Temperature interval method

Example 4.30 Pinch analysis by temperature interval method and grand composite curve Table 4.23 shows hot and cold streams. 

Table 4.24 Temperature interval method for an approach temperature of 10°C...

Carry out the temperature-interval method to locate the pinch temperatures and the minimum hot and cold utility loads. 

The temperature-interval method can also be used to produce grand composite curves, which show the variation of heat supply and demand in a system. These diagrams enable engineers to minimize the expensive utilities, network area, and number of heat exchanger units. 

Numerous procedures have been developed for graphical differentiation. A particularly convenient one (9), which we call the chord-area method, is illustrated using the same data (from Table A.2) to which we previously apphed numerical differentiation. It is clear from Figure A.2 that if we choose a sufficiently small temperature interval, then the slope at the center of that interval will be given approximately by A /Af. In this example, with an interval of 5°C, the approximation is good. Then we proceed to tabulate values of A °/At from 0°C, as illustrated in Table A.6 for the first few data. Note that values of are placed between the values of to which they refer, and the temperature intervals (5°C) are indicated between their extremities. Similarly, as L%°jis an average value (for example, —0.000484) within a particular region (such as 0°C to 5°C), values in the fifth column also are placed between the initial and the final temperatures to which they refer. 

This method was also employed by Chu et al. 2 2) who investigated the effects of long range correlation for a critical mixture of polystyrene in cyclohexane at small temperature intervals from the phase seperation temperature. 

The first method used a Pauling-Erdeli-Pauling thermobalance at temperature intervals between 20° and 700°C in an inert medium. The temperature was raised 12.4°C per minute, the results being compared with those of the standard polymer. With a Fe3+ complex synthesized for 96 hours, the polychelate reached a maximum loss of only 28 at 440°C and often showed a constant thermal stability. Unmodified polyethylene terephthalate continually lost weight showing a loss of 95.9 at 577°C. 

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