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Old 10-04-2012, 04:20 PM   #49
WDE46
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Join Date: Sep 2012
Location: Old Greg's Cavern
Posts: 9,036
My Ride: 2004 330Ci OBM
Quote:
Originally Posted by SeanC View Post
Just did a quick calculation. Apparently 1 bar cap is good enough. Take it for what it's worth:

Assuming 96 degree Celcius coolant temperature, which is pretty much the max, the pressure of an ideal gas (e.g. air) is 1.3516 atms.
I don't know where you got this number. Did you assume a 1 atm starting point at 25C for this and calculate the change in pressure given the same volume but higher temperature?

I mean did you do: P1/T1 = P2/T2

Anyway, we don't really have numbers to work with here. We need volumetric flow rates, temperatures at various points in the system, pressures at various points, and some system specs (total volumes of water and air/steam). It is more complex than doing some ideal gas calculations. You need data. I designed a model of a presurized water reactor for my senior design project. This cooling system uses all of the same fundamentals. To get the data we need will require at least $1000 in instrumentation, plus the time and know how to hook it up.

My opinion is that the system does not operate anywhere near its 3 bar absolute maximum. If it did, then that cap would be constantly releasing steam/air. That limit is only there for emergency pressure relief. It will release steam, not air (just a little from the ET), when triggered because to reach 3 bar the system must boil some water into steam. My thought is that the cap is there only to relieve pressure to save the engine's seals in the event of an over heat. The system likely runs just above 1 atm in normal operation due to the vapor that forms in the ET, but that is it. The numbers we have suggest this. You can't pressurize a system to 3 bar with 100C water/antifreeze (unless it's its a solid system), that's just basic physics.

Also, a note on pressure units: When the cap says it is a 1 atm or a 2 atm cap, it means 1 atm or 2 atm gauge. Gauge pressure is your absolute pressure minus your barometric pressure (1 atm, 14.7 psi, the air you breath). That's just a note for those reading, who may be confused by the term's usage.

Last edited by WDE46; 10-04-2012 at 04:24 PM.
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