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Abstract
Perforated plate heat exchangers (PPHEs) are made of alternately arranged high thermal conductivity perforated plates and low thermal conductivity insulating spacers. Fabrication of this type heat exchanger needs perfection in diffusion bonding. Copper (Cu) and stainless steel (SS) being the most commonly used materials for PPHE, sample studies have been done on diffusion bonding between Cu and SS under varying nickel interlayer thickness and bonding conditions. Based on these studies, the parameters such as nickel interlayer thickness, temperature, pressure, duration of bonding etc. are perfected. The other aspects such as cleaning, handling and stacking of the large number of plates and spacers inside the vacuum hot press unit are also discussed in the paper. Several heat exchangers have been fabricated and pressure tested for leak tightness. Main aim of this paper is to provide necessary guidelines to the manufacturers of PPHEs.
Keywords: Cu-SS diffusion bonding, fabrication of perforated plate heat exchangers
1. Introduction
Perforated plate heat exchangers (PPHEs) (Figure 1) are made of a stack of alternate layers of high thermal conductivity perforated plates and low thermal conductivity insulating spacers. Perforated plates and spacers are so ananged and bonded that a set of holes allows hot fluid to flow while the other set of holes makes the passage for the cold fluid. The plates receive heat from the hot fluid by convection and transfer the same by lateral conduction and by convection to the cold fluid. Spacers minimize the heat loss due to axial conduction and reduce the effect of flow mal-distribution by continuous reheadering between the plates.
Originally invented by McMahon et al.(1950), PPHEs come mider the category of compact heat exchanger. They have large heat transfer surface area per miit volume (as high as 6000m2/m '). high effectiveness and high porosity (Fleming, 1969). By adopting proper materials and fabrication technique, PPHEs can be used from a very low temperature (<4 K) to a temperature as high as 1000 K. PPHEs are used in many applications such as cryogenic refrigeration, power engineering, chemical industries etc. Because of their compactness and high performance, PPHEs are also suitable for flight applications. For example, this type of heat exchangers can be used in reverse Brayton cycle based cryocoolers which are used in spacecrafts for cooling...