Math question can you check me

[JRM]

I wanted to check and make sure i was understanding the formula correctly

cfm= 13.29 x diameter2 x sqroot of water test pressure

what i came up with is
2- 1 1/8" and 1 1 3/4: holes = 445 cfm @28"
2- 1/2" and 1 2" =345cfm @28
1- 2"= 255cfm @28
1- 1 3/4 = 112cfm @28
1- 1/4"=104cfm @28
8- 5/16" = 11 cfm @28
5- 5/16 = 5cfm @28

I went a little further
a 4" hole @10" of water =800cfm
@ 6" it = 600 cfm

and a 2" hole @ 10" = 200 cfm

is this right or am i doing the math wrong

[Mouse]

Let's look at a 2" orifice @ 28"wc

13.29 * 2" diameter^2 * sqrt 28"wc
13.29 * (2" * 2") * 5.29
13.29 * 4 * 5.29
13.29 * 21.16
281.3 cfm

I have found that 13.55 is a better fit for sharp edge orifices using a standard Cf of .62, and a standard air density of .075lb/ft^3

Another point if I may, I have strong reservations about using multiple orifices to achieve a flow rate, unless they are well seperated. But separating them has other effect too. This is because of the effect they may have on each other both on the leading side, and the exit. On the exit is a phenom called Vena Contracta (I think that is what it it called). It is the effect of turbulence on the trailing side of the orifice that makes the orifice appear to air flow smaller than it really is. Do a search for Vena contracta for more details. If the Vena Contracta is disturbed by a nearby orifice, the Cf, or coefficient of flow, may be unpredictable.

John

[86rocco]

[larrycavan]

I would have to agree with John. If I were making a new flow disk, there's no doubt about it that I would use a single hole for ech flow range. The less variables, the better. An argument could be made to the contrary because superflow uses multiple hole combinations on their 110 model [i've never seen the inside of any other model] but with the design of the MSD type of bench, I would think that the more you can simplify any variables in calculations and the associated problems of trying to determine if other variables apply to the properties of the air as it passes the flow disk, the sooner you'll be using the thing and not second guessigng your readings. If you are forced to use to holes, I'd space them at least and 1.5" apart from each other. Look at a 110 model.

Larry

[Mouse]

86rocco's spread sheet is very good. It takes into account a few more variables, but the results jive exactly with the flow rate calculator I use at Efunda.com.

My equation is for round sharp edge orifices in ideal conditions (no initial velocity on either side, existing on a large flat plane with no bounderies or objects nearby). A square or pie shaped orifice may not work with these equations. Also, I have found, as Tony has mentioned earlier, the larger the orifice, the less the thickness of the material will have an effect on the Cf (or Cd). A smaller orifice will be harder to achieve a standard Cf of .62 than a larger orifice. When I make orifices, sometimes they just will not flow correctly, and I end up throwing them away as I just can't figure out why. This usually happens with the smaller ones. Like Larry says, the less variables the better, and I can't think of many things besides a flow bench where this phrase better applies.

John

[larrycavan]

I was reading the flow calculation posts and got to wondering about some things. I have the actual P.H. mag with the article on building the flow bench. In looking it over, I can't find any reference to the pressure drop they used in calculating the flow of the orifices. It seems that most people don't agree on the flow numbers posted in it either. I stand somewhere in between. I thought I read somewhere that it was calculated at 28" so that's the pressure figure I used to calculate. It's sort of, kind of, well....let's say in some ways close.

Let's look at some of the numbers and hole sizes.

I calculated useing the following formula:
[cfm= 13.29 x diameter2 x sqroot of water test pressure]

According to the article:
(4).3125" holes = 28cfm [calculated = 27.47cfm (OK)
(8).3125" holes = 56cfm [calculated = 54.94cfm (OK)
(1) 1.25" hole = 104cfm [calculated = 109.88cfm (OK)
(1) 1.75" hole = 180 cfm [calculated = 215.37cfm (hmmmm)
(1) 2.125" hole = 255 cfm [calculated = 317.56cfm (wow)

Everything starts off fairly close until you get to the 1.75" hole. I won't even venture into how anyone is calculating the total flow when there are multiple holes used for a single flow range [except for the first two ranges above] because when I total up the hole sizes for those ranges, it completely runs away from the MSD article's posted flow figures.

