Retrofitting is the process of upgrading an existing system by adding a new component or equipment. Centrifugal fans used in industrial plants are very large and they are retrofitted to improve the performance and efficiency. In centrifugal fans impeller is the most important component as it gives the necessary direction and pressure to the airflow. Hence, in case of centrifugal fans retrofitting of an impeller is important to regulate and control the air movement in the industries. When an impeller is changed it facilitates the change in other parts such as casing, inlet cones and shaft of impeller.
As said earlier impeller is the most important component of a centrifugal fan and the overall performance and efficiency is dependent on it. Retrofitting of an impeller can be done in following two ways.
• Trimming or tipping the impeller diameter
Trimming involves reduction of impeller diameter and tipping involves increasing diameter of impeller by adding material to the ends. Change in diameter of impeller influences three main features, such as air-flow, pressure and power.
> Change in flow is directly proportional to impeller diameter
> Change in pressure is directly proportional to (change in impeller diameter)2
> Change in power is directly proportional to (change in impeller diameter)3
Trimming or tipping is less expensive method to retrofit and the overall halt time of the plant is less. If the diameter of impeller changed is very less, then the efficiency change may be negligible and therefore trimming or tipping is done for at least 10% of impeller diameter.
• Replacing with a new impeller
Impeller is replaced if the trimming or tipping procedures does not improve the efficiency of the fan. In this process we design a new impeller by considering the parameters such as air-flow rate, system resistance, density of gas at the inlet, site altitude and temperature of gases inlet of the existing fan. With the above points taking into consideration, an impeller is designed having a diameter which is almost nearer to the existing one, so that it fits perfectly in the casing without any necessity for modification or replacement.
Retrofitting of casing depends on the replacement of an existing impeller. If the newly designed impeller does not fit inside the existing casing then throat portion of casing is modified to let in the new impeller. The clearance between the impeller and casing vary from 5 to 20% depending on pressure and airflow.
Inlet cones are used in centrifugal fans to facilitate easy flow of gases into the impeller. So, inlet cones reduce the resistance of flow into the fan. When we design for a new impeller, it demands for a matching inlet cone as well, because the newly designed impeller can take more air into it.
Sometimes it may need to design a new shaft as the new impeller cannot be fitted in the existing one. Shaft is designed based on the total weight of the impeller which is being installed and the velocity of the inlet gases.
Retrofitting is a better alternative to improve performance than going for a complete new installation of fan. It saves money as well as overall halt period of the fan system and in some cases overall plant halt period.
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