Torque calculations

[ctenophore]

This problem is beyond my physics and math skills. Maybe one of the engineers out there can shed some light.

 

I am trying to estimate amount of torque (in inch-pounds) necessary to turn a 8" diameter, 5-bladed fan with a roughly 10deg blade angle at 120-150rpm in saltwater. Then, I'd like to see if I can develop a rough water flow curve (in gph x rpm).

 

I am working on a new flow system for my greenhouse tubs that is powerful enough to keep detritus in suspension, but gentle/slow enough to not harm baby fish or mobile anemones. I am trying fixed speed AC gearmotors driving large plastic fan blades, similar to how warehouses use giant, slow moving fans to exhaust air. I have a few gearmotors that I am experimenting with, but it would be nice to have a mathematical model that I can use to plug in variables to scale the device up or down.

 

Anyone want to take a shot at this?

[dandy7200]

I can give you the contact info for the engineering department at Crouzet Motors. They can solve it. It kind of makes my head hurt just thinking about it :P

[GaryL]

try here

[ctenophore]

try here

The tricky part is figuring out how much torque is required to spin the fan blade at a given rpm in saltwater. I found a few boat motor sites discussing it, but their props had very different characteristics and I didn't see how to apply their formulas even though there are similarities in the application.

[Der ABT]

Im sending an email to a good friend, ill let you know if he comes up with anything. I wish i had stayed with engineering, autocad is soo much fun but auditing has its perks too.

[ctenophore]

Thanks Evan.

[Coral Hind]

Did you get an answer yet?

 

 

[Der ABT]

haha i got the, ive been in management too long but has sent it on to some employee's/friends

also realized another friend deals with subs so i think he may be able to anwser me sorry its taking so long

[Chad]

I can answer this question, I dont have time right now, but I will work on it tonight, I think the solution just requires a couple of eqations.

 

More info may be needed, but I can see right away that fan blade width will be needed (blade area).

 

Also, some quick additional thoughts for you:

 

Plastic fan blades (I am assuming for an air fan) will be forced to react to a large bending stress and will most likely be the weakest point in the system, I would look for blades that have a steeper angle (perhaps something designed for a boat), I think "minn kota" (sp?) trolling motors have a plastic propellor and may work for you.

 

Also, a propellor that large will generate a lot of force that will need to be reacted by something (I can give an approximate amount), assuming it is placed on a glass aquarium wall or something similar, this might be a problem in the long term on the seams or glass itself.

 

I will get back to you later on this.

[Origami]

I'm glad to see that Chad is working it!

 

Justin, not seeing any other response (before Chad's), I tried sending this out to an engineer in our Fluids division this morning. They supply everything from sump pumps to pumps as big as your house, so I figured this was the place to go. You'll probably, like, Chad says need to provide more information about the propeller. In lieu of this, I've asked the engineer to make assumptions but I'm not sure how that will turn out. Perhaps we'll see.

[Leishman]

Sounds like he had it in the tank this morning for a test run with very good results - he said a video is to follow.....

[ctenophore]

Thanks guys for looking into this.

 

So far, I am using the "about right" engineering approach As Rik said, I have it working successfully with a used photocopier motor. I posted a video to youtube but it says that it is not yet processed.

 

Here is the link if/when it ever finishes:

[ctenophore]

I can answer this question, I dont have time right now, but I will work on it tonight, I think the solution just requires a couple of eqations.

 

More info may be needed, but I can see right away that fan blade width will be needed (blade area).

I'll try to measure the blade area, but there are 5 blades that measure roughly 2"x3", but they are curved on all edges. The pitch is about 30deg on average, the blades are curved in their attack angle. Overall diameter is 8", with a ~3" disk in the center to which the blades are mounted (see video)

 

Also, some quick additional thoughts for you:

 

Plastic fan blades (I am assuming for an air fan) will be forced to react to a large bending stress and will most likely be the weakest point in the system, I would look for blades that have a steeper angle (perhaps something designed for a boat), I think "minn kota" (sp?) trolling motors have a plastic propellor and may work for you.

The fan is attached with a 1/4"-20 nylon thumbscrew, which I suspect will the weakest point long-term. I may need to look into a better attachment method.

 

Also, a propellor that large will generate a lot of force that will need to be reacted by something (I can give an approximate amount), assuming it is placed on a glass aquarium wall or something similar, this might be a problem in the long term on the seams or glass itself.

My plan is to "float" the motor on an aluminum rail that hangs on a light mover across my fiberglass tubs. So tank stress won't be a problem, but I will keep an eye out for torque effects on the light mover trolley. The mover has about a 2 minute period, which will give a nice surge effect over the coral.

[Origami]

Here's what I got from our Fluids guys:

 

"I happen to have an ex-mixer guy in my group and he did a quick evaluation and came up with 0.0063 BHP. Since I cannot fathom this low of a horsepower I double checked this with an 8 in. axial flow pump and came up with the same value. Torque associated with this power at 150 RPM is a whopping 2.65 in-lb! Mechanical losses will be more than that. Flow rate will be approximately 200 GPM. I

[Chad]

I more or less validated what Tom said...

