A heat exchanger design is described which includes header plates, apertures, seals and flow tubes. A plurality of apertures in at least one header plate are configured to allow a flow tube to be inserted into or removed from the aperture as a whole at an angle. A seal is included within the aperture and is supported by differing diameters of the aperture wall. The seal engages the flow tube in a manner to allow high pressure operation of the heat exchanger when the flow tube is perpendicular to the header plate.