force exerted by air
force exerted by air
(OP)
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
A similar problem to this would be to try to calculate the force on a butterfly valve when only the velocity is known.
Thanks





RE: force exerted by air
force = velocity * mass flow rate [N]
where
velocity [m/s]
mass flow rate [kg/s]
http://www.engineering-4e.com
RE: force exerted by air
Fluid drag force is proportional to velocity [bold]squared[/b], more specifically,
http:
**********************
"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/
RE: force exerted by air
I do not see anything wrong with what I have put out there ...
Please let me know if something else needs to be considered or done in this case ...
Thanks!
http://www.engineering-4e.com
RE: force exerted by air
RE: force exerted by air
FOURe Do you see the velocity SQUARED term. You only put V^1. Little bit of a difference, don't you agree. Why not have a look at the link I posted to reassure yourself.
**********************
"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/
RE: force exerted by air
RE: force exerted by air
If I may add, velocity is squared in my input. It is contained in the mass flow rate.
mass flow rate = velocity * density * cross sectional area
force = velocity * mass flow rate
Therefore, you and I are in agreement -- it is a function of ~ v^2 ...
I am OK with where we are ...
Thanks!
http://www.engineering-4e.com
RE: force exerted by air
Your velocity pressure is .5*(density of air)*(velocity of air)^2
so if you just want the force that the velocity exerts on the dampers, just multiply that velocity pressure by the cross sectional area of the dampers.
P = F/A... F = P*A
If you want the actual force on the dampers, then you need the (static pressure + the velocity pressure)at full open multiplied by cross sectional area.
-Mike
RE: force exerted by air
force = velocity * mass flow rate + (air total pressure - pressure after the dampers) * cross sectional area
http://www.engineering-4e.com
RE: force exerted by air
How about:
force = (air total pressure - pressure after the dampers) * cross sectional area
http://www.engineering-4e.com
RE: force exerted by air
-Mike
RE: force exerted by air
**********************
"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/
RE: force exerted by air
The typical drag equation looks like:
1/2 * Cd * rho * V2
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: force exerted by air
**********************
"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/
RE: force exerted by air
I'd vote for mjpetrag a few posts above. It's just a stagnation pressure reduced from Bernoulli's:
P = (rho * V^2)/2
There's 0.016 psi above atmospheric acting on the dampers at a 30 mph wind. Just multiply by the damper area to get total force. If it's a 40 ft2 damper bank it would be around 90 lb force.
RE: force exerted by air
**********************
"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/
RE: force exerted by air
RE: force exerted by air
If the damper were closed (static pressure only on one side (fan discharge pressure x area), then the total force on the damper would reduce to only be pressure x area of the damper.
If the damper is at least partially open, the static component of pressure on each side of the damper is assumed equal (since there is no elevation change across the damper). Any differential pressure therefore must be caused by flow around the damper, which makes drag the only mechanism of interest, hence the Cd.
Come on guys. Differential pressure across a valve isn't figured only with P1, its P1-P2.
For a valve,
Q = Cv dP^0.5
(Q/Cv)^2 = dP
since Q/A = V
(V/Cv)^2 = dP
V^2 / Cv^2 = dP
dP = V^2 / Cv^2
P1-P2 = 1/ Cv^2 * V^2
so, looks like Cd is pretty much physically equivalent to the inverse of 2 IRstuff x Valve_Coefficient^2
Do you see how Cv for a valve is analogous to Cd/2 for some shape being hit by a flow.
**********************
"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/
RE: force exerted by air
RE: force exerted by air
**********************
"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/
RE: force exerted by air
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
RE: force exerted by air
experimental Cd is based on the stagnation pressure far up and downstream as being equal.
RE: force exerted by air
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: Eng-Tips.com Forum Policies
RE: force exerted by air
RE: force exerted by air
Really though he needs to check w/ the damper manufacturer, or google "sizing damper actuators" - there appears to be plenty of guidelines out there.
RE: force exerted by air
**********************
"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/
RE: force exerted by air
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?
RE: force exerted by air
30 mph "air speed"....
Unfactored pressure exerted by the "air speed" = 2.30 psf
0.00256xV^2
RE: force exerted by air
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
RE: force exerted by air
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: force exerted by air
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/
RE: force exerted by air
RE: force exerted by air
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/
RE: force exerted by air
RE: force exerted by air
**********************
"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/
RE: force exerted by air
RE: force exerted by air
**********************
"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/