Transient heat exchange

The discussions of three tjq es of heat exchange in Secs. 2.1.4-2.1.6 deal with steady-state heat exchange for reasons of simplicity and ease of understanding. Two fixed temperatures are assumed to exist at the two points between which the heat flows. In many applications, however, temperatures are in transition, so that the values shown for energy radiated from a target surface are the instantaneous 

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Let us consider a class of problems concerning transient heat exchange between convex bodies of various shape and the environment. At the initial time t - 0 the temperature is the same throughout the body and is equal to 2], and for t > 0 the temperature on the surface T of the body is maintained constant and is equal to Ts. The temperature distribution inside the body is described by the heat equation... 

Fypass Flow Effects. There are several bypass flows, particularly on the sheUside of a heat exchanger, and these include a bypass flow between the tube bundle and the shell, bypass flow between the baffle plate and the shell, and bypass flow between the shell and the bundle outer shroud. Some high temperature nuclear heat exchangers have shrouds inside the shell to protect the shell from thermal transient effects. The effect of bypass flow is the degradation of the exchanger thermal performance. Therefore additional heat-transfer surface area must be provided to compensate for this performance degradation. 

Distance-Velocity Lag (Dead-Time Element) The dead-time element, commonly called a distance-velocity lag, is often encountered in process systems. For example, if a temperature-measuring element is located downstream from a heat exchanger, a time delay occurs before the heated fluid leaving the exchanger arrives at the temperature measurement point. If some element of a system produces a dead-time of 0 time units, then an input to that unit,/(t), will be reproduced at the output a.s f t — 0). The transfer function for a pure dead-time element is shown in Fig. 8-17, and the transient response of the element is shown in Fig. 8-18. 

Heat exchanger-like, multi-tube reactors are used for both exothermic and endothermic reactions. Some have as much as 10,000 tubes in a shell installed between tube sheets on both ends. The tubes are filled with catalyst. The larger reactors are sensitive to transient thermal stresses that can develop during startup, thermal runaways and emergency shut downs. 

Design considerations should be examined by process design engineers when designing heat exchangers for stage 1 tube rupture transient effects, which includes the following ... 

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 ... 

Sumeria, V.H., J.A. Rovnak, I. Heitner, RJ. Herbert, Model to Predict Transient Consequences of a Heat Exchanger Tube Rupture, Proceedings-Refining Department, Vol. 55, American Petroleum Institute, Washington, D.C., 1976, p.63... 

Patankar, S. V., and D. B. Spalding. 1974. A calculation procedure for the transient and steady-state behavior of shell-and-tube heat exchangers. In N. H. Afgan and E. V. Schliinder (eds.). Heat Exchangers Design and Theory Sourcebook. New York McGraw-Hill, pp. 155-176. 

The earth itself is the reaction vessel and chemical plant. The complicated reaction chemistry and thermodynantics involve ntixers, reactors, heat exchangers, separators, and flnid flow pathways that are a scrambled design by nature. Only the sketchiest of flowsheets can be drawn. The chemical reactor has complex and ill-defined geometry and must be operated in intrinsically transient modes by remote control. Overcoming these difficulties is a trae frontier for chemical engineering research. 

Yavuzturk, C., J.D. Spitler, and S.J. Rees, 1999. A transient two-dimensional finite volume model for the simulation of vertical U-tube ground heat exchangers, ASHRAE Trans., 10592, 465-474. 

As with the other reactor configurations, vaporizers, heat exchangers, and a heat source are also needed for microreactors.Unless the hydrogen is 99.999% pure, the PEM fuel cell typically will utilize 70—80% of the diluted hydrogen fed to it. The unreacted hydrogen from the fuel cell anode, augmented with additional fuel as needed, can be used as fuel for the combustor. The use of anode off-gas requires special controls for transient operating conditions for example, a mechanism is needed to... 

One-dimensional models of a solid oxide fuel cell (see Chapter 9) and a methane-steam reformer [19, 20] were incorporated into the ProTRAX programming environment for transient studies. Lumped parameter ProTRAX sub-models were used for the remaining system components (heat exchangers, turbomachinery, valves, etc ). A schematic of the model is provided for reference in Figure 8.21. 

A modular reactor similar to the approach of Adler et al. [11] was introduced by Muller and co-workers [37, 38, 56], The screening procedure was separated into a number of process operations. In chemical process engineering, these so-called unit operations are essential components of every complex plant. As catalyst screening involves many different processes such as heat exchange, flow distribution, sampling, analysis and reaction, such a subdivision into unit operations is justified. The flexibility of such a system was demonstrated with two exemplified configurations later, one of which was used for transient studies and one for steady-state experiments (Figure 3.30). 

Power or thermal transients initiated in nuclear reactor SSC, stress on heat exchanger or other component in contact with balance of plant Transient causes stress on SSC induces possible transient in process facility... 

A scaling analysis of the hydrogen generation by the thermochemical plant was performed. For a steady-state power of 268 MWth in the heat exchanger, the simplified model hydrogen production rate was scaled to 142.3 mol/s. In a transient scenario, this steady-state production rate will change. 

HEAT EXCHANGER TEMPERATURE RESPONSE FOR DUTY-CYCLE TRANSIENTS IN THE NGNP/HTE... 

Heat exchanger temperature response for duty-cycle transients in the NGNP/HTE... 

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