel cage fan” are frequently used as synonyms. These kind of fans increase the speed of air stream with the rotating impeller. They use the kinetic energy of the impellers to increase the pressure of the air or gas stream which in turn moves them against the resistance, which is caused by ducts, dampers and other component. Centrifugal fan accelerates air radially by changing the direction of the air-flow. They are sturdy, reliable & capable of operating over a wide range of condition. The fans are constant CFM devices, meaning at a constant fan speed, a centrifugal fan will pump a constant volume of air rather than mass.
Facts about Centrifugal Fans & Blowers:
The centrifugal fan was invented by Russian Military Engineer Alexander Sablukov in the year 1832, and found its usage both in the Russian light industry and abroad. These fans are by far the most prevalent type of fan which is used in the HVAC industries. They are cheaper than axial fans and simpler in construction steps. Used in transporting gas or materials and in ventilation system for buildings, they are also commonly used in central heating & cooling systems. They are well-suited for industrial process and air pollution control system.
It has a fan wheel composed of a number of fan blades, mounted around a hub. The hub turns on a drive-shaft that passes through the fan housing. The gas enters from one side of the fan wheel, turns 90 degrees and accelerates due to centrifugal force.
1. Use of heavy-gauge welded components provide structural strength and durability.
2. The process of continuously welded housings provide the strongest possible construction.
3. Shafting is straightened to close-tolerance to minimize run out.
4. Lifting eyes provided on all such fans.
5. Impellers are properly balanced.
6. The bearings are selected to provide long service life through the entire operating range of the fan.
Principle of Working:
The centrifugal fan uses the centrifugal power. This power is generated from the rotation of impellers to increase the pressure of air. When a impeller rotates, the gas near the impeller is thrown-off from the impeller. It is due to the centrifugal force and then moves into the fan casing. As result the gas-pressure in the casing is increased. Now it is guided to the exit via outlet duct. After the gas is thrown-off, the gas-pressure in the middle region of the impellers decreased. The gas from the impeller eye rushes in to normalize this pressure value. The cycle repeats and therefore the gas can be continuously transferred.
Difference between fans and blowers:
The property that separates a centrifugal fan from a blower is the pressure ratio it can achieve. A blower in general can produce high pressure ratio. As per ASME (American Society of Mechanical Engineers) the specific ratio “the ratio of the discharge pressure over the suction pressure” is used for defining the fans and blowers.
Losses In Centrifugal Fans:
You can get the actual performance of the centrifugal fan by overcoming following losses.
1. Impeller entry losses: Friction and separation causes impeller blade losses. Since there is change in incidence. Generally these impeller blade losses are also included in this head.
2. Leakage loss: Leakage of air and disturbance in the main flow field is caused due to the clearance provided between the rotating side of the impeller and casing at the entry.
3. Diffuser and volute losses: Flow from impeller or diffuser expands in the volute which is having larger cross section leading to the formation of Eddy Current, which reduces head in turn. Friction and flow separation losses also occur due the volute passage.
4. Disc Friction: Viscous drag on the back surface of the impeller disc causes Disc friction.