After my first post / question above I made a deep dive into this forum and have seen many design errors with my forge
There is a sliding valve after the blower, to be able to limit the air flow, however, air blast direction must be changed. I have seen that Japanese charcoal forge designs, tuyere pipe makes approximately 30 degrees angle downwards and connects the forge wall horizontally.
As "Irondragon ForgeClay Works" mentions, charcoal size must be smaller. Wallnut-peanut size would be better. I've learned from the forum that the iron I'm trying to heat needs to mate not with the flame but with the charcoal itself. So smaller size is better, it means more surface contact.
Answer to "Irondragon ForgeClay Works"'s question, air gate valve can be shut off completely as needed. It is an easy to operate valve and I shut it off just before working on anvil. And thanks your compliment for the forge.
Charles, I'll completely change the design. As you said, air direction is so wrong for charcoal ( and especially charcoal that big) almost all the ash gone with the air. After burning around 20 kg's of charcoal in a batch, just a handful of ash piled in the ash gate
And Ok, I forget that firebrick issue. At least bricks won't have a direct contact with the fire. I'll build the new pot with 10mm steel (3/8"). Deep enough to accommodate enough properly sized charcoal. Additionally find a speed controlled blower. Some designs reduce the tuyere pipe diameter by half where pipe just enters the pot, to increase the air speed. Would you recommend this?
Thanks everyone for advices.
BTW, 20kg of charcoal for forging/bending 54 pieces of holdfasts from 3/4" St-37 steel. I have no idea if this amount of charcoal is to much for this project.