Theoretical Fan Questions in order to get myself a better understanding of how they operate

[CeaserMech]

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.

 

[DRWeig]

Here's a good start for you:

http://www.greenheck.com/media/articles/Product_guide/perf_basics.pdf

Best to you,

Goober Dave

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[ChasBean1]

sjbayer3, too much. I'd have to remediate. To try to keep it basic:

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.

 

[Waramanga]

1. All motors for hvac are synchronous, ie they rotate at a speed dependant on the line frequency and the number of poles the motor has. If you have a giant motor driving a tiny fan it will spin at the same speed as the right sized motor on the same fan (assuming the same number of poles.
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??

 

[willard3]

You should study some fluid/fan mechanics. I recommend Buffalo Forge Fan Engineering Manual.