Patent GB-575177-A (GB)
Title: Improvements in or relating to heat exchange devices
IPC: F28F3/02
Filed: 1944-02-28
Abstract: 575,177. Soldering radiators. STEADMAN, C. S., and IMPERIAL CHEMICAL INDUSTRIES, Ltd. Feb. 28, 1944, Nos. 3625/44 and 3278/45. [Classes 83 (ii) and 83 (iv)] In making heat exchangers of the kind comprising a number of thin walled tubular elements extending either in side-by-side relation to define two series of parallel passages or arranged alternately with a number of transverse corrugated plate like elements to define two series of passages extending in different directions or comprising a number of tubular elements provided with transverse fins, the elements are assembled and the tubular elements subjected to a pressure difference as between their respective internal and external surfaces, which is maintained whilst the elements are bonded by a furnace brazing operation, during which a non-oxidising atmosphere is maintained within the. tubular elements. The pressure difference may be produced by connecting one end of each of the tubular elements to a source of non-oxidising gas under pressure, such as nitrogen, hydrogen or cracked and burnt ammonia, while the other end is temporarily sealed or constricted or connected to a valved conduit or by evacuating the elements whilst admitting a limited flow non-oxidising gas into the elements through through a constricted opening therein or through a valve. In a heat exchanger such as shown in which flat tubes 1 containing longitudinally corrugated strips 2 are passed through apertures 4 in header plates 5 and engage intermediate corrugated elements 3, the elements 2 may be first brazed to the tubes 1 by brazing &c. material of higher melting point than that which subsequently unites the tubes 1 to the elements 3. Alternatively if the elements 2 are united last, brazing material of lower melting point is used in this operation. The elements 1, 2 may be of steel coated with brass, while the elements 3 may be of steel coated with silver copper eutectic alloy. Before uniting any of the elements to each other, the tubes 1 are first assembled in the apertures 4 in the header plates 5 and are united therein by copper brazing. Temporary headers 6 are similarly united to the header plates and the assembly placed in a brazing furnace 8 and heated to melt the brass but not the copper. During this operation an atmosphere of cracked and burnt ammonia is maintained in the furnace, and also in the tubes 1 at a pressure lower than the furnace atmosphere by connecting one of the pipes 7 projecting from the headers 6 to a vacuum pump whilst the other pipe is connected to an extension 7a terminating within the furnace so that a constant flow of the gas is sucked from the furnace through the tubes, the rate being controlled by a valve 9. When the tubes and internal elements have thus been joined, the elements 3 are inserted in position and the assembly placed in the furnace again and heating effected at 850‹ C. while the tube 7 is connected to a source of gas under pressure. The valve 9 'now contacts the pressure head within the tubes which are thus expanded into pressure contact with the elements 3 and gas is admitted into the furnace to maintain the required atmosphere. The silver copper alloy melts and unites the parts 1 and 3. Suitable combinations of brazing media for the two operations may be cupronickel and copper, copper and brass, brass and silver solder, and may be in the form of foil interposed between the parts or in the form of an adherent coating thereon. According to the first Provisional Specification the difference of pressure between the internal and external surfaces may also be obtained by partially or completely evacuating the exterior side or by temporarily sealing air, non-oxidising gas or volatile liquid within the elements at atmosspheric pressure and allowing the bonding heat to create the pressure within the elements. Also the bonding may be effected by soldering. According to the second Provisional Specification in the case in which different parts are bonded in different operations silver solder may be used in the first operation and soft solder in the second operation. Specification 5599 /02, [Class 83], is referred to.