Different motors... - in the same bench?

[What]

Like the lay-out of this new forum... first time poster and basically a newbie to flow bench as I have yet to build and operate one.

Due to a renovation at work I have access to 5 shop vac motors. Poor student = if it's free, I'll try it that way first. The problem is that none of the motors are identical. 2 of the motors are from decent industrial vacs and quite close in specs, the third is close but less effecient, and the last 2 I'm debating whether to use at all as they are from seperate smaller 2.0hp Craftsman vac that I can't find specs on. Will flow quality truly suffer in building a bench similar to the "MSD style" bench with 5 basically different motors? Just wondering if uneven pressure distribution may cause inconsistencies... or if it will just be as simple as 'they are each capable of flowing 'X' amount' through the orifice so just calculate flow for which ones when setting up the bench.

[Greg]

It really doesnt matter if the vac motors are different sizes as they are all running in the same box so they basically all add up to being one vacuum source.
The only thing you need to worry about is how many amps they draw when you start them all up.
Probably the best way to set them up would be to wire them in 2 pairs of 2 and the last one through some sort of speed controller for fine adjustment of your depression.
That way you can run either 3 motors with fine adjustment or 5 with a fine adjustment.
If the smallest motors are 2hp you should be able to drag a fair amount of air through.

[Shawn L]

All the motors will end up operating at one steady state vacuum.
To find out what performance you will end up with (ie CFM at X water lift) you will need to look at the vacuum vs cfm graphs for each motor you will be using. Something that is not easy to get in most cases.
Estimate your test pressure( say 28"water) subtract your measurement device loss( say 8" for an orifice) and any piping losses(so small usually ignore) to get 20" of vaccum at your source. Then look up all the CFM ratings at that pressure and add them up

[86rocco]

This is somewhat off topic but I'm in the process of gathering together the parts to build an MSD type flowbench, what should I be looking for in terms of vac motors? Would electric leaf blower motors be suitable?

[84-1074663779]

Leaf blowers usually have a single fairly crude plastic rotor. They blow lots of air, but they are not capable of generating any significant pressure. As soon as you begin to restrict the flow, the air volume falls away to nothing without much pressure being developed.

Vacuum cleaner motors have several rotors stacked in series, one behind the other, and can generate massive suction/pressure. They are going to be far more suitable, and can often be obtained fairly cheaply secondhand.

I still believe the most important first step in building a flowbench is to just test the bare blower all by itself. Plot the flow against pressure, and measure the power consumption to get a good feel for what you have. You will quickly realise how difficult it is to get a lot of airflow at a sufficiently high pressure differential.

It is best to sort all this out before spending a lot of time and money on the main flow bench enclosure, as it may not be easy to change things later on.