Development of Plate Heat Exchangers as Main Components of an Absorption Chiller

Abstract: Absorption chillers are energy saving, but low-capacity applications are not available yet. To become competitive with the established vapor-compression chillers, compact and inexpensive main component heat exchangers are required. In the respective capacity range, the conventionally used tube bundle heat exchangers can hardly fulfill that requirement - plate heat exchangers, however, could. The present work evaluates cycle and heat exchanger concepts for a four kilowatt lithium-bromide absorption chiller based on compact plate heat exchangers. The preliminarily planned external temperatures are 15/10C, 30/38C and 90/80C. Cycle calculations show: Recirculation provides enough solution to permit good wetting of the absorber. Two concepts perform similarly. At the generator, recirculation requires an additional pump. Alternatively, the generator could operate as a pool-generator, but only if the external temperatures are adopted. Making evaporator/absorber and condenser/ generator pairwise equal in size results in a low overall heat exchanger area increase. Consequently, two heat exchangers can be pressed on one plate, which lowers the cost and allows to make the chiller more compact. CFD-simulations of different plate heat exchangers show: Lowering the plate width, which is favorable as to vapor pressure loss, hardly impacts the water side. Rotating the channels towards the horizontal promises better wetting on the solution side, but increases the pressure loss on the water side. Smaller channels between the water channels supposedly increase wetting, too. Moreover, they increase the heat exchange performance and reduce the pressure loss on the water side. Integrating the wall of the pressure vessel and the solution distribution system into the heat exchanger plates is an approach to further reduce the manufacturing costs and size of the absorption chiller.