Air Pressure to Torque Question

[RSmith]

Hi Guys,

First time posting, sorry if this isn't the right place to post this kind of question. A personal project involving air pressure and propulsion has lead me to a question I hope somebody may know the answer to.

Imagine hypothetically you have a turbine that has paddles with a surface area of one square inch. It's held in an enclosed casing which has an inlet tube and an outlet tube - so if you blow through the inlet tube, the turbine spins and the air is explelled out of the outlet tube. A bit like a Turbocharger in a car. If you were then to blow air through the inlet tube at a pressure of 1000psi does that mean that the turbine would spin (friction on parts etc. excluded) with 1000 lbs of torque?

I figured this would be the case seeing as the paddles were 1 square inch and obviously psi is lbs per square inch.

Many thanks,



Richard

 

[mgb_phys]

Torque has a force and a distance so if the turbine had a radius of 1ft and you applied a force on the blade of 1000 pounds then you would have 1000 ft-lb of torque.
1000psi is quite high and 2ft diameter is large for a turbo!

Once it's moving the calculations are a little more complicated because it depends on the speed of the airflow and the speed of the blades.

 

[RSmith]

Awesome. Yeah I know - the variables are quite drastic! I'm just experimenting with different methods of producing torque (as you probably guessed to something similar to a crankshaft in a car). I just wanted to know that the pressure in the air in lbs per square inch or lbs per square foot would equal out roughly to how much torque it would produce if blasted into a turbine system.

 

[mgb_phys]

That's only the starting torque. Once the air is flowing into the turbine at the same speed as the blades are moving there is presumably no torque at all.
It's also going to be more complicated for real turbine shapes.
There are probably lots of resources on the web with figures for real units.

 

[RSmith]

If that's the case would there be an online calculator / a formula to convert the psi pressure on the turbine blades to HorsePower?

 

[mgb_phys]

Google turned up this http://www.turbofast.com.au/javacalc.html but I don't know of anything

 

[RSmith]

Thanks! That looks v useful... I'll have to punch in some numbers.

The more I think about it though, I don't think anything anything will substitute for a good old fashioned practical experiment. I may have to set this test up over the weekend.

 

[FredGarvin]

Take a look at a Pelton wheel. The actual force applied will be a function of the momentum change of the fluid as it strikes the paddles.

http://en.wikipedia.org/wiki/Pelton_wheel