From what I have seen, when in running condition, the tank needs to be filled at around 40%, and minimum level when stopped should be 60%, according to the SOPs where I have worked (But this is an emergency system, so you can keep it higher). So, that means that the volume of all the pipes...
Orifice can't give you control. It can just hold pressure in your tube. As pointed out above, a butterfly valve will give you decent control. What I wanted to point out is that your discharge looks too high. In your excel sheet, you have calculated 7.8 kg/s of air (8000m3/hr of hot air) flowing...
