by larrycavan » Wed Apr 06, 2005 10:34 pm
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