Centrifugal Fans: What’s In A Name?

You would have come across many fans like Pressure Blower, FD fans, ID fans, Raw Mill fans, Flue Gas Recirculation Fans etc. They are all technically centrifugal fans having different names and serving different purpose in the industry.

Common features in all centrifugal fan
A centrifugal fan is a mechanical device to displace air or other gases. Centrifugal fans utilise the kinetic energy of impellers to displace air which is moved radially. Centrifugal fans are termed as constant volume devices because at constant speed they displace constant volume of air and not constant mass of air. The rotation of impellers provides the kinetic energy needed to throw off the gas or air. The gas or air exits through outlet duct. Now the new air rushes into the impeller to fill the space and thus new air keeps coming in to be thrown off by the impeller. A diagram called velocity triangle is used to describe the dynamics of air at inlet and outlet.

The main parts of a Centrifugal fan can be:
1. Fan impellers
2. Inlet and Outlet duct
3. Drive shaft
4. Drive mechanism
5. Fan housing

Different names based usage in industrial process
Above mentioned features are common in every centrifugal fan. However, centrifugal fans have different names based on the method they are used to move the air in industrial process and by the system pressure they must operate against. The fans have different industrial application ranging from de-dusting to flue gas recirculation. Based on requirement of airflow, pressure, volume and speed, Centrifugal fans serve the industry in different forms. Combustion air fans maintain the good amount of airflow in the burner system for proper combustion of fuel. Its operation is regulated using accessories like louvers and inlet guide vanes. Similarly pressure blowers are used where high volume of air is required. Example – printing industry. De-dusting fans is yet another application where centrifugal fans find diverse market. Raw Mill fans and Raw Mill Baghouse fans serve cement industry. Flue gas recirculation fans aid in thermal combustion and in removal of pollutants like nitrogen oxide.

Some centrifugal fans have special construction and design elements
Sometimes Centrifugal fans incorporate special construction and design elements to serve the industry. Cast Aluminium Blower fans is one such example. This fan is housed in an aluminium casing with spark resistant construction. Forward Curved Volume blowers work at slow speed and are designed to provide large amount of volume of air at low pressure. For heavy duty, a Centrifugal fan is modified as Double Inlet Centrifugal fans. It has two inlets and one common outlet. They serve the purpose of supplying large airflow in the industrial process. In Chemical industry where large pressure is required for evaporation of chemical substances Multistage Centrifugal Fans are used. In Multistage Centrifugal Fans two impellers are fitted on a single shaft. The airflow from the first impeller is magnified by the other. In Airfoil Blade Fans, blades are made hollow and backward inclined. They are specialised in handling large volume of air at low pressure. They find application in heating, ventilation and air-conditioning.

Centrifugal fans named on their characteristics
Centrifugal fans can be named on the basis of their characteristics. For an instance, Limit Load Fans have Limit Load Characteristics. Limit Load Characteristic is adopted to prevent the fan from overload. They find their application in ventilation and air-conditioning in mid to high rise buildings. Induced Draft fans and Force Draft fans are widely used in industrial furnace applications. Draft means there is pressure difference between the atmospheric pressure and the pressure inside the furnace. Induced Draft fans create negative pressure inside the furnace whereas Forced Draft fans would create a positive pressure inside the furnace.

So, we have seen few examples where a centrifugal fan has been named differently on the basis of its applications, construction or design, and its characteristics or principle of operation. Further centrifugal fans can be customised to serve a special need. Accessories available with centrifugal fans stretch the application of these fans to an extra mile.