In one of my worlds i had an automated smelting system.
I had a row of 8 furnaces, each with 2 hoppers feeding them (one for fuel, the other for material to be smelted) and then hoppers under each furnace feeding into each other in a chain eventually dumping into a chest. What was in that chest was then dumped into a minecart which when full would depart for the surface, eventually dumping the finished goods in my castle on the surface.
The furnaces were underground, near where i was mining.
The hoppers for fuel and materials were fed by 2 minecarts that would keep travelling over them until empty.
Basically after every trip down the mines, i would dump everything i needed smelted into one minecart, and all fuel i'd collected into the other and set them both going.
A few minutes later finished goods start arriving on the surface.
My design has limitations, but it's the simplest (as in space required, ease of setup/modification, and materials required to build it) and it's really reliable.
There are certain limitations that cannot be avoided; chief among which is if a minecart heads for an automated unloading station, it can only unload if the chunk the unloading station is in is inhabited at the time.
If the chunk isn't "loaded" the cart will just bounce back, still full.
Luckily, the chunks are from sky box to bedrock, so you could have the system buried underground and deliver to the surface, as long as someone is in that chunk.
The other limitation, which someone smarter than i may work out a work-around for: the system of detectors and circuits used fails to let a minecart leave if material is still passing through or in a hopper, so the cart cannot leave. The only way to avoid this at present is to make sure no more than a single (or half) chest worth of material will ever be fed to the minecart that is to transport the goods, or include a button to force the minecart to leave.
I Hope that made some semblance of sense!
I will build a working model of my design under spawn if you'd like.
