Dynamic Analysis of Heat Exchangers

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. 

It would be very difficult to describe the model for a heat exchanger, since there are so marty different types. The most common type is the cooling or heating coil in a tank or reactor, meant to transfer heat in either direction. Inside the coil the temperature varies with time and axial direction outside the coil the temperatiue is usually uniform, since the tank or reactor contents is ustrally stirred. Therefore this type can usually be modeled fairly easily and hnearization of the model can give a good estimate of the dynarrrics of the heat transfer process. 

The second well-known type is the pipe-moimted heat exchanger in which the medium to be heated flows through the pipes and steam condertses outside the pipes. Sometimes this type is called shell and tube heat exchanger. Even though this type resembles the previous type somewhat, its dynamic behavior is different. 

A third type of heat exchanger is a cormtercurrent heat exchanger in which one fluid flows in one direction and another fluid flows in the opposite direction. In this case both fluid temperatures are truly time and location dependent and this is the most corrrplicated case in terms of an analytical treatment. 

There are still other types of heat exchanger however, we will limit the discussion to the three types mentioned. 

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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 ... 

In this initial analysis, the dynamics of the heat exchanger are assumed to be fast compared to those of the reactor, so simple gain transfer functions are used that relate the blended temperature Tm in FS1 or Tmlx in FS2 to the two inputs to the heat exchanger bypass flowrate Fby and reactor exit temperature 7 out ... 

JAEA conducted an improvement of the RELAP5 MOD3 code (US NRC, 1995), the system analysis code originally developed for LWR systems, to extend its applicability to VHTR systems (Takamatsu, 2004). Also, a chemistry model for the IS process was incorporated into the code to evaluate the dynamic characteristics of process heat exchangers in the IS process (Sato, 2007). The code covers reactor power behaviour, thermal-hydraulics of helium gases, thermal-hydraulics of the two-phase steam-water mixture, chemical reactions in the process heat exchangers and control system characteristics. Field equations consist of mass continuity, momentum conservation and energy conservation with a two-fluid model and reactor power is calculated by point reactor kinetics equations. The code was validated by the experimental data obtained by the HTTR operations and mock-up test facility (Takamatsu, 2004 Ohashi, 2006). 

These questions touch on the theoretical fundamentals of models, these being based on dimensional analysis. Although they have been used in the field of fluid dynamics and heat transfer for more than a century - cars, aircraft, vessels and heat exchangers were scaled up according to these principles - these methods have gained only a modest acceptance in chemical engineering. The reasons for this have already been explained in the preface. 

The job is not finished with steady state controllability analysis. Only dynamic simulation enables a reliable assessment of the control problem. The solution of the dynamic modelling depends on the dynamics of units involved in the control problem. Detailed models are necessary for the key units. The simplification of the steady-state plant simulation model to a tractable dynamic model, but still able to represent the relevant dynamics of the actual problem, is a practical alternative. Steady-state models can be used for fast units, as heat exchangers, or even chemical reactors with low inventory. 

The post analysis results from a plant wide dynamics code MIMIR-N2 are in excellent agreement with the experimental data as shown in Fig. 12. Various key parameters are clarified to improve the calculation accuracy through the study. In the short-term analysis, the evaluation of thermo-hydraulic behavior in the core is largely affected by the inter-assembly heat transfer effect, the pump flow coast characteristics and coolant flow distribution. For the long-term analysis, it is important to assess precisely the buoyant head effect in the IHX, the heat exchange effects in the lower plenum of the IHX and others. The experimental result is also applied to the assessment of natural convection characteristics in the MONJU reactor. 

Given the design decision to use < > = 0.25, based on the steady-state C R analysis, verification is performed by dynamic simulations with HYSYS.Plant. The hot stream of n-octane at 2,350 Ibmol/hr is cooled from 500 to 300°F using n-decane as the coolant, with F2 = 3,070 Ibmol/hr and F3 = 1,200 Ibmol/hr. Note that these species and flow rates are chosen to match the heat-capacity flow rates defined by McAvoy (1983), with F, slightly increased to avoid temperature crossovers in the heat exchangers due to temperature variations in the heat capacities. Additional details of the HYSYS.Plant simulation are... 

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