flow of air inside a PCP

[rumbleUU]

Hi, always want to know and learn, for example, will there be an restriction by the 3mm hole threw the manometer-piece? also does the small volume(6cc) has something to do with it?

Having hard time imagane if that hole resicts or not, hmmm?

Thanks

[rsterne]

Yes, but whether that reduces the power depends on many factors.... The Benjamin Discovers is like that, with a 4mm exhaust port, a 6mm inlet on the valve, but only a 3mm hole in the gauge block.... In stock tune it makes little or no difference.... but if you start trying to increase the power, eventually you get to the point that the hole in the gauge block is enough of a restriction that increasing it to 6mm, like the inlet hole in the valve makes a difference....

Bob

[rumbleUU]

ok, but how does the volume between gauge and valve matter?

if small it matters more, right? cause if that volume was very big the hole in the gauge would not matter....

Just to have an example, pretend the gun is shooting .22 kaliber 40 ft/lbs, is there some math of how much more energy you would get with a hole of lest say 4mm instead of 3mm? would it matter.... Im guessing the air in the reservoar will fill that space with 6cc inbetween really quick....

[rsterne]

You are correct, the larger that volume (including the volume inside the valve), the less the pressure drops during the shot.... assuming the hole through the gauge block is small.... If it is larger than the valve inlet, then the volume doesn't matter....

If the hole in the gauge block was really small (eg. 1mm or less), then the space between the block and valve would act like the plenum in a regulated PCP.... that would be the only air available for the shot.... In that case, it would be good to have 1 cc for each FPE (eg. 40 cc for 40 FPE) to minimize the pressure drop during the shot.... At least 1/2 cc per FPE is required to prevent large pressure drop.... However, if the inlet hole through the gauge block is the same diameter (or larger) than the valve inlet, the volume of that space doesn't matter, as it can fill faster than it can empty.... I like to see the valve inlet, and any upstream restrictions (eg. the hole in the gauge block) at least as large as the diameter of the valve throat (the hole under the poppet seat)…. to prevent any pressure drop inside the valve....

Bob

[rumbleUU]

Great explaning, youre the man on stuff like this, it was about the same i was thinking..

Sometimes isnt easy to understand how air under pressure flows and behaves, especially if there is a chamber inbetween....

One thing more i did wunder, what pressure will be behind a poppet ? Is it the hole in the valveseat or the area of the delrin piece on the poppet that creates the force. Example, the valveseat hole is 5mm and the delrin piece on the  poppet is 8mm, would it also matter if the valveseat is straight and plain or conical?

[rumbleUU]

better to draw

Is the above more difficult for the hammer to open?

[subscriber]

Sure the length of the passage through the gauge block matters as much (probably more) than the diameter?

If the passage is very short, like a restrictor plate, then pretty much only the area of the hole matters.  If the passage is 10 times longer than the diameter of the port, then a small diameter is worse than if the passage was the same length as its diameter.  No?

[rumbleUU]

Sure the length of the passage through the gauge block matters as much (probably more) than the diameter?

If the passage is very short, like a restrictor plate, then pretty much only the area of the hole matters.  If the passage is 10 times longer than the diameter of the port, then a small diameter is worse than if the passage was the same length as its diameter.  No?

thats a new one, didnt think of that, but have no idea if thats true?
Maybee Bob can

Thinking about it, it cant matter, think of it this way, you have a tube that is 4" long and one that is 10" long, blowing air threw must be about the same resistance?  Of course, if one length is 1" and the other is 100", then maybee something else matters, but dont think length up to 1" should matters for holes like 3-5mm big..

[subscriber]

Sebastian,

It is not the absolute length of the passage or tube bore that matters.  It is the ratio of length to diameter (area to be more exact) that matters.  So, for a tube that has a 25 mm bore, the length for airgun applications matters very little.

Now, compare that to a 4.5 mm bore that is 900 mm long, and the drag of the air flow down the barrel matters.  It matters more for high pressure air, as the effective viscosity is higher.  So, a barrel is a bad example, as the pressure decays towards the muzzle.

