Thanks for linking the post but I still don't quite understand. Is dead heading when the system resistance is so high that the fan is at essentially zero airflow?
Is there a scenario where the system resistance could be low and the fan dead heads?
Dead-heading would be zero flow. So that would be a damper being closed or some other blockage. Yes, a damper or valve being closed basically provides an infinite resistance.
in real life there will be minimum flow due to leakage between the fan and that damper. But for practical purposes you can assume no flow. All the fan energy will over time heat up the fan since there is no airflow to cool the fan.
Okay thank you, so the scenario I'm seeing is a fan which definitely doesn't have too high system resistance but is cutting out before maximum fan speed due to current monitoring on the fan motor getting too high.
However, applying additional resistance to the fan inlet helps to reduce the current and allow the fan to run higher fan speed. So is this still considered dead heading or something different?
Fan curves are very similar to pump curves. Consider:
A fan that can move 1000 CFM at 2" w.c. (static pressure) through some duct work. Now add an air filter that will increase this resistance, so the fan is now flowing 800 CFM at 2.5" w.c. of static pressure.
Now if you keep adding to this eventually you will reach a point where you are generating a high static pressure at zero (or close to) flow. If you are not moving the fluid into and then away from the mover then you will be increasing the energy in the fluid local to the pump/fan, causing the mover & fluid to increase in temperature and ultimately something to fail.
Most (all?) pumps have a MCSF or minimum continuously stable flow where they can still effectively flow the medium without any undesired effects, I am sure there is something similar for fans though I do not have the experience.
I guess if you were to operate a fan in a vacuum there would be low resistance on the fan but the fan would still be in a dead head situation of not moving any air, but you also wouldn't be accumulating any energy into a fluid.
So this particular fan isn't operating in a vacuum, and if anything is oversized for the system resistance. I think that's why I'm struggling to understand the issue because it should be the opposite of enough system resistance to cause dead heading
It doesn't make sense to me that increasing resistance, increases flow and reduces current since flow rate and power (& therefore current with constant V) are proportional. Could you be more specific about the system?
I assume you have a multiple speed motor fan or a fan with variable speed that you can adjust manually. As you speed the fan up more flow is produced and more static pressure is also produced at higher speeds. There comes a point where the pressure/flowrate exceeds the horsepower capabilities of your motor without any added resistance to the system such as throttling a manual damper at the outlet of the fan. Your fan over heats and then cuts out.
Now when you throttle a manual damper at the outlet of the fan the pressure may go up but the flow goes way down so you never exceed the horspower of your fan.
In summary your motor horspower is too low to handle your required flowrates at static pressure required.
The specifics are it's an EC centrifugal backward curved fan blowing against a resistance on the discharge side with very little resistance on the suction side. The fan is controlled by 0-10V and when getting up to 10V (after about 9.4V) the logic inside the motor is detection high current resulting in overheating on the motor and shutting the fan down.
There are multiple fans, I'm confident there's very low resistance per fan and if anything more fans than required to overcome the resistance. By partially blocking the suction side (fan inlet) the current reduces allowing the fan to run up to 10V.
It looks like your fan motor is not sized for end of curve operation.
I.e. there is too much flow going through it.
Either limit your system to 9.4V or put on a bigger motor. Or throttle the fan a little bit. To do it properly you need the fan curve and take measurements of flow and pressure. Up to you. We don't know what size you're talking her but 10V sounds like a computer fan....
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
EC = Electronically Commutated, combination of DC and AC.
Sorry if I'm coming across an idiot I still just can't get my head around how there's too flow going through it and it's not sized for the application. I would have assumed it would be able to go to 10V/max fan speed and provide whatever the resulting airflow is at that static pressure of the system resistance.
Unfortunately I'm unable to share most of that due to either not being with the application to take measurement and IP. All I can say it's that it's a HVAC application, multiple EC centrifugal fans blowing air against an air filter and heat exchanger with no resistance on the fan inlets.
I understand that. but what is the input to the controller and the control sequence? It could be based on meeting a static pressure or flow setpoint or something else.
Also need to know what the entire system is and does. Is it an open ductwork, or a controlled flow with VAV boxes? You need the whole system to find out what is going on.
" I still just can't get my head around how there's too flow going through it and it's not sized for the application. I would have assumed it would be able to go to 10V/max fan speed and provide whatever the resulting airflow is at that static pressure of the system resistance"
Well without data and only your description, that's what it is telling me.
The supplier might be able to throw some light on it. Can you post a link to a brochure or model number.
But it loos looks like the pressure resistance is too low and they made a fan with a smaller motor and sold it cheaper than one designed to handle end of curve operation. Maybe.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
