Blower/Fan Performance 1

[Webber]

I have the performance data of a small DC powered centrifugal forward curve fan. According to this data, the rpm increases and amp draw decreases as the static pressure increases. The voltage is constant at 13.5vdc. I would have expected the fan to run at full speed with no static pressure and the amp draw would be at lowest point with zero static pressure.

Fan laws dictate in general that cfm, static pressure and bhp increase with rpm increase.

Can anyone explain the reasoning for these results?

 

[MintJulep]

As the static pressure increases, the flow decreases, hence the fan is doing less work.

 

[Webber]

The same performance data provided for a vane axial or prop fan shows the static pressure increase, rpm decrease, air flow decrease and amp draw increase? This seems to contradict somehow. Is there a difference in performance between centrifugal blowers and vane axial fans?

 

[MintJulep]

Power has both RPM and current draw in the equation.

Different types of fans do behave differently as various factors change, however I'm pretty sure that power will always go down as mass-flow rate goes down. If you find a fan that doesn't follow this you are on to something big.

 

[MintJulep]

Take a look at thread403-65758

 

[Webber]

Thank you very much for your input. I will investigate the prop fan data I have further to confirm the readings. Nonetheless, I appreciate your input!!!!

 

[ChasBean1]

Webber, I applaud both you and Roach (from Mint's thread reference above) for the question. I understand the fan laws and relation of power and flow rate.

BUT...

A motor is a dumb device. If the motor spins at X RPM with minimal resistance (such as when static pressure drop across the fan is lowest, which occurs at highest flow), how could it draw a higher amperage than when it spins at X RPM with maximum resistance (such as when static pressure differential is highest, at lowest flow)? If you try to shut the suction or discharge of a running constant volume fan, it sure sounds to me like the motor works harder.

There are different fan curves for different fans, but all the ones I've seen show higher static pressures at low flows and lower static pressures at high flows.

This absolutely defies common sense (at least to me, you, and Roach) and I have yet to see a true explanation, in terms of friction, forces, etc., without referring to accepted thumbrules and the premonition that the motor "knows" when the fan moves more air.

 

[MintJulep]

Chas,

Motors are not only dumb, they are lazy too. They will only provide the power needed to drive the load that they are connected to at the moment.

AC motors are NOT constant power devices. A 1 HP motor only provides 1 HP if it needs to drive a 1 HP load. If you connect a 1 HP motor to a 1/2 HP load, the motor will provide 1/2 HP. If you connect a 1 HP motor to a 5 HP load, the motor will try to provide 5 HP and burn up. Its a conservation of energy thing. If a motor always provided its rated full-load power, where would the excess power go when the load was low?

The resistance you are refering to in your question is the resistance to air flow. The motor doesn't see this, it only sees how much work it is being asked to do, which is a function of how much mass per unit time the fan is moving.

An synchronus AC motor will try to turn at it synchronous speed, which is a function of the frequency of the power source, and the number of poles in the motor. As more load is applied to the shaft of the motor, more torque is required to attempt to maintain the RPM. The motor generates more torque by drawing more current from the power supply. Power is torque x RPM. In real life, motors do not spin at a truely constant RPM. AC motors slow down a bit as they are loaded up. This is because of slip, electronically within the motor. We'll need a real motor guy to explain that to us all.

 

[twainagain]

I'm not an expert, but, after a few minutes with the equations of state, I recall that we find

work produced by a fan or a pump is change in head (corresponds to your change in "static pressure?&quot times mass flow rate...

in other words, for fans, it's the product of cfm and delta p times some correction factor to get your favorite units.

The fan curve, then, describes a family of conditions where the fan's work out should factor to a narrow range of work output. We expect various losses would increase with cfm, so the gross power required from the motor would go up (disproportionately) with cfm as well.

For a shut suction, the only work is that of heating up the air, scroll, bearings, etc as the air spins around with the fan instead of flowing. The fan develops the shutoff head (max) at zero flow (min); this defines one end of the fan curve.

As to motor sound, don't we only hear armature speed? I think it can make almost as much noise free-wheeling as at full load. Without some duct hum, would we have much of any way to know what it's doing?

A motor is not like an engine where the higher cylinder pressures tell you of the load even when the tach doesn't change (for example, an engine gets louder starting up a hill, even a stick shift with cruise control.....because it's more work at the same speed).

Don

 

[twainagain]

I'm not an expert, but, after a few minutes with the equations of state, I recall that we find

work produced by a fan or a pump is change in head (corresponds to your change in "static pressure?&quot times mass flow rate...

in other words, for fans, it's the product of cfm and delta p times some correction factor to get your favorite units.

The fan curve, then, describes a family of conditions where the fan's work out should factor to a narrow range of work output. We expect various losses would increase with cfm, so the gross power required from the motor would go up (disproportionately) with cfm as well.

