[Dsg-rtpc] Backpressure

[Bob Miller]

Hi Sebastian,

Based on our Hall model, the exhaust line exit on the gas pad is 2.5 m above the beam line.

If you want pressure lower than 0.1 IWC, just remove the oil from the bubbler.

The exhaust line is 1" ID and about 40 m long.  With a flow of 170 ccm (mixed gas and buffer gas flow for 1 volume exchange per hour), the pressure drop due to flow is 0.001 IWC.

I can see if we have some 1" nylon tube to set up in the EEL and have the exhaust 2.5m higher than the detector if you like.

Regards,
Bob




________________________________
From: Sebastian Kuhn <kuhn at jlab.org>
Sent: Tuesday, January 14, 2020 5:49 PM
To: Bob Miller <rmiller at jlab.org>
Cc: Jiwan Poudel <jpoudel at jlab.org>; Mohammad Hattawy <hattawy at jlab.org>; Cyril Wiggins <cwiggins at jlab.org>; dsg-rtpc at jlab.org <dsg-rtpc at jlab.org>; Silviu Covrig Dusa <covrig at jlab.org>; Stephen Bueltmann <bueltman at jlab.org>; Eric Christy <meric.christy at gmail.com>
Subject: Backpressure

Dear Bob,

we have been pondering how we can reduce the backpressure from the bubblers for the drift gas exhaust in the BONuS12 RTPC gas panel. We find that the pressure in the gas return line has to reach at least 0.1 IWC (about 25 Pa or 0.00025 atm) before a bubble can form and leave the exhaust pipe. The overpressure inside the RTPC is undoubtedly even larger, taking flow through the chamber and the pipe connecting it to the DMS (and the DMS itself) into account.

On the other hand, we will be operating the RTPC in Hall B, where the exhaust port will be connected to an exhaust gas line that leads out of the Hall and up to ground level, which I believe is several meter higher (does someone have that number?) This actually HELPS, in the sense that the drift chamber gas (80% He / 20% CO2) is less dense than air, so that the hydrostatic pressure differential should be somewhere around 7 Pa per meter, or about 0.1 IWC for a 3.5 m height difference between the RTPC location and the final exhaust exit port. This is akin to a chimney effect which would significantly help with the gas flow as it more or less could mitigate the extra 0.1 IWC backpressure we are battling right now. (It’s right at the limit of what the RTPC can handle, and furthermore increases its leak rate.)

If my reasoning is correct (I am cc’ing Silviu in case he wants to weigh in), we could simulate the final behavior in the Hall by attaching a (at least) 4 m long exhaust tube to the gas panel in the EEL that we somehow rig vertically so that its end would be > 3.5 m above the RTPC. My question is
a) is this reasonable,
b) do we have suitable piping available
c) and if the answer is “yes” to both, could we either ask the DSG for help with this or rig it ourselves?

Thank you, and best greetings - Sebastian

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