This project report presents the application of Kern's Method for the optimal design of shell-and-tube heat exchangers. A primary objective in the heat exchanger design is the estimation of the minimum heat transfer area required for a given heat duty, as it governs the overall cost of the heat exchanger. Since many discrete combinations of the design variables are possible, the design engineer needs an efficient strategy in searching for the optimum design of heat exchanger with minimum cost and maximum operating efficiency.
In the present study, we have tested many design configurations obtained by varying the design variables viz. outer tube diameter, tube pitch, tube length, number of tubes, number of shell and tube passes and heat transfer area. Kern's method is used to find the optimum heat transfer area for a given set of constraints. For a case study taken up, it is observed that this method has a simple evolution strategy, significantly faster and accurate enough for preliminary design calculations. It is also used for designs where uncertainty in other design parameters and it is such that the use of more elaborate methods is not justified.
A heat exchanger is process equipment used for transferring heat from one fluid to another fluid through a separating wall. Usually heat exchangers are classified according to the functions for which they are employed. The most widely used heat exchanger is the Shell & Tube heat exchanger. It consists of parallel tubes enclosed in a shell. One of the fluid flows through the shell & the other flows through the tubes. The one, which flows through the shell side, is called as shell side fluid & the one flowing through the tubes is called as tube side fluid. "When none of the fluid condenses or evaporates, the unit is called as Heat Exchanger." In this only the sensible heat transfers from the one fluid to another.
This project report presents the application of Kern's Method for the optimal design of shell-and-tube heat exchangers. A primary objective in the heat exchanger design is the estimation of the minimum heat transfer area required for a given heat duty, as it governs the overall cost of the heat exchanger. Since many discrete combinations of the design variables are possible, the design engineer needs an efficient strategy in searching for the optimum design of heat exchanger with minimum cost and maximum operating efficiency.
[...] Deficiencies of the segmented baffle include the potential for dead spots in the exchanger and excessive tube vibration. Baffle enhancements have attempted to alleviate the problems associated with leakage and dead areas in the conventional segmental baffles. The most notable improvement has resulted in a helical baffle as shown in Figure 2. Van der Ploeg and Master17 describe how this baffle is most effective for high viscosity fluids and provide several refinery applications. The author further describes how the baffles promote nearly plug flow across the tube bundle. [...]
[...] Start with one shell pass and two tube passes. [pic]ºC [pic] [pic] Ft = 0.88, which is acceptable ΔTm = 0.88 x 80.7 = 71.0 ºC STEP 5: Heat Transfer Area [pic]m2 STEP 6: Layout and Tube Size Using a split-ring float head heat exchanger for efficiency and ease of cleaning. Neither of the fluid is corrosive and the operating pressure is not high, so plain carbon steel can be used for the shell and tubes. The crude is dirtier than the kerosene, so put the crude through the tubes and the kerosene in the shell. [...]
[...] Modified Design The tube velocity needs to be reduced. This will reduce the heat transfer coefficient, so the number of tubes must be increased to compensate. There will be a pressure drop across the inlet and outlet nozzles. Allow 0.1 bar for this, a typical figure (about 15 % of the total): which leaves 0.7 bar across the tubes. Pressure drop is roughly proportional to the square of the velocity and ut is proportional to the number of tubes per pass. [...]
[...] Length: The preferred lengths of tubes for heat exchangers are 6, 8, 12,16, 20, 24 ft. the optimum tube length to shell diameter ratio falls within the range of 5-10. Pitch: The recommended tube pitch i.e. the distance between centre of two tubes is 1.25 times the tube outside diameter. For a square pattern, the recommended minimum clearance between the tube is 0.25 inch (6.4 mm). Tube-side passes: Heat exchangers are generally built with from one up to about sixteen passes. [...]
[...] • Excessive temperatures in heat exchangers. • Lack of control of heat exchangers' atmosphere to retard scaling. • Increased product temperature over a safe design limit. • Unexpected radiation from refractory surfaces. • Unequal heating around the circumference or along the length of tubes. Temperature Profile Distortion An important issue that has not been considered so far is the temperature profile distortion. As noted earlier, the leakage and bypass streams are less efficient for heat transfer than the main cross-flow stream. [...]
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