Heat exchanger assembly, comprises a first heat exchanger being a high pressure heat exchanger and a second heat exchanger being a low pressure heat exchanger, the first heat exchanger and the second heat exchanger forming a main heat exchanger in a cryogenic air separation plant.and a subcooler; a first heat exchanger cold box, for accommodating the first heat exchanger and heat exchange fluid pipelines, with a first opening being disposed in a side of the first heat exchanger cold box (1, 1', 1"), and a first group of pipelines extending through the first opening; a second heat exchanger cold box (2, 2', 2"), for accommodating the second heat exchanger and heat exchange fluid pipelines, with a second opening being disposed in a side of the second heat exchanger cold box, and a second group of pipelines extending through the second opening; a subcooler cold box (3, 3', 3"), for accommodating the subcooler and heat exchange fluid pipelines, with a third opening (4, 4', 4") and a fourth opening (5, 5', 5") each opening being disposed in a side of the subcooler cold box, and a third group of pipelines and a fourth group of pipelines extending through the third opening and the fourth opening respectively, wherein the first group of pipelines and the third group of pipelines are connected and encapsulated in a first thermally isolating casing, and the second group of pipelines and the fourth group of pipelines are connected and encapsulated in a second thermally isolating casing characterised in that the first and second heat exchangers and the subcooler are all plate fin heat exchangers, the subcooler is a heat exchanger that is separate from the main heat exchanger and formed by one or more heat exchanger units, adapted to use one or more cold-state fluids from a low-pressure column of the air separation plant to cool one or more fluids from a high-pressure column of the air separation plant, and connected to send these cold-state fluids to the main heat exchanger, to cool a hot fluid and the first and second thermally isolating casings are packed with thermally isolating material. a second heat exchanger cold box (2, 2', 2"), for accommodating the second heat exchanger and heat exchange fluid pipelines, with a second opening being disposed in a side of the second heat exchanger cold box, and a second group of pipelines extending through the second opening; a subcooler cold box (3, 3', 3"), for accommodating the subcooler and heat exchange fluid pipelines, with a third opening (4, 4', 4") and a fourth opening (5, 5', 5") each opening being disposed in a side of the subcooler cold box, and a third group of pipelines and a fourth group of pipelines extending through the third opening and the fourth opening respectively, wherein the first group of pipelines and the third group of pipelines are connected and encapsulated in a first thermally isolating casing, and the second group of pipelines and the fourth group of pipelines are connected and encapsulated in a second thermally isolating casing characterised in that the first and second heat exchangers and the subcooler are all plate fin heat exchangers, the subcooler is a heat exchanger that is separate from the main heat exchanger and formed by one or more heat exchanger units, adapted to use one or more cold-state fluids from a low-pressure column of the air separation plant to cool one or more fluids from a high-pressure column of the air separation plant, and connected to send these cold-state fluids to the main heat exchanger, to cool a hot fluid and the first and second thermally isolating casings are packed with thermally isolating material.