Heat Exchangers temperature efficiency factor

Because of the need to operate more efficiently at low teiTmera-tures, the simple heat exchangers have generally been replaced with more sophisticated types. Guidance for the development of such units for low-temperature service include the following factors ... 

To determine the true overall temperature difference, the correction factors, F, shown in Figure 10-34 are used to correct for the deviations involved in the construction of multipasses on the shell and tube sides of the exchanger. Note that R of the charts represents the heat capacity rate ratio , and P is the temperature efficiency of the exchanger. 

The added capability of calculating unknown values based on measured inputs will greatly enhance the system capabilities. For example, the neither fouling factor nor efficiency of a heat exchanger can be directly measured. A predictive maintenance system that can automatically calculate these values based on the measured flow, pressure and temperature data would enable the program to automatically trend, log and alarm deviations in these unknown, critical parameters. 

When the temperature of 3He returning to the MC is higher than the MC temperature for more than about a factor 3, the dilution process does not work. Equation (6.13) clearly shows the importance of having efficient heat exchangers. For example, if h3 = 100 p,mol/s and the power demanded to the MC is 1 xW, we get ... 

Oxygen uptake is not very useful in studies of energy expenditure, since 30-40% of it is not used for phosphorylated oxidation or ATP resynthesis, but for other varieties of oxidation such as peroxic and enzymatic oxidation which results in the production of heat (Khaskin, 1981). In any case, the efficiency of oxygen uptake as measured from the PIO coefficient varies according to various eco-physiological factors (Verzhbinskaya, 1968 Arsan et al., 1984 Savina, 1992). The heat produced is dissipated into the environment, except in the case of certain very active fish that possess a heat-exchange mechanism (Stevens et al., 1974) permitting the body temperature to rise and thereby increase muscle power (Love, 1980, p.321). 

The heat is available at 1200 K, but there will be temperature differences in the heat exchanger, so more available work will be lost in the heat exchange process. What can we learn from this example If we examine the Carnot factor, the answer seems to be clear. If we increase the operating temperature of the combustor, we can increase the efficiency and lose less work in the process. For example, if we had chosen an operating temperature of 2000 K, as could be possible in the suspended bed, we would have obtained an efficiency of 0.79, which is quite considerable. However, any gain in efficiency could be offset by the increase in work necessary to pulverize the coal For the sake of simplicity, we have not included these in this analysis. From the point of view of efficiency of combustion,... 

Furthermore, the cross sectional average of the i and 9i outlet temperatures and the average of q over the heat transfer area are needed. All these manipulations show that the temperature distributions and heat transfer in cross-flow heat exchanger are mathematically more complicated than those in parallel-and counter-flow heat exchangers. Because of this fact, practical problems associated with cross flow and other heat exchangers of similar complexity are handled by an approximation of mathematical results such as Eqs. (7.26) and (7-27) or by simpler results obtained from approximate formulations. These results are usually expressed in terms of a correction factor relative to the counter-flow (which is the most efficient) heat exchanger. ... 

The activity of a biocide will also be affected by other factors in the system in particular temperature and flow velocity will influence the biocidal efficiency in a heat exchanger (say a power station condenser). 

Air-cooled heat exchangers are employed on large scale as condensers of distillation columns or process coolers. The approach temperature - the difference between process outlet temperature and dry-bulb air temperature - is typically of 8 to 14 °C above the temperature of the four consecutive warmest months. By air-humidification this difference can be reduced to 5 °C. Air cooled heat exchangers are manufactured from finned tubes. Typical ratio of extended to bare tube area is 15 1 to 20 1. Finned tubes are efficient when the heat transfer coefficient outside the tubes is much lower than inside the tubes. The only way to increase the heat transferred on the air-side is to extend the exchange area available. In this way the extended surface offered by fins increases significantly the heat duty. For example, the outside heat transfer coefficient increases from 10-15 W/m K for smooth tubes to 100-150 or more when finned tubes are used. Typical overall heat transfer coefficients are given in Table 16.10. The correction factor Ft for LMTD is about 0.8. 

The difference between the total thermal output from the fuel and the electrical output is made up of the heat rejected to the heat sink (condenser), plus any heat losses in the reactor circuit, for example from the heat exchangers and the turbine. In practice, these heat losses are small in comparison with the heat rejected to the condenser. The most important factors in determining the thermal efficiency are the temperature of the steam entering the turbine and the temperature of the coolant in the condenser. The efficiency increases as the difference between these two temperatures increases. Since the condenser coolant temperature can vary over only a relatively small range, depending on the ambient temperature of the coolant water, the efficiency depends in practice on the temperature of the steam fed to the turbine. 

In terms of passive decay heat removal systems, a major difference is noted between the liquid cooled AHTR and gas cooled reactors. The AHTR can be built in very large sizes (>2400 MW(th)), while the maximum size of a gas cooled reactor with passive decay heat removal systems is limited to -600 MW(th). The controlling factor in decay heat removal is the ability to transport this heat from the center of the reactor core to the vessel wall or to a heat exchanger in the reactor vessel. The AHTR uses a liquid coolant, where natural circulation can move very large quantities of decay heat from the hottest fuel to the vessel wall with a small coolant temperature difference ( 50°C). Unfortunately, under accident conditions when a gas cooled reactor is depressurized, the natural circulation of gases is not efficient in transporting heat from the fuel in the center of the reactor to the reactor vessel. The heat must be conducted through the reactor fuel to the vessel wall. This inefficient heat transport process limits the size of the reactor to -600 MW(th) to ensure that the fuel in the hottest location in the reactor core does not overheat and fail under accident conditions. 

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