Test pressure control

[spoofty]

I am in the design stage of my first orifice type flowbench. I've searched but not found the exact answer I'm looking for.
What is the best style of test pressure control?
1: A valve between the vacuums and the plenums.
2: An air bleed.

I was thinking that the valve might be harder on the vacuums keeping them under load.
But maybe with an air bleed the vacuums might surge if they are running wide open all the time.
Or maybe it doesn't matter.
I know speed control for the vacuums is probably the best but this is a very low buget build.
Thanks for your help.

[Thomas Vaught]

I have personally used a "Restriction control" type valve to control test pressure. The SF 300 and 600 benches use this method.

I have used a "Bleed control" type valve to adjust test pressure. The motors like this method over the restriction method. The get continous cooling air.

I have used a motor controller to adjust the test pressure. Flow Com SF benches have this feature.

For a low budget build, grabbing a throttle valve off of any junker automobile and using that as a bleed valve is as cheap as you can get. Get the ones that are cable driven and rig up a pull cable/screw adjustment deal.

Tom V.

[Tony]

Given a choice, motor speed control will always be be the best method.

Quite apart from, heat, noise, and wear and tear on the motors, it will give excellent control right down to zero flow.

[DaveMcLain]

I have a motor controller on my Superflow bench and it's the best way to control the pressure. When I first bought the bench I used the restriction method whereby the blowers are running full force all the time. This works fine and it gives good control but it makes the bench very noisy at low valve lifts and flows and it makes a lot more heat. Now on most of my testing the motors are just cruising along. I think you could build something very similar pretty easily where while it doesn't automatically keep it at a preset pressure you can still adjust the amount of flow with the twist of a knob.

[larrycavan]

A motor controller can deceive you when pitot testing a port for velocity. When probing for velocity profile, it's best to allow youself to see the true picture of what is happening in the port. A motor contoller could mask things on you.

[jsa]

[larrycavan]

You want to be able to see sudden pressure changes. If the controller recovers too quickly, you won't see them.

Example: Maybe the pitot has shifted and is touching the port wall and you didn't notice it.....end result..misleading data.

[jsa]

[larrycavan]

[jsa]

Larry,

Ok thanks, now I follow.

[DaveMcLain]

I think I understand what Larry is saying, the depression pressure jump could be caused by the distrubance in the port from the placement of the pitot tube. You might not notice this with the motor controller because it would see the pressure rising and then slow down the motors slightly, reducing flow and keeping the pressure the same.

[86rocco]

So if I'm understanding Larry's point, that would apply to only to a speed control system which automatically adjusts the test pressure but not to a system where the operator manually controls the motor speed. Is that correct?

[Tony]

That is how I see it too.

Manually setting motor speed, or manually adjusting a flow control device are functionally identical.

Closed loop control by either method is a whole different ball game.

[jsa]

Yeah manual speed control would still allow dynamic changes from the probe effects to show up.

One might also delay the closed loop response to allow dynamic changes to appear but still adjust for supply voltage fluctuations automatically.

With the speed control I have been playing with, I've noticed it is effected by voltage changes from the mains supply, hence some variation in test pressure. This would also apply with non speed controlled motors.

[riga team]

Who can will prompt the price for this converter ? hitachi L100 075 HFE : in usa .. Or cheaper on the motor 7,5 kw .Gratitude Anatolij .