Some question on basics and settling chambers

[turbotuner]

First off let me say hello as this is my first post, this is an awesome forum with a wealth of knowledge. I understand the concept fairly well (I think), as to how the inclined manometer is scaled to what the orifice will flow with no restrictions in the air tract, therefore the scale will be 100% when test pressure is achieved (28" in my case) on one side of the orifice and atmospheric conditions on the other side. So now the item to be flowed will provide another restriction in the tract and the change in flow is measured as a percentage drop from the max flow of the orifice. Ok so here is where I have some questions, If a settling chamber is built, wouldn't this provide an additional restriction on the system? So in other words you would no longer have atmospheric conditions on the settling chamber side of the orifice, and you would no longer achieve 100% on the scale? I work for a HVAC company and we have perforated steel that we use to diffuse airflow into a room, that way the air is more laminar and "falls" into the room as opposed to being turbulent. I was planning on basically using the Mercdog plans, but covering the top of the original bench with this perforated steel, and building a 3 inch settling chamber on top of that. But once I got to thinking about it, that plate would essentially add hundreds of orifices to the tract. The pictures I have seen people post up show the test pressure tap in the settling chamber, with the first differential tap in the chamber below it. Wouldn't these two pressures now be different since you would be creating a pressure difference through the perforated steel? Please let me know if I'm way off base here. Thanks in advance!

[larrycavan]

Hi and welcome to the forum,

Probably the best think is to do up a simple sketch and post the drawing. It's so easy to draw conclusions that are wrong from a description of something like a flowbench with all it's pecularities.

In the Mercdog design, the top plate on the bench with the 4" hole imposes a restriction in itself. "Generally", as soon as the motors are energized and up to speed, the top plenum should fall below atmospheric pressure BUT it will have a higher pressure than the plenum below the flow disk. The imballance of the pressure is what you are measuring and combined with the area of the orifice and it's Cd will ultimately result in the CFM value.

Adding anything between the top intake hole of the bench and the orifice itself can have several effects including:

1. Causing turbulence
2. Imposing a restriction to the maximum potential flow of the orifice.
3. Improving laminar flow characteristics of the air stream.

As with all air flow situations, it's dependent upon positioning, locality in relation to the orifice, etc.

What's been found to be of help in this design is to install a baffel to direct the air away from directly blasing on the orifice itself. Simply causing the air to flow out and around before it passes through the orifice indeed helps.

With regard to achieving 100% on the manometers, you most definitely can peg the guage during testing. What I think you are referring to is the ability of the bench to achieve full flow characteristics of a given orifice size if the additional restriction of the peforated steel plate were introduced into the design. Again, it may impose some restriction to the calculated full flow of the orifice BUT that would depend upon the size of the perforated holes. You don't want to choke it down but calming the air before it reaches the orifice can be a good thing.

Best Regards,

Larry C.

[turbotuner]

Thanks for the reply, I'll whip up a drawing when I get home this evening. My intention with the perforated steel is to prevent the direct blast of turbulent air onto the orifice. Same principle as the baffle. Basically I'd replace the baffle with the perforated metal. And you read me correctly on my manometer question. I didn't really take into account the hole in the top of the bench, since is will always be covered by a cylinder head etc, but you're right it will be a restriction. What I am worried about is if you have an orifice that flows say 300 cfm at 100% scale (28" water rise on both the U-tube and total vertical rise on the incline) and now add a restriction such as a baffle or perforated plate, our equations will not account for that, and it will make it seem that the lost cfm is attributed to the head. Or can this be corrected by adjusting the Cd of the oriface?

[larrycavan]

That's part of the calibration process. You calculate, speculate, and take educated guesses at what the flow through the orifice will be. Calibration will reveal what your careful planning and preparation actually produced.

If you have access to a flowbench already, why not cut some sample pieces of the perforated steel and test it. At least you'll gain some insight as to what you might expect from it should you decide to use it.

I'd consider using a flat baffel plate with perhaps sections of the perforated steel attached to the edges of the plate. Rather like a box with the bottom being solid and the sides being perforated. The flat plate will initially redirect the flow and the perforated steel can help to calm the flow down after it bounces off the baffel. I can't say what the actual outcome will be but in theory it seems proper.

You can alter the Cd of any orifice [or so it seems logical] by tweaking the edge of the hole.

Larry

[turbotuner]

Ok, please excuse my paint skills but heres a little drawing to give you a better idea.

So you can see that the perforated plate will go inbetween what used to be the top for the mercdog bench, and the settling chamber. Larry, thats a good idea to flow the plate and see how well if flows. Although I dont have a flowbench, we do have a system that uses and orifice meter to measure the air loss in a sealed HVAC duct system, I could attach it to one of the perforated grilles and see what kind of flow I get. I was also thinking along the same lines of a solid baffle in the middle with the perf. on the sides. Maybe I'll just get off my butt and build the thing, and do some testing with different setups.

[larrycavan]

That's pretty much the way I had it pictured from your description. It should work just fine.

Larry

[Mouse]

One thing that needs to be considered in any application of this type, is that all panels need to be ridgid enough not to flex. Any flexing or bending of large surfaces can cause resonance or pressure pulses which will affect your manometer readings.

John

[84-1074663779]

turbotuner, you are definitely on the right track. Placing a perforated screen in there is an excellent idea, although I have not tried this myself. It should certainly break up any large swirls or eddies into lots of little ones which should quickly dissipate.

Pressure drop through the perforated screen is not an issue if your sloping manometer "sees" the pressure directly either side of the orifice plate.

[turbotuner]

Awesome thanks for the replies guys. Tony I hoped you would chime in, I noticed in reading as many post as I can that you definitally seem to be a fluid dynamics wizard. I plan on not only placing the perforated metal on the top of the bench and the orifice, but also between the orifice and the flow valves, to break up any turbulence cause by air entering. I would imagine that would equal out any differences.