Porous media compact heat exchanger unit––experiment and analysis

Abstract

The steadily increasing use of fossil fuel now represents the greatest danger for planet Earth, as a result of the ever-increasing CO 2 emissions in the atmosphere. Development and use of modern technology for combustion of fossil fuels and of very efficient systems for heat exchange (water–smoke gasses) in a useful form permit increased energy efficiency while maintaining or increasing the level of comfort, with decreased emission of CO 2. Successful application of low-temperature (condensate) boilers depends on the heat exchanger (water–smoke gasses), and technical and economical suitability. Construction of the heat exchangers with smooth or ribbed tubes is well known. The tubes are made of high-alloy stainless steel. Applications of low-temperature boilers are limited due to high cost and large dimensions of the heat exchangers. The cost of the heat exchanger is equal to or even greater than the cost of the boilers. Low heat transfer coefficient from smoke gasses on tubes requires a large heat transfer surface, creating the need for the large dimensions of the heat exchanger. Further development can be expected in application of heat exchangers on the basis of porous medium. Development of heat exchangers can go in two directions: as exchangers which are a supplement to existing boilers and those which are an integral part of the boiler itself. The paper describes the boiler with the built-in heat exchanger and the construction of the test rig and measuring equipment that was employed in the study. This work presents the advantages of the burner with a built-in heat exchanger. These are in the high specific thermal load of the porous medium and of the heat exchanger (7 MW/m 3), the extremely good thermal conductivity of the porous medium and very high thermal capacity, the very stable functioning of the burner–heat exchanger, the high efficiency of heat exchange (use of higher heating value fuel). Through increased energy efficiency there were lower CO 2 emissions. Because of these advantages, the system is very applicable to central heating systems for apartment or office buildings, the heating of water for sanitation purposes, etc. In addition, the paper presents the concept of heat exchangers for supplemental installation in existing boilers for use higher heating value of fuel.