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Fans and Blowers - Saylor.org

Fans and Blowers - Saylor.org

5. Fans

5. Fans and BlowersImpeller Inlet Seal Clearances• Axial overlap – 5 to 10 mm for 1 metre plus dia impeller• Radial clearance – 1 to 2 mm for 1 metre plus dia impeller• Back plate clearance – 20 to 30 mm for 1 metre plus dia impeller• Labyrinth seal clearance – 0.5 to 1.5 mmThe inlet damper positioning is also to be checked regularly so that the "full open" and "fullclose" conditions are satisfied. The fan user should get all the details of the mechanical clearancesfrom the supplier at the time of installation. As these should be strictly adhered to, for efficientoperation of the fan, and a checklist should be prepared on these clearances. A check on theseclearances should be done after every maintenance, so that efficient operation of the fan isensured on a continuous basis.System Resistance ChangeThe system resistance has a major role in determining the performance and efficiency of a fan.The system resistance also changes depending on the process. For example, the formation of thecoatings / erosion of the lining in the ducts, changes the system resistance marginally. In somecases, the change of equipment (e.g. Replacement of Multi-cyclones with ESP / Installation oflow pressure drop cyclones in cement industry) duct modifications, drastically shift the operatingpoint, resulting in lower efficiency. In such cases, to maintain the efficiency as before, the fan hasto be changed.Hence, the system resistance has to be periodically checked, more so when modifications areintroduced and action taken accordingly, for efficient operation of the fan.5.5 Flow Control StrategiesTypically, once a fan system is designed and installed, the fan operates at a constant speed. Theremay be occasions when a speed change is desirable, i.e., when adding a new run of duct thatrequires an increase in air flow (volume) through the fan. There are also instances when the fan isoversized and flow reductions are required.Various ways to achieve change in flow are: pulley change, damper control, inlet guide vanecontrol, variable speed drive and series and parallel operation of fans.Pulley ChangeWhen a fan volume change is required on a permanentbasis, and the existing fan can handle the change incapacity, the volume change can be achieved with a speedchange. The simplest way to change the speed is with apulley change. For this, the fan must be driven by a motorthrough a v-belt system. The fan speed can be increased ordecreased with a change in the drive pulley or the drivenpulley or in some cases, both pulleys. As shown in theFigure 5.10, a higher sized fan operating with dampercontrol was downsized by reducing the motor (drive)pulley size from 8” to 6”. The power reduction was 12kW.Motor8”1470 RPM6”1470 RPMFan10”1181 RPM10”882RPMFigure 5.10 Pulley Change26 kW14 kWBureau of Energy Efficiency103

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