# Thread: Curious Solution.

1. OK y'all, the question is not done yet.
Let's say we change the medium from a fluid to air. An air tight duct, say glued PVC, is run
around the same world. Remembering fan laws and pump laws apply. Would you expect any air to come out the other end?

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The first thing you have to know is that fans and pumps have no brains, they do what the system allows. If the system allows a fan to put air into a duct that is leak proof and it doesn't come out the other end where does it go? A fan or pump can approach shut-off but friction loss by it's definition cannot stop all airflow because you HAVE to have velocity to develop friction. It may take 8 or 10 years but without leaks the air has to come out the other end.

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Originally Posted by hvacker
OK y'all, the question is not done yet.
Let's say we change the medium from a fluid to air. An air tight duct, say glued PVC, is run
around the same world. Remembering fan laws and pump laws apply. Would you expect any air to come out the other end?
I still stand by my first rebuttal. Friction loss cannot cause flow to stop completely. If you were to graph it out, it would appear hyperbolic, getting infinitely closer but never reaching "zero flow".

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When you calculate and show friction loss graphically it becomes a system curve. That doesn't tell you where the flow would end up being in this case. What it would tell you is the dynamic static pressure you need for various airflows. Once the system curve crosses the fan curve you are above the design static pressure needed for the fan to deliver design airflow. There are two system curves calculated for a fan which are design and actual. The actual system curve is calculated from field measurements. I don't know of any way to calculate airflow that causes friction loss equilibrium.

5. Originally Posted by WAYNE3298
The first thing you have to know is that fans and pumps have no brains, they do what the system allows. If the system allows a fan to put air into a duct that is leak proof and it doesn't come out the other end where does it go? A fan or pump can approach shut-off but friction loss by it's definition cannot stop all airflow because you HAVE to have velocity to develop friction. It may take 8 or 10 years but without leaks the air has to come out the other end.

I'll play with this awhile. What I'm pondering is we know that we can create system in a duct where static pressure doesn't exist and only velocity exists as In some factual situations the air will collapse into a jet. We've talked about this earlier. With a vena contracta situation, where duct friction won't normally apply (I said normally as you might have some secrete sauce where it does) All that's needed to deal with the ?? is when the velocity pressure reaches zero does all air movement cease? Or is your position that the velocity pressure never reaches zero? If there were a super instrument that could measure any pressure would I expect to see movement? Maybe a super thermometer as where there's movement there should be some temperature.
This exploration is interesting. Thanks to all for indulging me.

6. Originally Posted by shellkamp
I still stand by my first rebuttal. Friction loss cannot cause flow to stop completely. If you were to graph it out, it would appear hyperbolic, getting infinitely closer but never reaching "zero flow".

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About your curve: Never reaching zero has a built in problem. In our experience everything has a zero.
I remember a question where a ball was being held four feet off the ground and dropped. The question was does the ball ever hit the ground? In our experience, of course it does because we can set up experiments to prove it does.
But because mathematically every distance can be cut in half, the math says the ball never does reach the ground.
One solution said that there is a moment in space that can no longer be divided. Not all agree.
Because the graph has no zero I'll take that to mean your on the side of the ball never reaching the ground.

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hvacer,
I agree this is an interesting conversation. Another theory about dropping the ball is if you drop it enough times it will eventually go up instead of down. I don't believe that for a second but some professor came up with it and I thought you might find it amusing. The ball falling half way to the ground is one way used to explain half life of uranium ect. The system curve zero is without the fan running so the curve starts at the zero-zero point on the graph. What the system curve tells you is how much pressure you need to obtain a specified amount of airflow. The curve itself never ends but has no use beyond desired airflow. The flow at shut-off head can also be zero but can only happen if you have a shut-off valve. Yes there will be velocity pressure in our example albeit pretty small. Shut-off head (or zero airflow with the fan running) doesn't fall on the system curve but instead is represented on the fan curve where it meets the Y axis.
The orifice flanges that create the vena contracta do have pressure drop. Orifice flanges are not only used for flow measurement but also to restrict air and water flow. The only place I know of where restrictive orifices are still used is in commercial de-aerators for non-condensables venting.

8. Originally Posted by WAYNE3298
The first thing you have to know is that fans and pumps have no brains, they do what the system allows. If the system allows a fan to put air into a duct that is leak proof and it doesn't come out the other end where does it go? A fan or pump can approach shut-off but friction loss by it's definition cannot stop all airflow because you HAVE to have velocity to develop friction. It may take 8 or 10 years but without leaks the air has to come out the other end.

