Machined Flow % Scale - Where can I find one?

[larrycavan]

Gentlemen,

If you could, please take a minute to consider the following. I"d to get your thoughts on it.

MSD bench, modified in the following way:

Let's say I add a new top section right on top of my existing one. It would be a rectangular piece about 3" high with the discharge hole placed in the center like the current MSD design.

I would seal the current 4" discharge hole and create a new pathway to the top plenium by making new a new opening or openings in the old top piece.

What would you suggest for the new openings?

I've considered several possible ways:

[1] A long narrow rectangular opening with more area than the current 4" hole, located toward the front of the bench about an inch inward from the front edge. Something perhaps 1" wide by 18" long.

or

[2] Placing holes off toward each end of the old top section where the air would hopefully balance out the pressure in the plenum. As for sizes, shapes or patterns, I'm uncommitted at this point. [again, the old discharge hole will be sealed]

What I'm looking to do is calm the air down before it passes the flow disk by distributing the pressure in the top plenium in a more uniform way.

A baffeling system between the new top and the old one would be easy to create as well. I don't want to overkill it so bad that the motors begin overheating either.

What's your opinion on using a flow straightener in the pathway before the air enters the top plenium?


Best Regards,
Larry

[RRBD]

So lets say I have 6" of rise from the 0 to 100% on my inclined manometer and lets say I set my depression at 28" on the vertical when testing. When I do the calculation to determine orifice size for a particular cfm, do I use 6 or 28 in the calculation (like in Rocco's spreadsheet)? ???

[bruce]

28", that is the pressure drop across the orifice

[Mouse]

Larry,

I have been playing with some orifices and have found that they seem to draw most of their air from the sides. A large flat board can be placed quite close to the orifice from above without effecting flow, but place anything on the sides and flow will be effected. Even placing a 4" ring that is .25" tall around a 2.8" orifice increased flow.

So, if I were you I would place a baffle between the measuring orifice and the discharge hole that will evenly distribute air to the sides of the orifice. Then the pressure measuring point for the top side of the orifice should be placed between the orifice and that baffle, but not too close to the orifice.

John

[larrycavan]

John,

Thank you for the feedback. A couple of questions.

Would you place the baffel parallel with the flow disk?

Do you know aproximately how close to place it?

What thickness would you suggest making it?

Should the edges be rounded or left square?

Larry

I"ll include an image

[Mouse]

Yikes! I was afraid you were going to ask.

These are my opinions based on some experiments. I have no practical application experience.

Parallel with the measuring orifice. The spacing depends on the size and total flow through the orifice. But I would venture 1" for every 100 cfm you expect through the measuring orifice would be more than enough. So an orifice you expect to flow 600cfm through, I would give it 6". You should be able to test the flow through your orifice, install baffle, and retest to see if anything was lost or gained.

For thickness and edges, I don't know. Maybe sharp edges because you don't want the air to follow the baffel surface. But make it thick enough so that it won't flutter or flex causing pressure fluctuations.

Just my thoughts.

John

[fchp]

[bruce]

The 6" is the difference between the left side of the orifice and the right side of the orifice. So you can measure a difference of 6". You are measuring differential pressure with an orifice not static pressure. The orifice is calibrated to read a specific cfm based on a given static pressure.

Make sense now?

[fchp]

It's clear as mud now!! LOL. Actually, I understand what you are saying now... which means I have my CFM's all wrong for the size plates I have. But other than converting %flow numbers to real CFM numbers... I guess it really does not matter as long as my scale and orifices stay the same.

In the mean time... I started using the software from mercdog so I just input my hole sizes into the software and it did the calcs for me. However, I think I probably have it calculating for the wrong pressures.