force exerted by air

[Yulian24]

I am trying to size an actuator for an air damper. My entrance velocity is 30 MPH. I would like to know how to calculate the force the air will exert on the dampers when they are closed.

A similar problem to this would be to try to calculate the force on a butterfly valve when only the velocity is known.

Thanks

 

[BigInch]

There's enough information to solve it any way he wants.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[FOURe]

Bribyk:

That was my way of thinking ...

BigInch:

I agree with you. There is plenty of information and input provided. Number crunching needs to be done and the results need to be shown and compared with different ways o thinking and looking at the problem that is on our hands ...

To All:

Thanks!

http://www.engineering-4e.com

 

[sailoday28]

Use of Cd. My understanding of its use is that--
experimental Cd is based on the stagnation pressure far up and downstream as being equal.

 

[IRstuff]

Hmm...I just re-read the OP. If the damper is fully closed, then there shouldn't be any airflow across that interface.

If that's the case, then the drag equation isn't applicable, regardless of value of the drag coefficient, because the airstream velocity would be zero.

This is where I think you need the fan curve, which gives you the static pressure at zero flow.

You would then still need to calculate the converse case, which is what the actuator needs to move the damper when there is air flowing, which would involve the drag equation, but in a condition where the drag coefficient is unknown.

For a standard butterfly valve, I would think that you'd also need to split the area across the axis of rotation, since the air pressure on one side is helping you turn the valve, while the pressure on the other side is working against you. The drag coefficient will be different on the two sides of the valve, since the separation point of the airflow will be different.

TTFN

FAQ731-376

 

[Bribyk]

That's true with the two drag coefficients but this is getting pretty technical for an HVAC flapper.

 

[ARenko]

Because the OP said it's closed is why I said use the fan total pressure, which is static pressure at zero flow (assuming fan inlet and duct discharge are at the same atmospheric pressure).

Really though he needs to check w/ the damper manufacturer, or google "sizing damper actuators" - there appears to be plenty of guidelines out there.

 

[BigInch]

Ya, it was one of those trick questions. So, tell me how do you have a velocity of 30 mph when the damper is closed? I keyed off the "30 mph", djv on the "closed" half. So then Julian what is it, 30 mph or closed?

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[sailoday28]

I repeat my previous question?

Is this "flow" analagous to that with a branch of a piping system where say a gate valve is shut and flow continues to other branches?

 

[Ron]

Hey guys...this is just a wind load problem...

30 mph "air speed"....

Unfactored pressure exerted by the "air speed" = 2.30 psf

0.00256xV^2

 

[rmw]

It must be one heck of a leaky damper with that type of entering velocity. Somehow, I think that that is the velocity at design flow and the OP is asking about shut off pressure for his/her damper actuator. Of course, a little static pressure operating and closed would make this much easier.

I notice that the OP has the thread marked so he/she is watching all this engaging conversation go by as we gnash our gums over his problem.

rmw

 

[IRstuff]

With a 1.28 drag coefficient, it's more like 2.9 lbf/ft^2

TTFN

FAQ731-376

 

[BigInch]

sailo, As far as I'm concerned, its all your's.
I'm outa' here.


**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[ChasBean1]

How did a stagnation pressure become such a matter of opinion? There's no drag because there's negligible flow-though. Cd = N/A. There is about 0.016 psi at 30 mph. If you're worried about multiplication factors, size the actuator for 0.024 psi. Multiply by damper area for total force. Guys (not all of you), 30 replies and no clear answers makes this an AOL chat room vs. a place where people can trust in good engineering advice.

 

[BigInch]

Well looking back, that's a fair comment.
BTW, where were you on 28 April?


**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[ChasBean1]

Look back. I chimed in a 29 April. That's not good enough? Jeez...

 

[BigInch]

Ah.. yes. I also see that was when I still thought it was not a stagnation pressure problem. I admit I had a hard time picturing how you could have 30 mph and a closed damper at the same time. Actually, I still don't see how, but apparently that IS the case.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[hacksaw]

stagnation pressure is in the equations that is no surprise, but the greatest force on the damper is not when it is closed, but partly opened. if the actuator is not sized properly to get you past that you literally have to shut the system down to reposition it, assuming that you've not already broken the shaft!

 

[BigInch]

Now that makes sense. Now back to Cd ....

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/