Heat Transfer Enhancement And Tubesheet Strength Study Of Shell And Tube Heat Exchanger

As having strong structures, high pressure and temperature bearing capability and more maturity in design, manufacture and application,shell-and-tube heat exchangers are the most widely used heat exchangers and play critical roles in many engineering plants. But compared with the new types of heat exchangers, shell-and-tube heat exchangers are less effective in heat transfer. On the other hand, in the shell-and-tube heat exchangers, tubesheet is one of the most important parts and its reliability in strength is critical in the safety operation of the exchangers. Therefore,it is important to study the heat transfer efficiency and tubesheet strength of the heat exchangers.In this paper, after verified by some experiments, numerical simulations were carried out on the fluid flow and heat transfer in shell-and-tube heat exchangers constructed by some special baffles and tubes. Lightweight design method of tubesheet and thermal stresses at the tubesheet of U-tube or float-head heat exchangers were also studied. The main work and conclusions are as follows.(1) Experiments were carried out on the heat transfer and the pressure drop in six shell-and-tube heat exchangers constructed with large-and-small-hole baffles, segmental-large-and-small-hole baffles,traditional single segmental baffles, corrugated tubes and plain tubes.Results show that by comparing with traditional single segmental baffles,large-and-small-hole baffles and segmental-large-and-small-hole baffles have lower flow resistance, although the heat transfer coefficient was decreased a little. By using corrugated tubes, the heat transfer efficiency can be raised by 30?38%.(2) After verified by some experiments, systematic numerical simulations were conducted on the fluid flow and heat transfer of the six heat exchangers. It is found that with large-and-small-hole baffles or segmental-large-and-small-hole baffles, the longitudinal flow on the shell-side of the heat exchangers are significantly raised. The jet flow near the large hole can effectively enhance the heat transfer and the field synergy. The dead-flow zones are also decreased. The turbulence of fluid flow in the corrugated tube is higher, and the field synergy is better,which significantly enhanced the heat transfer. If using the ratio of heat transfer coefficient and pressure drop to measure the performance of heat exchangers, the heat exchanger with corrugated tube and segmental-large-and-small-hole baffles have the best properties. This kind of heat exchangers will be of the vast application in engineering.(3) Lightweight design of the tubesheet of a high pressure U-tube heat exchanger was carried out based on the design-by-analysis method and finite element analysis. It is found that by reasonably establishing parameterized paths, the stress classification method according to the design-by-analysis can be directly embedded in the optimization design to improve the efficiency of lightweight design of the tubesheet. The results also show that the thickness calculated by design-by-rule method is too conservative and with the method proposed in this paper, the thickness of the tubesheet studied here can be thinned by 31% . The key factor that determine the thickness of the tubesheet is the membrane plus bending stress intensity at the center of the tubesheet. Compared with the membrane stress, the membrane plus bending stress intensity of the tubesheet is more sensitive to the tubesheet thickness.(4) Thermal analysis for the temperature distribution field at the tubesheet in heat exchanger was carried out numerically. Results show that the distribution of temperature field at the tubesheet is mainly controlled by the heat transfer inside tubes. The tubesheet thickness affects the temperature field at the perforated region on the tube-side and the unperforated region. The temperature gradient at the tubesheet is related to the heat transfer coefficients on both tube-side and shell-side.Compared with the heat transfer coefficient on the tube-side, the influence of the heat transfer coefficient on the shell-side is more significant. The thermal contact resistance between tubes and tubesheet will lead to the decrease of the temperature gradient on the perforated region of the tubesheet.(5) Based on the thermal analysis, the thermal stress field at the tubesheet of U-tube or floating head heat exchangers was numerically simulated and the stress distribution with its influence factors was analyzed. Specifically, the tubesheet thickness and the heat transfer coefficients on both tube-side and shell-side were studied for their effects on the thermal stress at the tubesheet. With sufficient numerical results,regressed formulas for evaluating the thermal stress at the surface center of the tubesheet were obtained to facilitate the engineering application.