Fan Airflow vs Power Consuption

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Rick Christopherson

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And the full flow amperage was?
I don't know. I no longer have my original notes. As I said, the numbers were published in the manual, and that it where I took them from. As I said, I didn't agree with the whole concept of recording actual voltage/amperage results from a single machine, so that is why I forgot that I even did it.

.......without any over speed control servos to keep the DC motor from running away it destroyed the blower wheel on the first try as soon as it hit the eggs......
Yes, and that affirms what everyone here agrees with. When you block the inlet to a pump/vacuum/blower, it will be at its lowest load. No one disagrees with that (as far as I know).

My original posting was regarding the difference between blocking the inlet versus the outlet. The original postings prior to mine were oversimplifying the situation and claiming that it made no difference. Bob (Iwire) has shown that with a reverse-curve impeller it makes no difference. My results show that with other types, it does make a difference. That was the whole point of my posting when I said it was being over simplified.

I never said it was totally wrong. I said it was being over simplified.
 
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hurk27

Senior Member
I don't know. I no longer have my original notes. As I said, the numbers were published in the manual, and that it where I took them from. As I said, I didn't agree with the whole concept of recording actual voltage/amperage results from a single machine, so that is why I forgot that I even did it.

Yes, and that affirms what everyone here agrees with. When you block the inlet to a pump/vacuum/blower, it will be at its lowest load. No one disagrees with that (as far as I know).

My original posting was regarding the difference between blocking the inlet versus the outlet. The original postings prior to mine were oversimplifying the situation and claiming that it made no difference. Bob (Iwire) has shown that with a reverse-curve impeller it makes no difference. My results show that with other types, it does make a difference. That was the whole point of my posting when I said it was being over simplified.

I never said it was totally wrong. I said it was being over simplified.
And I agree as I can see if the inlet is blocked there is no mass to take the place of the mass that was slung out of the impeller so the impeller is no longer moving a load or much of a load, but with the output blocked you will still have mass in the impeller that will add to the load, but this mass will be just turning with the impeller and not flowing out so the load would or should be less then full flow but more then the blocked input, if I get some time (been very busy trying to save my house from a sheriff sale and trying to keep jobs going) I will try to take some amp readings of that yard blower in all three modes and see if this holds true to a flat blade impeller type blower?
 
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don_resqcapt19

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I would assume that there would be some difference in the current between blocking the inlet and blocking the outlet.
There is more liquid or gas in contact with the impeller when the outlet is blocked than when the inlet is blocked, and possibly more leakage, but unless you have some type of positive displacement device, the current with either the inlet or the outlet blocked will be significantly less than will both fully open.
There have been cases of steam explosions on fire pumps when they have been run for long periods of time with the outlet blocked. If no work is being done when the outlet is blocked where does the heat come from to turn the water into steam?
 

kwired

Electron manager
Location
NE Nebraska
I would assume that there would be some difference in the current between blocking the inlet and blocking the outlet.
There is more liquid or gas in contact with the impeller when the outlet is blocked than when the inlet is blocked, and possibly more leakage, but unless you have some type of positive displacement device, the current with either the inlet or the outlet blocked will be significantly less than will both fully open.
There have been cases of steam explosions on fire pumps when they have been run for long periods of time with the outlet blocked. If no work is being done when the outlet is blocked where does the heat come from to turn the water into steam?
No useful work that the pump was designed to do is being done when outlet is blocked. There are still friction losses from media being moved around in the pump.

Place I worked at had a small lecethin tank they heated by pumping hot water through the jacket of tank. It only needed small amount of heat, the product began to gel around room temperature and only needed heated to 100?F or so to be able to flow better. All we used to heat it was a small pump and a balance tank that only held about 10 gallons of water. No external heat was needed the pump heated the water enough to get the job done. If they forgot to turn it on far enough ahead of time they would give the water in the tank a shot of steam to speed up heating process but otherwise it would do fine on its own.
 

Besoeker

Senior Member
Location
UK
I would assume that there would be some difference in the current between blocking the inlet and blocking the outlet.
There is more liquid or gas in contact with the impeller when the outlet is blocked than when the inlet is blocked, and possibly more leakage, but unless you have some type of positive displacement device, the current with either the inlet or the outlet blocked will be significantly less than will both fully open.
There have been cases of steam explosions on fire pumps when they have been run for long periods of time with the outlet blocked. If no work is being done when the outlet is blocked where does the heat come from to turn the water into steam?
If the pump runs against closed valve the impeller just churns the water and heats it up.
As I mentioned in an earlier post, we do a fair bit of work on centrifugal fan and pump applications. The pumps are mostly in water pumping stations and the valves are electrically operated. We get contacts back from the valve to indicate status i.e open, opening (in transit), closing, and closed. The control is such that pump can run unless the valve(s) is/are open to avoid the problem you mention.
 

kwired

Electron manager
Location
NE Nebraska
Some positive displacement pumps are specifically designed to operate in a "bypass" mode when discharge is blocked. This is very typical for a high pressure washer pump. The media will have increase in heat when in this mode and usually there is a temperature activated relief system of some type to prevent too much heating.

