Ran Out Of Ideas - Manometer Bounce Issue

[stef-1]

Hi All,

I've spent all day trying to sort out a manometer bounce issue that's been plaguing my new flowbench.
Details of the bench are here

This is what I have tried today;

1. Installed filter on bleed valve - no effect.
2. Made bellmouth on bleed valve (plasticine) - no effect on bounce but improved bleed valve capacity.
3. Moved baffle plates closer to orifice - small improvement.
4. Removed baffle plates - worse effect.
5. Mesh under head mounting hole - no effect.
6. Made several probes with an array of 1.5mm holes in them to pick up static pressure only. Placed in loads of different positions - no real improvement.
7. Cotton wool in lines - no effect other than to slow response.
8. Small orifice in lines, 0.6 - 0.3mm sizes - as above.
9. Needle valves in lines - as above.

I tried all the above singlely and in combinations. The best improvement still had 1/8" - 1/4" of bounce in both manometers.

I don't know what to try next ???

I'm lucky my wife has patience with me spending weekends in the garage lol

[stef-1]

I wonder if it's worth fitting extra pickups in each chamber so that they can be linked together and provide an average reading in the chamber - like thry do with some pipe orifice plates ??? Anyone tried this?

[thomasvaught-1]

Put a 1.5" or 2" x 3" long "accumulator in the lines.
This will dampen out the pulsations and will still give you very accurate readings. There will always be air in the accumulators so the response time will barely change.

You can make the "accumulators" out of PCV pipe and end caps glued together and a hole drilled in each end for the manometer lines.

Tom Vaught

[stef-1]

Tom,

I will try out the accumulator idea again. What sort of diameter should I be looking at?

[thomasvaught-1]

Try 1.5" ID PVC Pipe about 3" long with the rounded caps on the ends. Drill the rounded caps first and either install nipples or tap for a screw in fitting then glue them on the tubing.

Some put cotton inside but if the manometer accumulator ever sees fluid the cotton is trashed. Unless you have a removable end piece (and that might throw off your readings if it leaked).

Tom Vaught

[Tony]

Manometer bounce can only be caused by pressure variations and unstable airflow in your bench.
In other words the bench is very likely surging.

You can play around with your manometer by greatly slowing down the response, but is that really a good idea ?
Much better to fix the surging problem if you possibly can.

After reading through the construction thread, I would say almost for certain, your surging problem is because you are throttling the air instead of controlling the speed of the blowers.

All centrifugal blowers have the potential to surge to a greater or lesser degree, if the airflow is greatly restricted.
I strongly suspect it is your throttle body flow control system that is causing the centrifugal blowers to go into a surge condition.

It is far more efficient to control blower speed, than to always run the blowers flat out and then throttle the air. Less heat, noise, and power consumption as well. And it will give a far better control characteristic.

[stef-1]

I would rather fix the surging or instability problem. Although the problem has happened to a few members on here there does not seem to be much information on this issue, especially the causes. I've not heard it happening to the "Big Blue" benches or the proper PTS bench. I have been thinking that there was, or is, a fundamental problem with the design of my bench. Although the pitch of the motors does not change in operation I have heard "woofling" or "chuffing" sounds from the bench. Is this surging or flowstreams seperating and re-attaching inside the bench?
I wonder why a throttle type depression control would cause problems rather than the "Big Blue" method?

I will build a motor control unit for two of the motors. I did have the foresight to order a couple of the Harbour Fraight router speed controllers ages ago. I did not use them originally as I wanted to keep the bench simple in construction. I will report if this solves the issues but I'd like to fully understand just what it is about my design that is causing the problem. Hopefully others can learn from my mistakes

[bruce]

Are you running all your motors at one time and bleeding off to get your static?

If you are not running all at one time then I'd lean towards having a back flow issue (motors in-reverse spinning up and slowing down when not under power) if you did not install check valves.

I'm with Tony, you have something going on with your motors.

[stef-1]

[stef-1]

Some more thoughts after spending a while reading on this forum; the heights of the plenums on both sides of the measuring orifice are 9". Is this too short? Also the height of the bleed chamber is around 4". I'm wondering if these dimensions may be causing turbulance inside the bench?
What do you more experienced guys think?

[Tony]

Stef, if you can hear "chuffing", your bench is definitely surging.
Fix that, and your manometer bounce problem will go away.

Throw away those throttle bodies, open up the air path both into and out of the blower compartment, and speed control your blower motors.

I have seen this exact same problem before in one of my own very early flow bench projects.

[stef-1]

[Tony]

[49-1183904562]

Steff,

Remeber you will need to install the flapper vlaves on the motors that will not be running once you move toward a motor speed controler.

Rick

[blaktopr]

Stef, I'm with Tony. The more I close my bleed valve the less surge there is. Completely closed, steady as a rock. It doesn't matter about the size either. I have a 12X30X30 settleing chamber only for where the motors feed and to bleed before the rest of the bench. There is still some funky turbulence in there playing with the meters.