how close is to close

[DaveW]

Anybody do any testing by putting something close to the orfice hole and see if it did make a difference in flow. I am going to make hold downs to seal my orfice plate and was wondering how close I could get. Thanks

Dave

[bruce]

I just happen to be testing some orifice plates I machined up sometime this afternoon/evening on my bench I'll do some "playing", I'll report back.

[bruce]

Ok had a chance to quickly play with a 160 cfm orifice plate 6" x 6" square on top of my flowbench. I added a 4" dia 3" high peice of turned wood with the orifice hole located in the center and flow increased to 170cfm's. I then used two peices of 3/4" high steel plate on either side approx 1" in from the sides and flow was not effected.

So my non-scientific results would suggest that something lower than 3/4" and as far out from your orifice hole would be the ideal hold-down. My orifice hole dia was approx 1-1/2". I'm sure others who have actually built an orifice style bench would be better at offering a more knowledgeable answer

[larrycavan]

The basis of the question is related to bench construction techniques. It's a good question and I personally like the concept of the application. Some consideration to certain aspects should be kept in mind. Should the construction technique involve the fastening of the orifice plate to facilitate quick removal of various plates and the fastening technique being the influencing factor in altering the flow, that may well carry presidence over concerns of the calculated flow of a given orifice.

Since we can calibrate to determine what any given orifice flows, it becomes evident that weather the orifice actually flows 100% of it's calculated value or 90% or 105% is less important that achieving the design goal. It all, always comes down to one specifice piece of criteria, Cd of the orifice.

Also keep in mind that fastening techniques that affect the flow on the top side of the orifice could be non-influential when air passes the other way through the orifice.

Example:

A 6" x 6" removable orifice plate held down with studs and wing nuts. Will the wing nuts be an influencing factor on flow through the orifice? Even if they are, it is a calculable influence. It is determined during calibration. As such, facilitating the fast change orifice design, even though flow is influenced by other factors is still a viable method.

JMO

Larry C

[SWR]

This was the closest relating post I could find while searching,so here goes: If you run a 3" diameter orifice inside a short section of pipe (6" from the entrance to the orifice) with a diameter of 500mm's (App. 20") would the orifice hole consider such a 20" disk to be "big enough" to act as the flat wall of a settling chamber,or would the pipe walls still influence the flow through the orifice? How big is "big enough" when it comes to the flat area around the flow orifice?

The bench would have a 3x3x3 ft. settling chamber on either side with one wall cutout to attach the big pipe leading to the orifice. Reason for asking is that I think I found a very good way of making an orifice bench design that should be 100% leakproof across the orifice,while still having a very easy access to the orifice for changing sizes... that is,unless the pipe walls of even such a big pipe affect the orifice..

[larrycavan]

[SWR]

[Thomas Vaught]

Quote by Bruce:

"Ok had a chance to quickly play with a 160 cfm orifice plate 6" x 6" square on top of my flowbench. I added a 4" dia 3" high peice of turned wood with the orifice hole located in the center and flow increased to 170cfm's. I then used two peices of 3/4" high steel plate on either side approx 1" in from the sides and flow was not effected."

Bruce, I was thinking about your post and wondered if the piece you
machined up of turned wood was possibly a "sectioned donut". If it
was then you might have created a possible ASME nozzle effect which
made the airflow go up by 10 cfm.

Some extra info would be helpful.

I have a fixed orifice plate (24" x 24") that has 5 holes with the spread between centerlines of the holes about 8" and even with stoppers extending over 3/4" above the surface, I see no flow interference between holes.

Tom V.

[SWR]

[bruce]

I would simply reply that my testing was probably flawed and not real scientific looking back now.

[Thomas Vaught]

My plate has 5 holes of different orifice diameters.

Imagine a plate that is square and 24" by 24"

Draw a line from one corner to the opposite corner thru the center

Draw a second line from the remaining two corners through the center.

At the center of the plate I have the smallest hole.

In the middle of the line from the center position to the top left corner
I have the next larger hole.

In the middle of the line from the center to the top right corner I have
the third largest hole

In the middle of the same line but going the opposite way from the
center to the bottom left corner I have the 4th largest hole

And finally in the middle of the line from the center to the bottom right corner I have the largest hole.

The plate now looks like a "dice" cube with a Number 5 showing.

From the center of the center hole to the center of any other hole is 8"

Hope that helps. Hole diameters are: 1.263, 1.783, 2.525, 3.575, 4.375
so the holes are pretty large but the plate was created to flow large carbs if necessary. For a head flow bench I would make the diameters even smaller so even less interference.

Tom V.

[SWR]

[Thomas Vaught]

The plate edges are flush with the sides of the cabinet. The edges also have mounting holes and screws that secure the plate to cabinet. The plate is also sealed with rtv. It is a "divider" located midway (top to bottom) in the plenum chamber.

Tom V.