Exchange effective

Binsch G 1969 A unified theory of exchange effects on nuclear magnetic resonance lineshapes J. Am. Chem. Soc. 91 1304-9... 

Hence, the heat-exchanger effectiveness can be written as follows ... 

The importance of equations 37—39 is that once the heat-exchanger effectiveness, S, is known for a given heat exchanger, one can compute the actual heat-transfer rate and outlet stream temperatures from specified inlet conditions. This process is known as rating a given heat exchanger. 

Fig. 4. Heat-exchanger effectiveness where numbers on the curves represent the ratio flow (b) counterflow (c) parallel...

There are two relative temperature changes for a heat exchanger. 1 he greatest of them is the heat exchanger effectiveness, c ... 

The heat exchanger effectiveness shows how close the heat exchanger is operating to the maximum heat transfer performance. Equation (9.5) is valid for any type of heat exchanger. 

Counterflow Heat Exchanger and the Heat Exchanger Effectiveness... 

This heat exchanger effectiveness is one of the important parameters that describes the performance of a counterflow heat exchanger. 

Heat exchanger Effectiveness (or thermal ratio) e = Temperature rise (cold side (/maximum temperature difference between entry (hot side) and entry (cold side)... 

For the closed recuperative cycle [CHTXJi, with states 1,2,X,3,4, Y as in the T,s diagram of Fig. 3.10, the net specific work is unchanged but the heat supplied has to be reassessed as heat qj is transferred from the turbine exhaust to the compressor delivery air. Using the heat exchanger effectiveness, e = (Tx — — Ti) the heat supplied... 

A set of calculations using real gas tables illustrates the performance of the several types of gas turbine plants discussed previously, the [CBT]ig, [CBTX]ig, [CBTBTX]ig, [CICBTXIig and [CICBTBTX]ig plants. Fig. 3.15 shows the overall efficiency of the five plants, plotted against the overall pressure ratio (r) for = 1200°C. These calculations have been made with assumptions similar to those made for Figs. 3.13 and 3.14. In addition (where applicable), equal pressure ratios are assumed in the LP and HP turbomachinery, reheating is set to the maximum temperature and the heat exchanger effectiveness is 0.75. 

Before eonsidering the effects of water injection in an EGT type plant, it is worthwhile to refer to the earlier studies on the performanee of some dry recuperative cycles. Fig. 6.6 shows the T..s diagram of a [CBT i X r cyele, with a heat exchanger effectiveness of unity. It is implied that the surface area for heat transfer is very large, so that the outlet temperature on the cold side is the same as the inlet temperature on the hot side. However, due to the higher specific heat of the hot gas, its outlet temperature is higher than the inlet temperature of the cold air. 

In practice, however, the heat exchanger effectiveness will not be unity for these dry cycles, but the above analysis does suggest that for practical plants ... 

The discussion of the last section is then useful in considering the evaporative cycles. We shall see that the effect of water injection downstream of the compressor (and possibly in the cold side of the heat exchanger) may lead towards the [CBTJiXr type of plant, with increased cold side effective specific heat and hence increased heat exchanger effectiveness. Water injection in the compressor may lead to a plant with isothermal compression. 

We also give calculations of the performance of some of these various gas turbine plants. Comparison between such calculations is often difficult, even spot calculations at a single condition with state points specified in the cycle, because of the thermodynamic assumptions that have to be made (e.g. how closely conditions in a chemical reformer approach equilibrium). Performance calculations by different inventors/authors are also dependent upon assumed levels of component performance such as turbomachinery polytropic efficiency, required turbine cooling air flows and heat exchanger effectiveness if these are not identical in the cases compared then such comparisons of overall performance become invalid. However, we attempt to provide some performance calculations where appropriate in the rest of the chapter. 

We often split the exchange-correlation term into a sum of one part for exchange effects and one part for correlation effects. 

In designing a unit, each side of the exchanger is independently tailored to the duty required, and the exchanger effectiveness (discussed in Section 9.9.4) can range from 2 5 per cent to values in excess of 98 per cent without fundamental design or construction problems arising. Countercurrent, co-current and cross flow contacting can be employed individually or in combination. 

The principal use of this method is in the rating of an existing exchanger. It can be used to determine the performance of the exchanger when the heat transfer area and construction details are known. The method has an advantage over the use of the design procedure outlined above, as an unknown stream outlet temperature can be determined directly, without the need for iterative calculations. It makes use of plots of the exchanger effectiveness versus NTU. The effectiveness is the ratio of the actual rate of heat transfer, to the maximum possible rate. 

Hess O, Caffarel M, Huiszoon C, Claverie P (1990) Second-order exchange effects in intermolecular interactions. The water dimer. J Chem Phys 92 6049... 

Figure 5.15 Liquid-vapor exchange effect on CHF (circular tubes). (From Tong, 1975. Copyright 1975 by American Society of Mechanical Engineers, New York. Reprinted with permission.)...

Discussion of the Observed Exchange Effects 3.4.1 The CObuIk/COad Exchange... 

Further improvements to the theory require the incorporation of quantum factors such as exchange effects,42 although, even then, the theory still fails to predict the correct magnitude of the cross section for several fairly simple atoms such as Ne, N, and F. 

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