What puzzles me even more is this. The inclined manometer for that project has a 12" rise. Therfore, if my reasoning is proper, a 12" pressure drop is 100% flow for any given orifice. Actually I know that reasoning is correct. All the inclined manometer is doing is telling you what percent of 12" of pressure drop you are seeing.

That being said, then wouldn't it stand to reason that the flow figures [calibibrated figure]should be calculated at 12" as well. What I'm seeing here are holes calibrated at a 28" pressure drop but being measured for 100% flow at at 12" pressure drop. Therfore, if you saw 100% in the 104cfm range on the inclined manometer, you would't be anywhere near 104cfm but rather have if the orifice is calibrated at 28". So let's recalc the formula using 12" and see what we get.

(4).3125" holes = [calculated = 17.98cfm
(8).3125" holes = [calculated = 35.97cfm
(1) 1.25" hole = [calculated = 71.93fm
(1) 1.75" hole = [calculated = 140.99cfm
(1) 2.125" hole = [calculated = 207.89cfm

Wouldn't these number actually be more representative of what we would be truly flowing in cfm if we're using 12" as 100% on the inclined manometer? How else can you reason this?

In other words if 28" is set as the 100% figure for each flow range, then why wouldn't you make your inclined manometer read 100% at 28" as well? The obvious reason is the inclined manometer would be HUGE. So, as an alternative, why not calibrate the flow ranges at 12". Either way you go, shouldn't 100% actually be 100% of the same pressure drop?

Like it is designed, we're seeing what we're calling 100% is some value that is actually significantly less than what 100% really is, in terms of what the flow range capacity was labeled [calibrated for].

Think about it. What we're saying her is OK, I'm selecting a hole that should flow 104cfm at 28" of pressure drop BUT since my inclined manometer is designed for 12" [reaches 100% at 12" pressure drop], I'm not really measuring a full 28" of pressure drop. Instead I'm saying that when I reach 12" of pressure drop, it's 100% of the capacity that it's claimed to flow at 28" which is incorrect. It actually would flow closer to 68cfm at a 12" pressure drop [100% on the inclined manometer].

I'm looking for a reply on my logic far more than a discussion on why the advertised hole sizes don't calc out to what the forumla says. That's it's own can of worms.

All things considered we're just performing comparisons when we flow test [what we had to what we have now] but if we do not use the same actual pressure drop on the inclined manometer that we did when we calculated the flow rate of the hole, then we are not comparing apples to apples. Well maybe large apples to small apples....

Best Regards,
Larry

[JRM]

how i figured my cfm with the multi hole set up was figure the cfm of each hole then add them together.
larrycavan you bring up a very interesting point about the incline manometer.

Im starting to think that finding the real answer is along the lines of what came first the chicken or the egg. :laugh:

[larrycavan]

JRM,

Mysteries like this have surrounded this project since day one for me. What I do know is that back in 93 there was nowhere to reach out for answers. This forum didn't exist. Fortunately these days we have this forum and some very sharp people that read and post here. Someone will come back with some reasoning to my question and hopefully we'll all learn something.

Larry

[Ugly Duck]

Larry - I've been wondering about this myself, but something just triggered a thought:

If your inclined manometer was 28" tall, you'd need to draw 28" of depression across the orifice, and if you're drawing 28" across the test piece, the motors will need to draw 56" of water. That is, assuming you're using water in the inclined manometer...

Am I way off base here?

Also, if you go back and perform Pythagorean's theorem to PHR's inclined manometer angle, you'll come up with a discrepancy. At least I did, so I wasn't sure if the manometer should be installed at the specified angle based on the tan of the sides, or the length of the hypotenuse, or should just be set to measure a 12" drop.