I'll play with this awhile. What I'm pondering is we know that we can create system in a duct where static pressure doesn't exist and only velocity exists as In some factual situations the air will collapse into a jet. We've talked about this earlier. With a vena contracta situation, where duct friction won't normally apply (I said normally as you might have some secrete sauce where it does) all needed to deal with the ?? is when the velocity pressure reaches zero does all air movement cease? Or is your position that the velocity pressure never reaches zero? If there were a super instrument that could measure any pressure would I expect to see movement? Maybe a super thermometer as where there's movement there should be some temperature.
The reason in my mind as to why the air/water never reaches the other end is also air. Atmosphere would become the major resistance. Just like my race car only able to ge so fast because it can't overcome air resistance. For the race car there are many other limiting reasons.

9. I think I ended up with two partial posts. Teach me to eat dinner in between posts. Or I'm getting old and am repeating myself.

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Overcoming atmospheric air pressure is not a factor but remember air resistance doesn't stop your race car but does limit how fast you can go. Remember in a vena contracta the airstream quickly recovers from the diameter reduction, if it didn't it couldn't be used for flow measurement and the airstream contracts because it hits a sudden reduction in the duct which temporarily increases the velocity because the same amount of air is moving thru less area. You need to reduce the duct area suddenly to generate the vena contracta and the reduction creates the velocity needed but also adds pressure drop. I have heard several engineers argue that the beta ratio dictates the outcome. That is one discussion I stayed clear of because that statement told me they had never calculated the size and related pressure drop expected to determine air or water flow. Beta ratio is the baseline for calculations but there are empirical values that are part of the final sizing. Remember also that the air entering the fan is at atmospheric pressure and you add pressure at the fan discharge in order to move it.

11. Originally Posted by WAYNE3298
hvacer,
I agree this is an interesting conversation. Another theory about dropping the ball is if you drop it enough times it will eventually go up instead of down. I don't believe that for a second but some professor came up with it and I thought you might find it amusing. The ball falling half way to the ground is one way used to explain half life of uranium ect. The system curve zero is without the fan running so the curve starts at the zero-zero point on the graph. What the system curve tells you is how much pressure you need to obtain a specified amount of airflow. The curve itself never ends but has no use beyond desired airflow. The flow at shut-off head can also be zero but can only happen if you have a shut-off valve. Yes there will be velocity pressure in our example albeit pretty small. Shut-off head (or zero airflow with the fan running) doesn't fall on the system curve but instead is represented on the fan curve where it meets the Y axis.
The orifice flanges that create the vena contracta do have pressure drop. Orifice flanges are not only used for flow measurement but also to restrict air and water flow. The only place I know of where restrictive orifices are still used is in commercial de-aerators for non-condensables venting.

I know students need to watch what they say to a Professor. I remember saying to my Philosophy Professor, when a question about consciousness came up and I suggested that consciousness is in everything. "Do you mean that this chair has consciousness?"He asked me. I said "It seems to have enough consciousness to remain a chair."
Then he kicked the chair across the room.
He had everyone's attention at that point.
Any student that argues with a Professor has a distinct disadvantage. The student is not aware that they are a mouse and the Professor is a cat.
The question of how high is up? Some might say up goes on forever only to find out up is 274 miles. After that it becomes "out".
Maybe just hold fast to the idea that there are just tendencies to exist.

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The statement I liked was "if you reach the end of space what is on the other side". Another one said what is electricity and one guy raised his hand to give him an answer. The prof yelled "the smartest person in the world doesn't know what electricity is and your going to tell me". He was really a fun person and it made sense when he explained what he meant.

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Originally Posted by hvacker
About your curve: Never reaching zero has a built in problem. In our experience everything has a zero.
I remember a question where a ball was being held four feet off the ground and dropped. The question was does the ball ever hit the ground? In our experience, of course it does because we can set up experiments to prove it does.
But because mathematically every distance can be cut in half, the math says the ball never does reach the ground.
One solution said that there is a moment in space that can no longer be divided. Not all agree.
Because the graph has no zero I'll take that to mean your on the side of the ball never reaching the ground.
I'm certainly not speaking of theoretical physics here. So long as that fan is running and airflow is not fighting gravity (going up) - the flow will never actually get to zero.

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