Use of Swagelok needle valves

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Jerry Biehler
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Re: Use of Swagelok needle valves

Post by Jerry Biehler » Wed Mar 21, 2018 7:35 am

Density is irrelevant for a MFC for basic gas conversion, all that matters is the conversion factor. Hydrogen is 1.01 D2 is .995 , so a 10sccm MFC for N2 will be basically a 10sccm MFC for H2/D2. The one for BCL2 will give about 22.5sccm.

Page 4-13

https://www.brooksinstrument.com/en/~/m ... -5850e.pdf

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Rich Feldman
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Re: Use of Swagelok needle valves

Post by Rich Feldman » Thu Mar 22, 2018 4:47 am

Thanks for the handy reference, Jerry. Best detail I've seen about how the proportional valves work.

I have learned things and come to agree with you, about those gas conversion factors actually applying to standard volume flows, not mass flows! Took me a while to make sense of that description on page 4-13, because it's sloppy about distinguishing mass from volume.
mass_flow_factor.JPG
Maybe it's traditional in gas-flow jargon to misuse the term "mass flow" for parameters that are given in units of standard volume / time.

The first scale shift they mention is in "relation between output signal and mass flow rate... due to difference in heat capacities". True, but for that scale shift the Cp values need to be true specific heats, e.g. in kJ/(kg-K). The values for N2 and H2 are 1.04 and 14.3. The heat transported in MFC sensor capillary by 1 gram of N2, for a given delta T and time, is matched by 0.072 grams of H2. (Giant alternators in electric power plants are often cooled with pure hydrogen gas, to minimize power loss from windage.)

In the next sentence they switch to comparing the "molar specific heats", e.g. in J/(mol-K). The values for N2 and H2 are both 29. It's 21 for monatomic He and Ar, 37 for CO2, and 62 for BCl3. The Sensor Factors in Brooks's Table 4-3, and the table I cited in a previous post, are proportional to the inverse of molar Cp ratios.

So the control sensor output from 10 sccm of N2 (18 g/day) would be matched by
10.1 or 9.95 sccm of H2 or D2 (1.3 or 2.5 g/day)
13.9 sccm of He or Ar (3.6 or 36 g/day)
7.4 sccm of CO2 (21 g/day)
4.4 sccm of BCl3 (33 g/day).

All those cases transport about 0.21 mW of thermal power per kelvin. I wonder what fraction of the flow is routed through sensor, in a 10 sccm MFC.

While we're on the subject, the Orifice Factors in Table 4-3 are proportional to square root of gas densities. For hydrogen with respect to nitrogen, in wide open valve with a given pressure drop, volume flow ratio is 2.63 and mass flow ratio is 0.38.
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Rich Feldman
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Re: Use of Swagelok needle valves

Post by Rich Feldman » Fri Mar 23, 2018 11:31 pm

The answer to my last question jumped out from the document Jerry pointed us to.
mass_flow_sensor.JPG
Looks like for that line of MFC's, and maybe many others, the raw sensor works with around 10 sccm Max. when carrying gases like nitrogen and hydrogen.

Any guesses how hard it would be to modify a bigger-flow MFC, to plug path B? Then it might serve as a sensor for 10 sccm, but not usefully control the flow.

I still won't call 10 sccm a mass flow rate. 10 scc is 12.5 milligrams of nitrogen or 0.9 milligrams of hydrogen.

In the Theory of Operation section of Brooks doc (page 3-1), "The temperature difference T2-T1 is directly proportional to the gas mass flow." They define mass flow in kg/s and Cp in kJ/kg-°K. Here the gas correction factor for hydrogen w.r.t. nitrogen is about 14.
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Jerry Biehler
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Re: Use of Swagelok needle valves

Post by Jerry Biehler » Sat Mar 24, 2018 8:51 am

My 5 sccm units still have the restrictor in it so they still use them at that low of a flow. You can change the orifices in the field, the manuals say how to do it. A friend has a ton of big units, like in the 10s to 100's of liter per minute range but they are just useless for anything I want to do.

I think it is mass flow since they are using mass to sense the flow unlike other methods like impellers, ultrasonics, etc which for gasses are much more affected by pressure and temp. Not controlling the mass of the product through the sensor.

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Re: Use of Swagelok needle valves

Post by prestonbarrows » Sat Jun 23, 2018 3:05 am

Bob Reite wrote:
Sun Mar 18, 2018 5:58 am
You can find 0-10 SCCM mass flow controllers on Ebay if you wait long enough. Most of my fusion is done in the 4-6 SCCM region. Then again, I'm not using expensive bottled D2. But as I recall, when Richard tried to be too miserly with his D2, he did not get really great neutron numbers.
It is fairly simple, if your input D2 flow rate is on the order of your leak rate + outgassing rate, you will have a bad time. Either you need to man up and fix your system or lay down the bucks to flood your tank with silly amounts of fuel.

A fraction of an SCCM of D2 is plenty to fuel fusion if you don't have other garbage swamping the reaction.

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