"主铁水沟" can be translated as "main iron runner" or "main iron channel" in English.
"Main iron runner" is a more technical and commonly used term in the metallurgical industry, referring to the primary channel through which molten iron flows.
"Main iron channel" is also a valid translation, conveying a similar meaning but perhaps with a slightly broader connotation.
In most contexts, especially within professional or technical discussions, "main iron runner" would be the preferred choice.