Here goes,

You need to put your air into a plenum that you divide to go to 2 places in your arch,then by using dampers to control your airflow to those 2 areas being your grates and your top air. Duct work is built into the arch to carry air to the front to cool and add air to the frame around the door and the door itself. The frame work is usually made out of 3x3 box steel with black iron pipe for nozzles and I use blanket insulation to insulate it so that it doesn't burn the steel. I transfer the air to the door through the hinge that is basically one pipe inside another which has been lathed to fit of which there are 2 of these both top and bottom. The door has a air chamber on the outside to allow air to get to the whole door for cooling purposes. On the inside there is 3 layers of 2600 degree blanket insulation, there are nozzle coming from the air chamber to the fire on 6 inch squares. On the door frame across the top there are air nozzles pointed down at approx. 20 degrees every 3 inches.

The back of the firebox has vertical wall going to the bottom of the flues at 2/3's the total length of the pans. basically rectangular box. Just under the pans, pointed forward are three rows of nozzles built into an air chamber that comes from the same duct work that runs the front air. These nozzles are welded in on 3 centers.

The other air goes to the grates that I make out of firebrick by drilling 3 - 3/4 inch holes in each one of which I have very good luck with. The reason I don't use the cast grates is simply they have to many and to big of holes in them which puts to much air to the bottom of the fire making it very inefficient. A good arch such as the above mentioned Hurricane or the intens-o-fire only put 25 percent of the total air to the fire with the remaining 75 percent going over the fire. The bottom fed forced air units will boil just as hard but their would pile will be smaller at the end of the year.

You have asked me something that is easy for me to build but hard for me to explain as there is shortage of brainwaves between my head and fingers. I hope this helps.

I also use high pressure blowers to run the arch based on 15 to 20 cfm per square foot of arch depending on how many restrictions in the duct work system.