Thank you for your comments and kind offer for helping with redwire.
I will expriment on my own, and then probably come back with questions.
My smelting area in the first factory was not very structured, so today (at work
) I couldn't help to think about a compact design with the following goals:
1: It should be extendable in 2 dimensions.
2: It should be possible to build early, i.e. without advanced research.
This is what I came up with, and it only requires long inserters and underpasses to be researched.
So to explain:
Coal and ore are travelling on the belts to the south.
Short and long inserters grab it and places it on belt with either coal and ore for the iron smelters or just coal for the steel smelters. The number of inserters can be increased as needed.
The iron smelters are fed from the mixed belts, and deliver iron plates either to the output iron belt, or directly into the steel smelters that are fed coal from the other side.
The finished products are passed out under the feeding belts to the south.
If I need more steel, then I can extend the production by removing a section of the iron belt...
...and then add a new steel section and rebuild the iron belt.
The design can be extended both upward and sidewards by copying the sections as needed.