Shell and tube exchangers design

Conventional shell and tube exchanger designs are used, with one shell pass and two tube passes, when the process fluid is on the shell side and one shell and one tube pass when it is in the tubes. High tube velocities are used to reduce fouling, 3-9 m/s. 

Another example from oil refineries is in crude preheat service with vacuum resid again on the shell side (with the helical baffles). Once again, similar results are seen as described above, with less fouling, reduced rate of increase in pressure drop, and better maintenance of heat-transfer coefficient as compared to the conventional shell-and-tube exchanger design. Our more direct experience of this comes from current practice in the United States, but we have also seen evidence of similar applications in Australia, as discussed in a recent article on the subject of crude preheat exchanger train redesign. ... 

Induced-draft units are less likely to recirculate the hot exhaust air, since the exit air velocity is several times that of the forced-draft unit. Induced-draft design more readily permits the installation of the aircooled equipment above other mechanical equipment such as pipe racks or shell-and-tube exchangers. 

The shell-and-tube exchanger is by far the most common type of heat exchanger used in production operations. It can be applied to liquid/liquid, liquid/vapor, or vapor/vapor heat transfer services. The TEMA standards dcTine the design requirements for virtually all ranges of temperature and pressure that would be encountered in an oil or gas production facility. 

Engineering thermal design of heat transfer equipment is concerned with heat flow mechanisms of the following three types—simply or in combination (1) conduction, (2) convection, and (3) radiation. Shell and tube exchangers are concerned primarily with convection and conduction whereas heaters and furnaces involve convection and radiation. 

The procedures for designing exchangers using the finned tubes are generally specific to the types of fins under consideration. The 16 and 19 fms-per-in. lowfm tubes (Figure lO-lOA and lO-lOB) are uniquely adaptable to the conventional shell and tube exchanger (see Table 10-39) and are the type of tubes considered here. 

The simplest and cheapest type of shell and tube exchanger is the fixed tube sheet design shown in Figure 12.3. The main disadvantages of this type are that the tube bundle cannot be removed for cleaning and there is no provision for differential expansion of the shell and tubes. As the shell and tubes will be at different temperatures, and may be of different materials, the differential expansion can be considerable and the use of this type is limited to temperature differences up to about 80°C. Some provision for expansion can be made by including an expansion loop in the shell (shown dotted on Figure 12.3) but their use is limited to low shell pressure up to about 8 bar. In the other types, only one end of the tubes is fixed and the bundle can expand freely. 

The usual practice in the design of shell and tube exchangers is to estimate the true temperature difference from the logarithmic mean temperature by applying a correction factor to allow for the departure from true counter-current flow ... 

Design a shell-and-tube exchanger for the following duty. 

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. 

This section covers the design of shell and tube exchangers used as condensers. Direct contact condensers are discussed in Section 12.13. 

The approximate method given below can be used to size an exchanger for comparison with a shell and tube exchanger, and to check performance of an existing exchanger for new duties. More detailed design methods are given by Hewitt et al. (1994) and Cooper and Usher (1983). 

The design procedure is similar to that for shell and tube exchangers. 

Emerson, W. H. (1973) Conference on Advances in Thermal and Mechanical Design of Shell and Tube Exchangers, NEL Report No. 590. (National Engineering Laboratory, East Kilbride, Glasgow, UK). Effective tube-side temperature in multi-pass heat exchangers with non-uniform heat-transfer coefficients and specific heats. 

Design a shell and tube exchanger to heat 50,000 kg/h of liquid ethanol from 20°C to 80°C. Steam at 1.5 bar is available for heating. Assign the ethanol to the tube-side. The total pressure drop must not exceed 0.7 bar for the alcohol stream. Plant practice requires the use of carbon steel tubes, 25 mm inside diameter, 29 mm outside diameter, 4 m long. 

The shell-and-tube exchanger is the most common type of exchanger used in the chemical and process industries. The basic heat exchanger design equation was developed in Chapter 15 ... 

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