Has anyone had success at large scale sous-vide at home?
My scale was not quite as big as yours, but I cooked my Thanksgiving turkey for the extended family this year sous vide. Not only was the outcome a huge hit among the guests, but it was a huge relief for (your truly) the chef.
I want to go to party scale, say 30 to 40 servings, what are my chances of success?
In my experience, scaling up is one of the biggest advantages of cooking sous vide. Cooking 30-40 servings on a home stove would be a daunting prospect, to say the least. With sous vide, 15–20 servings is only marginally more work than 4–6. Again, I've not done 30–40, but it should scale accordingly.
Should I just get three or four chilly bins or just one large bin?
If your food will fit in one cooler, it will work, but I think you'll find it much easier to use more. The more water you have, the slower it will lose heat, and thus, the less you will have to pay attention to it. I also typically cook chicken and beef at separate temperatures, so I would probably have two coolers for chicken and two for beef.
Best Answer
You could use oil, but I don't think it's worth it; realize that you're going to be using additional energy for obtaining, cleaning, and disposal of it.
Regarding using a broth with a porous bag, well, that isn't sous vide. That's more akin to a slow-cooker. The whole point of sous vide is the airtight barrier between food and heat.
Update
Oil does not have a higher thermal mass than water. Ammonia does, but I'd suggest avoiding that. ;)
Source: http://www.engineeringtoolbox.com/specific-heat-fluids-d_151.html