Centrifugal fan which is also called as squirrel cage fan because of its hamster wheel design is widely used to move air/gas continuously in industries and commercial applications. The speed of the air stream entering into the fan, is increased by the impeller thereby it gains kinetic energy and accelerates radially changing the direction by 90? As these fans are capable of displacing constant volume of air/gas at increased speed, they are widely used in various process applications in industries such as in material handling, boiler applications, transporting gas or materials, ventilation and combustion, air circulation and in other HVAC systems.
Apart from small accessories like bearings, couplings, hoods, guards, flanges, isolators etc., centrifugal fan also contain some basic parts that play a major role in system working and performance.
Some important parts of centrifugal fans are:
Impeller:
Impeller is a rotating device consisting of number of fan blades mounted on a hub with drive shaft that forces the air/gas in the desired direction with increased pressure. In centrifugal fan, the air stream enters the impeller in an axial direction, acquires acceleration due to centrifugal force as it flows through the fan blades and gets discharged radially from the fan housing.
To meet the demands of large scale applications in industries, centrifugal fans are custom designed as double inlet impeller and multistage impellers where two or more single inlet impellers are mounted back to back on the same shaft. Industries with large capacity plants which require high pressure airflow use these types of fans in their process applications.
Fan Blades:
Based on the blade configuration such as forward curved, backward curved, radial, airfoil etc., centrifugal fans are divided into various types and differ in their functionality. The efficiency and fan performance basically depends on the shape of the blades that are mounted on a hub of the fan wheel. Some fans can handle larger volumes of air with high static pressures where as some are meant for low flow rates. Hence, it is important to select the centrifugal fan design that matches with the required pressure and flow-rate of any industrial process, so that the efficiency will be high for the given operating conditions.
Fan Housing:
Housing in a fan system improves efficiency as it reduces noise and acts as a protection for moving parts like impeller, hub etc. In centrifugal fan, the air from the inlet enters the impeller axially through fan housing, where it picks up the acceleration and is radially sent out in 90?direction. The design of the housing has to be optimized so that it withstands the centrifugal action of spinning of the air inside it.
Drive shaft:
It is a rotating device that holds the impeller & blades and determines the speed of the fan wheel through various drive mechanisms like direct, belt or variable. Shaft design depends on the installed impeller weight and the velocity of the inlet gases.
Direct: In direct driven fans, the fan speed is directly proportional to the speed of the motor as the shaft is straightaway connected to the fan motor.
Belt driven: In belt driven mechanism, the wheel shaft and the motor shaft are connected by belt which is mounted on the sheave. These fans have fixed wheel speed unless the belt gets misplaced from its position.
Variable: To get the desirable variable speed of the fans, variable driven mechanisms use couplings to establish a connection between the motor shaft and the wheel shaft. Some of the fan systems are equipped with electronic variable speed drives that can automatically control the fan speed at the desired level.
Inlet and outlet ducts:
Ducts that are attached at the fan inlet and outlet will affect the airflow pattern and influence the performance of the fans. Ducts at the inlet control the flow of air/gas by admitting only required amount of air whereas ducts at the outlet impose a resistance to the air stream. Improper inlet and outlet airflow conditions, and ducts with sharp cornered turns result in inefficient fan operation, high system noise and poor performance with undesired outputs.
Dampers & Louvers:
Dampers and louvers are the plates or sheets inside the ducts which are used to control or alternatively block the airflow at the inlet or outlet of a fan. The main difference between them is damper blades are movable or adjustable manually or automatically to control the volume or regulate the airflow whereas louver blades are fixed and there is no control over the air moving through them.
Centrifugal fans and blowers provide customized solutions to many industrial processes and air pollution systems for ventilation, dust collection, drying & cooling, exhausting gases, fume extraction, air recirculation, pollution control etc. The amount of the airflow and the efficiency of the fan system will decide which type of the fan has to be used in the industrial process.
