In theory you could comfortably use a very small bore pipe, you are only looking at a minute flow, so friction losses will be just about nonexistent. BUT the danger I perceive would be the pipe becoming blocked. Something like a brake pipe could quite well end up with a solidified drip of glycerol blocking the end, or crud simply entering the pipe and getting stuck. What ever way you look at it the head inside the pipe will be the same if the pipe is 3mm or 300mm diameter.
Could you weld a vertical socket to the lid? Save messing up the nice paint job on the tank. The lid could still be removable as once above the fill level, you can use small bore plastic hose to connect to the pressure switch.
With regards to in-processor bubbling, I don't want to raise a scare, I'm just trying to err on the side of caution. The statement comes from the time I was working on Inert Gas systems in the marine industry. In the late 60's early 70's several oil tanker fitted with these systems suffered catastrophic loss and the only explanation anyone could come up with was gas being supplied to the cargo tanks at too greater velocity, causing static. After this design parameters were changed to limit the velocity of gas in the entire system, but I can remember what the limit was.
I've also had static shocks when using a Henry to suck up brick dust and small, general building debris. however, both these instances are very different to what we are doing in the processor. The Henry scenario is a plastic pipe with debris and the IG system was handling many thousands of cubic meters in pipe work up to 30 inches diameter.
I've used the idea for many months with no ill effect, other than the twin PD cant cope with the throughput and it's very easy to emit Methanol fumes from the vent. I can imagine people not fully understanding what they are doing, stuffing a full flow air supply into the processor with possible static dangers, emitting vast amounts of fumes with neighbours complaining to the Authorities and reflecting on all home brewers.
GL's design is beautiful in it's simplicity and safety and I don't really want to promote anything that is a retrograde step in those respects.
Keith and I have been looking at coalescing filters (bet Keith is spending all his time on the new processor and not coalescing filters!) but I've only got as far as putting a sintered exhaust silencer on the vent and over supplying with air. This worked to a degree, roughly doubling the liquid condensate from the vent, but fumes were still snifable in the vicinity.
As a side note, I've made a rig to try the idea on wet bio, but my feeling is that coalescing filter are only good for free water and won't be effective on mixtures or emulsions ... still only one way to find out. I'll post the results once I've run the test.
Keith, I've found a good source of fine aluminium mesh. If you are taking out any kitchens, have a look at the filter in the cooker hood. The one I took apart comprised several layers of fine Al mesh sandwiched between two courser layers.
This is getting rather lengthy, sorry, but back to the in processor bubbling ... if you are going to try it, keep the pipe work large and the air flow low. I did experiment with my aquarium pump when I first tried the idea, but it's really weedy and appeared to make no difference at all. The compressor I'm using is a small diaphragm jobbie which can just be seen, bottom right, in this photo
http://www.biopowered.co.uk/wiki/File:Simple_fat_melting_tank.jpg Unfortunately there's no rating plate on it but as a workshop compressor it would be a complete waste of time, the flow is so low.
Currently I'm using a sintered brass exhaust silencer to introduce the air, but I'm not convinced that it's the most effective method. At full tilt (adjusted on the needle valve) and when drying oil, you can hear a "gluging" sound within the processor which I believe to be the bubbles joining together and rising to the surface as series of large bubbles. I think a sparge pipe may be more effective, but I haven't got round to trying it yet. As you two are building it would be quite easy to try a sparge pipe with many widely spaced small holes so that the bubbles don't have a chance to combine.
Finally I think it would be a advisable to arrange things so the bubble stream is nowhere near the heating element. If the element was exposed to a constant bubble stream I think it might cause it to over heat.
