Originally Posted by tufLTD
I think the reason they use lower flow rates is that there aren't many flow benches here in Oz big enough to flow large cfm at 28''. Quite a few people I've spoken to confirm this. Not that it really matters. As you say, it's easy enough to convert flow rates.
You mention that you can only assume it's measured in inches of water. Is there any other measurement? I thought all flow benches measured in inches of water.
And, yes, as I wrote, the 7" results are corrected to 10". I know the charts don't say this, but I can assure you that it is the case.
The reason I posted the charts was for an independant comparison between CHI & AFD, regardless of what flow rates were used on these charts.
I've attached a flow conversion chart for anyone interested.
Cheers, from Mal.
Well, of course, they can't flow enough at higher pressures. My point was that the quality of the product (the data chart) is lacking due to the inadequacies of the data. Take your lastest flow chart conversion table as yet another example of inadequate representations. The conversion table does not indicate what operation is performed to achieve the conversion! It is obvious to someone who knows how to convert pressures from any number of standards, say in bars, pascals, atomspheres or inches of mercury. Computerized flow benches can easily adjust output to any of these and more known constants based on correction figures entered before the data acquisition phase. In other words, why hand a human a sheet of paper and a conversion chart so that s/he must then come up with the corrected and normalized results without so much as even the mention of the mathematical conversion operation required? To me, that is simply poor publishing.
I think that I got the idea behind why you posted the charts. I'm not trying to insult you for doing it, either. I'm just saying that the quality of the data and the presentation of the charts is lacking. It is like looking at a very old map of the world, in some respects. While perhaps very intriguing and certainly worthy of some extended review, they're not what you're going to use to plot your course against if you're running an oil tanker, right? The same goes for these charts in terms of the usefulness of the data and its presentation. Perhaps a better choice may have been to convert all of the units to like terms and post the results along with the "raw" representation.
What is most funny about all of it to me is that "mechanism" used in the product. The product is a flow number based on HP, the "like term" that all of the dissimilar unit values are converted to. This HP "value" is the "component" of the product that is being sold to you Aussies and New Zealanders (I wouldn't want to say Kiwis in case that may insult anyone), it appears. A very slight modification of the formula used to produce the "HP" value, and suddenly, your flow bench is much better than the bloke's down the street!
What is more stupidly funny is that the "HP" selling point shows exhaust and intake for the same port to make different amounts of HP. What is more stupid than that? With this intake port your engine will make 600-blah-blah horsepower, mate...but, too bad it will only make 300-blah-blah horsepower with that bloody exhaust port! It also raises the question of what method was used to convert some flow number into some HP number, which is clearly not readily apparent from a quick glance at the non-normalized data. It would be a fun math problem should anyone care to spend one's time playing math games, but for someone who wants to immediately see the data set the only choice is the graph component of the product. And, it is that graph, which you say is normalized to 10" water, not the data points that produced the graph.
Look Mel, I'm not trying to get down on you in anyway at all. I'm just saying that the usefulness of the product is questionable at best. That isn't to say that it isn't entirely accurate or even 100% truthfully corrected. What I'm saying is that the presentation method used to produce the product and the lack of certain data elements open it up to all kinds of speculation about data integrity and overall correctness for any applicable use of the data. If I were a detective and this data were a suspect, I would have a "funny feeling" about this suspect after the interrogation (reviewing the data). A better quality product would remove the funny feeling from the outset by being more complete, normalized and clearer by stating the correctedness of it. The conversion chart, for example, would indicate that the conversion from X to Y would require a multiplication of X by the number found at the intersection of the X and Y in the table. That's all I'm trying to say in what is probably the most longwinded way.
If this data source is something that you dug up from the web somewhere, I'd try to find better sources. If this data source is a product handed to you by a flow bench operator, I'd try to encourage that person to produce a better representation of the corrected and normalized data. If this is a product that you've paid for, I'd demand better quality simply because it is about as worthwhile as marketing hype in a comparison of Coke to Pepsi--as presented. It is kind of like the ricer with the 4 different colors of primer and the unpainted fiberglass hood. Kind of difficult to respect it and who cares if it runs fast?
Additionally, the conversion chart is interesting. To convert from 7" water to 28" of water you multiply by 2. 7:28 as 1:2?
What it is not telling you is that it is calculating for CFM. Yet another little tidbid of useful information left off of the chart.
A flow bench can be simply described as shown in the following image:
Where P1 is the pressure before the orifice, the part being "flowed" is the orifice and P2 is the pressure after flowing through the orifice. Due to the inconsistency of the part shape, we can not easily describe the frictional losses except as a component of the re******t flow decrease in pressure from P1 to P2.
However, in most flow benches, the flow direction is similar to the photo if turned 90* clockwise, or from top to bottom. However, the higher pressure is not at P1, it is at P2, which is the source of the low pressure area. Then delta P is P2 - P1, and not P1 - P2.
If we assume the differences and substitute the Ps...we get:
Then if we want to get rid of the velocities and calculate for volumetric flow "Q":
...and solve for Q:
...and now we must solve for the discharge coefficient, since there is going to be some turbulence in the port and viscosity of the air:
...and then solve for the flow coefficient:
...assuming that for A0, we measured the volume of the port, we can calculate the true volumetric flow rate Q:
...and because we undoubtedly have differing inlet and outlet orifice dimensions:
...of course, sometimes too much information is just as bad as not enough! <grin>