I’m strruglling to find the diameter for the heat exchanger of one pipe as the velocity is unknown, and the mass flow rate should not exceed 20kg/s which is the only parameter i have. Problem assumption dont work as i have to design this. i tried to calculate but i cant seem to get the diameter which i want it to range between 9mm and 12mm as i dont want a bigger heat exchanger. When i try to use the 2m/s velocity i get diameter of 110mm which very big for this cause for tube the diameter should be between 9mm and 12mm. So if anyone can help with this it’ll be appreciated.