Fabrication of wire mesh heat exchangers for waste heat recovery using wire-arc spraying

Abstract

Waste heat can be recovered from hot combustion gases using water-cooled heat exchangers. Adding fins to the external surfaces of the water pipes inserted into the hot gases increases their surface area and enhances heat transfer, increasing the efficiency of heat recovery. A simple method of increasing the heat transfer surface area has been developed by using a twin wire-arc thermal spray system to generate a dense, high strength coating that bonds wire mesh to the outside surfaces of stainless steel pipes through which water passes. At the optimum spray distance of 152 mm (6 in), the oxide content, coating porosity and the adhesion strength of the coating were measured to be 6.6%, 2.1% and 24 MPa respectively for 152 mm spraying distance since they affect the performance of the heat exchanger. Experiments were done in which heat exchangers were placed inside a high temperature oven with temperature varying from 300°C to 900°C. The temperature rise measurement of the water between the inlet and outlet of the heat exchanger was measured for four different water flow rates 0.013-0.032 kg/s (0.2-0.5 GPM). Using an energy balance, the total heat transfer to the water was calculated. Several different heat exchanger designs were tested to estimate the heat transfer due to convection and radiation in each case. The maximum water temperature rise and total heat flux measured were 13°C and 57 kW/m2. The efficiency of heat transfer was found to depend strongly on the quality of the bond between the wire meshes and pipes, and the size of openings in the wire mesh. It was possible to produce significant increases in heat recovery efficiency by using wire meshes.