Calculating an orifice opening size - air flow
Calculating an orifice opening size - air flow
(OP)
I need some help with this one. It has been a while since I played around with fluid dynamic and differential equations.
I have an air tank with a free air volume of 10.7 ft^3. I would like to know what size of an orifice is required to change the pressure in this tank from 5 PSF to 25 PSF in exactly 1.5 seconds. Is this possible to figure out or am I missing a variable?
I have an air tank with a free air volume of 10.7 ft^3. I would like to know what size of an orifice is required to change the pressure in this tank from 5 PSF to 25 PSF in exactly 1.5 seconds. Is this possible to figure out or am I missing a variable?





RE: Calculating an orifice opening size - air flow
For non-choked flow:
Q=C*A*P2*(2gM/RT)^.5*(k/(k-1))^.5*((P1/P2)^2k-(P1/P2)^((k+1)/k))^.5
Q=mass flow rate,lbm/s
A=orifice size,ft^2
C=coefficient, assume .75
P2=upstream pressure,lbf/ft^2 absolute
P1=downstream pressure,lbf/ft^2 absolute
k=Cp/Cv
M=molecular weight
R=universal gas constant,1545 ft-lbf/lbmole-dR
T=gas temp dR
In your case I would have P2=25 lbf/ft^2 and P1=5 lbf/ft^2 and I would consider the compression to be isothermal.
I don'thave time to check out the result and if it seems unreasonable interchange the values of P1 and P2.
RE: Calculating an orifice opening size - air flow
RE: Calculating an orifice opening size - air flow
No offense, but that's what can happen when you ask someone to solve your problem for free. People only do what they have time and interest to do -- after all, you're not paying them.
BTW, I believe you're missing at least one variable, if not more. What is your charging source? Why do you believe that an orifice can control the charging rate (as it is more like a function of the pressure of the charging source rather than the receipt tank)?
Patricia Lougheed
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RE: Calculating an orifice opening size - air flow
RE: Calculating an orifice opening size - air flow
Let's look at discharging the tank then. Assume there is 25 PSF in the tank, what is the orifice size required to discharge the tank to 5 PSF? This is what I need to get figured out.
RE: Calculating an orifice opening size - air flow
RE: Calculating an orifice opening size - air flow
RE: Calculating an orifice opening size - air flow
RE: Calculating an orifice opening size - air flow
In so far as the equation that I presented, rearange the terms to get A; also Q=mass/time; also this equation is for isothermal conditions and considering the relatively short time span the error should not be significant.
RE: Calculating an orifice opening size - air flow
I think I follow your post. However, I am unclear on how I am going to calculate the mass of air that is moved in that period of time to suffice the Q variable. Also I am not 100% clear for the values I need to calculate k.
Can you help me here?
PS: I also found this article that I am looking through on the same subject: h
RE: Calculating an orifice opening size - air flow
mu - mu(initial)= integral of hodm
where m mass in receiver
u specific internal energy
ho stagnation enthalpy of source
dm represents change in mass flow
integral of dm = m - m(initial)
Does ho, the source stagnation enthalpy vary?
What is initial temp of air in the receiver?
Classic problem. Filling an evacuated tank with a perfect gas and constant specific heat from a fixed source.
mu =ho*m
CvT= CpTo
T=gamma * To gamma =ratio of specif heats, T= abs temp