meanfreepath (![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png)
meanfreepath) wrote2005-06-03 07:25 pm
meanfreepath) wrote2005-06-03 07:25 pm
SCBA and Joule-Thomson throttling
It never ceases to impress me just how much two of the things I think about a lot (and probably write way too much in this LJ about), physics and the fire service, are quite closely connected.
Last night at our weekly drill (my last for quite a few weeks, because of fire school starting up) we did search and rescue operations. We pulled the apparatus out of the engine bay and set up the gear racks and folding tables to simulate the aisles in a store. A few obstacles were set up along with some electrical cords for entanglement, and a simulated victim was hidden in the room. Two three-person teams were set in blindfolded, in full gear and breathing from airpacks.
The drill was a tough one; I was rather surprised at how quickly I became disoriented, even in a familiar environment. We didn't manage to find the dummy before we ran low on air and had to exit the building, but we came close (the other team did get him).
Here's where the physics comes in. We made it out of the building, whence I proceeded to take off my gear. It was at this point that I noticed that my air bottle was cold, indeed cold enough that water had condensed on its surface (we had started with 4000 psi 1-hour bottles and in about half an hour were down to about 1/3 of the bottle). I should note that SCBA bottles are filled with compressed air, which like the atmosphere is mostly nitrogen.
After some thought, I realized that what was going on was the Joule-Thomson throttling process, which is used for liquefying gases like helium. The whole thing is supposed to be adiabatic, but basically there's a pressure gradient with gas flowing through a constriction, and somehow the gas cools through a constant enthalpy process. I'll admit I don't remember the physics of it that well (stat mech/thermo remains one of my less favorite areas in physics). The really cool part is that it's a very nice demonstration of non-ideal gas behavior, as ideal gases do not cool when throttled.
I believe Reif's derivation assumed a constant pressure gradient, which isn't the case with the SCBA. It would be cool to make a good analytical or numerical model of this, though - maybe I'll check the Scott webpage and try to figure out some relevant figures. Experimentally - I'm not sure how well one could measure the gas temperature as it flows out (at least not without compromising the integrity of a $1000 air bottle).
Last night at our weekly drill (my last for quite a few weeks, because of fire school starting up) we did search and rescue operations. We pulled the apparatus out of the engine bay and set up the gear racks and folding tables to simulate the aisles in a store. A few obstacles were set up along with some electrical cords for entanglement, and a simulated victim was hidden in the room. Two three-person teams were set in blindfolded, in full gear and breathing from airpacks.
The drill was a tough one; I was rather surprised at how quickly I became disoriented, even in a familiar environment. We didn't manage to find the dummy before we ran low on air and had to exit the building, but we came close (the other team did get him).
Here's where the physics comes in. We made it out of the building, whence I proceeded to take off my gear. It was at this point that I noticed that my air bottle was cold, indeed cold enough that water had condensed on its surface (we had started with 4000 psi 1-hour bottles and in about half an hour were down to about 1/3 of the bottle). I should note that SCBA bottles are filled with compressed air, which like the atmosphere is mostly nitrogen.
After some thought, I realized that what was going on was the Joule-Thomson throttling process, which is used for liquefying gases like helium. The whole thing is supposed to be adiabatic, but basically there's a pressure gradient with gas flowing through a constriction, and somehow the gas cools through a constant enthalpy process. I'll admit I don't remember the physics of it that well (stat mech/thermo remains one of my less favorite areas in physics). The really cool part is that it's a very nice demonstration of non-ideal gas behavior, as ideal gases do not cool when throttled.
I believe Reif's derivation assumed a constant pressure gradient, which isn't the case with the SCBA. It would be cool to make a good analytical or numerical model of this, though - maybe I'll check the Scott webpage and try to figure out some relevant figures. Experimentally - I'm not sure how well one could measure the gas temperature as it flows out (at least not without compromising the integrity of a $1000 air bottle).