Nectarine
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Registered: 19-4-2014
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Vacuum, pressure, and capillary diameter... wut
I've never done anything close to this, its a grad level course so he just dropped it on us. I don't need the answer, but I do need the equations:
"You are designing an electrospray system. You have a vacuum pump that displaces 37 m^3/h of air. You must purchase a capillary that is 7.8cm long.
What should the inner diameter of the capillary bne fpr a [ressure of 1.3 torr in the first differential pumping region of your instrument. Consider
only gas flow through the capillary to the pump (i.e. assume other gas flows are negligible).
So far, some equations I've found are
Conductance: C=(pi*d^4)/128nl * (P1+P2)/2 for 'large pressures'
C=(pi*v*d^3)/12l for kn>1 (knudsen #)
v = average velocity of the gas
l = length of tube
d = diameter
n = ?
P1 and P2 are the pressures outside and inside
Also Qout=PPSP
Pp= internal pressure (1.3 torr, I think in this case)
Sp = vacuum velocity (37 m^3/h I think in this case)
Then... I believe you can do Q = C(P2-P1)
..... I have no idea what the f*ck to do.  Is 1.3 torr going to use a Kn > 1
or is this considered a large pressure??
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Zombie
Forum Hillbilly
Posts: 1700
Registered: 13-1-2015
Location: Florida PanHandle
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Copy/paste this into Google search box. The first link on the page is a Perdue University PDF w/ everything you need
Numerical Calculation of Mass Flow Rate in
Capillary Tubes Using ‘Art’, an Advanced
Simulation Softare
Also my second favorite site on the net. (after pornhub)
http://www.engineeringtoolbox.com/
It's not that difficult. See the trees inside the forest. Take what you know, and apply that. The rest falls into place because it HAS to.
They tried to have me "put to sleep" so I came back to return the favor.
Zom.
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Nectarine
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Posts: 44
Registered: 19-4-2014
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I ended up doing the problem and getting an answer that is at least 1 order of magnitude too high.
I used different equations, but I'm still unsure what mistake I've done. Everywhere I go rounds off different numbers and uses different conversion
values (ending up with different sig figs and rounding).
Unsure, but since there is a r^3 value, any change in rounding can be substantial.
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Zombie
Forum Hillbilly
Posts: 1700
Registered: 13-1-2015
Location: Florida PanHandle
Member Is Offline
Mood: I just don't know...
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It's over my head brother but like I said before... Take what you do know, and the rest has to fall into place.
That's how I approach everything.
Hopefully someone will know the formula that applies for you. If push comes to shove I have a friend I will see on Saturday that made a career in thin
film tech. and high vacuum. Perhaps he will look at this thread with me.
They tried to have me "put to sleep" so I came back to return the favor.
Zom.
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