For a shut suction, the only work is that of heating up the air, scroll, bearings, etc as the air spins around with the fan instead of flowing. The fan develops the shutoff head (max) at zero flow (min); this defines one end of the fan curve.

As to motor sound, don't we only hear armature speed? I think it can make almost as much noise free-wheeling as at full load. Without some duct hum, would we have much of any way to know what it's doing?

A motor is not like an engine where the higher cylinder pressures tell you of the load even when the tach doesn't change (for example, an engine gets louder starting up a hill, even a stick shift with cruise control.....because it's more work at the same speed).

Don

 

[ChasBean1]

Mint, where would the excess power go? Heat dissipation. I remember slip and torque, but the relation between torque and air volume is where it becomes disconnected.

I don't disagree with any of what you or Don said. But the original point (the intent anyway) was beyond the accepted engineering equations and known facts. Take the fan off the motor shaft altogether. Start and run the motor, then slowly, incrementally, increase the pressure of a brake against the shaft. To me, this would simulate added friction that might be caused by pinching off the air flow by closing a damper, adding pressure, which should increase amperage if RPM were held constant.

There is a mechanical phenomenon that I'm missing, that I'd need to fully understand the 'why' behind it. One thing that Don brought up above was "losses," which might be the key behind it. More air volume per unit time, more air molecules impacting the fan blades, more molecular force lost in normal (vs. tangential) motion of molecules against the shroud, etc., leading to a net effect of added braking on the motor shaft.

Although higher D/P would appear to be the main cause of higher turning friction, losses at higher mass flows would (counter-intuitively to me anyway) outweigh the friction reduction from lower D/P and have a higher net braking effect.

It's like other things in life - you hit a point where you just accept it as fact. Does this make sense?

Thanks for your above inputs. -CB

 

[MintJulep]

Fan laws are to be used to predict the performance of a fan if you know the performance characteristics of a similar fan. They do define the performance curves of a single fan.

To know the performance of any given fan, you need to test it and plot the curves.

Once you know the curve for one fan of a particular design, you can generate new curves for a similar fan of say a different diameter.

To paraphrase Mark's Handbook, "Fan laws are useful, but they are dangerous if misapplied."

Chas, the flaw in your thinking is that you have convinced yourself that adding a restriction to airflow is identical to adding braking toruque to the motor shaft. They are different things.

Get a bunch of fan catalogs. Look at the performance curves. You will see that for any fan (in the stable region) flow decreases as pressure increases, and that power decreases as flow decreases.

Power is work per unit time. Work is moving a mass. If you increase the pressure across a fan LESS air goes through the fan, this means less mass goes through the fan and the fan does LESS work and hence draws less power.

 

[Webber]

MintJulep,

It seems the more I know, the less I understand. I am certainly enjoying the discussion, nonetheless.

I did look at some fan curves and you are correct regarding the stable condition of the fan performance. However, this trend changes with a prop fan for example. I looked at the fan curves for a Lau H, S and heavy duty prop fans. They completely contradict the centrifugal blower data. According to these fan curves, the CFM increases with reduction in static pressure, but the break horsepower also decreases gradually.

 

[MintJulep]

Well crap! Webber, you are correct. Thanks for keeping me honest.

I don't see any efficiency numbers on the Lau web site. I suspect that this is because the efficinecy of a prop fan falls rapidy as static pressure increases.

 

[ChasBean1]

Mint, I understand - you may have misunderstood my last post. Accepting work as a function of mass flow and not further thinking of what occurs mechanically to cause this, it would hold true that the maximum fan power is at maximum air flow. Most fan curves I've seen show a drop in power at maximum flow, with the power peak at about 2/3 to 3/4 of max flow.

 

[zakh]

Mint..the static efficiency curve of a prop fan is bell shaped, peaking at approx. 2/3 of max. flow.

Webber...for a forward curve centrifugal fan, the BHP curve rises continuously as the CFM increases...IOW, its an overloading type fan. So when you increase the static pressure, the CFM reduces, and the BHP reduces. A motor will just try to meet the power requirement it sees from the fan...and I would not be surprised if the RPM for a DC motor with constant voltage rises with a reduction in power. However, as the RPM rises, so will the fan curve (because its running at a higher RPM). It is unlike a fan powered by an async induction motor which will try (slip increases as load increases...therefore speed reduces slightly as load increases) to maintain synchronous speed. A fan powered by an induction motor will "ride" the fan curve as you change the static pressure, while a fan powered by a DC motor will generate a new fan curve.

For a FC fan, the static pressure will generally increase as the CFM is reduced..i say generally is because at the surge region, the static pressure decreases as the CFM decrease.