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Simple Surge

@348joey, thanks. You have great ideas to improve this. My design is definitely out of date. I intend to play again sometime soonish and will look at your adjustments when I get back into the game.

Oh, right, forgot to explain how the junctions waste six surge alloy each. An odd quirk about junctions is that they will hold up to six items in each direction if there is ANY block on the opposite side to the input, even if that block cannot accept those items. This is only a short-term issue, as it will reach full output speed once each of the junctions fills up with six items, but that's still rather wasteful. You can observe this if you use your original design, let it run for a while before turning it off, and then replace the unloaders with regular conveyor belts.

Also noticed I miscopied the design code. That bottom right overflow gate should be a sorter set to surge alloy. Sorry about that.

Also the reason you stall without the junctions/inverted sorters is because of how the unloader works. If it's attached to two smelters at the same time, it will try to balance the total between them. This can be a problem when you want to distribute a large amount of resources from one end of a chain to another, because it will try to balance most of it between the first two, then only about half of it between the second and third, then half of that between third and fourth, and so on. The inverted sorters/junctions help by having it always unload in one direction rather than trying to balance it.
That's everything, hope it helps.

You can even stack two of these with a shared output bridge like this:
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

The way you have your lead input at an angle is a bit unwieldy. Since the best layout for a Nucleus Core is more like this: isbXNjaAF4nCWMSw5CIQxFLyDmNbgIN+AqTNyEcYDYAUkFw8f1W3zp4CQn5xYBweFQ4ptBtylyvtbGCC/uqeXPyLUAOEp8snTY+8PBf+OUAUq1jJgLN5ySbi5lJuHZNQ+AgdVT+B0brIFb2qryS7kl9fe/VdAC7TvSxCxsMD8KXRhT you're more likely to have flat sides available. (Note that in version 7, containers can actually contribute to the core's total capacity, but this still works in version 6 without the containers.)

Ultimately, I think this would be the best layout for your design:
bXNjaAF4nFWQTU7DMBBGxz9J2zRNWnEB1kjZsOAU7FgiFmnjVpZcGzkJqKflIkDw5EOieOGxR2++J5tuKFOkfXs2VD6djRtMvH203tC6M/0h2tfBBk9EuWv3xvUkn19yWo7ehbYzkep9tN3JNIfg38wlRKpGn/pHF96bUzsY2rTOhUvTI5p2gx1ab8fz30Rt0yEOpmv6EJkhuideGZEQfJg3Iomi0NCLaZoepun7Q0oQKhGJ0QA1g4wqoF9zRJb687zGDMPiF9ZsZG2WWoKLRFEkJOU8p2jBEp3KHLNA9uecrWmJ+RUUq7RJLhJFMVIAKaAooCigWENR8oBIt3/ZJYwlunf8cKFpg7gKxgrGCsYKxhpIDWMNYw3jFsYdjNtro0hdPJO/SfD1yvwDAklYLQ==

Instead of the junctions, consider using unset inverted sorters. They function similarly to junctions, but they have no internal inventory. Also, bridges are a better option for the output. It shortens the design by one unit, uses no titanium, gives a bit of extra space and just makes it look cleaner overall.
Now that you have inverted sorters in place of junctions, you can actually unset the unloaders that are directly between the smelters. This will actually let you add a fourth smelter to your line, as you have all the unloaders working constantly rather than having the titanium unloader working ~67% of the time.
Character limit. Let me post the rest in separate comments.

Okay, I've made a significant improvement for your smelters, let me organize it a bit for explanation.

Let me fix this quickly, hang on.

Your junctions between the smelters are going to waste 6 surge alloy each.

@peppermancy that is really interesting why do the junctions stop the third one from being starved?