Mass Flow Controller

27 09 2009

So I’m making good progress building out the computer control and data acquisition system for the reactor. Currently we have a manually controlled needle valve to regulate flow on the deuterium handling system. I want to replace this with a computer controlled mass flow controller.

A new mass flow controller costs roughly $1,100. There are plenty available on ebay starting from $70.

A mass flow controller must be calibrated for a specific gas. Some can be calibrated on demand with a digital interface, and some are hard wired to work with a specific gas. I’m wondering if we can get away with using a hydrogen calibration for our deuterium system?

The next parameter is flow rate. Here is some background on flow rate nomenclature: flow rate nomenclature. Although mass flow controllers meter based on mass flow rate, they confusingly are rated using volumetric flow rate metrics like SLPM (standard liters per minute), SCCM (standard cubic centimeters per minute) or SCFH (standard cubic feet per hour). Conversion between the two must be done at standard temperature and pressure, and must take into consideration the density of the gas: details.

So before I can spec a mass flow controller I need to calculate our midpoint flow rate needs.

Some facts:

The chamber holds ~6,500 cm^3

The turbo pump removes gas at the following rates:

N2 -> 56 liters/sec

He -> 48  liters/sec

H2 -> 36  liters/sec

So the mass flow controller meters how much fuel is flowing into the chamber. We also want to control how much fuel the vacuum pump is pulling out of the chamber. This requires a gate valve or butterfly valve. Many of them include an integrated bellows connection (which is great), and are pneumatically actuated (not so great).

Adding a gate valve will require us to redesign the welded sled; and I bet it won’t be the last design change. To keep it flexible I’m going to take another page from Andrew and switch to 80/20.

Now my naive attempt to calculate flow rate needs:

Without a gate valve the pumps will remove 2160 l/min. Now the typical mass flow controller seems to max out at 10 SLM (l/min)…. so the pump will remove fuel far faster than the mass flow controller can supply it. And this will waste expensive fuel.

I think these calculation work for pressures in the laminar flow region (> 1e^3 torr). We are working with pressures between the laminar flow domain and the free molecular flow domain – I don’t know how the calculations change at this point.

ps. Wolfram Alpha seems to be useful for density calculations.



6 responses

29 09 2009

You will need a very low flow mfc. Around 10SCCM should be ample from experience .Typically less is more in fusion. Mean free paths need to be long enough to ensure fusable accelerations. You would have found with the needle valve that you can never get it quite low enough before it shuts off completely.

7 10 2009

This is interesting. This chart shows the correction factor for various gasses in a mass flow controller.

It shows air and deuterium both having a correction factor of 1.

I guess this means I should get a mass flow controller calibrated for air/n2.

8 10 2009

So here are the specs for our ideal mass flow controller:

Gas: N2
Connectors: VCR
Pinout or manual available.
I would prefer a D-sub electrical connector, but not a deal breaker.

10 10 2009

OK. Just bought this mass flow contoller:

· Model: SEC-4400M
· Gas: N2
· Range: 20 SCCM

I contacted the manufacture previously and they emailed me the manual.

10 10 2009
M. Simon

You have to adjust your flow calculations for STP. i.e. a flow (arbitrary units) of 1 at STP is equal to a flow of 1,000 if the pressure is 1/1,000th standard and the temperature is constant.

And don’t be shy about coming to talk polywell. Love hearing about your progress and/or troubles.

10 10 2009
M. Simon

BTW the arbitrary units are volume/time.

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