"入口集合管" can be translated as "inlet header" or "inlet collecting pipe" in English.
"Inlet header" is a more concise and commonly used term in engineering contexts, especially when referring to a pipe or channel that collects incoming fluid (such as in a heat exchanger, boiler, or other similar systems).
"Inlet collecting pipe" provides a more literal translation and might be used in contexts where a more detailed description is needed.
In most technical and engineering discussions, "inlet header" would be the preferred choice.