Static Pressure Control?

[Brucepts]

Been giving this some thought the past few days and was wondering what others might think, I know most use an electrical motor control but if one is not able to go this route and has to go manual what size bleed valve might be required to maintain a given static pressure?

I plan on doing some testing on my PTS Style flowbench in the next week or two.

I know it's not the preferred way of setting static pressure but I'm trying to get back to the root goal of this forum . . . and offer all options so anyone can build a flowbench.

[blaktopr]

I had this in the old forum. Used a 3" pvc pipe cap. It has a radius on the closed side. Tapped a 1/4-20 thread through the center of the closed end with a 1/4-20 bolt through it. Made a 3" radiused hole in the bench. Took a 1" by 4" long piece of steel and tapped the center for the 1/4-20 bolt. Bolted this piece of metal to the inside of the bench. Then screw the pvc cap into that. You start the bench with the cap unscrewed open about 2 inches (?) then close it down until you reach the pressure.
I did not have much power then, so having it allmost closed was the norm. Plus because of the size, there was turbulence at larger openings. I would try it again with a larger venturi section, like with adding a hole in a 2X4. Any turbulent air shown at the bleed, shows up on the inclined.

Oh and it cost me the price of the cap and 1/4-20 bolt. I had the steel and the tap.

[jfholm]

another easy way may just be an inexpensive PVC ball valve - they also come in all sizes. The only thing that may be of concern is the one I have is fairly stiff in movement and may be hard to control.
John

[SWAirflowServices]

Gate Valve maye work better.

Gate Valve.jpg

[jfholm]

Hey that gate valve does look better. Looks like it would be easier to turn and a finer control. I agree that gate valve looks good. Do they turn fairly easy?

John

[SWAirflowServices]

It is a little stiff but not as bad as the ball valve. I would be willing to bet that if you took it apart and greased it up with some Vaseline it would be acceptable. I do feel that it would be hella easier to fine adjust. I think I saw them at The Home Depot.

I've thinking if doing this just to have the option.

Shane

[SWAirflowServices]

I'd prefer one with a srew adjustment not a pull.

Gate Valve.jpg

http://flexpvc.com/cart/agora.cgi?produ ... alves-Gate

[Tony]

If you wanted to bleed off 500 CFM, and hold 28 inches test pressure, then the orifice formula says you might need around a 2.64 inch (67mm) bleed hole.

My choice would be a throttle body with a fairly long quadrant lever to operate it directly.

The non linear flow with opening angle should give a very good range of control.
Commonly available throttle body sizes should fall into our required flow range quite nicely, and these are very well made precision units, and should cost almost nothing.

It may even be possible to use a motorized throttle body and have automatic test pressure control ?

[1960FL]

Bruce,

So as not to bastardize your bench a simple test fixture could be made by sacrificing a single Beckson cover and epoxy a PVC tube stub to it say a 3” then you could adapt all you want and test away. Then replace the cover to go back to normal.

I also think in this light we may want to go back to our roots and look at true low cost single motor speed controllers like the HF unit maybe adapted with a multi turn pot???

Just food for thought.

Rick

[Sir Yun]

i know a mechanical bleed is simple to make but i kind of think it is sort of wastefull to dump several hundred cfm (powered by your hard earned money), heat up to the room and drive you nuts with the noise

As I build my motor controller for about 35 bucks including everything (socket, plug, box..etc) from ready made phase control modules (made in ze germany even..not china) that even a total electronics dimwit can stick together I think that that is affordable enough for just about everyone.

btw when you combine / stack two pots with a large and a smaller value you get coarse and fine controls.

it's 4000Va@220v.. so for 110v it's a bit meager if you want to power up a big bench (18 amp continuous) but you could use a second or third module in parallel I guess. I use 115519 Ametek motors and those only draw about 5.5 amps (downside is that they are rather expensive)