You haven't answered my question........but it sounds like you are talking about an open fire??
The tubes would start at the front of the fire just above the hearth level. (see below) they would go back towards the back of the fireplace and curve up and travel up the back of the fireplace about 400mm-500mm or so depending on the height of the fire you would normally run or the height of the fireplace, then curve over and point back towards the room. They need to be on an upward angle to where the air exits the pipe into the room. It is kind of like a square letter C the bottom leg is straight and flat and the top leg is straight but on an angle upward at about 5-10 degrees. This is about enough to create the thermo-syphon effect to move the air through the pipe. the steeper this angle the better the pumping effort, ie air flow. This shape is achieved in practice simply with two bends in a pipe bender. Each tube takes about 5 minutes to form once you get the hang of it. The top leg is a bit longer than the bottom one so that the tubes can protrude past the fireplace opening into the room by a 100mm or so, so the hot air discharges into the room. About 6 or so is good, work it out for the width of your fireplace. I used 2inch steam pipe about 80mm apart from memory.
I then welded them all to two horizontal members across the bottom ie the are elevated above the hearth, and two more across the top. Just use the same tube that you are making the unit from. The idea is not to have the heat tubes buried in ash..... 2-3 inches above the hearth is good.
From memory, i was fabricating 1 unit from 1 length of steam pipe to make it convenient and economical.
There you go now you can see what was inside my head, ...about 40 years ago....gees its dusty and dark in these back rooms.