[rumbleUU]

Sebastian,

It is not the absolute length of the passage or tube bore that matters.  It is the ratio of length to diameter (area to be more exact) that matters.  So, for a tube that has a 25 mm bore, the length for airgun applications matters very little.

Now, compare that to a 4.5 mm bore that is 900 mm long, and the drag of the air flow down the barrel matters.  It matters more for high pressure air, as the effective viscosity is higher.  So, a barrel is a bad example, as the pressure decays towards the muzzle.

ok, but we are talking lengh up to 1" and hole 3-5mm big, do you really think there will be any restriction of airflow threw that short distance?

[subscriber]

Making the hole diameter through the gauge block helps, not so much because a larger hole flows easier, but because if large enough, the air in that passage at the start of the shot cycle can be added to the plenum volume nearer the valve.

I would be confident that if you could open the gauge block "passage" to 10 mm diameter, the gains in power would come almost completely from the larger plenum effect; rather from easier flow. 

It would also make the airgun ping louder when fired, as the air damping effect of the narrow channel through the gauge block would be removed.

The flow through the gauge block occurs mostly after the valve has closed, to refill the plenum.  So, a 25 mm long gauge body with 2 mm diameter hole in it won't affect power by restricting flow.  It will reduce power because the plenum is "too small".  Making the passage in the gauge block larger will increase the effective plenum volume; boosting power potential.

[rumbleUU]

Making the hole diameter through the gauge block helps, not so much because a larger hole flows easier, but because if large enough, the air in that passage at the start of the shot cycle can be added to the plenum volume nearer the valve.

I would be confident that if you could open the gauge block "passage" to 10 mm diameter, the gains in power would come almost completely from the larger plenum effect; rather from easier flow. 

It would also make the airgun ping louder when fired, as the air damping effect of the narrow channel through the gauge block would be removed.

The flow through the gauge block occurs mostly after the valve has closed, to refill the plenum.  So, a 25 mm long gauge body with 2 mm diameter hole in it won't affect power by restricting flow.  It will reduce power because the plenum is "too small".  Making the passage in the gauge block larger will increase the effective plenum volume; boosting power potential.

ok, so if the hole is 3mm , making it 4mm wouldnt add nothing to power, as the small 6cc "plenum" volume is what restricts the power anyway, is that right?

So the rule of making every hole the same all the way to the pellet, does not apply the hole threw the gauge, am I right?

[rumbleUU]

but does the 6cc space in between gauge and valve really matter , ? Im not trying to understand how regultaed plenum works with a small tiny hole to fill it up works, that I know of..

In other words, is there no befinit AT ALL making the hole threw gauge from 3mm to 4mm?

[subscriber]

The 6 CC plenum is almost all that matters in terms of potential power and efficiency (at a given starting air pressure; other than the valve size and action).  It is a portion of the air available in the plenum that drives the pellet down the barrel.  The valve opens briefly on firing.  As it does this, the pressure in the plenum drops; and is then refilled from the rest of the air tanks via the 3 mm channel in the gauge; after the valve is closed. 

The degree to which the plenum pressure drops depends on the volume of the plenum and the bore volume behind the pellet when the valve closes.  The smaller the plenum, the more the pressure drops while the valve is still open.  The less the pressure drops while the valve is open, the more power and efficiency you can get.

If the effective plenum volume is only 6 CC and you want more than 12 ft.lb from this airgun, then anything you can do to enlarge the effective plenum will increase power potential and efficiency.  This includes making the air channel through the gauge larger.

If it were me, I would ask if 4 mm is the maximum you could go?  Remember, the hole does not have to be the same size all the way through the gauge body, end to end.  If the end of the hole facing the valve was drilled 12 mm in diameter by 10 mm deep, you have just increased your plenum volume by about 17%.  A little large and a little deeper and the gains could double.

Drilling the hole in the gauge body on the opposite side of the valve, while leaving a 4 mm restriction in the middle would not help power or efficiency; as it cannot be considered part of the plenum volume.

