If offset angles are used - why do so many commercial gas forges place them vertically?
One word..... PROFIT! It's far less time and labor intensive to build forges with burners set at 90 degree angles. To me it also indicates that the company designing/offering a given forge doesn't have a clue about forging or how a forge should operate.....they're just interested in the $$$$. (In the current FIF era, the same holds true with many other Bladesmithing/Knifemaking tools..... there are a ton of people and companies who've jumped on the bandwagon, hoping to make a quick buck, offering everything from forging hammers to grinders.......without a clue as to what they are building/offering)
There's a reason that most experienced Bladesmiths build their own forges is because there really isn't a well thought out, well designed, Bladesmithing forge produced/available commercially. There are a few that come close.....but even those models fall short as far as burner placement is concerned.
Just a quick primer on what most full time/Professional Bladesmiths tend to look for in a forge...
1. Round in design
2. A SINGLE, properly sized burner, for the forge size
3. If the Bladesmith has only a single forge in their shop(s), it's usually a "blown" burner design. (that way the forge can be used for everything from general forging to forge welding) Personally, I have 3 forges, 2 of which are used most of the time......a single burner/venturi forge, with ceramic fiber lining that is used for general forging, and NEVER sees any type of flux. The second is a vertical welding forge that I describe in this WIP:
http://www.legacystudioproductions.net/knifemakertraining/blog/page/3/
4. A burner that is inserted in such a manner as to force the flame pattern to "swirl" along the ID of the forge, and avoids the burner flame being directed at the work piece(s).
5. And likely the biggest "trade off" choice for a forge...... castable versus ceramic fiber insulators. Each has their pros and cons.
There are also other aspects that will vary based on personal preference from Bladesmith to Bladesmith, but the most common attributes are those I listed above.