However, I suspect that the answer to your other question might lie in something simpler and slightly less clear - that is, the holes used in the PHR article are VERY close together. That's gonna mess up the flow measurements, is it not? I'm suggesting that instead of calculating the flow through those holes at a given drop, they were actually tested and awarded those values. The guys at PHR didn't seem too hot at calculating things...

So the bottom line is that if you're going to build a truly accurate bench, you could go get your orifice plate tested to whatever depression you plan on using for your inclined manometer, and use that CFM value as 100%.

[larrycavan]

Ugly Duck,

I've done a lot of thinking about the situation as well. I agree with you.

I thought I was on the right track with my thinking and thanks to Tom V. posting the orifice sizes and other pertinent data from a SF600, my thoughts were supported.

You should calculate your orifice flow based on what you are calling 100% verticle rise on your inclined manometer and you absolutely must account for your well ratio on the inclined manometer as well.

I ran the superflow numbers for the SF600 and it all fell into place. Yes there are some variances in pin-point precision of the actual CFM in the calculations but that's all to do with the coefficient of flow of the holes. Using the common formula I found here and 13.55 as the constant, it proved to me that Superflow calculates based on the verticle rise of the inclined manometer's correlation to 100%.

I've also found the answer to another puzzling mystery regarding connecting the inclined and the verticle manometers together and checking the inclined against the verticle to see if they match up at various pressure differentials. Mine was off and I suspected it was do to my inclined well diameter. While testing I marked my well at zero and again when the inclined reached 100%. I stuck a ruler up to measure the difference and it measured what looked like an inch.

In looking through some of the posts I found a formula that allows you to compensate for what Dwyer calls "well drop effect". The formula was spot on. With 1/8" tubing, a 5/8" well and a verticle rise of 12", I was not seeing my inclined reach 100% until the vertical manometer was at just over 13". Again, the formula calculations are right on the money.

The flow rates published in the MSD article are not

My next steps are to redo the top of my flowbench to install a baffel system and to calculate my flow disk at 13.14". I don't even use the ranges with more than one hole except for the first two ranges.

I'm expecting the baffel system will provide more believable readings. I'll post my results.

Best Regards
Larry

[84-1074663779]

Larry, good to hear it is all falling into place for you.

Well drop will always exist, but it can be reduced to insignificant proportions by having a large enough well. It is the relative surface areas of the tube bore, and the well that matter. The volume of fluid in the well can be quite small, as long as the surface area is kept large. It is fairly easy to get a 1000:1 ratio of areas if you work it all out. In that case, the "well drop error" could probably be ignored.

My well is made from a length of two inch diameter copper tube, four inches long, laying on its side and half full. The fluid surface area will be just under eight square inches.

Another little trick is to fit an upper well too. In the event of an accidental manometer overpressure, the fluid will be blown into the upper well where it can then drain back all by itself. There is nothing more annoying than getting half way through a test session and having a little accident which blows all the manometer fluid out of the manometer into the bowels of the bench. This happens to all of us sooner or later !

[larrycavan]

Thanks Tony,

Now that I have the well ration formula, I'm going to eliminate my U Tube and make a well type for the test pressure manometer too.

I like your idea for the over pressure protection.

Best Regards,
Larry

[84-1074663779]

I have done the same thing here. My test pressure manometers use identical large wells. The advantage is you can then use a standard fixed steel ruler or steel tape as an inches of test pressure scale directly against a single rise manometer tube. Beautiful !

A standard U manometer has one leg go up and the other down, and having to measure test pressure between the two manometer leg heights is a damned nuisance.

[86rocco]

For my U-tube manometer, I'm thinking about using one of these steel centre finding rules, it's calibrated in half scale. I'd hold it in place with a magnet and a single hold down screw so that it's easy to adjust the zero point.

[larrycavan]

Last night I set out to remove the 1/4" aluminum top to my flowbench so I can do the modifications to the discharge hole location. It was held on with countersunk wood screws and GE clear, 20 year silicone. It was a battle all the way. After about an hour, I considered waiting the additonal 8 years but after about 2 hours, the top came off. That stuff is incredibly strong.

Larry