Help - Mining Refinaries?

Essentially, refineries give a small bonus to resource extraction in the planet you build it in and adjacent planets. If you mouse over a resource asteroid, it will show a "Refinery Quota: x/3," (x/4 for neutrals) x being the number of refineries you have in the asteroid's system and adjacent planets.
If you really want to, you can give yourself a headache trying to figure out the optimal arrangement, but one per planet generally works out fairly well.

You can build as many as you want. They will harvest from the planet they are based on and the surrounding planets (connected directly by phase lanes- no I don't know if the Vasari can exploit this). However each asteroid can only support three refineries- extras are redundant. So the best way to use them would be to place them in places with multiple connections.

And just in case you don't know they give you their own crystal and metal income (based on the asteroids they harvest- I think it takes time).