Valve design question - energy calculations for dump chambers

[Siress]

I'm kicking a spool valve design around and wondering if I can estimate the volume and pressure of the dump chamber needed to reach 280fps. I've calculated the minimum avg. pressure required to propel the paintball itself, but I have no idea what the typical losses are when firing a paintball. Anyone have any insight here? My hail marry is to back-calculate from any known designs that I can get my hands on and see what the delta is... it'll be non-linear and have some dependence on the resistance to flow out of the chamber, so it'll be a little difficult to apply to my own design...

Is this something we've even done the science on, or has every one just solved it through trial and error?

[Jordan]

I sort of remember reading 1 cu.in at 400psi. Maybe in relation to Automags?

[Spider!]

Yeah, the last time I did those calcs, I think I used the total energy from a 4.5/45 tank between 4k and 1k divided by the number of shots I would typically get in my different markers, (650 to 800 depending on efficiency). There was indeed a thread on it in the last MCB.

[latches109]

1.12 cubic inches @230psi is one that could work, depending on the surface areas of the moving parts. Consider to account for Bernoulli's principle. I've unscientifically found the longer bolt tip (after the dump) has a more laminar flow, and I dare say better trajectory.

[Siress]

1.12 cubic inches @230psi is one that could work, depending on the surface areas of the moving parts. Consider to account for Bernoulli's principle. I've unscientifically found the longer bolt tip (after the dump) has a more laminar flow, and I dare say better trajectory.

That sounds low. Where'd you get the data point? The image isn't working for me, unfortunately

[latches109]

It’s been more than a decade since any design stuff, that figure stuck in my head. As butter said The 1cu.in. @400psi is “a” starting point. I know 2cu.in. @200psi works but is not great. Depends on your design. Spider brings up a good point with the HPR.

[Spider!]

Even if I had an accurate CFD model that gave me a good solution, I would be wanting to design around an envelope of different o-ring friction factors.

Is your dump chamber isolated? If it's open to a regulator during the shot, it has the connectors and reg top volume added, but then the regulator response becomes part of the shot. The condition of the connection to the reg (elbows and constrictions) goes into that as well.

[Siress]

Gotta start somewhere. I expect true friction to be the same for given materials and compression rate. Diameter and lubricant choice will definitely play a role in the flow restrictions and surface tension that adds drag to the system. Hopefully most test cases are using Dow 33 light (or one of the many rebrandings), anodized aluminum, and buna 70A orings so that we can estimate the other contributing factors in aggregate.

Isolated dump chamber, btw.