TECHNICAL AND ECONOMIC OPTIMIZATION FOR AN “AIR COOLER” HEAT EXCHANGER, INVOLVING THE USE OF INTERNAL MECHANICAL “TWISTED TAPE” INSERTS

A fluid flowing through a tube does not have a uniform velocity profile because the fluid portion near the wall flows more slowly than the portion in the core, due to wall drag friction. In “air cooler” heat exchangers, this phenomenon is more pronounced, because the fluid fillets near the wall /close to the internal surface of the tubes, i.e. inside the “boundary layer”, are colder and thus more viscous. The boundary layer effect may be mitigated by promoting greater mixing of the fluid through the introduction of mechanical inserts, known as “static mixers”, inside the tubes. The type of insert examined in this paper is a helical tape, better known as “twisted tape”. Adoption of this technical solution generally brings a number of significant economic benefits, in terms of reduced heat transfer surface and, consequently, a reduction in the number of tube bundles, as well as in electrical power consumption, plot area, weight and foundation loads of the equipment. Another significant benefit is the reduction in fouling inside the tubes caused by particulate deposits, resulting in a lower frequency of routine maintenance. This paper presents a technical and economic analysis/comparison in terms of heat transfer surface, weight, electrical power consumption, plot area and cost of four different design solutions featured by the same project input data. The first solution is based on an air cooler without the use of twisted tapes, while the only variation between the other three solutions is represented by the twist ratio of the static mixers introduced inside the tubes of the air cooler.