Cleaning Filters increase Amp Draw in Fans

[xtragr8]

Can someone explain the operation of a constant speed fan that would make a constant speed fan increase in Amp Draw after the filters have been cleaned? Is it because there is more air to blow? I would think that the lower static pressure on the system would cause the opposite, but that is not the case.

 

[rmw]

You called it right. The clean filters have less pressure loss, and pass more air, and it takes additional HP to make it flow.

rmw

 

[imok2]

If you have a partial blockage on the suction side of a pump (less flow) the motor amps will go down, the same priciple applies to pumping air

 

[friartuck]

It depends on the type of fan you are using and the point of operation on the fan curve.

Forward curved fan have a rising power characteristic so that when more air is passed, the more power is drawn. If you take a side panel off an air handling unit when it is running, the fan motor may overload. However, if you use a backward curved fan, (depending on the manufacturer), you may find a slight rise and then a fall as the airflow rises.


We in the UK use both types in AHU's. FC fans are cheap, but I prefer BC curved fans because they are more tolerant to airflow etc.

Friartuck of sherwood

 

[EagleCove1]

Dirty Filters add resistance and lower airflow volume. Clean filters decrease resistance thus more air volume. Standard air weighs .075 lb/Cubic Foot.

Units of Power are based on effort to move weight at a given pressure and volume. While air is relativly light in comparison to say water (8.33 lbs/gal)electrical energy will still increase to accomplish this additional level of loading on the fan motor.

The fan laws are based on Forward Curved fans that state the the volume of increase has a expondential impact on motor BHP [Cube Root of the (New CFM divided by the Old CFM)times the old BHP equals the New BHP ]

The type of fan wheel that you use gives some better qualities as stated in other responses.

 

[friartuck]

Fan laws aren't based on any particular fan type. Other wise they would be called the forward curved fan laws or the backward curved fan laws.

Fans also exhibit odd behaviour. Try measuring the fan speed of a direct drive FC fan and then closing the duct damper to throttle it. The fan impellor speed up dramatically. This characteristic varies between fan types.

When you take a side panel off an air handling unit when it is running, the reduced system resistance allows the fan to push more air and absorb much more power (FC Fans). The net result is that unless the motor is suitably sized, or protected, it will overload. BC fans don't do that. Try using a current transformer to measure the variations, and you will get a few surprises.


Friar Tuck of Sherwood