Experimental analysis of a finless gas-liquid heat exchanger

Water heating for domestic applications is a major component in energy usage. Amongst the various technical alternatives, instant water heating represents one of the alternatives available. In this, no storage is present and hot water is produced on demand. In addition to an automatic ignition system (usually driven by a pressure differential resulting from the water flow), the boiler includes a heat exchanger and a burner. Their power band is within the range of 19 through 30 kW and it can provide a temperature differential of up to 50 oC. The heat exchanger is paramount to the optimal performance of the unit because of: the heat transferred to the water (a minimum net efficiency of 84% is required) and of the maximum admissible gas side pressure drop. This restriction is of high relevance for the low cost units where the draft of combustion gases occurs by natural convection. Usually the heat exchanger is of the finned tube in cross flow variety where the water is preheated by a pass along the combustion chamber walls. The material of choice is copper which results in a heavy and potentially expensive construction. There is a strong drive to find alternatives to this universally adopted design involving a different configuration and new materials. This paper describes the design and testing of a gas-liquid cross flow heat exchanger. The design consists of thin wall stainless steel tubes, arranged in 80 tubes staggered in 6 rows; the water flows in two passes. Such an arrangement did not required fins. A prototype was manufactured and tested with a natural draft combustor according to the applicable normative. The results show that by appropriate selection of the tube dimensions and layout the conflicting criteria of low pressure drop and high heat transfer rate compared favorably with the traditional design. The overall thermal efficiency was in excess of 86%.