Fan Airflow vs Power Consuption

Status
Not open for further replies.

Rick Christopherson

Senior Member
Pressure is force per unit area.
Still with it?
Well, yes and no. Pressure is force per unit area, but the term "area" will cancel out when dealing with a vertical column of fluid. I will humor you and say go ahead with this line of discovery. Maybe I am just jumping ahead.

That being said, YES, I confirm that pressure is force per unit area.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I happen to have this squirrel cage fan in my basement.




With it operating as unrestricted as possible at full speed. 4.8 amps

With it operating with only the the inlet blocked at full speed 3.8 amps

With it operating with only the outlet blocked at full speed 3.7 amps



I have to assume you have switched the topic to positive displacement pumps and not centrifugal blowers.
Another fan example, a shop vac, unrestricted inlet and restricted:



 

Rick Christopherson

Senior Member
Careful!
You'll have him again asserting that it relates to pumps and vertical lift.

I gave a formula for power. The basic physics is fairly simple.
Power is force times distance divided by time.
During this lull in discussion (it's late at night in the UK) I would like to reiterate my point of contention here, just for clarity.

I am not disputing Besoeker's equation. Not at all. It is quite valid for what it is intended.

What I am disputing is that he firmly and repeatedly asserted that his equation is not the equation for vertical lift. (as affirmed in his quotation above.)

The equation is absolutely valid for vertical lift, where the only pressure differential across the pump is the result of the weight of the vertical column of fluid (liquid or gas). I do not contest this aspect.

Besoeker is denying this, and that is all that I am contesting. It is a vertical lift equation and doesn't apply universally to other applications.
 

Rick Christopherson

Senior Member
The unfortunate part of this "vertical lift" discussion is that it distracted from the original discussion. I have wanted to discuss this, but the topic got lost in the heat of this ancillary argument.

My original posting was this:
I believe the discussion is being a little over simplified.
I stand behind that and I want to address it.

Before I made that posting, I did at least go down to the workshop at 1 am and confirm my thoughts before making the posting. So when Iwire came back with his own findings that contradicted me, I took that quite seriously. At first I couldn't explain it without a B.S. reply. (so I didn't reply at all.)

However, today I took a closer look at his picture and realized it was a "backward curve" impeller. These are also called "Non-overloading" impellers. They get that name because they will not overload the motor when fully blocked--regardless whether it is the inlet or outlet. They are designed such that changes in pressure differential does not change loading.

My original posting (#11) said this was being over simplified, and was universal to all blowers and pumps. I didn't say the concept was wrong, just over simplified.

As we can see here, the reason why this original statement gets made so frequently is because it applies to "backward curved" impellers, which are very common. However, it does not apply to other types of blowers or pumps.

Reverse curve blowers are the exception, but if you try my experiment with any other type of blower/pump, then you will see that what I said is correct.
 
Last edited:

Rick Christopherson

Senior Member
Opps.....Bes's posting came in while typing. I haven't read it yet,

By the way, I actually took a video of what I desscribed. I haven't decided whether it is worth the huge hassle of uploading. It exists, but it takes more effort than just putting up a picture.
 
Last edited:

Rick Christopherson

Senior Member
Good.
So let's look at units for pressure.
I prefer SI so let's go with that.
Force per unit area is thus Newtons per square metre.
N/m2
Or the Pascal.
Pa.

Still OK with that?
Yes, I am ok with this. CONTINUE!

However, you may wish to read my other responses before you go down this path.
 

Rick Christopherson

Senior Member
But you accept that pressure is force per unit area so how can you reasonably exclude area as part of that definition?
Gee?!?! How did I know you would pull something liike this?

Could this be the reason why I would not let you force me to redact my caveat from last night?

Well, yes and no. Pressure is force per unit area, but the term "area" will cancel out when dealing with a vertical column of fluid. I will humor you and say go ahead with this line of discovery. Maybe I am just jumping ahead.

That being said, YES, I confirm that pressure is force per unit area.
You hammered against this caveat several times last night, and I refused to budge. Now you want to pretend I never said it?

Where is your honesty in debate?
 
Last edited:

hurk27

Senior Member
However, today I took a closer look at his picture and realized it was a "backward curve" impeller. These are also called "Non-overloading" impellers. They get that name because they will not overload the motor when fully blocked--regardless whether it is the inlet or outlet. They are designed such that changes in pressure differential does not change loading.
I have never run across a centrifugal pump, or blower that does not unload when input or output is blocked regardless of the impeller type, I have worked with water pumps as well as blowers and in all cases if either is blocked the current on the motor will reduce, I have a yard blower that you can block the input or the output and while it is hard because of the volume of air it moves, it always speeds up when either is blocked (universal motor) and the blower in this case is a flat bladed half open wheel so that it can pass material, I used to call these types of pumps and blowers "slinger" in my younger days as I could never remember how to spell centrifugal, but it physics involved are the same as it uses the mass acceleration from the center of the wheel to the edge of the wheel to sling the mass outward, like the Earth would do to us if it ever started rotating faster.

While I'm not intending to get into the discussion of the math part, I wanted to point out that I believe your above statement is not correct, now if we were discussing positive displacement pumps and or air compressors, this post would not have been written.

For those who don't know, positive displacement pumps are a pump or compressor that will keep pumping up pressure even if you block the output to a point the motor stalls or the pressure results in a catastrophic failure of the point up to the blockage such as the holding tank.

These type of pumps or compressors come in a wide verity such as piston, off set vane, gear, screw, and a few others.
 
Last edited:

Rick Christopherson

Senior Member
I have never run across a centrifugal pump, or blower that does not unload when input or output is blocked regardless of the impeller type, .......
I want to thank you for the reminder, because I had completely forgotten about this until I read your posting. :thumbsup:

Last year I wrote a service manual for a piece of equipment, where I did this very experiment. As part of the service manual, I (we, it took 2 of us for these readings) took voltage and amperage readings throughout this machine while it was operating. One of those readings was for the vacuum motor, and I decided to take extra readings with both a blocked inlet and blocked outlet. These readings are actually published in the service manual. I forgot that I recorded them until now.

Blocked Inlet amperage = 14A
Blocked Outlet Amperage = 17A

Here is a picture of the vacuum motor from the manual I wrote. Balloon #22 is pointing to the exhaust, which can be fully blocked, unlike a typical household vacuum cleaner.

VacuumMotorConnector.jpg
 
Last edited by a moderator:

hurk27

Senior Member
And the full flow amperage was?

Not saying that in every instance my example would be true, but I would think that most "slinger" type blowers or water pumps would hold true to this.

At one time I tried to use a 0-90 VDC motor (AC motor burned up and didn't have a replacement) with just voltage control to drive a vacuum pump to suck eggs out of there key flats and load them onto the in-feed conveyor to the wash machine, well 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, well as soon as I realized the mistake I wired a servo board into the voltage control (after installing a new wheel) and it worked fine till we got a replacement, talk about a patch job, but it worked:happyyes:

The Machine was a Diamond Automation egg processing system if any one is familiar with them.
 
Status
Not open for further replies.
Top