Radio Controlled Engine Flow Bench

[Mike Rappold]

I need some clairifcations. I have been reading a few other posts about test pressure(static) and pressure drop across the orifice plate.

Is it best to first pick how much of a pressure drop across the orifice plate is desired and then pick a test pressure? If so, what is the advantage of a higher or lower pressure drop?

I understand the CFM flowing through the orifice will be higher with a high drop but I do not understand why I would or would not want a higher drop across the orifice - especially considering the extremely small size of the flow bench I am building.



Rick,
You mentioned using a low depression of 3 -5 ". By depression, do mean the pressure drop across the orifice?

Thanks in advance.

Mike

[bruce]

Your build presents some unique challenges and throws some theories out the window. A large Differential Pressure (DP) will be more stable on an incline manometer and allow a smaller orifice plate hole but use more blower power to achieve. A smaller DP will have more "bounce" on the incline but will require less blower power.

Now on your bench a higher DP will mean you will have some small dia orifice plates so going with a lower DP will get your plate diameters larger and allow your bench to operate more efficiently. The blowers will run cooler as the bench itself is moving more air to keep them cool.

This is just my opinion and observations. . .

[49-1183904562]

[color=#000000]Mike,

No I mean your test pressure, my thoughts here are that at the size of your ports and minimal flow (a couple CFM) if you tried to test at say 28

[slracer]

[color=#000000]Mike & Rick, I have been following this thread (quietly) and now wonder if it isn

[49-1183904562]

Doug,

I love the way you think, We might even convert Bruce back to a Pitot Guy :p

Your dead on on your thoughts.

Rick

[bruce]

[Mike Rappold]

I need some assistance in figuring a few things out. I have read several items on the forum but that only gets me more confused at times.

I have built the flow bench and am now trying to set the manometers up and get results. The manometers are water based to start - may eventually switch to light oil. I have two identical built ones - one for the test pressure and the other for the differential across the plate. Both are angled and the angles are fully adjustable. The manometers are based upon a 1 meter yard stick, use .125" ID acrylic tubing, and have wells on them that are cylinders on their side. The well area comes out to 12.3 sq inches each. There is an "overflow/overpressure" well at end of the manometers.

I also have an Omega handheld digital manometer that I acquired some years ago. I have been using this as a "check" against the homemade manometers.

I am trying to use a 0.449" ID orifice. At 3" of differential pressure, this should equate to 4.738 CFM. This was calcluated using the spreadsheet on the forum. I have also set the test pressure up to 3".

What is happening:
- On the test pressure, the homemade manometer reads lower than the hand held digital. At 1000mm on the scale at a 3" rise, the handheld digital reads 2.62 in H2O. Shouldn't this be closer to 3.0", assuming the handheld digital is calibrated correctly? I am not sure where the loss is coming in.

- On the inclined manometer I am the most confused. I set the height of the end of the scale at 3". I am assuming this is supposed to equal the 3" differential pressure I used in the orfice diameter calculations???

- When I try to run a test, using the 0.449" orifice and set the test pressure manometer at 3.0", the homemade inclined manometer goes off the scale. It is set for a rise of 3" also. If I check the differential pressure using the handheld digital manometer, it will read 12.21 in H2O.

- So the part I do not understand is how to properly set the inclined manometer rise and how the desired differential pressure across the orifice plate factors in.

Please help. I will post a few pictures later today.

Mike

[200cfm]

Will try and help assuming your orfice bench is plumbed the way mine is. Your test pressure manometer is 3". Your digital shows over 12" across the orfice plate. That means the motors are pulling a combined sum of 15+ inches. You are dropping 3" across the test chamber manometer and 12 inches across the orifice plate chamber. Is your test pressure manomter 100% vertical?

[slracer]

Hi Mike, I was thinking about your problem and thought I would pass along what Rick told me. When you are measuring your rise, be VERY careful to do so when the board is perfectly level. With a 3" rise, an error of 1/8 inch is over 4%! :O Since it looks like you will be moving your bench out when you want it and putting it away until you need it again, you might want to mount the high end of the manometer with an eccentric like Bruce shows on one of his threads. (Which I can't find right now. Maybe someone else can find it?) That allows you to move it a small distance easily. Keep up the good work. -- Doug

[bruce]

Looking at the pictures you posted in another thread you have the wells (PVC pipe) laying over on it's side since the wall is a curved surface you are going to have a change in the well surface area as the well level drops. If you have them standing straight up you would have the same surface area for the whole well drop. Now granted the well does not drop that much and this might not be a problem but it is something I noticed.

