In thermal power plant practices, the speed of PA & FD fans are high 900 to 1400 rpm, speed of ID fan that handles more volume and more mass of flue gases as compared to PA & FD Fans combined is only 500 rpm. What are the design criteria for this practice?
hi dear,
Normally in power plant fans with belts and pulley systems are used in such cases to reduce the speed. or can use variable speed motors for this purpose but for large capacity it may be expensive. for balance the draft dampers are used for flow volume control.
Dear Asim,
You are talking about low capacity cases.
First, as flue gas contain fly ash which are abrasive in nature and would erode the ID Fan blades fast at high speeds in large thermal power units.We can keep the ID Fan's speed low as a design criterion to save these blades from erosion and vibrations/shut down.
Secondly, ID Fan has to deliver the flue gas at top of the chimney at a low pressure where high speed is not required. For low speed and less power consumption, Hydraulic coupling and variable frequency drive motors are deployed.
For above two reasons, speed of ID Fans is kept low.
hi dear,
thanks for reply. i know all what you have explained. i gave what i have observed being an O&M Engineer at co-generation Coal, Baggage, Oil and gas fired power plant. the standard motor speed were 900rpm, 1100rpm and 1800rpm.
As explained, higher volume and low pressure demands can be met by lowering speed, increasing the diameter of impeller and opting to go for double suction radial fans, are best preferred. This automatically takes of erosion , life of fan components and also lesser dynamic loads and vibrations apart from lesser noise levels . If axial fans are chosen then material selection for blades shall be carefully done and hence ferrous materials are best compared to normal Aluminium materials.
To take care erosion and extended life, ID fans of radial types, are designed with variable speed methods like Hydraulic couplings, Variable Frequency Drives, etc. If we go for Axial flow type fans for ID applications, speed variation is not feasible. Fans selected for maximum speed operation so as to meet the maximum parameters called Design Point/ Test Block at Best Efficient Point. If mechanical flow controls like butter fly damper/ Inlet damper control/ Inlet Guide Vane controls are selected, then fan efficiency will be far low compared to Best Efficient Point (BEP), when fans are operated for lower parameter requirements like 100%MCR, 60%MCR, etc..
Thanks for permitting to share
Anantharaman - Ex BHEL
An experienced fan engineer
Low discharge pressure fans are designed with large volume and slow speed criteria for less erosion of moving and stationary components.
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