On Saturday, May 6, 2017 at 11:27:49 AM UTC-4, Ecnerwal wrote:
In article ,
rangerssuck wrote:
So, given that I have a system that's shut down and pumped out, and that I
can isolate the rink floor pipes from the rest of the system and there are
service valves accessible, does anyone know how I might determine the volume
of these pipes?
Nitrogen is cheap and cheerful. CO2 might be another option, not sure if
it plays havoc with refrigerants (but you'd be pumping it back out
anyway - if you can handle the pressure sit's a nice cheap refrigerant
itself, IIRC.)
I also don't know what the ramifications of filling the system with CO2 would be. I was planning Nitrogen or Argon - Nitrogen is almost certainly cheaper, but Argon would be good to have for other (tig) reasons. [while we're on that subject, here's a guy who is making his own liquid nitrogen.
http://www.instructables.com/id/Home...gen-generator/ ]
If you have the underfloor pipes evacuated/isolated and they are not
where the leaks are (which would seem like "throw in the towel" time to
me, or at least a much more expensive repair) you should be able to
carefully weigh a cylinder, connect it to the system, fill to the
pressure of your choice (using more cylinders if needed, just be sure to
weigh them) and then weigh the depleted cylinders - initial weight minus
final weight is the mass of gas you moved (far more precise than a CFM
meter, AFAIK) and if you know the temperature of the floor, the
pressure, and the mass of gas you should be able to get the volume
(metric units on the weight will make that easier.) Obviously (I hope)
you can't have leaks in your fill rig and have this work, nor will it
work with significant leaks in the pipes.
Excellent plan. There BETTER not be significant leaks in the rink piping. I don't believe that there are - the pipes were replaced and a new slab was poured 20-something years ago. There were initial problems with this rink design - the R22 would find its way out of tiny leaks and react with the water in the concrete, eating the carbon steel pipes to death. - newer rinks use stainless pipes, or some other material that's run in continuous 200 foot runs.
PV=NRT is the formula that springs to mind from a long time ago, where N
is the number of moles of gas (mass will get you to that, temperture is
in C, R is the gas constant, and Pressure and Volume.
V=(NRT)/P would be the form you want. Or, y'know, cheat:
http://www.webqc.org/ideal_gas_law.html
Nitrogen's molar mass is 28.014 g/mol
If this part of the system is truly leak free you might get better
numbers by waiting a day for the temperature to stabilize, since
expanding from the compressed tank will of course cool it somewhat,
though the mass of the floor will mostly shrug that off. And you
obviously want the most accurate/precise scale you can still weigh a
nitrogen tank on.
I do have a good scale - will weigh down to grams in that range. The temperature measurement is part of the new control system I'm installing. There's already a sensor in the slab. I'm also installing new pressure transducers, so the bases really are pretty well covered.
I could actually hook up a data logger and watch that the temperature and pressure keep making sense over time. I'd expect that there will be some change as it settles, and then it would hold steady (assuming no leaks). If the pressure (vs temperature) slopes down over time, then we've got a problem..
So long as pressures are not too high, the deviation between the "ideal
gas law" and real gasses is quite small - should certainly be enough to
get you a good solid estimate.
Thanks loads for this. It's been a great help.
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