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TCC contractor, we always loop the air mains. The more air usage in the main the larger the pressure drop is at the end. Looping the main gets the same pressure everywhere. Till someone cuts one a branch.

2. Pretty sure Wayne has already explained it to you.

Here is the ultimate argument that you trying to make: Friction loss becomes so great that all flow ceases.

You are trying to claim that there is friction loss with no flow [friction with no flow?], and you will not get flow even though you have a pressure differential.

Love to see your formula for that.

Originally Posted by hvacker
Your example isn't equal as your describing a closed system where pressure is added to to it. Your mention of water being non-compressible does apply to your closed system as far as pressure only. A static system. All pump power is consumed adding pressure. If you want to move that water around in a loop, all the resistances in the system apply and consume the power. Dynamic system. The terms static and velocity apply.

Our talk was about an open system (dynamic or moving)and what happens to pump/fan power.
You know my point is the power is consumed by the resistance in the duct/pipe friction and the added weight of the added water by pump displacement. In the case of air, the added air from displacement is compressed by the existing air. In both cases the motor energy becomes heat low grade heat.
I use the term "Low grade heat" as heat that's not easily reclaimed.

3. Originally Posted by BBeerme
Pretty sure Wayne has already explained it to you.

Here is the ultimate argument that you trying to make: Friction loss becomes so great that all flow ceases.

You are trying to claim that there is friction loss with no flow [friction with no flow?], and you will not get flow even though you have a pressure differential.

Love to see your formula for that.
Friction losses only exist if there is flow. That's obvious. When a fan is run the static pressure will roughly indicate how efficiently the fan is over coming the system resistance.
A fan delivers xx fan HP. That fan power has to overcome all the resistance in the duct. The fan power will only do so much. Add a bigger motor, speed it up and the fan will overcome more.
Fan HP through resistance becomes heat.
Friction loss is built in the system in ducts but total resistance includes the column of air that is in the duct. The fan has to move that column and include the friction of the duct.
The fan can only push so much and the energy diminishes to zero.
As long as there is a differential there will be flow. What I'm suggesting is the resistance totals consume the differential.
Are you suggesting that as long as a fan/pump runs there will be flow regardless of the system?
Total resistance in a system has to be overcome for flow to happen.
GTOGO

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hvacker,
What you are missing is that because airflow from the fan is resisted by friction loss the airflow diminishes accordingly but friction head loss cannot totally stop flow. There is no column of air to consider overcoming because the duct is flat. You only have to overcome the weight of the air passing thru the fan which the fan does or there is no airflow to begin with. As the air progresses down the duct friction loss occurs which starts reducing airflow from the fan. As the airflow drop so does friction loss per foot of duct. There is no limit other than zero as to how low the friction loss can go and the same goes for the velocity. The velocity due to friction loss could fall to 10 to the -6 but you still have airflow and the friction loss per foot would be miniscule because of the reduced airflow. Heat input to the airstream from the motor only exist if the motor is in the airstream and wouldn't be more than 2-1/2 F.

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One more try
Water depending on the use is normally pumped thru a pipe between 4 to 10 feet per second. Friction head is calculated with the expectation that the entire hydraulic radius is wetted (pipe is full). Water if poured onto a flat surface will by gravity not accumulate in one spot but will flow by it's own weight until the surface is level enough that flow ceases. When leveling flow ceases without being contained the water thickness is a thin film. If more water is added the film will spread until level. Friction does not become a factor because the weight of the water is all that is needed to advance the water. If water at the rate of one foot per second is pumped into a ten foot diameter pipe that is level and open on the end the pump will never see pipe friction loss because you can put water into it at that rate forever and never fill it. The head required to put water into the pipe will never increase and resistance to flow (friction) within the pipe is overcome by the weight of the water. The temperature of the flowing water cannot be increased enough under these conditions to have any measurable impact on the water. The water will seek it's own level continuing to flow as long as it takes to become level. The heat taken by the water do to friction per foot is so miniscule it can be ignored. The only way to dissipate the water is evaporation and the heat of friction in this case can't even approach that.

6. In fairness to my original post introducing another variable is not part of my statement. The fact that liquids seek their own level as stated ignores the the fact that it has nothing to do with the energy's in the model.
Anyone can look up the Principle of Conservation of Energy or if ambitious, go to Bernoulli's equation and figure the steady flow of liquids and see what part friction plays.
My post had more to do with understanding losses rather than creating a model that has nothing to do with energy added to a fan/pump system. In an air system would being level matter? Seems an UP system would loose energy faster than a Down system. Elevation is a factor in Mr.B's equation.
I know my original model had a level hose/duct/pipe but I guess we could turn the argument upside down and give the pipe/duct/hose a .005% grade up just to make sure were talking about the energies involved and not a water level.
Kinetic energy from:
relative position
Lost energy Friction and system effect

BB wanted an equation OK Bernoulli is fine. It's tooo much work to type it all out when it's readily available. Don't worry, it's simple math.
One more try: At some point in a system of infinite length ( lets say for this model the pipe/duct is not being subjected to properties that would prevent accurate calculations of fan/pump energy. At some point in the system energy added will not be able to overcome the losses.
We won't have a perpetual motion device.
I hope this horse is dead.

