CFM

[Sups]

I was looking at Grainger.com and noticed that the motors listed on that site are listed as saying

Air Flow @ 2 Inches Orifice XXX.0 CFM

What I thought was that cubic feet was cubic feet and inlet or outlet holes did not matter on this. But then I started to see the motors as an object that "produced" a volume of air and if enclosed in a system with a hole, the amount of volume that it can be pushed out through that hole per unit of time.

Is that correct?

So then my question would be, can a 100 CFM motor at 2 inch orifice produce double that (200 CFM) if the orifice diameter is adjusted? I would say no since the orifice disk is used as a restrictor of the flow and so able to adjust the flow rate by decreasing the maximum amount of flow (the "pie hole" in the MSD orifice disk).

Am I right to assume that the orifice in the:
Air Flow @ 2 Inches Orifice XXX.0 CFM

the diameter of the orifice disk and not the orifice opening where the adapter is attached to?


How is the diameter of the orifice disk determined? I think I saw an equation before like:

13.55*sqrt(water height)*diameter^x = CFM

x being 2 or 3

so how does the motor size and power a factor in this equation? like how is a motor that produces 100 CFM differ from a motor that produces 200 CFM or the amount of motors needed to produce 500 CFM. I'm just not totally understanding the equation and how it works with the motors/blowers.

[84-1074663779]

The whole situation of rating blowers is extremely complex, but there are a few guidelines to steer by.

First, there is no definite reliable relationship between flow and developed pressure, that will depend on the actual type and design of the blower rotor(s). For centrifugal blowers the shape and design of the fins in the rotors, and the number of stages connected in series will create some sort of pressure/flow characteristic curve that will be quite different for different blowers.

For example a large diameter single stage industrial centrifugal blower will produce an almost constant pressure over a very wide flow range.

A very small high Rpm multistage (vacuum cleaner type) blower can develop massive pressure at low flow, but as flow increases the pressure it can develop falls off very quickly.

The effect of this is that if you take two blowers that can both deliver say 200 CFM at thirty inches of water. If you completely seal the inlet (or outlet) of the big single stage industrial blower, measured pressure may go up only to thirty five inches at zero flow. On the other hand your multi stage vacuum cleaner motor might go to well over one hundred inches of developed pressure at zero flow.

On the other hand if you ran both blowers completely open with no back pressure at all, the big industrial might go to 500+CFM, the vacuum cleaner struggle to only 300 CFM.

You can never estimate the pressure versus flow characteristic, it must be tested and measured. *Hint* a blower with a very flat pressure characteristic makes a much better flow bench blower. It will hold a constant stable test pressure much more easily than something with a very peaky pressure/flow curve.

It all comes down to tip speed, internal flow area, and number of stages in series. A big single is going to be quite different to a very small multistage with small internal flow areas.

The second factor is blower efficiency. Air versus amps, if you like to think of it that way.

This can be worked out, as it is a fairly straightforward thermodynamics problem. Assuming an impossible 100% blower adiabatic efficiency, where all mechanical work is converted to moving and compressing the air, required blower shaft power would be:

Hp = CFM x psi / 229

This makes a few assumptions about air inlet conditions, but is close enough for us. (Note 1psi is about 28" of water).

That figure is impossible to achieve in practice, we need to allow for blower design efficiency. That can be anything from 10% to 70%. So you divide the above Hp figure by the efficiency to come up with REAL required shaft drive power.

The electric drive motor comes into it as well, how efficiently it can convert amps into shaft Hp. So again you need to divide by motor efficiency. 80% might be a rough guess, but it will vary quite a bit depending on the type of motor.

Working through a rough example for a single stage industrial forge blower with an 18" rotor diameter, driven by a 10Hp three phase motor (via pulleys) at roughly 6,000 Rpm.

Measured performance might be 500 CFM at 60" water pressure. Theoretical "air" horsepower 500 x 2/229 = 4.4 Hp

Estimated blower efficiency 45%. Real required shaft drive power 10Hp. While the mathematics are a bit shonky, those are pretty much the actual figures I am getting with my bench.

If you plan on using multiple vacuum cleaner motors, I strongly suggest plotting a pressure/flow curve, and measure the motor current at various operating points. Both the curve, and efficiency will vary over a wide range. From that you can then plan on how many motors will be needed, or how many can be run from total available power.

One advantage of using an old supercharger blower is that they are fairly efficient (>50%), and curves are usually available for them.

I have no idea what the expected efficiency of a vacuum motor is, it has been many years since I used them. Perhaps someone here can run some tests to find out?

It takes a lot of Hp and a lot of amps to build a high capacity bench. If by using a different type of blower, you could quite conceivably double the efficiency, that is going to make a rather large difference to what you can finally achieve. So it is well worth taking the trouble to go through all this in some detail.

