I kindly need your input on a project I am doing, pardon me for the long text.
I have 2 furnaces with a common fume-extraction-duct. The duct split in to two (like a ‘Y’) and a fan is fitted on each split for the purpose of redundancy. The fans sit in the ducts but shafts, mechanical torque converter and 1 speed 3 phase D.O.L (direct-on-line) motors sit outside.
A controller compares atmospheric pressure to the pressure in the ducting and commands the mechanical torque converter to drive the fan slower or faster so as to cause a flow of fumes from the furnace to the atmosphere.
I need to upgrade this system by replacing the torque converter with a V.S.D. that will control the speed of the motor. Pneumatic controls will also be replaced by electrical signals. The main purpose of doing this project is because we can’t get spares of the torque converter. The pneumatic loop is also susceptible to leaks and the present motors were poorly selected – open motors instead of closed motors. Besides, this system is 40 years old.
The current motors are rated at 380V, 15KW, 30A, and 960RPM. The fan operation speed ranges between 540RPM to 870RPM.This is a high mass/high inertia fan by design so as to coup with the chemicals and temperatures in the fumes.
Although the current motors are working properly, we can’t just duplicate their sizes because we are going to take out the torque converter that is sitting between the fan and the motor and all what we will be left with is this heavy fan, shaft and VSD controlled motor. We are also not sure how the motor sizes were determined – the designers might have made a mistake (lower or higher) simmilar to the one they made of selecting an open motor instead of a closed one.
We are in doubts as to how to size of the motors and whether the VSD will be able to perform as good as the variable torque converter is.
An engineering manager wants us to use fan-curves characteristics as a guide. I say this are not an ordinary fans i.e. low starting torque and high torque at high speed - therefore characteristics of an ordinary fan will not apply.
A mechanical engineer wants us to wrap a rope on the shaft a couple of times, pull it to turn the shaft, with a clock on his hand. Then he will work with time and length of the rope and the thickness of the shaft to determine momentum. I said that is a joke.
I suggested that we determine the torque in Nm then multiply it by 960RPM then divide the product by constant 9550 to determine motor KW but I have not been able to convince anyone. What I am not sure of is what to do with this KW. All what I know is that this is the pull-out power required, after that the fan will require little power to move it. We also know that the VSD will mitigate the starting torque but we can’t put all this facts together. This is a critical part of the plant so accuracy is paramount. If this system fails we will loose about US$500,000 a day and hopefully no employees.
May some one kindly shed some light.
Thank you.
I have 2 furnaces with a common fume-extraction-duct. The duct split in to two (like a ‘Y’) and a fan is fitted on each split for the purpose of redundancy. The fans sit in the ducts but shafts, mechanical torque converter and 1 speed 3 phase D.O.L (direct-on-line) motors sit outside.
A controller compares atmospheric pressure to the pressure in the ducting and commands the mechanical torque converter to drive the fan slower or faster so as to cause a flow of fumes from the furnace to the atmosphere.
I need to upgrade this system by replacing the torque converter with a V.S.D. that will control the speed of the motor. Pneumatic controls will also be replaced by electrical signals. The main purpose of doing this project is because we can’t get spares of the torque converter. The pneumatic loop is also susceptible to leaks and the present motors were poorly selected – open motors instead of closed motors. Besides, this system is 40 years old.
The current motors are rated at 380V, 15KW, 30A, and 960RPM. The fan operation speed ranges between 540RPM to 870RPM.This is a high mass/high inertia fan by design so as to coup with the chemicals and temperatures in the fumes.
Although the current motors are working properly, we can’t just duplicate their sizes because we are going to take out the torque converter that is sitting between the fan and the motor and all what we will be left with is this heavy fan, shaft and VSD controlled motor. We are also not sure how the motor sizes were determined – the designers might have made a mistake (lower or higher) simmilar to the one they made of selecting an open motor instead of a closed one.
We are in doubts as to how to size of the motors and whether the VSD will be able to perform as good as the variable torque converter is.
An engineering manager wants us to use fan-curves characteristics as a guide. I say this are not an ordinary fans i.e. low starting torque and high torque at high speed - therefore characteristics of an ordinary fan will not apply.
A mechanical engineer wants us to wrap a rope on the shaft a couple of times, pull it to turn the shaft, with a clock on his hand. Then he will work with time and length of the rope and the thickness of the shaft to determine momentum. I said that is a joke.
I suggested that we determine the torque in Nm then multiply it by 960RPM then divide the product by constant 9550 to determine motor KW but I have not been able to convince anyone. What I am not sure of is what to do with this KW. All what I know is that this is the pull-out power required, after that the fan will require little power to move it. We also know that the VSD will mitigate the starting torque but we can’t put all this facts together. This is a critical part of the plant so accuracy is paramount. If this system fails we will loose about US$500,000 a day and hopefully no employees.
May some one kindly shed some light.
Thank you.
