formulas for how many motors i need

[wheeljack545]

Ok looking for formulas to find out how many motors I need to do what I want. I know the bigest head that I will have, will flow around 550 cfm. I dont know how to count for loss. Also How do you figure for wc. If i want 550cfm at 100in.wc. Thanks

[Brucepts]

Well for that flow you are going to need something other than vacuum motors!

A rough idea of how many vacuum motors to use on a PTS Style flowbench build goes something like this; look at the flow chart for your motor, find the total depression your flowbench will pull, this will be your testing pressure plus your differential pressure across the orifice plate, on a PTS flowbench that would be 44" (28+16) go to the cfm column on your flow chart and read what that motor will flow at that depression. Multiply that number or divide to find your answer. It's not exact but it does give you a good idea.

Keep this in mind though from a real-world finding as CFM increases more motors become less effective on the flowbench ie when 2 motors are running you might have 150-200 cfm but adding in 2 more motors won't net you 300-400 cfm and 2 more won't get you 450-600 etc . . . 8 - LH9563 motors on my PTS flowbench gets you just around 600@28"

This all assumes an orifice style flowbench

[Tony]

Bruce is right, most vacuum motors will be struggling to have much flow at all up at that very high developed pressure.

One hundred inches test pressure is just short of 3.6psi.
Maybe a supercharger producing 4.0 psi boost, and 550 CFM.
But it will need about a 15 Hp motor to drive it, which is getting rather large.

[wheeljack545]

Bruce and Tony Thank you very much!!
This is what Im looking for. One place I worked we had some blowers.That would blow plastic regrind up to the silos. I was thinking I would have to go with something like that. Thanks

[budman]

If you stick strickly to the physics, the flow (at the same pressure) goes up with the cube of the power. The actual equation is based on the same principles as the formula to figure 1/4 mile times and speeds from horsepower. In other words if you want to double your flow, you need 2 cubed times the power (8). If you want to triple the flow then it would be 3 cubed or 27 times the power. Graingers catalog used to haave this information, I don't know if they still do. This is a good starting point, but real conditions seldom for the theoretical data.
If you go to the "blue" web site and look at the power requirements for the various commercial benches, you can get an idea what is needed.

[Tony]

Actually power is DIRECTLY proportional to mass airflow.
Double the mass airflow, double the required blower shaft drive power.

In fact, doubling the number of blowers will not double the resulting airflow, because of increased pressure losses created by trying to force twice as much air through every part of the flow bench.

If you look at the entire bench as being just a restriction on the blowers, as you keep adding extra blowers the effective restriction increases, and you gain less and less flow increase for each additional blower added.

Drive power also increases directly with developed differential pressure.

The formula for air horsepower is: Hp = Psi x CFM divided by 300
So 300 CFM at 1 psi is one air horsepower.
One psi is very close to twenty eight inches of water.

Typical blower efficiencies are all over the place, but something between 30% and 60% covers most.

If you take 50% blower efficiency as being a nice convenient stab in the dark average, then you might expect around 150 CFM at 1psi for each blower shaft horsepower.

Earlier I (guessed) 4.0 psi at 550 CFM might require about a 15 Hp drive motor, and that is where that figure came from.

These figures never turn out exact, but they are a pretty good guide to estimating how much total horsepower, and electrical load, a proposed bench is going to require.

[budman]

Try Graingers HVACR Terminology and tech section
http://www.grainger.com/tps/hvacr_terms ... nology.pdf

[wheeljack545]

Budman and Tony Thanks
This is what I need. I will have plenty to read over the weekend. So I can start figuring tis out. I want to start ordering parts next month or sooner. I just need to get a better understanding of how to get what I want. How much it will cost and how easy to up grade to what I want if I can not get it there this time. Now to get to my next classes. Thanks again