Dynamics of a shell-and-tube heat

HEATEX - Dynamics of a Shell-and-Tube Heat Exchanger... 

Gas-Liquid Mixing and Mass Transfer 385 Dynamics of a Shell-and-Tube Heat Exchanger 511 Semi-Batch Manufacture of Hexamethylene-triamine 353... 

Guideline 8 The manipulated variables should affect the controlled variables directly, rather than indirectly. Compliance with this guideline usually results in a control loop with favorable static and dynamic characteristics. For example, consider the problem of controlling the exit temperature of a process stream that is heated by steam in a shell and tube heat exchanger. It is preferable to throttle the steam flow to the heat exchanger rather than the condensate flow from the shell, because the steam flow rate has a more direct effect on the steam pressure and on the rate of heat transfer. 

The determination of the process mathematical model is often the most difficult and time-consuming step in control system analysis. This is a result of the dynamic nature of the process in other words, how the system reacts during upsets or disturbances. The problem is further complicated by process nonlinearities and time-varying parameters. To illustrate the modelling procedure we will look at developing a model for a shell and tube heat exchanger with temperature control [7], shown in Figure 3.29. 

In this chapter different types of heat exchanger will be analyzed for their dynamic behavior. The first type is the shell and tube type, where steam condenses inside the tubes and the contents of a well-mixed tank or reactor have to be heated. The second type is also a shell and tube type heat exchanger, where the steam condenses outside the tubes (shell-side) and the liquid to be heated flows through the tubes. The last type is the countercurrent heat exchanger, in which the liquid to be heated flows countercurrent to the heating medium. 

Eluor Daniel has the ability to perform a heat exchanger tube rupture transient analysis consistent with the method referred to in RP-521 ("Model to Predict Transient Consequences of a Heat Exchanger Tube Rupture," by Sumaria et ah). This methodology accounts for effects such as the inertia of the low-pressure liquid, the compressibility of the liquid, the expansion of the exchanger shell or tube chaimels, and the relief valve dynamics. Dynamic simulation can be used to meet the following objectives ... 

At each point in the heat exchanger, heat passes from the hotter fluid to the tube wall, and then from the tube wall to the cooler fluid. The tube wall will have its own dynamic response, and, just as it separates physically the two fluids, so it separates mathematically the calculations of the two fluid temperatures. Heat will also flow between the shell fluid and the shell wall, which will normally be heavily insulated to prevent heat flow to the environment. The shell wall will be relatively massive and cause a signiflcant slowing of the response of the shell-side fluid outlet temperature. 

Loop seal. In some low-pressure steam reboilers the condensate pot is replaced by a loop seal (Fig. 17.Ih). In this arrangement, increasing the flow to the reboiler raises the pressure in the reboiler shell, which in turn lowers the liquid level in the reboiler and exposes more tube area. The dynamics of this system are similar to that of the condensate outlet scheme. The height of liquid in the loop is typically 5 to 10 ft. The system can be troublesome when the reboiler heat load or the steam mains pressure tend to fluctuate, and it is usually best avoided. 

However, this model can only be realistically used when the reboiler heating medium is a hot liquid stream, and the holdup of this liquid on the shell or tube side is significant. If the reboiler is heated with a condensing vapor, which is much more frequently the case, this dynamic model is not applicable. 

The cryostat is shown schematically in Fig. 2. In order to eliminate the necessity for dynamic cold seals exposed to LH2, the load-carrying wall concept was utilized. The cryostat outer shell is fabricated from 304 stainless steel tubing of 4.50 in. OD with a 0.125 in. wall thickness. The inner shell is made from the same material, with a 3.00 in. OD with a 0.083-in. wall thickness. The purpose of the thin (2.50 in. diameter x 0.020 in. wall) tube inside the cryostat is to present a longer heat path and increase the efficiency of the apparatus with respect to the amount of cryogenic fluid consumed. The inner shell is wrapped with aluminum foil for reflective insulation, and the space between the two shells is evacuated through a high-vacuum seal-off valve provided in the outer shell. 

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