A fluid is cooled and liquefied in an apparatus with a heat exchanger. A plurality of flow passages has two or more primary groups of one or more primary flow passages. Each primary group carries a part of the fluid stream through the heat exchanger and indirectly cools the part of the fluid stream against a refrigerant in the shell side of the heat exchanger to provide a liquefied fluid stream. A primary inlet header connects the two or more primary groups of primary flow passages to a source of the fluid, and is arranged to split the fluid stream between the two or more primary groups of primary flow passages. Valves are provided for selectively blocking at least one of the two or more primary groups of primary flow passages whilst allowing the fluid stream to flow through the remaining unblocked primary groups of primary flow passages.