Examples heat exchanger design

The solution to this example illustrates the iterative nature of heat exchanger design calculations. An algorithm for the design of shell-and-tube exchangers is shown in Figure A (see p. 684). The procedure set out in this figure will be followed in the solution. 

At the first level of detail, it is not necessary to know the internal parameters for all the units, since what is desired is just the overall performance. For example, in a heat exchanger design, it suffices to know the heat duty, the total area, and the temperatures of the output streams the details such as the percentage baffle cut, tube layout, or baffle spacing can be specified later when the details of the proposed plant are better defined. It is important to realize the level of detail modeled by a commercial computer program. For example, a chemical reactor could be modeled as an equilibrium reactor, in which the input stream is brought to a new temperature and pressure and the... 

EXAMPLE 11.2 OPTIMAL SHELL-AND-TUBE HEAT EXCHANGER DESIGN... 

Example 11.2 Optimal Shell-and-Tube Heat Exchanger Design 422... 

Example 8.12 summarizes the results of such calculations made on the basis of data in Heat Exchanger Design Handbook (1983). Procedures for the design of kettle, thermosiphon and forced circulation reboilers also are outlined by Polley (in Chisholm, 1980, Chap. 3). 

Although final heat-exchanger designs will be made on the basis of careful calculations of U, it is helpful to have a tabulation of values of the overall heat-transfer coefficient for various situations which may be encountered in practice. Comprehensive information of this sort is available in Refs. 5 and 6, and an abbreviated list of values of U is given in Table 10-1. We should remark that the value of U is governed in many cases by only one of the convection heat-transfer coefficients. In most practical problems the conduction resistance is small compared with the convection resistances. Then, if one value of h is markedly lower than the other value, it will tend to dominate the equation for U. Examples 10-1 and 10-2 illustrate this concept. 

Calculate the pressure drop for the water flowing through the air-cooled heat exchanger designed in Example 7.37 if the number of tube-side passes is 10. The density of the water is 60 lb/ft3 (961.1 kg/ m3), and the viscosity is 0.74 lb/(ft)(h) (0.31 cP). Assume that the velocity in the nozzles is 10 ft/s (3.05 m/s) and that the viscosity change with temperature is negligible. 

A specific example of this can be found in the evaluation of composites for heat exchanger applications.13 In this case, the heat exchanger design calls for a tubular construction which will be pressurized. Under these conditions, a flexural stress will be present in service, and consequently, a C-ring test specimen configuration provides a reasonable way to examine the properties of the composite. 

Optimize the heat exchanger design of Example 4.5 to minimize the total surface area required. 

Fig. 1.24 shows a particularly simple heat exchanger design, a coiled tube inside a vessel, for example a boiler. One fluid flows through the tube, the other one is in the vessel and can either flow through the vessel or stay there while it is being heated up or cooled down. The vessel is usually equipped with a stirrer that mixes the fluid, improving the heat transfer to the coiled tube. 

Process integration and system synthesis require a skillful manipulation of system components. For example, heat exchanger network synthesis requires the utilization of very specialized methods of analysis [111, 112]. The search for an efficient system operation requires a multidisciplinary approach that will inevitably involve simultaneous utilization of heat transfer theory and thermal and mechanical design skills as well as specific thermodynamic considerations and economic evaluation. The optimal design of a system cannot be achieved without careful thermo-economic considerations at both system and component (i.e., heat exchanger) levels. 

The inside diameter of the shell is used as the reference dimension for heat exchanger design. For example, the unit shown in Figure 2-4 has a shell with an outside diameter of 20 inches and an inside iameter of 19-1/4 inches. These are the dimen-sons of 20-inch, Schedule 20 pipe, which is .eaper and easier to use than flat steel plate which wDuld have to be rolled into a cylinder and welded together to form the shell. 

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