Fan Motor Current

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Inifinity,

The blades will be moving their own turbulent air, a smaller volume,
and the load will Decrease.

So, the RPM should Increase measurably.

IMO,
You could physically measure the air flow,
but the problem defines its own solution.
 
The power required for doing work in a centrifugal machine (pump or fan) is related to the "q", of the flow (volume) passing through it. The actual formula is P = dp * q where P = Power (watts), dp = change in pressure (d = delta or change, p = pascals, a unit of pressure) and q = volumetric flow in cubic meters per second. Do if you drop either the pressure or the flow, the power required to do the job drops. When you close a damper, you are doing both.

This is related to how VFDs save money by the way. Some people like to claim that VFDs do not save any real energy by pointing ut this fact. You use less energy when you reduce flow in an air duct no matter how you do it, VAV (dampers) or VFD on the motor. But with dampers, you also get turbulence, so that then increases the pressure in the duct (or the fan box if they are inlet dampers), meaning that your dp in that formula goes down. By using a VFD instead, there is a difference in the AMOUNT of total energy savings that you gain by reducing flow, because you no longer create the turbulence that was robbing your duct of efficient air flow.
 
Jraef,

""
Do if you drop either the pressure or the flow, the power required to do the job drops.
""

In the Fan Example, I suggested that covering the blades would cause more turbulance, because the blades are no longer slicing air volume as designed to do.
This would remove some of the load on the blades, allowing increased speed, and drawing less current. This is different from dampers causing back pressure and turbulance.

But, I think both approaches are going in the same direction.
Less work, Less current.
 
Jraef said:
The power required for doing work in a centrifugal machine (pump or fan) is related to the "q", of the flow (volume) passing through it. The actual formula is P = dp * q where P = Power (watts), dp = change in pressure (d = delta or change, p = pascals, a unit of pressure) and q = volumetric flow in cubic meters per second. Do if you drop either the pressure or the flow, the power required to do the job drops. When you close a damper, you are doing both.
Click to expand...
See post #11.
;)

Jraef said:
This is related to how VFDs save money by the way. Some people like to claim that VFDs do not save any real energy by pointing ut this fact. You use less energy when you reduce flow in an air duct no matter how you do it, VAV (dampers) or VFD on the motor.
Click to expand...
A VSD reduces both head and flow at reduced speed.
 
I have been working with a customer to reduce the size of a fan motor from the a now overloaded 250 hp to a properly loaded 150 hp by changing the rpm of the fan. I think it is finally going to happen. What helped was downloaded specs from a similarly sized fan and the affects rpm had on the required bhp. In this case the reduction in fan speed and subsequent 100 hp reduction could amount to about $10,000 per year in savings. After the system is engineered correctly a VFD may be added to fine tune instead of the damper they now use. A drive for 150hp is considerably cheaper than for 250hp.
 
ptonsparky said:
I have been working with a customer to reduce the size of a fan motor from the a now overloaded 250 hp to a properly loaded 150 hp by changing the rpm of the fan. I think it is finally going to happen. What helped was downloaded specs from a similarly sized fan and the affects rpm had on the required bhp. In this case the reduction in fan speed and subsequent 100 hp reduction could amount to about $10,000 per year in savings. After the system is engineered correctly a VFD may be added to fine tune instead of the damper they now use. A drive for 150hp is considerably cheaper than for 250hp.
Click to expand...
The power for a centrifugal fan is proportional to the cube of the speed so it won't take much of a speed reduction to get down to the required load.
 
In this case the duct system was changed a few years ago. We don't know what the CFM is now. We don't know what the existing fan curves are. We do have a good idea what the linear feet per minute is required to move the product but that changes as the temp of the incoming air changes from ambient to 500 degrees plus or minus. As I understand it cold air is harder to move than warm but it does more work because it is denser. In short I am an electrician and am glad to pass the details and responsiblity on to someone that knows.
 
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