PTS Flowbench Forum

Bakerlite wrote:Thanks for the replys, and I have had a look at the PTS bench, but I'd rather try to build the MSD style one. I do like the idea of the multi hole orifice as opposed to having to unscrew a decklid and fiddle around inside the machine.

F.W.I.W.

That feature is nice in concept. Execution can be troublesome.

To make it effectively leak proof when flowing exhaust, it's got to seal tight. Intake flow isn't generally an issue because the disk sucks down tight.

Anyway, something to take into consideration while you're putting it together.

Is your rotating disk going to be precision machined? or just hole saws?

I have to add to this, an accurately machined orifice will give you precision cfm readings. I prefer the extra effort to open the cover and flip the orifice for intake to exhaust flow test.

With the rotating orifice disc setup, you'll always have doubts about the accuracy of your bench.

Ray

Thanks again for some replies. Yes I will be using a motor controller to set the test depression.
I know I could make some valves that would work better than the stainless bowels but being able to control the motors is a neater idea to me and gives less places for air leaks and mechanical wear.
The orifices will be precision machined. I'm just thinking about how to seal them at the moment..
I've already got some ideas on reversing the flow but I'll have to build some test boxes to evaluate how well they work.

Keep us up to date on your build, we always enjoy seeing pics!

Enjoy your build and most of all have fun . . . if you get stuck please feel free to ask questions.

Bakerlite wrote:Thanks again for some replies. Yes I will be using a motor controller to set the test depression.
I know I could make some valves that would work better than the stainless bowels but being able to control the motors is a neater idea to me and gives less places for air leaks and mechanical wear.
The orifices will be precision machined. I'm just thinking about how to seal them at the moment..
I've already got some ideas on reversing the flow but I'll have to build some test boxes to evaluate how well they work.

A motor controller is a nice feature to have for standard air flow testing. It's also very handy to have a manual control when you're probing the port for velocity readings. Having the bench auto correct the depression during those types of tests can mislead you.

Everyone faults the salad bowls but they work. Whatever type of valve is used, one thing that is not obvious until the bench is operational is the size of the valve's effect on adjusting depression. A large valve can be sensitive to adjustments.

Thanks Larry, when I first looked at this bench I made a note to look at the size of those valves if I was going to use that route. I'm still thinking of using a speed contoller to set the pressure. If I don't then I will pay some close attention to their size and how they function.
I've worked out my reverse flow situation and also an effective seal on the orifice disc.
My question is now, just how may orifices I'm going to need in the disc to flow heads between 30 to 400-500 CFM at 28". How many plates would you use on a PTS style bench to be able to cover that range?

You have to know what your delta P is for the flow bench. On my version of the PTS I measured the test pressure in the upper chamber above the test orifice in the bench and below your test piece (head whatever). In My case 10" H2O. Then I measured the pressure in the lower chamber below the test orifice inside the bench. In my case it was around 26" H2O and the difference is your Delta P In this case 16" H2O. Or you could take a vertical manometer and hook the high pressure side to the lower chamber and other side to the upper chamber at the same time. Then crank your bench up to your test pressure and the manometer should be reading you Delta P.

There is a spreadsheet on the forum where you can then figure out what each orifice will flow at your Delta P and then you can calculate what orifices you will need to cover your flow range. You need to guess at what your CD for your orifices will be. On mine since they were square edged I used .605 as my CD.

Also try not to flow below 50% capacity of the orifice if you are using a water manometer. If you use a digital manometer like Bruce's then you can get away with one or two orifices.

On my bench since I flow at 10" and mathematically convert to 28" right now I am using the following:
#1 = .903" dia. @ 16" DP = 43.2 CFM

#2 = 1.298" dia. @ 16" DP = 89.2 CFM

#3 = 1.520" dia. @ 16" DP = 122.3 CFM

#4 = 2.144" dia. @ 16" DP = 243.36 CFM

John

Thanks John, I'm thinking I could use around 4to 5 different holes to get me from 30 CFM to about 500 CFM. The MSD bench uses 7 sizes to get from 28 to 425 but I think this can be reduced. I figure at the moment I will go for a Delta P of 16".

jfholm wrote:You have to know what your delta P is for the flow bench. On my version of the PTS I measured the test pressure in the upper chamber above the test orifice in the bench and below your test piece (head whatever). In My case 10" H2O. Then I measured the pressure in the lower chamber below the test orifice inside the bench. In my case it was around 26" H2O and the difference is your Delta P In this case 16" H2O. Or you could take a vertical manometer and hook the high pressure side to the lower chamber and other side to the upper chamber at the same time. Then crank your bench up to your test pressure and the manometer should be reading you Delta P.

There is a spreadsheet on the forum where you can then figure out what each orifice will flow at your Delta P and then you can calculate what orifices you will need to cover your flow range. You need to guess at what your CD for your orifices will be. On mine since they were square edged I used .605 as my CD.

Also try not to flow below 50% capacity of the orifice if you are using a water manometer. If you use a digital manometer like Bruce's then you can get away with one or two orifices.

On my bench since I flow at 10" and mathematically convert to 28" right now I am using the following:
#1 = .903" dia. @ 16" DP = 43.2 CFM

#2 = 1.298" dia. @ 16" DP = 89.2 CFM

#3 = 1.520" dia. @ 16" DP = 122.3 CFM

#4 = 2.144" dia. @ 16" DP = 243.36 CFM

John

On your figures a 3.1 dia @16" DP should get me to around 500 CFM.

I love it when a plan comes together.

Also one other thing regarding the orifice, the disc uses steel and the orifice holes looks to be around 60 to 80 thou thick, how does this compare to a sharp edged orifice? would the hole have to be larger to have the same flow?