GB568659A - Improvements in or relating to heat exchange devices - Google Patents

Abstract

568,659. Making heat exchangers. COLTMAN, J. L., and IMPERIAL CHEMICAL INDUSTRIES, Ltd. Oct. 17, 1941, Nos. 13378/41, 16392/41 and 338/42. [Classes 83 (ii) and 83 (iv)] Brazing.-In joining parts of heat exchangers formed of tubular or plate-like elements composed of or provided at least at the joining surfaces with copper or a copper base alloy, a thin layer of metal or alloy melting below 350‹ C. and capable of forming an alloy with the copper &c., is interposed between the surfaces, the parts are assembled and heated in a non-oxidising or reducing atmosphere to a temperature which is below the melting point of the copper &c., but is sufficiently high to cause the layer to alloy with the copper &c. to form a bonding alloy with a melting point not lower than 500‹ C., and then allowing the assembly to cool whilst still in the said atmosphere. The base of a copper &c. coated element may consist of steel or ferrous alloy or other metal having a melting point not lower than the temperature used in forming the joints. The coating may be rolled on the base metal. The interposed metal or alloy may consist of tin, cadmium or their alloys, or alloys of lead, e.g. 50 per cent each of tin and lead, or 95 per cent lead and 5 per cent silver. It may also contain up to 3 per cent of copper. The layer may be 1/50 to 1/5 of the thickness of the elements adjacent the joining surfaces. The layer may be applied electrolytically, by hot dipping, by spraying, as a paste consisting of a comminuted metal in a medium such as a solution of nitrocellulose, or as a thin foil inserted between the surfaces. The atmosphere may comprise nitrogen or hydrogen, or a mixture thereof obtained by admixture, or from cracked ammonia. It may also comprise burnt coal gas which may be previously purified. In one method, the steel sheets may be electrolytically coated with copper then dipped in molten tin and then assembled and heated in an atmosphere of burnt coal gas to 800‹ C. for 10-20 minutes and then allowed to cool in such atmosphere. The parts of a heat exchanger which require liquid-tight joints and non-liquid-tight joints, may be coated adjacent the parts to be liquid-tight with silver instead of with the low melting point metal or alloy which is applied to the other parts. Heating is then effected as in the previous case to a temperature below the melting point of the surface metal, but sufficiently high to bond all the elements together. The silver coating may be effected by electrodeposition or rolling. When the two coatings are to be formed on a single element, the silver coated elements are clamped together and the edges sealed with molten wax or lacquer to prevent access of the electrolyte thereto when electrodepositing tin on the exposed surfaces.