"入口集合管" can be translated as "inlet header (pipe)" or "inlet collecting pipe" in English.
"Inlet" refers to the entrance or opening through which something (such as a fluid) enters.
"Header" or "collecting pipe" in this context refers to a pipe or duct that collects and distributes the incoming fluid to multiple branches or outlets.
The choice between "header" and "collecting pipe" may depend on the specific context or industry terminology. In some engineering or technical fields, "header" might be more commonly used, while in others, "collecting pipe" could be more appropriate.