Pressure Blower with restricted intake

[Roach]

I have a pressure blower with a restricted intake.
When I open the intake fully (no restriction) my motor pulls FLA (I have blown fuses). When I close it (restrict air flow) it goes down to idle (normal amperage on the motor).
I would think that restriction to intake causes the motor to work harder. Why is this not the case?
Thanks.

Roach

 

[fredb]

Hello,

By way of your thinking, as an automobile goes faster, it requires less horsepower. Horsepower is work, generally speaking if you move more of anything per unit of time, it will require more work.

 

[Roach]

Please correct my thinking....
A direct drive fan will turn a constant (within reason) RPM, and the fan will try to emit a set air volume per revolution (CFM).
If I restrict the intake, it seems to me that it would reqiure more mork to move this volume of air.
Pumps try to deliver a certain amount of GPM at a certain head.
Do fans not try to create a certain CFM at a certain DP?
I am sure the compressibility of the fluid plays a role in this, but is that the only difference?

Roach

 

[pablo02]

Roach -- what is happening is that the fan turns at a fixed RPM; how hard it works depends upon how much air it moves and it only moves the air that gets to it -- you restrict the intake, LESS air is moved consequently, the motor amps will drop -- the rated CFM of such a fan is based upon free intake

 

[Roach]

Is this true for all centrifugal fans (squirrel cage, forward curved, backward curved, etc.)?

What about centrifugal pumps?

Roach

 

[pablo02]

You guessed it -- it applies to both centrifugal fans as well as pumps... one reason that large centrifugal fans / pumps usually require the dampers or discharge valves throttled or closed when starting...

 

[MintJulep]

Different types of fans behave differently to changes in pressure. For example, forward curved blades typically tend to respond as you describe. As the pressure that the fan is working against increases the MASS flow of air decreases, or conversely as the pressure decreases the mass flow increases, leading the higher current draw.

BAckwards inclined fans, sometimes refered to as "non-overloading fans" do not do this.

Study the fan curves for various types of fans to see what happens. Remember, fans really work on a mass flow basis. CFM ratings assume standard air conditions. If the density of the air you are moving differes significantly from standard you MUST correct for density.

 

[Roach]

Great explanation.
Thanks for the clarification.
"Common sense" would make me think that increasing the static pressure(fan)/head(pump) would require more torque from the motor, but as I understand it, it is all related to the total work being performed by moving the mass.
Maybe my sense isn't so common.
Roach

 

[chicopee]

Because there is less air mass flowing thru the fan; consequently less load(or torque) on the motor and therefore less current( less electrical energy) useage by the motor.

 

[gnordo]

The phenomina you describe is characteristic of a forward curve centrifugal fan [ i.e. SQUIRREL CAGE}.

Typically the motor HP is selected for the fan pressure duty. So if the fan is allowed to operate at less than design Static Pressure, it will move more air, and WILL move towards overload of motor. At free flow [no back pressure and inlet pressure to piush against, the fan will move far more than design flow and impose an overload condition on the motor, UNLESS THE MOTOR WAS MATCHED TO THE FAN WITH THE HP SELECTED TO MEET FREE FLOW CONDITIONS.


It is not uncommon for squirrel cage motors to become exposed to this type overload when the unit it is installed in, is opened to be serviced. The opening of the unit can allow the air to be pulled in, and/or discharged out of the unit without being restricted by the ductwork/filter/coil pressure drops.

A classic example of this is in a residential HVAC unit. lmost universally, they employ a forward curve blade. If the unit is opened for servicing any length of time, the motor may run overloaded until it's thermal overload element safety heats up enough to turn the power off through it, or an overload element in the fan circuit will trip.

A FORWARD CURVE BLADE MUST BE USED WITH THE DESIGN STATIC PRESSURE FOR WHICH IT WAS SELECTED!

As someone else mentioned, the classic non overloading blade is the backward curve centrifugal.

Gnordo