The fact that the rest of the air channel through the gauge body is only 3 or 4 mm in diameter would only matter if this airgun fired fully automatic, at such a rate that the air pressure could not be fully restored in the valve before the next shot was fired.

The devil is in the details.  Bob will no doubt be along shortly to give you the most accurate input about this.

[rumbleUU]

Ok , lets skip the fiction, my first post was just something I made up to understand how the air flows and all works, I want to learn....

Lets take a real gun and discuss, as fiction can be wrong.

lets take the Artemis PR900W .22 as an example, its pretty similar to my first example.

Airtube 100CC big, gauge with 3mm hole, valve with 5mm hole intake, 3.5mm holes all the way to the pellet, transferport is 3.5mm in FAC version making 30Ft/Lbs and 0.7 to 1.0mm here where I live producing 7Ft/Lbs..
I think it has a "plenum" volume between gauge and valve that is not much bigger then 6CC.

From what Ive read the gun is not bad in efficiency, even in FAC version, so HOW can this PR900W produce 30Ft/Lbs efficient with only a 6CC plenum if thats the only thing that matters? you said a small "plenum" of 6CC is only good for 12Ft/Lbs......  Im not arguing against what you said, Im sure you know what you are saying, Im just trying to understand....

[subscriber]

Fair questions.  Perhaps the answer is a 3000+ PSI or 200+ BAR operating pressure.  The energy in the compressed air depends on its volume and pressure.

6 CC at 3000 PSI is more than equivalent to 18 CC at 1000 PSI.  More than, because higher pressure usually produces greater efficiency.

[rumbleUU]

one thing: isnt the air "filling up" the "plenum" throw that smaller 3mm hole even in the start where the hammer opens the valve up?
And also, if all holes where 3.5mm all the way threw from the tube to the pellet, would there really be any restrictions even with a small size plenum like 6CC?

[rumbleUU]

this is a simple way to look at it, if its wrong, well, the I will learn again

lest think of the volumes, 1,2 and 3, where 1 is airtube, 2 is the "plenum" and 3 is the air goin OUT from Valve that drives the pellet in the barrel forward...  if the hole is the same IN and OUT from volume 2, I cant see how there would be a restriction? No matter how fast the air is sucked out from volume 2 it should be filled up again as fast as the hole IN and OUT are the same size, Am i thinking right?

[subscriber]

Sebastian,

In theory, the 3 mm hole through the gauge will start to refill the plenum as soon as the valve starts to open.   It is a matter of degree.  The amount of air flowed then is small compared to the air leaving the valve, because the valve area is larger than the gauge port flow area.  There simply is not enough time while the valve is open to refill the plenum to a meaningful degree with a 3 mm tube feeding the plenum.

The idea that the restriction depends only on the tube diameter; and that if the diameters match there is no restriction, is incorrect.

Yes, steps in the flow path create flow losses, so a near constant diameter path is better.  However, if that path is narrow and long, then there is a "restriction" in the form of friction.  It is like the pressure loss seen at the end of a 50 m garden hose; or the electrical resistance of a 50 m extension cord.

If all the holes through gauge, valve port and barrel all match, then the actual plenum exists only on the air tank side of the gauge.  As in, very far away.  Now, all the air leaving the valve has to accelerate through a very long narrow path.  That will cause a significant pressure drop and reduce the acceleration of the pellet.

If you can get it, you want a fat pipe all the way to the valve.  The port between the valve and barrel should be as short as possible. It should match the bore area where it meets the barrel (if possible).  The effective valve area (considering the valve stem is in the way) should be a little larger than the port area.  Very much larger and it starts reducing efficiency.

"Restriction" is measured as pressure loss.  The higher the flow velocity of the air, the greater the friction losses.  The longer and narrower the path, the greater the losses, or restriction.  Wide and short are better, all the way to the front of the valve.  After the valve, match the bore area and keep the path short.

[Atom]

I widened that hole to 6mm, and valve inlet at 6 mm and outlet at 5 mm  on mine PR900W 5.5mm
Now the plenum is the whole cylinder, and I'm going to put a 0.3 l 300 bar bottle and a regulator outside...