Coolant Flow Variations

This system can be made to work in several cases if designed carefully. The key point to consider is low-rate operation, and to ensure that under such conditions both velocity and cooling water outlet temperature are satisfactory. The following guidelines for using this technique have been recommended (68)  

Provide an override control from the cooling water outlet temperature that will not allow it to rise excessively. 

Provide a limiter, or even better, a flow override, that will prevent cooling water velocities from falling below the minimum recommended above. 

If keeping a high velocity is a problem, consider the system in Fig. 17.7a. This, however, may induce excessively high temperatures. 

It is important to have at least some degree of subcooling, or the system will lack sensitivity. It has been recommended (67, 68) to have at least about 5 percent of the total heat load utilized for subcooling. 

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Control techniques. Pressure and condensation control techniques are classified into four categories vapor flow variations, flooded condensers, coolant flow variations, and miscellaneous methods. These techniques are described below. 

Figure 17.6d shows a coolant flow variation arrangement for air condensers. The controller varies fan speed or fan pitch to control pressure. This arrangement is energy-efficient and minimizes fan power consumption, but it also entails high maintenance and requires the use of a variable-pitch fan or a variable-speed motor. Fan pitch... 

Flooded condenser schemes shown in Fig. 17.5a to c and f can be used instead of coolant flow variations. In such cases, the inerts normally leave finm the top of the condenser instead of the reflux drum (Fig. 17.8e). If the reflux drum is not flooded, a pressure balance line must be included otherwise, a stable pressure will be impossible to keep in the reflux drum. An overflow line should also be included in this arrangement. 

Table Type This is a simple flat metal sheet with slightly upturned edges and jacketed on the underside for coolant flow. For many years this was the mainstay of food processors. Table types are still widely used when production is in small batches, when considerable batch-to-batch variation occurs, for pilot investigation, and when the cost of continuous devices is unjustifiable. Slab thicknesses are usually in the range of 13 to 25 mm (V2 to 1 in). These units are homemade, with no standards available. Initial cost is low, but operating labor is high.

Optimisation may be used, for example, to minimise the cost of reactor operation or to maximise conversion. Having set up a mathematical model of a reactor system, it is only necessary to define a cost or profit functionOptimisation and then to minimise or maximise this by variation of the operational parameters, such as temperature, feed flow rate or coolant flow rate. The extremum can then be found either manually by trial and error or by the use of a numerical optimisation algorithms. The first method is easily applied with ISIM, or with any other simulation software, if only one operational parameter is allowed to vary at any one time. If two or more parameters are to be optimised this method however becomes extremely cumbersome. 

In this situation, a periodic variation of coolant flow rate into the reactor jacket, depending on the values of the amplitude and frequency, may drive to reactor to chaotic dynamics. With PI control, and taking into account that the reaction is carried out without excess of inert (see [1]), it will be shown that it the existence of a homoclinic Shilnikov orbit is possible. This orbit appears as a result of saturation of the control valve, and is responsible for the chaotic dynamics. The chaotic d3mamics is investigated by means of the eigenvalues of the linearized system, bifurcation diagram, divergence of nearby trajectories, Fourier power spectra, and Lyapunov s exponents. 

It is well known that a nonlinear system with an external periodic disturbance can reach chaotic dynamics. In a CSTR, it has been shown that the variation of the coolant temperature, from a basic self-oscillation state makes the reactor to change from periodic behavior to chaotic one [17]. On the other hand, in [22], it has been shown that it is possible to reach chaotic behavior from an external sine wave disturbance of the coolant flow rate. Note that a periodic disturbance can appear, for instance, when the parameters of the PID controller which manipulates the coolant flow rate are being tuned by using the Ziegler-Nichols rules. The chaotic behavior is difficult to obtain from normal... 

Eq.(50) shows the variation of the equilibrium dimensionless temperature as a function of the maximum value of the dimensionless coolant flow rate X6max- Plotting XQmax versus X3e a bifurcation curve can be obtained, from which it is possible to determine the value of xsmax which gives a different behavior of the reactor in steady state. It is interesting to note that Eq.(50) is equal to Eq.(47) when we make the substitutions of Eq.(49) into Eq.(47). 

Figure 7 shows the variation of the coolant heat duty and the sensible heat duty of the vapor with absorber pass. As the coolant flows upward through the absorber, the heat duty increases due to an increase in the LMTD between the coolant... 

Figure 17.6 Pressure control by condensing medium flow variations, (a) Liquid coolant (5) refrigeration coolant, level control (c) refrigeration coolant, outlet flow control (d) air coolant, fan motor speed or fan pitch variation (e) turboexpander inlet guide vanes variation.

The temperatures at motor end-winding, stator and case are simulated under different coolant flow rate 2L/min, lOL/min, 22L/min and 30L/min. The temperature of end-winding and stator will become lower gradually with the increase of the coolant flow rate. The reason for this phenomenon is that the heat transfer coefficient of cooling channel becomes larger with the increase of coolant flow rate, thus the temperatures of each section of PM motor decreases. It can be found that the variation of the temperature of end-winding and stator is not obvious... 

Although the drying process progresses on the basis of the preprogrammed profile of shelf temperature and chamber pressure with time, there can be variation in terms of temperature within a shelf and across the chamber itself. The mapping of the variation in temperature across the freeze-dryer is an important part of the qualification of a new plant and should be repeated periodically, especially if problems are suspected in coolant flow path etc. 

Slow LEMs can provide both negative and positive reactivity insertion with a moderate thermal response. Thirty-five slow LEMs provide the variation of reactivity between -2.79 and +2.88. The slow LEMs are used for automated bum-up reactivity compensation. In addition, slow LEMs partially realize the function of power control in accordance with the primary coolant flow rate. In nominal operation, the gas-liquid interface is placed in the active core region as shown in Fig. XVII-4. In case the core outlet temperature decreases, the gas-liquid interface goes up and positive reactivity is added, and vice-versa. To avoid quick positive reactivity addition, slow LEMs also have double-enveloped reservoirs (shells) with vacuum insulation. Therefore, only moderate thermal transients resulting from bum-up reactivity swing and primary flow rate variations affect slow LEMs. The design parameters of the LEMs are given in Table XVII-3. 

As discussed above, during normal reactor operation, full reactor power is removed by natural circulation. The necessary flow rate is achieved by locating the steam drum at suitable height above the centre of the core. Figure 5 shows variation in coolant flow rate, void fraction and quality with power for design configuration of the reactor. 

III-39. In the case of noble gases, the fraction of the releases from the fuel that is discharged to the atmosphere is determined by the half-life of the isotope and the rate of depressurization of the reactor. For iodine and caesium nuclides, which are released in molecular form, deposition on reactor surfaces reduces the concentration in the coolant and hence the discharge to the atmosphere. It is necessary to take account of both deposition and subsequent desorption. Important factors determining the deposition and desorption rates are the variations of coolant flow rate and surface temperatures with time and the extent of mixing of coolant in the reactor. 

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