How many vac motors for 300 cfm @28"

[rusty105]

Does anyone know how many of these surplus motors, I will need to drarw 300 cfm @28" ??

Thanks

[2seater]

I get 500+ cfm @ 28" H2O with six of them. Theoretically four should do the trick, although some will depend on how many bends in the piping, which cost flow rate.

[wing64k]

How are you testing to measure your 500 cfm?

[Tony]

Easy, just make up a whole series of very simple orifice plates. Just squares of thin gauge sheet metal with different odd sized round holes. Whatever selection of drills, hole saws, or metal punches you happen to have on hand will be fine.

Then see how much pressure your blower (or blowers) can pull through each different sized hole. Each hole size will then have a pressure reading to go with it for a particular blower. The flow can then be worked out by knowing the hole diameter and the developed pressure.

These points (one for each hole size) can be plotted on a piece of graph paper to generate a pressure versus flow curve. From that you can read directly how much flow there is at 28 inches, or any other pressure.

These simple sheet metal plates stick to the blower intake themselves by suction pressure alone, and a small diameter hollow probe can measure the pressure on the down stream side by inserting the probe through the hole. All pretty simple and easy to do. But it is an excellent way to compare various blowers and judge what you actually have, or what may be needed before constructing the actual bench.

[rusty105]

I was thinking 4 would do it. BUT I am leaving room for more, just incase I have the need later on.

Thanks

[2seater]

The orifice plate is the best method. I have three plates from Bruce, and two homemade ones. The largest was flowed by Bruce for over 500 cfm and I showed 514 cfm @ 28" and could pull up to 29.75" H2O through the 2.68" orifice with six motors through a 3" pitot flow tube.

[Tony]

For initially selecting vacuum motors, no great accuracy of measurement is required, almost anything flat with a hole in it will work fine.

[rusty105]

Wouldn't sticking the probe through the hole mess up the calculation of flow?? Or did I miss something?

[Tony]

A 1/8 inch diameter probe sticking through a two inch hole does not rob a lot of area.

For a rough sort of initial estimate of blower capacity, it is close enough. You only really want to know if there is maybe 40 CFM or 140 CFM per vacuum motor to get you into the ballpark for sizing.