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You started this discussion off with WATER on page two. I'm for putting this to bed because right now you have a mental block. It will come to you when you least expect because you have shown you are a thinker not a dumb butt. The key is in the transformation of energy as it relates to the fan.

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Originally Posted by hvacker
In fairness to my original post introducing another variable is not part of my statement. The fact that liquids seek their own level as stated ignores the the fact that it has nothing to do with the energy's in the model.
Anyone can look up the Principle of Conservation of Energy or if ambitious, go to Bernoulli's equation and figure the steady flow of liquids and see what part friction plays.
My post had more to do with understanding losses rather than creating a model that has nothing to do with energy added to a fan/pump system. In an air system would being level matter? Seems an UP system would loose energy faster than a Down system. Elevation is a factor in Mr.B's equation.
I know my original model had a level hose/duct/pipe but I guess we could turn the argument upside down and give the pipe/duct/hose a .005% grade up just to make sure were talking about the energies involved and not a water level.
Kinetic energy from:
relative position
Lost energy Friction and system effect

BB wanted an equation OK Bernoulli is fine. It's tooo much work to type it all out when it's readily available. Don't worry, it's simple math.
One more try: At some point in a system of infinite length ( lets say for this model the pipe/duct is not being subjected to properties that would prevent accurate calculations of fan/pump energy. At some point in the system energy added will not be able to overcome the losses.
We won't have a perpetual motion device.
I hope this horse is dead.
Do you know what a perpetual motion device is?

"You keep using that word. I do not think it means what you think it means" :grin:

Sent from my SCH-I545 using Tapatalk

9. Originally Posted by shellkamp
Do you know what a perpetual motion device is?

"You keep using that word. I do not think it means what you think it means" :grin:

Sent from my SCH-I545 using Tapatalk
Of course I know what PM is. It's why people get kicked out of a patent office because it is against the law.
I don't know what you're grinning at because your implication is insulting. As if I just fell off the turnip truck.
Instead of throwing crap around why don't we see what You really know.

10. Originally Posted by WAYNE3298
You started this discussion off with WATER on page two. I'm for putting this to bed because right now you have a mental block. It will come to you when you least expect because you have shown you are a thinker not a dumb butt. The key is in the transformation of energy as it relates to the fan.
Why are you going to come when least expected? What are you , a sniper?
My so called "mental block" can be blamed on teachers much smarter than myself.
And I would have to conclude that I'm dealing with very stubborn people who's ideas of argument become desperate. GTOGO

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Originally Posted by hvacker
Of course I know what PM is. It's why people get kicked out of a patent office because it is against the law.
I don't know what you're grinning at because your implication is insulting. As if I just fell off the turnip truck.
Instead of throwing crap around why don't we see what You really know.
There's no need to be butthurt. I was grinning because I quoted Inigo Montoya from "The Princess Bride". :grin:

What you're talking about has nothing to do with perpetual motion, though.

I feel like I've demonstrated what I know in this debate, and unfortunately we are at a standstill since not one of us can produce a hose or duct long enough for empirical data.

Wayne has offered some very good explanations using proper terminology and sound principles of physics to back up what he says. You give examples that don't fit well with the subject of discussion (a car rolling to a stop after the engine was shut off).

This is no longer fun to debate, so I'm with Wayne that we need to just let the thread die.

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12. Originally Posted by BBeerme
When physicists come up with a newfangled formula, they put in extreme variables to see how the equation will respond. Those extreme variables are: One, zero, and infinity.

In the case of your airflow question, and using the above parameters, let's increase the density of your air to equal that of water.

Now let's fill the entire hose or duct with water. Of course we are making assumptions like there is no expansion or contraction in this hose, pipe, or duct.

If pressure is applied at one end, do you think you will or will not [eventually] see an increase of pressure at the other end?

Seems I've heard that water does not compress. It may at some level not discernible with simple instruments, but that is a topic for another discussion.
Originally Posted by hvacker
Your example isn't equal as your describing a closed system where pressure is added to to it. Your mention of water being non-compressible does apply to your closed system as far as pressure only. A static system. All pump power is consumed adding pressure. If you want to move that water around in a loop, all the resistances in the system apply and consume the power. Dynamic system. The terms static and velocity apply.

Our talk was about an open system (dynamic or moving)and what happens to pump/fan power.
You know my point is the power is consumed by the resistance in the duct/pipe friction and the added weight of the added water by pump displacement. In the case of air, the added air from displacement is compressed by the existing air. In both cases the motor energy becomes heat low grade heat.
I use the term "Low grade heat" as heat that's not easily reclaimed.

I think you just admitted defeat.

If you can get pressure to the other end of that closed system, then you can and will get flow, albeit extremely low.

13. Actually that is a pretty common design for Unico systems.

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