Fan Airflow vs Power Consuption

[Besoeker]

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

Demonstrates the real world situation nicely, if I may say so.
As you are probably aware we supply drives mostly for industry and some of these are on large fans. It is common for these to be started against closed dampers.
The fans, being large, have a high inertia. Closing the dampers reduces the load during starting allowing an easier run up.
For those who haven't experienced this, it might seem counter intuitive. Block the outlet thus it has to work harder might be the cause of any such misconception.

 

[Besoeker]

No. The question stands. Why does your equation need a weight?

How else would you calculate mass?

And the offer still stands:
Instead of disputing the formula, why don't you simply provide the one you believe to be correct.

 

[Rick Christopherson]

oh screw it! I concede. Your equation is right. I don't care any more. tell people whatever you wish. I'm going back to playing games with my son.

 

[Besoeker]

oh screw it! I concede. Your equation is right.

Thank you.

 

[iwire]

tell people whatever you wish.

Nice of you to allow that.


(And yes, that is somewhat 'snarky' )

 

[Rick Christopherson]

Thank you.

I guess sarcasm doesn't play well in the UK.

 

[Besoeker]

I guess sarcasm doesn't play well in the UK.

Oh, I understood your point. Unfortunately mine seems to have been missed.

OK, slightly more seriously, I deal quite a lot with drives on centrifugal loads. We often have to provide guaranteed operating efficiencies at the bid stage of a project. These efficiencies have to demonstrated after build. Failure to meet them invokes harsh financial penalties or possible total rejection of the equipment. This puts you between a rock and a hard place. Bid the job with the efficiency too low and you'll lose it to a competitor. Make it too high, and fail to demonstrate that then suffer the consequences.

The point is that it makes you very, very careful when it comes to doing the calculations and for us, it usually falls to me to do them. And, as I said, getting them wrong isn't an option.
The formula that I gave for P_i has worked for me for many years. And it's just basic physics.

 

[readydave8]

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.

I may be wrong but this post appears to answer the original question and can be understood, I like the electrician's answer better than the engineers'

 

[Besoeker]

I like the electrician's answer better than the engineers'

Good point.
The engineer gave an explanation.
The electrician gave a demonstration.

 

[kwired]

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.

I may be wrong but this post appears to answer the original question and can be understood, I like the electrician's answer better than the engineers'

Electricians live in the real world and see these things and have to correct any problems. Engineers know what they have been taught yet seldom get to see if what they design actually works as intended.

 

[Besoeker]

Engineers know what they have been taught yet seldom get to see if what they design actually works as intended.

See post #47.
Some of us have to deal with the harsh realities of what we design.

 

[kwired]

See post #47.
Some of us have to deal with the harsh realities of what we design.

And that makes you a little more like us guys in the field, as obviously you are in the field sometimes also.

Then there are designers that never see much of what they have designed in operation. They may never know what has been changed that did not work, and just assume they did right.

Someone had a good story a while back about a toothpaste factory and an application to eliminate empty packages from being sent out. May or may not be true story but faces what reality there is at times. I will try to find a link to it.

Found it:http://forums.mikeholt.com/showthread.php?t=142010&p=1362405#post1362405

 

[Besoeker]

And that makes you a little more like us guys in the field, as obviously you are in the field sometimes also.

Well, thank you.
If you're dealing with practical applications of what you design, you need to see the practical application.
And I know which end of a screwdriver is which.

Then there are designers that never see much of what they have designed in operation. They may never know what has been changed that did not work, and just assume they did right.

Probably true. Though I've not seen many of those in my field.

Someone had a good story a while back about a toothpaste factory and an application to eliminate empty packages from being sent out. May or may not be true story but faces what reality there is at times. I will try to find a link to it.

Found it:http://forums.mikeholt.com/showthread.php?t=142010&p=1362405#post1362405

Excellent story whether true or otherwise.
Simplest is best.

 

[Smart $]

No. The question stands. Why does your equation need a weight?

Not going to debate the validity of the formula for the purpose desired, but the answer to your question is to establish a pressure value.

The pressure exerted by a column of liquid [fluid] of height h and density ρ is given by the hydrostatic pressure equation p = ρgh. Fluid density and local gravity can vary from one reading to another depending on local factors, so the height of a fluid column does not define pressure precisely.

 

[jumper]

53 posts and no one has commented that Bob was so bored that he took his AHU apart at 4:00AM just to a test a theory.:lol:

I do stuff that like, but I assumed he was a little more normal.

 

[jumper]

Good point.
The engineer gave an explanation.
The electrician gave a demonstration.

:thumbsup:

And someone, not either or two persons above, made his usual posts with a BS threatening tone.

Hint: the aforementioned person did not post a formula that was politely requested of him.

 

[Johnmcca]

Define normal. I'd say with over 43,000 posts here, a 4:00am post is normal for Bob. And I for one, appreciate his dedication to our continuing education and understanding of the code.

 

[jumper]

Define normal. I'd say with over 43,000 posts here, a 4:00am post is normal for Bob. And I for one, appreciate his dedication to our continuing education and understanding of the code.

Please tell me you realize that I was making a joke. Did you see the smileys?

Heck, I often post at wee hours. Insomnia.

I agree that Bob is smart and a valuable member.

 

[Johnmcca]

I'm far from normal myself. I'm addicted to this site. I also appreciate your contributions, humouros and technical.:thumbsup:

 

[iwire]

53 posts and no one has commented that Bob was so bored that he took his AHU apart at 4:00AM just to a test a theory.:lol:

I do stuff that like, but I assumed he was a little more normal.

Define normal.

Yeah, I did laugh to myself that I went out in the dark driveway at 4AM to grab my meter, clamp and camera to check out a fan.

Now FWIW I did not have to take anything apart, I picked up that fan on a job and use it as an exhaust fan for my basement.