Wow, haven't seen this thread before. Very interesting idea. Has anything like this been tried before?
I noticed you mentioned the equivalent would be an 8" bore pipe. I'm not sure. Eight pipes have the same cross sectional area as one pipe with square root 8 (2.8 ) times the diameter. So in terms of flow you will be getting the same as 2.8 x 2" = 5.6" (142mm) pipe. Where I live 159mm copper pipe is £65 per metre and 54mm pipe is £11 per metre. So for me it would be cheaper to go for a single 6" pipe. That's before taking in to account 8 lots of fittings for the boiler and 8 needle valves.
Just because the 5.6" would have the same CSA as your 8 x 2" that doesnt mean they would perform the same. Until you try it I don't think anyone can really know. If for example the centre couple of inches of a large tube performed much better than the outer few inches then there could be big benefits to having several smaller tubes. If however the cooling the vapour from being near the column reduces performance then the large bore column would out perform several smaller ones. There could be dozens of factors at play. Finding out which is better probably has to be done by experiment.
Balancing the vapour flow up 8 columns might be impossible. Your suggested approach may work for dry air but mixed vapour and liquid behave very differently to dry air. Consider what happens when one tube starts to get close to flooding (where you want to be for effective reflux). The pressure needed to push vapour up that tube increases. In your situation vapour will simply stop passing up that tube and us a different route unless all the other tubes are at the exact same point of almost flooding. The key is going to be ensuring that all 8 are at the same equilibrium state at the same time. It could be that its self regulating. So as one backs up vapour favours others until they back up etc and it all settles down to a nice stable equilibrium. Alternatively you could have more chaotic behaviour. One backs up and vapour goes elsewhere, that tubes pressure collapses and the tube clears, all vapour gushes up that tube and others collapse etc.
Its all mathematics and pertubation theory. Compare to a pendulum. It has 2 equilibrium points. The dangling one is stable, small nudges and it will settle back. The other is with the pendulum ballanced vertically. It is in equilibrium but its unstable. Any nudge will cause it to fall out of that equilibrium. A single pendulum is very easy to model. If you build a double pendulum its much harder to model because the two pendulums interact. A triple pendulum will exhibit chaotic behaviour and is impossible to model. It will never settle in to a stable equilibrium.
Triple Pendulum 01:
http://youtu.be/scI1GIkIWuk" onclick="window.open(this.href);return false;" rel="nofollow
You may be building an octuple pendulum. Seriously crazy stuff.
Good luck. I'm really loving where this might end up.
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