Bypass streams

The primary water specifications for a PWR are given in Table 1 (4). Rigid controls are appHed to the primary water makeup to minimise contaminant ingress into the system. In addition, a bypass stream of reactor coolant is processed continuously through a purification system to maintain primary coolant chemistry specifications. This system provides for removal of impurities plus fission and activated products from the primary coolant by a combination of filtration (qv) and ion exchange (qv). The bypass stream also is used both to reduce the primary coolant boron as fuel consumption progresses, and to control the Li concentrations. 

Tube-Bundle Bypassing Shell-side heat-transfer rates are maximized when bypassing of the tube bundle is at a minimum. The most significant bypass stream is generally between the outer tube limit and the inside of the shell. The clearance between tubes and shell is at a minimum for fixed-tube-sheet construc tion and is greatest for straight-tube removable bundles. 

As discussed in Section 9.4.4, the complex flow pattern on the shell-side and the great number of variables involved make the prediction of coefficients and pressure drop very difficult, especially if leakage and bypass streams are taken into account. Until about 1960. empirical methods were used to account for the difference in the performance... 

The shell-side leakage and bypass streams (see Section 12.9) will affect the mean temperature difference, but are not normally taken into account when estimating the correction factor Ft. Fisher and Parker (1969) give curves which show the effect of leakage on the correction factor for a 1 shell pass 2 tube pass exchanger. 

Stream C is the bundle-to-shell bypass stream. The fluid flowing in the clearance area between the outer tubes in the bundle (bundle diameter) and the shell. 

The complex flow pattern on the shell-side, and the great number of variables involved, make it difficult to predict the shell-side coefficient and pressure drop with complete assurance. In methods used for the design of exchangers prior to about 1960 no attempt was made to account for the leakage and bypass streams. Correlations were based on the total stream flow, and empirical methods were used to account for the performance of real exchangers compared with that for cross flow over ideal tube banks. Typical of these bulk-flow methods are those of Kern (1950) and Donohue (1955). Reliable predictions can only be achieved by comprehensive analysis of the contribution to heat transfer and pressure drop made by the individual streams shown in Figure 12.26. Tinker (1951, 1958) published the first detailed stream-analysis method for predicting shell-side heat-transfer coefficients and pressure drop, and the methods subsequently developed... 

This factor corrects for the main bypass stream, the flow between the tube bundle and the shell wall, and is a function of the shell to bundle clearance, and whether sealing strips are used ... 

Ns = number of sealing strips encountered by the bypass stream in the cross-flow zone,... 

A salient example from the process sampling realm is therefore given here. In the routine PAT laboratory it was desired to perform a mass reduction as part of the process of transporting a powder material. A bypass stream was established and on this the device depicted in Fignre 3.5 is snpposed to deliver one 10% snb-sample, another 25% snbsample, with the remaining 65% ( reject ), fed back to the main stream. 

Transfer optics The interface of the IR beam with the sample, especially in a classified hazardous environment, can be a major challenge. Gas samples are not too difficult although it is important to pay attention to the corrosivity of the gases, relative to the windows and any internal optics, as with a folded path length cell. Liquids offer different challenges. For on-line applications users prefer to minimize the use of valves, bypass streams and auxiliary pumps, especially over long distances between the stream and the analyzer. At times there is a benefit to sample the stream either directly or as close as possible... 

Automatic on-line monitoring of inhibitor reserve. This involves sampling a bypass stream of cooling water and is usually achieved by the measurement of a specific chemical inhibitor component, such as molybdenum. Other monitoring methods include azole fluorescence (as in the well-established Nalco TRASAR system) or... 

The amount of bypassing Fby is determined by blending the 475 K stream coming from the FEHE with the 310 K bypass stream to achieve the 400 K reactor inlet temperature. The bypass flowrate is 0.0119 kmol/s, and the flow through the heat exchanger is... 

There are two sources of complexity in this flowsheet. The first is the split of the feed between the bypass stream and the stream going through the heat exchanger. The second is the recycle stream from the reactor back to the heat exchanger. The final flowsheet is shown in Figure 7.15, but to get to this converged flowsheet, these issues must be handled. 

Bypass-stream transport is a method for maintaining high sample transport velocity to minimize transportation lag. This method is used when samples are vaporized at the sampling tap and no facilities exist for returning the vapor to the process. If the sample bypass is piped to a drain or vent, this will not only waste the process material but might also pollute the environment. Therefore, the use of a fast bypass-return loop is preferred. After selecting the appropriate sample transport method, the sample time lag should be calculated and used in the tuning of the analyzer controller. 

The measurements were carried out as follows the compound to be tested was filled in one or both of the stainless steel saturators, which were held thermostatically at 25°C. The flow of the gas carrier was divided in two, and each of these streams were passed through a flow controller [90], One stream went through a tube with a sinterplate in the base into the saturator where the carrier gas bubbles into the liquid adsorbate. This gas flow, that is, the stream which has passed through the saturator and becomes saturated with the test substance, was then mixed with the bypass stream of pure carrier gas at the outlet of the saturator, and the unified stream was then passed through the IR-cell [90],... 

We consider a plant designed to convert a feed stream rich in compound A (of molar concentration cao) into compound B in a high-temperature, mildly exothermic, first-order reaction carried out in an adiabatic reactor (Figure 6.8). For improved operability, the plant features a heater that is used at full capacity in startup mode and as a trim heater during operation, as well as a bypass stream that is used to regulate heat recovery in the FEHE. 

Figure 6.8 A reactor-FEHE process with a fired furnace and an exchanger bypass stream.

An inspection of the process parameters in Table 6.1 reveals that a/e is of 0( 1) (i.e., the energy flow associated with the bypass stream is much smaller than the amount of energy recycled to the process). Equations (6.53) can thus be rewritten as... 

We can solve some of the control difficulties associated with Alternative 6 by adding auxiliary utility coolers and reboilers to the process as suggested by Tyreus and Luyben (1976) and Handogo and Luyben (1987). In Figure 5.18 we show a control configuration that uses three new reboilers and three utility coolers to improve controllability. The coolers are located in bypass streams around the process-to-process reboilers so that disturbances in the heat balance can be dissipated... 

Waste of energy in bypass stream. Added piping for spill flow. 

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