Use your digital manometer as a check on your manometer design, "T" it inline and verify that your manometers rise and fall the same as the digital. They should all be linear. Once you have determined that, then work on why the incline is not reading correct. You just might find though that once you work out the other things your incline might be working correctly.

[Mike Rappold]

Bruce,
On the manometer PVC wells, I tried to make them "large" so the drop would be minimal. I was hoping that if I made them with enough area, that the drop I might be a minor amount. My solution with the side mounted cylinders is not the best do the varying drop as you point out. However when I did the math, it came out to roughly a 0.031" drop if the water in the .125" ID acrylic tubes goes the full 1000mm length.

Not to confuse the problems I have but for some unknown reason I also have the lower manometer read a several mm (~50mm) difference than the top one. It is lower. This has me baffled. I measured both manometers with the digital inline also at every 100mm and plotted it in Excel. The two track same - just offset slightly. I did not do the T fitting but rather tested one at a time with the digital manometer on the other port.

200CFM,
The way I have it plumbed is that the bench has two chambers. One chamber (A) has the item being tested attached to it. The other chamber (B) has the shopvac connected. The orifice plate is inbetween. The Test pressure manometer is connected to chamber A. It connects to the manometer via a tube to the top of an overflow well. The water supply well at the other end of the scale is vented to atmosphere.
Chamber B has one hose connected to the overflow well of the incline manometer. The other end of the incline manometer has the supply well. This supply end has a hose coming out of it that goes to Chamber A.
The test pressure manometer is not vertical. Instead I chose to make it an inclined one as well due to the low rise number (3").



Should the pressure reading on the incline manometer be closer to what the orifce size is designed for? In this case 3" drop across the orifice plate.

Mike

[bruce]

Swap manometers with each other and see what happens, if they stay the same as they were before then you have a manometer problem if the readings change you have a pickup location problem.

I agree that the well area is probably not a problem just wanted to point it out that it "might be" a problem. I did not work the math so only assumed it could be a problem.

Personally I'd have my pressure pickups for the orifice plate as close to the orifice plate panel as possible. I'd move the static pickup close to the top panel and locate it back in a corner were it will not "see" any air movement.

Another way to check this is do what Larry did on his bench and add a tube to your pickups so you can move them inside the bench to a location that works might take some trial and error . . .

You need to have another orifice plate the same size as the one in your bench to test on top of the bench. Set the bench for 3" static and set your incline to 100% rise across the internal orifice. From there you can do the math to get your other percentages.



Edited By bruce on 1232282402

[slracer]

Mike, On your Readers Projects thread you said "On the "top" well (test pressure), the vent hole to atmosphere is under one of the black nylon straps. It was not supposed to be but I had a brain fade when drilling it one night and put it in the wrong spot." Be sure the vent is actually venting as that could really foul up the measurement. However, I also think this is backward from your problem! :p -- Doug

A couple of questions from a pitot guy to all reading this:

If your test pressure is lower than what you want, change your bleed valve until you get what you want. That is your "variable". Then check the orifice drop value. That is how a pitot type works (except we read the pitot value and calulate flow from that). Is an orifice different? Should Mike adjust his bleed to get 3" on the "vertical/test pressure" manometer, then check orifice drop?

Also, can the test pressure and the drop across the orifice plate be the same? That is not how I thought you guys on the "other side" did things! ???

[thomasvaught-1]

The pick-ups for the orifice plate on the blue bench are basically 1/8" "off the deck" and within 1/2" of the OD of the largest orifice hole diameter. In a 2 foot by 2 foot plenum, the pick-ups will be still very close to the orifice (volume wise) as far as signal goes.

Tom Vaught

[Mike Rappold]

So if I am reading some of the suggestions correctly, it would better to move the orifice plate pickups closer to the orifice location. So in my bench case, this would be by the divider between the two chambers - correct? I do have some extra acrylic tubing I could make an "extension" to get he pick up points on both sides of the orifce plate closer to the orifice hole. Is this recommended?


Mike