Theoretical Fan Questions in order to get myself a better understanding of how they operate
Theoretical Fan Questions in order to get myself a better understanding of how they operate
(OP)
So you have a 3phase direct driven plug fan, blowing air into a VERY large space with an exhaust route such that the fan can run in a steady state.
1. What determines the RPM of the fan/motor? I can see a motors come often at fixed RPM from the catalogue, does this mean that when a motor turns on it always wants to operate at the same RPM? So, for example, if the pressure is higher, the fan will work harder (thus drawing more power) in order to get to that same RPM?
2. If we were to close the exhaust route in this very large space, what would be the temporal affects? I suspect the Fan will continue to pressurize until the pressure in the space is the maximum the motor can handle and it would just burn out? How would the RPM react during this time? What about the current?
3. Does having a belt driven fan change this (besides that the fan may operate at a different RPM than the motor)?
4. Lets say we added a VFD that we operated based on a static pressure setpoint in the room. What would happen after we closed the exhaust route? I suspect the fan speed would reduce as we reached the setpoint, until shutting off?
I may have more questions, but thank you for taking the time, I suspect I am asking a bit but I want to make sure I have the best knowledge and these are some questions I have thought of multiple times and wanted to get clarified.
1. What determines the RPM of the fan/motor? I can see a motors come often at fixed RPM from the catalogue, does this mean that when a motor turns on it always wants to operate at the same RPM? So, for example, if the pressure is higher, the fan will work harder (thus drawing more power) in order to get to that same RPM?
2. If we were to close the exhaust route in this very large space, what would be the temporal affects? I suspect the Fan will continue to pressurize until the pressure in the space is the maximum the motor can handle and it would just burn out? How would the RPM react during this time? What about the current?
3. Does having a belt driven fan change this (besides that the fan may operate at a different RPM than the motor)?
4. Lets say we added a VFD that we operated based on a static pressure setpoint in the room. What would happen after we closed the exhaust route? I suspect the fan speed would reduce as we reached the setpoint, until shutting off?
I may have more questions, but thank you for taking the time, I suspect I am asking a bit but I want to make sure I have the best knowledge and these are some questions I have thought of multiple times and wanted to get clarified.





RE: Theoretical Fan Questions in order to get myself a better understanding of how they operate
Best to you,
Goober Dave
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RE: Theoretical Fan Questions in order to get myself a better understanding of how they operate
1. Yes, fan power drops if static pressure drops for the same airflow. RPM: Yes, fixed RPM does mean same RPM.
2. No. If the space is large as you say, continued operation of supply after exhaust shutoff will not continue to build pressure in the space. The change in pressure after losing supply or exhaust flow will happen inside of 5 seconds. The resulting pressure can be estimated by the difference in supply and exhaust (cfm): Q = 2610 * A * dP^5. (Q is cfm difference between supply and exhaust, A is total leakage area in ft2, and dP is in inches w.c.).
3. No.
4. Yes.
RE: Theoretical Fan Questions in order to get myself a better understanding of how they operate
2. Assuming the room is sealed, your fan will ride its curve until the motor overloads or the fan is ripped apart (overloading fan) or the fan falls off the end of its curve and the blades stop shovelling air and you get no flow at some low pressure (non overloading fan).
3. No
4. The fan will reach the setpoint but not shut off, just change speed to find some point that the fan produces the required pressure at no flow. You wont be able to find any data for no flow situation though but you can try it next time you are playing with a small fan, just cap it off and there is still pressure there even at no flow. I am not sure if this would be a stable condition though, anyone else got thoughts on that??
RE: Theoretical Fan Questions in order to get myself a better understanding of how they operate