Problems flowing the plates

[Rick360]

First I'll give a brief description of my orifice bench. It is built similar to a SF300/600 except the rotating orifce plate is horizontal, not diagonal inside the front chamber. The orifice has 5 square edged holes machined evenly spaced. I also use a waste gate type pressure control rather than the plug type restrictions in a SF. There is another internal orifice on the same board as my rotating orifice that slides to add another 250cfm to the range, which is not shown on the drawing.
See diagragm.


When I had the test plates, I flowed each plate on different ranges and at different pressures to cross calibrate each range ( 5 ranges total). I have determined approx flow values for each range from testing and comparing to other benchs. Not exact, but has been close enough and very repeatable. When I tested the test orifices, they didn't flow the same on different ranges using my range values. The numbers were 15-20cfm off in some cases. For example with the "300cfm" plate I got 239 on my 354 range at 28", 252 @ 15" on my 248range converting back to 28" and as much as 288 on a 298range at 28" using the extra sliding orifice and the smallest rotating 50cfm orifce.

I have been using this bench for 15+ years with virtually no changes. With a head on the adapter and flowing a port, I have always been able to test on a higher range or lower range and get very close to the same CFM number using my range values, usually less than 2 cfm difference.

When testing the plates I removed the adapter and flowed them right on the top plate. I think the problem may be with how close the higher velocity air thru the orifice is to my internal orifice and the diffuser discs. Possibly causing some backpressure to alter the flow. When a head is attached to the adapter, the air is slowed down thru the cylinder before reaching the top chamber.

I'm not ready to tear it up and start over, I am still very happy with how it works. I am just curious what you guys think the problem might be.

Rick

[larrycavan]

My questions would be...How did you arrive at your range values without a calibration plate or plates?

If you feel your test stand corrects flow for your particular design, did it occur to you to flow the plates on the test stand and see if it was closer?


If you run the numbers through conversion factors your ranges come out higher than what you currently label them.

Since the values you posted for the second set of values actualy comprise 2 ranges it's difficult to apply conversion factors.

In any event here's some info to consider.

You got:

67.5% of 354 = 239cfm
82.7% of 348 = 288cfm

Ok, we can say with reasonable confidence that the 300cfm plate flows 296cfm.

So applying corrections to your ranges we get:

Your 354 Range = 438.5
Your combined 298 + 50 range = 357.9

Now, standard numbers for a SF plate flowd with the sharp edge opposing the flow on average is 152cfm@10" and 252@28"

Those plates are relatively easy to come by.

For the sake of somewhere to start, you could flow a SF Plate and calculate it's flow against the recalculated values just to see how things stack up.

If it does indeed fall into place then you've got a good indication of what to do.

If it doesn't fall into place then you've got some more data to work with anyway.

Regarding the dynamics of the flowpath in your bench. Consider this. My bench and Chad's bench are similar in cabinet construction, both being MSD benches.

Two differences are:

1. I use a single fixed in place orifice.
2. I have a difuser plate, Chad doesn't use one.

Similarities are:

Both use FP1 flow measurement.

Of the three SF benches in the pool, two were 600s and one was a 110. The 110 had to use converted values. Converted values can be misleading.

Just like testing a head at higher depression values and having the dynamics of the flow pattern change, the same can happen in a flowbench.

I "believe" Nick uses a horizontal mount for his orifice plate in his bench. His results were pretty darn good and he's using a home made water manometer....Entirely different cabinet layout though.

Just some things to think about

[Nick]

Yep horizontal. The air has to turn 90 degrees to flow thought the orifices. There is a large setting chamber before and after the orifice plate also.

And I like my water gauges, they don't lie.

I really wanted to take my mic to the plates, but I was in a hurry and knew I would get caught. Although It looked like someone did it any ways. Get out the magnifier Bruce.

Nick