Can anyone who has a forced air ribbon burner on their forge tell me how hot  the back of it..........the connection to the incoming piping of air and propane gets?

It shouldn't get too hot as the air is cooling it as it goes through. That said, I have had one of mine glowing slightly which I took to mean that the flame was inside the burner. I shut down and let the air cool things down and restarted. It was fine after that.

 But can you tell me if you could lay your hand on the back of the burner without being burned?

You should be able to Chris. The NARBs get uncomfortably warm but you can hold your hand on the plenum without problem. Cooler than a fresh cup of coffee. 

Frosty The Lucky.

Thanks, Frosty.  Then I won't have to "distance" my static mixer any further from the burner than it is.  WHEW!  One less thing to obsess about.  When my ribbon burner cures, I'll be ready to get going.

You're welcome Chris, my pleasure. You find almost as many things to worry about as Deb does but you're easier to convince things are okay. Wondering if the plenum could get hot enough to be a problem is a good thing to wonder about. 

Asking here is a good idea. You're not the only guy out there in the IforgeAsphere trying to figure out how to make one of these and are (hopefully) reading along. You ask some good questions they may not have thought of or are asking themselves but unlike you are too bashful to ask.

Frosty The Lucky.

Never been afraid to expose my ignorance concerning a subject I don't know about.  I make enough mistakes even WITH all the help you guys freely give me!

My plenum is getting hot to touch also. I packed the opening for the ribbon and glazed with kastolite. It’s still getting hot. I’m wondering if I don’t have enough air flow.  If I get the air or fuel too far out of adjustment I get a loud whooo sound.  

The plenum will get hot but you should be able to touch it for a little while. Enough fuel air flow keeps the burner block cool enough to prevent it burning back. You wouldn't expect the plenum to get as hot as it does but it's conducting heat from the block AND getting crazy strong IR shining in through the outlet holes. My first plenum with the diffuser has a rectangular cool spot where the diffuser shades it from the outlet IR. 

When I mounted the inlet on the side of the plenum 90* from the outlets I left the diffuser out, the NARB worked great but the back of the plenum is hot, hotter than the sides. 

Frosty The Lucky.

On the topic of air flow I have an observation with my NARB.   I can see the outlet holes (nozzlettes) in my NARB through the front opening and I've noticed that if there is enough fuel/air flow the holes all appear dark/black when everything else is glowing hot.  If there is not enough air flow then the outlet holes (at least at the surface) are glowing just like everything else in the forge.  When that is the case it's only a matter of time before the plenum starts to warm and backfiring will commence.

This is something that is likely to be specific to each burner/forge combination so there's no point in sharing the psi at which this happens.  Suffice it to say that the higher pressure you are running the less likely it is that your plenum will heat up and backfiring will begin.  For that reason imho it's always a good idea to try to match your burner output with the size of your forge.  If you have too much burner for the forge you will be either wasting fuel out the exhaust openings or you will have to turn the burner down so far that it's more likely you'll get backfiring and/or constant combustion inside the plenum.