Possibilities of Intensifying Heat Transfer in Heat Exchangers for High Temperature Applications

A high temperature heat transfer application actually represents the case of a heat exchanger operated within a process with high temperature. In every industrial domain, a different value of temperature may be considered "high". We are active in the field of chemical, petrochemical, waste-to-energy, power and process energy recovery heat transfer applications. In these applications a tube-fin exchangers are successfully used for gas or liquid and/or aggressive fluids with temperatures up to 350 and/or 400 degrees C. They are also frequently used in combustion systems with air preheating applications. Tubular heat exchangers, especially those with U-tubes, helical and straight tubes are most frequently used for high-temperature applications with working temperatures above 650 degrees C. Extended surfaces are used as an intensification approach to decrease the area requirements on flue gas side. Selection of an extended surface depends on type of fuel. In the case of combustion of fuels producing flue gas with fouling tendency, studded tubes are preferred. More efficient finned tubes may be used if fuel burnt produces relatively clean flue gas. Generally enhanced surfaces are used for gaseous media with low heat transfer coefficient. Fins substantially enhance the heat transfer area and consequently heat duty of the equipment. Improving heat transfer performance is commonly referred to as heat transfer enhancement. Enhancement is usually represented by increasing the (film and overall) heat transfer coefficient by so called "passive" (surface extension) or "active" (increasing fluid turbulence) way. This paper presents a possible selection of novel types of longitudinally finned tubes intensifying the heat transfer utilizing both passive and active principles. It means that fins not only increase heat transfer area but also make the fluid flowing around fin to change the flow direction, i.e., to increase the turbulence. This allows increasing the film heat transfer coefficient on fin-side.