 

Differences in our calcs I am sure are in the assumptions (I used 65% for fan efficiency), if you know the fan manufacturer, you can request the fan curves and find a better number. Also, if you shroud the propellor, efficiency will rise.

 

I calculated 212 GPM at 150 RPM and 170 GPM at 120 RPM (It is linear so you can extrapolate).

 

I calculated a different power though... an order of magnitude off (20 in*lbs), I will recheck numbers and repost or edit if I find an error. I think a .1-.25 BPH driver will be fine to covor errors and unknowns. Also, the pitch angle on the blades doesnt affect the output at steady state, but does affect the 'wind-up' speed. With the two minute pulse time I would overpower it somewhat to minimize this effect.

 

If you want, I have my calculations in an excel spread sheet that Ill send to you.

 

 

[Mountaineer]

I have a friend (also a reefer) that is an engineering officer on a submarine that I will forward this to. He loves this sort of stuff.

 

JR

[Origami]

I more or less validated what Tom said...

 

Not me. Just some of our Fluids guys up in New York....

[Mountaineer]

This is from my submariner engineer friend. He indicated that this is not an easy solve and that there would be significant calculus behind the answer. More that I want to take in right now, but maybe these links will help.

 

 

"Check out this website: http://www.navweaps.com/index_tech/tech-034.htm

 

http://www.google.com/url?sa=t&source=web&ct=res&cd=52&ved=0CAkQFjABODI&url=http%3A%2F%2Fcafefoundation.org%2Fv2%2Fpdf_tech%2FNoise.Technologies%2FAIAA.1980.1240.Prop.Effic.Korkan.pdf&rct=j&q=uniform+flow+propeller+equations&ei=lhagS-TODNOWtged88A4&usg=AFQjCNH6qNdknssHJr14dPPpOTA4IxAaqg&sig2=wk9TOwfth1fBUB94rZGfLw

[dschflier]

Well I am the furthest thing from and engineer, but I once got a tour at the coney island aquarium. I am not sure if this idea would work for you but for one of their tanks they actually used an electrical trolling motor for the water movement in one of the tanks. It worked pretty well but I dont recall the prop size.

[flowerseller]

I'd use a canoe paddle and a couple girls in bikini's with big life jackets and be fun with it.

[ctenophore]

I more or less validated what Tom said...

 

Differences in our calcs I am sure are in the assumptions (I used 65% for fan efficiency), if you know the fan manufacturer, you can request the fan curves and find a better number. Also, if you shroud the propellor, efficiency will rise.

 

I calculated 212 GPM at 150 RPM and 170 GPM at 120 RPM (It is linear so you can extrapolate).

 

I calculated a different power though... an order of magnitude off (20 in*lbs), I will recheck numbers and repost or edit if I find an error. I think a .1-.25 BPH driver will be fine to covor errors and unknowns. Also, the pitch angle on the blades doesnt affect the output at steady state, but does affect the 'wind-up' speed. With the two minute pulse time I would overpower it somewhat to minimize this effect.

 

If you want, I have my calculations in an excel spread sheet that Ill send to you.

I'm thinking about building a shroud of some sort, but I like the way it produces a very wide, gentle flow now. It's strong enough to blow corals around in at least a 2' radius, and keep detritus in suspension. I think the fan turns slow enough to not harm the clownfish, but a shroud would prevent that even more. Empirical/anecdotal evidence agrees with the ~200 GPM figure you both arrived at. It is easily doing twice the output of a tunze 6200.

 

Not sure what you mean by 2 minute pulse time? I am planning to run it continuously.

 

I am using a 30 in-lb motor, which seems to be handling the load fine. I still need to put a kill-o-watt on it to check power consumption, but unless it is crazy, I will probably stick with it as it only cost $50 delivered. So far I have ~$53 in parts to build this prototype, a tenth the cost of a tunze 6200!

[Chad]

Hah! $53! Got to love DIY!

 

I read back through, I misunderstood my mover has a "2 minute period" to mean the fan has a "2 minute pulse" I understand now, *please ignore me while I catch up!

 

BTW, Im not concerned anymore about torque effects... the numbers are low enought that I would be comfortable.

 

Also, for a bounding value on power usage, 30 in-lb at 135 RPM is a maximum 400w (seems high to me, but it is a unit conversion, not a calculation).

 

And because I think Im rambling now... have you looked at a periodic displacement type of system, similar to what Simon Garrat describes in his interesting tidal build thread on page 2?

 

Edit: being trick and inserting a fancy link isnt working for me, http://www.manhattanreefs.com/forum/tank-threads/66115-intertidal-reef-flat-mixed-biotope-system-2.html is the link.

Edited March 17, 2010 by Chad