Welding, mechanical expansion, or pulsed pressing is generally employed for joining tubes with tube plates in shelland-tube heat exchangers, and welding followed by expansion or pressing is employed in critical heat exchangers [I].With decrease of tube diameter and increase of tube plate thickness the joining process becomes complicated and the reliability of joining declines. The tube and the plate can be joined reliably by high-temperature brazing. However, high-quality joint brazing is hindered due to mismatching of the gap between the tube and the hole in the plate. Mismatch between the hole and tube diameters, deshaping of the tube, and misalignment and warping render formation of matching concentric capillary gap difficult. The probability of gap mismatch increases with increase of tube plate thickness.This defect can be eliminated by vacuum-brazing with the wide brazing gap filled with a metal powder (filler) before brazing ( Fig. I). For this purpose the following conditions must be met;where D is the hole diameter, ram; d I is the diameter of the sealing strip, mm; d is the diameter of the robe, mm; and a is the powder grain size, mm. The first requirement is dictated by the fact that a powder with a grain size larger than 50/zm is suitable as filler. Vacuum-removal of gas from smaller-grained powder is difficult [2]. If the plate is very thick, the powder may be blown out during evacuation or incompletely soldered parts may be formed as the gap is filled with the melted solder. Moreover, narrow gaps are difficult to fill while preparing for brazing. Fulfilment of the second condition ensures closing of the gap with the filler and prevents its efflux from the gap during relocation of the assembled tube bank before brazing.When heated in vacuum to the brazing temperature, diffusion welding occurs at the points of contact between the particles themselves as well as between them and the base metal. In the filler whose particles are spongy and crystalline with sharp projections, diffusion engagement occurs at temperatures above 900~ If the particles are spherical, a welded cage is formed at temperatures above 1000~ Thereupon the stationary cage of the filler particles creates a uniform network of capillary channels for the movement of the solder and prevents appearance of shrinkage defects as the melt crystallizes.For brazing tube banks of high-alloy and carbon steels, iron and iron-nickel alloy powders are suitable as fillers. Preferred are low-oxygen (less than 0.06%) powders that are produced by dispersing the melt with nitrogen or an inert gas [3]. The dispersed powders consist of spherical particles, have a high creep, and are an excellent filler of the brazing gap. Strong, plastic, and dense joints are formed when such a filler is used.The choice of the filler and solder depends on the composition of the base metal and the requirements placed on the heat exchanger. Copper, copper-manganese, copper-nickel, and nickel alloys are good solders for brazing steel tube bands. The compositions of the filler me...

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