I am a new to blacksmithing, and am trying to build my first smithy. I have basically everything done but a chimney. Previously I have been hung up on a super sucker or other side draft/sheet metal style chimney. This last week I was discussing with my dad what makes a chimney a good chimney, and the point came up if the outlet draws more than the inlet (to create the suck) it would work. So my question is I have a piece of 22" steel casing about 5' long, if I took about 2' of it and put lids top and bottom with a 8" arch in the side/bottom with a 12" pipe venting it...would that work? Thanks in advance for y'all's advice. Here's a couple of pics

Not sure what you're asking. The pipe is the chimney in that set up. and the casing would just be the hood. at that point.

 

The way a chimney works is the hot gasses raise up through the pipe creating a low pressure area behind it sucking more air and gasses into the pipe. these gases are also hot and thus rise up through the pipe creating low air pressure behind it. a metal chimney will heat up and thus create more suction as the air around the pipe warms up and rises.  

I would be setting the casing on end beside the fire pot. Basically a round super sucker. Do you think that would work?

YES your on the correct track

   I might use 30" instead of 2'. Your 8" arch might be a little too small but it could be enlarged with a torch until you got it right.

Dutmac, what do you mean by "lids top and bottom" ? Are you building a side draft or as ThorsHammer82 suggest, a hood?
The 12 inch pipe six to eight inches above your fire pot will pull just fine. No hood needed as long as it extends above the roof line. This sounds too close to the fire, but just how thick is the iron you are heating?
If its a side draft you are going for there is another element you will need. A throat opening just above the top of your opening. This will require some math but will provide positive results.
1. Opening at fire box in square inches
2. Throat opening reduced by 20% just above
3. Chimneny opening slightly larger than first opening.
These two options will work. I have had both with no problems. I hope this helps some and you get to forgeing soon.
Chuck

By lids I mean put a top and bottom in it.
I am wanting a side draft.this is what I had in mind.

As I said your on the right track. Keep your 12" pipe to the back of your casing it'll work better.