As mentioned on an air compressor the pressure continues to rise - that is what relief valves are for is to prevent rising too high, usually if a control device has not stopped the pump motor for some reason when it was supposed to.
 

kwired

Electron manager
Location
NE Nebraska
Just a quick reminder to all that the OP specifically asked about squirrel cage blowers. :)
He also said "This maybe more of an engineering / physics question" and he is on the way to getting an engineering degree from this thread:happyyes:


FWIW I think his question was answered to the level he was concerned within the first 10 posts or so.
 

don_resqcapt19

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Location
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retired electrician
If the pump runs against closed valve the impeller just churns the water and heats it up.
...
But while that is not useful work, it is using energy mechanical energy that is supplied by the motor and I would expect the load to be somewhat greater with a blocked outlet as compared to a blocked inlet.
 

Besoeker

Senior Member
Location
UK
But while that is not useful work, it is using energy mechanical energy that is supplied by the motor and I would expect the load to be somewhat greater with a blocked outlet as compared to a blocked inlet.
I don't know.
If the fluid is just being churned around I can't immediately see why one would use more than the other although iwire's practical tests would seem to support your view.
 

don_resqcapt19

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retired electrician
I don't know.
If the fluid is just being churned around I can't immediately see why one would use more than the other although iwire's practical tests would seem to support your view.
I think there may be more fluid to churn with a blocked outlet as compared to a blocked inlet. With a blocked inlet you create a partial vacuum.
 

realolman

Senior Member
I always thought this whole concept of this thread was hard to understand, and I don't know that I do, but it does seem to be that the motor runs easier with the inlet or outlet blocked.

I don't quite know why it wouldn't act similarly to a motor that had it's load seized up so it wouldn't turn, but it doesn't seem to be the case.

When you stick the hose from a vacuum cleaner to the palm of your hand the motor speeds up... seems like it's "trying harder" .... but I guess it's because it's easier for the motor to turn.

go figure.

... no, wait ...we already got guys doing that.:)
 

Smart $

Esteemed Member
Location
Ohio
I think there may be more fluid to churn with a blocked outlet as compared to a blocked inlet. With a blocked inlet you create a partial vacuum.
Essentially correct... but more so because of pressure differential. With only the outlet blocked, the pump (or fan) can maintain full pressure on the outlet side... but power drops off because there is no flow. With inlet fully blocked, pressure on the inlet side drops to less than atmospheric (i.e. partial vacuum) while outlet drops to head-only pressure.
 

kwired

Electron manager
Location
NE Nebraska
I always thought this whole concept of this thread was hard to understand, and I don't know that I do, but it does seem to be that the motor runs easier with the inlet or outlet blocked.

I don't quite know why it wouldn't act similarly to a motor that had it's load seized up so it wouldn't turn, but it doesn't seem to be the case.

When you stick the hose from a vacuum cleaner to the palm of your hand the motor speeds up... seems like it's "trying harder" .... but I guess it's because it's easier for the motor to turn.

go figure.

... no, wait ...we already got guys doing that.:)
Go back to what I said in post #3:

Does not matter if inlet or outlet is what is blocked. Same with a centrifigal pump.

The fan or pump is loaded by the volume of media it is moving. Any restriction limits the volume that is available to move.

If you have a boat with a leak in the bottom are you going to work harder by bailing out with one gallon buckets or five gallon buckets? (Assuming you bail same number of bucket per minute in either case) In which case is more work accomplished in same amount of time?

Now lets say you have to pour the bailed water into a restricted opening - you are now limited in how much you can move no matter how fast you can scoop it up and will accomplish less work in same time period.
As others have pointed out there is a little more to it than what I have said in that post, but what I said is still true when it comes to the majority of the loading on a centrifugal fan or pump. If you run the impeller in a true vacuum you have no resistance to movement from the impeller itself and only load on the driving motor would be what it takes to drive the motor itself, whatever enrgy is needed to turn the impeller and other mechanical inefficiencies like friction in bearings, seals, etc. In the absence of gravity it should only need enough energy applied to accelerate to speed and (if there is no opposition from anything else) should continue to rotate indefinately.
 
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