PTS Flowbench Forum

ok. my bench is going to be built around 8" of depression, hope i'm saying that right. the reason why is the motors I have will generate about 400 cfm at 36" of vacuum. so if I need a plate it have to be 8" + 28" = 36" right? if I machine a plate to go in my bench with a square edge would it be 2.56 inches @ 8"? or 1.76 inches @ 36"? or if i'm completely wrong what would I need?

hubba wrote:ok. my bench is going to be built around 8" of depression, hope i'm saying that right. the reason why is the motors I have will generate about 400 cfm at 36" of vacuum. so if I need a plate it have to be 8" + 28" = 36" right? if I machine a plate to go in my bench with a square edge would it be 2.56 inches @ 8"? or 1.76 inches @ 36"? or if i'm completely wrong what would I need?

You would machine it for the 8" as that will be you delta P. The 28" will be your test pressure. That means your orifice inside the bench will only have a 100% capacity of 242 cfm if you machine it to 2.56". Now if you used a higher delta P, say 16" that same 2.56 orifice would have a 00% capacity of 342 cfm.

Now another question are you using a digital manometer or a water manometer?

Also what motors are you using and how many?

John

water manometers. using 8 motors. http://www.surpluscenter.com/Brands/Pan ... 6-1448.axd

what is the formula I would use to figure out what I need?

If you have not already go to this thread in the forum and download Ed's spreadsheet. http://www.flowbenchtech.com/forum/view ... ?f=14&t=73

You do need a program like Excel to run it but there are plenty of free ones out there that will run it. That spreadsheet will give you all the calculations you need for your bench. There are some tabs at the bottom that you can choose from the different spreadsheets to use. The first one at the bottom, Orifice flow and size, is where you will want to start. You can change the numbers that are black but you do not want to change the blue areas. First enter the orifice diameter to see it's 100% range. Then you would enter your pressure difference, which in your case will be 8". Then you would enter your discharge coefficient. I would recommend using .597 then you will see the orifice area and the flow of the orifice.

Once you get that set up then right below that area is the Calculate Orifice Size tool which is extremely useful for what you are doing. Keep in mind since you are going to be using water manometers you will need to have a range of orifices for the inside of the bench as you really should keep an orifice at 50% or above of it's 100% capacity. I have included what orifices I was using in my old bench for you to see. They are at 16" Delta and using a CD of .620 as I was doing something different with the old bench.

There is also a a tab at the bottom of Ed's sheet for inclined scale compensation to help you make your inclined manometer.
John

I down loaded that a few days ago. but thanks for the .597 that helps . I made my incline meter 16" long have it mounted on a 30 deg angle

hubba wrote:I down loaded that a few days ago. but thanks for the .597 that helps . I made my incline meter 16" long have it mounted on a 30 deg angle

Sounds like you are most of the way there, congratulations.

yeah. going to make the internal plate and maybe order me a check plate. so I check my work.

but do I need to make my internal plate 50 cfm larger that what flow I am testing for. like a head that will flow 250@ 28 would my inside plate need to be 300 cfm @ 8?

hubba wrote:but do I need to make my internal plate 50 cfm larger that what flow I am testing for. like a head that will flow 250@ 28 would my inside plate need to be 300 cfm @ 8?

Not with water gauges you can measure right up to the plate rating