new to the forums

[coulterracn]

Yes you are starting to understand the concept of a flowbench and how to use one.

I was the one who stated you needed to use and orifice with a known cfm. What I failed to mention was the Dp and other known values determine how many cfm will flow thru an orifice.

As you search the forum you will learn an orifice used in a pitot style flowbench will flow a different number in the PTS bench due to the difference in design and calculated known values.

This confused me in the beginning. I could not find the information on the forum. I had to ask for help, I received good answers and was guided to the information on the forum. I should have ask for help sooner. I learned a lot from our fellow members. I'm sure I'll learn more as I progress. I thank each member who has corrected me, guided me and taught me.

Ray

[Tony]

Once you get your head around the basic concepts, it will all suddenly fall into place for you.
You will have a kind of "Eureka moment" where the light of understanding suddenly turns on.

The basic idea is that your sloping manometer can be calibrated in percentage of flow, and that very same percentage flow scale will work with any orifice, or orifice pressure drop that you finally decide to use.

The trick is, to decide on an appropriate orifice size and pressure drop at the 100% flow point. There is no single correct or "best" pressure, it is a compromise.

You can run a smaller orifice that has a higher pressure drop at the required 100% calibration flow. Suppose you decide to use an orifice that dropped twenty inches of water at xxx CFM. The sloping manometer is then set so that the high end is twenty inches above the low end. So at xxx CFM the water rise is twenty inches. That corresponds to xxx CFM being 100% on your flow scale.

Or you can run a larger orifice that drops only five inches water at the same xxx CFM. In that case, the high end of your sloping manometer would only be five inches above the low end. The 100% flow marking on your scale still corresponds to the exact same xxx CFM, and everything still works the same.

But.......... There are some things to think about here.

First thing is that a very low manometer slope is much more prone to error if the manometer is not exactly straight or level. A completely vertical manometer will read right, even if the manometer tube is a bit off vertical, or the manometer tube a bit curved. A nearly horizontal manometer needs a LOT more care in construction and levelling.

Second thing is, a large orifice running at a very low pressure drop will be a LOT more intolerant of any up stream air turbulence, or slight imperfections in your flow bench.
A small orifice running at a high differential pressure drop will have a lot more violence and fury right at the orifice, and will read more accurately even if the entering up stream air is a bit turbulent, or your bench internals have flow instability or imperfections.

So basically it is a trade off. A higher design orifice pressure drop across your measurement orifice generally leads to a better performing bench.
But unfortunately this also makes your air blower work a lot harder, and it will ultimately reduce the maximum flat out available flow volume, due to the extra back pressure load placed on your air blower.

My advice would be to use the highest orifice pressure drop you, (and your air blower) can live with, depending on how much blower power you have, and what you intend to test with your bench.

[rb209]

Hey Tony Thanks for summing it all up and explaining it very clearly. Im pretty sure I understand it now. You must be a mind reader because I was going to ask about the orifice sizes. I still have some more questions though. Lets say I have an orifice rated for 300cfm @ 12" pressure drop Could I use this same orifice @ 6" pressure drop. By using the formula to convert between pressures It should flow 212cfm. Would this be accurate or would you need a separate orifice plate(larger one?). Im guessing you would need some way to slow down your blowers to achieve the 6" pressure drop with the original plate. This isnt something I am planning on doing but would just like to know if its possible.

Another question: Is it possible to have a pressure drop across the orifice that is higher than your test pressure? Assuming you have enough blower power. right? Sorry if these questions dont make any sense Just thought I'd ask anyways :laugh:

[jfholm]

Your delta p is not your test pressure. Your delta p is just the inches of H2O that your internal orifice is calibrated for. As an example, a 2" diameter sharp edged orifice claibrated at 12" H2O flows approx 183 cfm. So if your delta p was 12" H2O then the 100% on your incline would be 183 cfm no matter what your test pressure is. So if your port you are testing hits 100% on the inclined manometer and your test pressure is 28" H2O then the port is flowing 183 cfm at 28" H2O for that lift.

Now if your test pressure were 10" H2O and it hits 100% then you still have 183 cfm but it is done at 10" H2O test pressure. What this means though is if that port flow 183 cfm at 10" and you raise the test pressure to 28" H2O then that same port would be flowing much more cfm at 28" H2O than at 10".

The delta P is just the pressure that it takes to get your incline to the 100% reading no matter what size orifice is inside the bench.

Now here is something I would like to get set straight. Just because your inclined has a rise of 12" does not mean that is your delta P. It should be around 12" but many things affect your delta p. The specific gravity of the fluid will affect the delta p. Water is usually set to a specific gravity of 1.00. Now the Dwyer gauge oil is .826 specific gravity so it is lighter and takes less pressure to get it to the 100% mark on the inclined.

You also have to take into account the diameter, area, of you resevoirs and id of your tubing. This is the nice thing about Ed's spreadsheet. It does all the math for you.

Now let me just use my planned incline to show the different delta p values that can be had.

No keep in mind my inclined is 36" long and the tubing I am using is 1/4" id with a 12" rise. My resevoir ie is 1.430" and I am going to use water and that creates a higher delta p. With water this incline will cause the orifices to use a delta p of 13.10" H2O even though I have a 12" rise.

Now if I changed to the Dwyer gauge oil with a .826 SG then the delta p would be changed to 10.82" H2O. So with the only change being made, the gauge fluid, the delta p changes by 2.28" H2O.

So if I had a 3" diameter orifice in the bench and I was using water my delta p would be 13.10" H2O and the 3" orifice would be flowing 431 cfm at the 100% mark.

Now if I changed to Dwyer oil with a SG of .826 then I would calibrate that 3" orifice at 10.82" H2O and that would mean if I was using the oil then at 100% on the inclined my orifice wouild be flowing 391.7 cfm.

You can now see why it is so important to plan ahead what you are going to do. Using Ed's spreadsheet has made this so much easier.

John

p.s. added after I thought about this post. Tom Vaught also suggested that after getting your inclined built and the fluid of choice in it, hook up a vacuum source like a mighty vac or something else that can accurately measure how much vacuum it takes to get the fuild to the 100% mark. That would also tell you what your delta p is.

[Tony]

[rb209]

I was thinking that the test pressure affected the delta p across the orifice. This is still a little confusing but hopefully when I actually start using a flow bench I will understand it.

I plugged the numbers posted by jfholm into the spreadsheet and I got different results. For 3" orifice @13.10" H20 I got 441.7cfm and @10.82" H20 I got 401.4 Is this right?
So even if you have a 12" rise with a delta p of 13.10 you could just use the 13.10 in the formula to find what 100% flow would be? Or would you need to actually have the orifice plate calibrated at the 13.10" pressure drop.

Thanks again for all the replies. I really appreciate it

[Tony]

[jfholm]

[86rocco1]

[coulterracn]

[jfholm]

[rb209]

You were right jfholm, I was using a Cd of .62

It is all starting to make sense. I will probably be ordering the plans and hopefully I will start to build my flowbench soon. In the mean time I will keep on learning as much as I can.

Thanks again