[Shawn]

Tony,
so with vacuum motors that are offered on surplus center which of the two would you recommend for my application? I want to be able to pull 60"+ at 500cfm. I was thinking 12-16 motors, but now i'm a little confused at to which one of the two motors that they list would be best for my application.Do you have a good "guesstimate" of which one would fit best?





here's some info. on each of them from another post-

Specs for 16-1234


The 16-1193-J is the better one if you're trying for more depression than 28", and its cheaper. 10 friggin bucks.


thanks,
shawn

[84-1074663779]

Hi Shawn,

Going by those specifications, the 076 blower can deliver 50 CFM at sixty inches, and require about 10.5 amps to do it. The efficiency peaks at around 30% at that combined flow and pressure too, so it is seemingly fairly well matched to the required application.

Maybe ten motors would be needed to reach 500 CFM, and that is 105 Amps !!! Is that going to be a problem ?

The 013 blower can only deliver 6 CFM at 8% efficiency at the required sixty inches of pressure, so it just doesn't have the balls to do the job. (It would work reasonably well at only thirty inches, but that is not how you plan to use it).

[Shawn]

That is what i was kind of thinking too. About the power, i have a buddy who is an electrician and he told me we could wire them in series or parallel(?) 220 and bring the draw way down. I don't know enough about what he was talking about to give you a good answer on that, but he seemed fairly confident that it wouldn't be a problem.
thanks again
shawn

[crazypj22]

Hello, I'm new here.
I usually work on much smaller engines ( motorcycles) but,
I worked on Ingersol-Rand compressors, and other construction equipment.They measured scfm at a given pressure through a specified outlet. needed at least 30hp diesel for 100cfm with 1.5" hose @110psi, constant delivery. 110hp for 600scfm through 3.0" outlet hose @110psi. dont know how exactly this relates to vacuum?
Should I be looking at a specific section of this board?

One thing I've noticed, all automotive/large engines use real high water inches. I realise that works for largish single manifold (added all airflows per cylinder) but, if your testing cylinder heads/ port flows the piston can only 'suck' so much and 30" or 40" water pressure not only gives unrealistically high numbers but could actually lose power/drivability at actual operating pressures.
A friend used to work on piston engined aircraft, they could use up to 30 feet water pressure, but were usually around 30 litre engines operating at relativly low rpm ( less than 3,000rpm) and a single manifold.
Finally, my point is, wouldnt it be easier to work out what flow you need for the tests your going to do, at realistic ( not extrapolated) pressures and build flow bench to work with those figures?
Thanks for your time
PJ

[84-1074663779]

Welcome to the Forum crazypj22

Interesting questions, and I really don't know the answer.

But it seems historically most people have tested at around 25" to 28" of water, and that particular range airflow roughly corresponds to full power engine air consumption. The problem is of course that the testing is done at steady flow, but a real engine has pulsing flow.

As engine port flow is continually changing there is probably not any really "correct" constant air velocity or pressure to test at. While some people might argue that 60" or 100" might be better, practical problems make that a rather difficult thing to actually do.

While higher flow may be better, there are people that argue that the nature of the flow (swirl and tumble) is also important, as well as port volumes and so on. Lots of different ideas about all of this, but only the dyno and stopwatch are going to tell you for sure.

[larrycavan]

Crazypj22

Welcome to the forum. As Tony says, the accepted standard for most is 25" to 28" on the flowbench. Still, much development on motorcycle engines [and a lot of cars too] is carried out at 10". Opinions vary and one thing is for certain. A lot of good development has been done and good performance results obtained at all of those test pressures. There seems to be a definite trend toward higher pressures these days.

My Superflow 110 manual states the following on page 10:

When and engine is operating, the pressure drop across the cylinder head ranges from 0 up to about 145 inches of water at 550 CFM. The average pressure drop is about 23 inches of water or about 2 inches of mercury at the 220 CFM flow rate. When testing with a Superflow, it is not important whether a test pressure of 5, 10 or 15 inches of water is used, provided the same pressure is used for each subsequent test that will be compared to the original test. A head that measures 10% better at 5 inches will also measure 10% better at 10 or 23 or 145 inches of water.
-end-

Notice though, the new 1020 Superflow and the pressures it is designed to provide. It gives an appearance that Superflow wants to provide the tools the pros want and listens to the industry engine development leaders. [Not meant to exclude Superflow as a leader in the industry]

It's not too difficult to take a standard internal combustion engine and raise it to an impressive level from stock configuration. IT IS howver, another matter when you are taking already developed race engines to the next level and that is precisely the level where the trends of the industry are formed. That being said, if the big boys say that you need XXX pressure to obtain yyy results, I believe them.

PS what kind of bikes are you working with? Not many of us on here...mostly car guys.

Best Regards,
Larry

[crazypj22]

thanks for the replies. I never did much with automotive, mainly because it was too expensive in Britain ( I'm originally from South Wales, UK) I've worked on most bikes over the years, japanese, european and Harleys. got to make it faster whatever it is. used superflow 110, ( 185cfm) dont really need higher flow rates for small engines.
If the cylinder capacity and max rpm are known engine has to flow at least that volume of air (steady state, pulsed flow is lower at most rpm but can get stupid high when everything works right, around 130% cyl capacity)
I've seen some 'strange' numbers which were extrapolated from 10" to 28". makes guy doing tests/porting look good, his numbers are more than double mine but percentage wise we are doing the same numbers (about 10%)
PJ