Flow bench efficiency

[Tony]

Hmmmm, interesting.....

I guess the basic problem is that "something" needs to be put into the flow path to develop some flow related pressure signal that can then be accurately measured.

That developed pressure signal must ultimately come from additional blower back pressure, no matter how it is actually done.

I cannot see any advantage in say, using a smaller diameter streamlined orifice, compared with a slightly larger diameter sharp edged orifice, if both develop identical pressure drops at identical flows. I also agree with 86rocco, a sharp edged round orifice is about the simplest and easiest shape to make, and the results will be predictable within a narrow range.

Anything other than a sharp edged round orifice fed from non turbulent up stream air is just adding further unnecessary complication.

[Scott]

It seems that the Pitot bench guys are able to flow more CFM through a similarly sized bench. I am planning on back to back tests using a appropriately sized SE orifice to see the benefits if any. In my research, a Quadrant Edged Orifice, similar to what I will try, is said to be more accurate in measuring lower velocity flow levels as well as having a greater range of accuracy over a Sharp Edged Orifice. I agree SE orifice are easy, but also want to investigate why these other forms have been developed as well.

Scott

[Rick360]

A flow nozzle will have a better pressure recovery in a pipe application and requires the tap locations to be in specific locations on the nozzle to get the DP measurement.

Most orifice benches are using the permanent pressure loss as the DP because the taps aren't at the orifice. In a large box the DP taps are normally put back out of the airstream away from the orifice in calmer air for consistency. If the taps are located this way, the type of restriction is irrelevent and the permanent pressure loss will be the entire amount of the DP reading.

That's how I see it anyway.

Rick

[Nick]

I read somewhere that flow nozzles are more repeatable than SE orifices. It may have been the site that Tom posted about the Ford bench.

Here is some interesting calculators I found surfing one night.




Nick

[larrycavan]

[Tony]

My take on this is that orifice plates and nozzles located in pipes are mainly suitable to monitor flows in industrial processes. It is the only convenient and economical way to do it. It works fine where a single, fairly limited flow measurement range is involved, and where a "flowmeter" needs to be installed into an already existing pipe.

Our problem is rather different. We need to accurately monitor flows over a very wide range, and usually one orifice size is simply not up to the job, because of the square law output pressure relationship to flow. If an orifice is mounted in a pipe, switching ranges becomes far more difficult mechanically. However, a sharp edged orifice, being thin can be slid or rotated fairly simply on a large flat surface.

Pitot measurement offers no real advantage if a fluid filled manometer is used. There still needs to be sufficient fluid rise, at least enough rise to be easily read along a scale.

As soon as you introduce electronic pressure measurement, the whole game changes. Even orifice plates could be made to read with the same very low overall pressure drops as are commonly used with pitot systems.

[Scott]

Larry,
I've actually included provisions for just that, just in case. I'm going to try the set up I have, I think it'll will be okay. I gave this stuff a run through on my big bench as test pieces, but will reserve my comments just in case my tests dont prove reliable under working conditions. I will build Venturi elements before I'm through, and they will be tested on my last and final bench which will likely be a Averaging Pitot design. From the turn down ratio, accuracy and power requirements stand points, Avg. Pitot comes out ahead of everything else.

This is all just education, I went to only two years of a formal college degree, when life got very real, and I quit. Now that was the best thing for me, because, since then,I, have chosen the professors I wanted to learn from, in this case, its you guys..........

Tony, I agree 100%. Electronics are almost a must as well. I've been meaning to place my order with John, but keep hearing about improvements/upgrades, so have kinda been waiting to hear more before I pull the pin on it.

Thanks guys

Scott

[Mouse]

Pipes and orifices are tricky because the placement of the taps are critical. Pitot tubes are at-home in a pipe.

Scott, I just realized your nozzel might be a perfect receptacle for a Pitot tube flow element. What's the inside diameter?

Your basic setup is very flexable, you can always throw SE orifice plates in there too.

John

[Scott]

The I.D. is about 1.440"

[Jesse Lackman]

These pitot flow elements have the best accuracy and turndown rates I have seen;



They use averaging pitots in a flow nozzle and come complete with built in flow straighteners.

[riga team]

I have a leaf for flow straghteners,and I shall apply mine bench. nzp is good what price ? http://foto.inbox.lv/gabtmotors clik ' skatit slaidus

[Mouse]