Investigation of a twisted-tube type shell-and-tube heat exchanger
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This master thesis investigates twisted tube type shell-and-tube heat exchangers with emphasis on thermal-hydraulic characteristics, fouling and vibration properties. An extensive literature study has been carried out in order to map all published research reports written on the topic. The mapping of performed research shows that the available information is limited.Mathematical correlations for twisted tube thermal-hydraulic characteristics are extracted from the research reports found in the literature study. Correlations for convective heat transfer coefficients and pressure loss for both shell side and tube side are presented. The enhancement of heat transfer by swirl flow in a twisted tube bundle is also discussed.Measurements on thermal-hydraulic characteristics are collected from a twisted tube heat exchanger installed at the process facility LNG Hammerfest situated at the northern end of Norway. These measurements are then compared to output from the heat exchanger design software HTRI Xchanger Suite. The calculation accuracy of the twisted tube program module is predicted on this basis.HTRI Xchanger Suite can evaluate heat exchanger design in two different modes. Simulation mode calculates two outlet parameters and thereby the duty, while rating mode calculates only one output parameter where the duty is specified by the user. In rating mode the software has an accuracy of +/-4% for temperature, flow and duty calculations. The overall heat transfer coefficient is miscalculated by approximately +50% and the shell side pressure loss is miscalculated by +40%. In simulation mode the duty is not specified by the user and the program may calculate two output parameters. The predicted accuracy depends on the calculated parameters. Calculation accuracy for two unknown outlet temperatures is +/-7% while calculation accuracy for two unknown mass flow rates is up to +200%. The accuracy on the overall heat transfer coefficient and shell side pressure loss is the same as for rating mode.The StatoilHydro twisted tube heat exchanger 25-HA-113 design on LNG Hammerfest is evaluated with respect to thermal-hydraulic characteristics. 25-HA-113 is operated with other flows and temperatures than it was designed for. By use of a mathematical method the expected performance in design operation point is predicted. By analysing the measured overall heat transfer coefficient and by use of this mathematical method, it is stated that the heat exchanger has insufficient thermal design. However, investigation of the pressure loss in the same manner shows that the unit performs better than specified in the design basis. The fouling characteristic is examined by evaluation of the overall heat transfer coefficient over a time period. This analysis shows that the fouling tendency is minimal.By use of the computer program HTRI Xchanger Suite and field measurements a quantitative comparison of a twisted tube, a helix baffled and a single segmental baffled unit is performed for the application of 25-HA-113. The alternatives are compared to each other by evaluation of heat transfer coefficients, pressure loss, fouling characteristics and vibration risks. The conventional and the helix alternative are equipped with low fin tubes which show a somehow higher heat transfer capacity per volume unit. However, the twisted tube provides a higher overall heat transfer coefficient than the two other alternatives. The single segmental baffle concept appears to have the lowest shell side pressure loss, followed closely by the twisted tube. With respect to vibration risk the twisted tube has a superior performance compared the two competing concepts due to a very rigid bundle construction. By considering the thermal hydraulic characteristics and the vibration risk connected to the evaluated application the twisted tube is the recommended alternative.