947 words - 4 pages

Energy Balances

From an energy balance around a heat exchanger, it is clear that the heat duties

calculated on the hot and cold sides should be equivalent. The data obtained

throughout this experiment were consistent with this assertion. This is shown in Figure

4.4. In the figure, it can be seen that the heat duties on the hot and cold sides are

equivalent (within experimental error) for all runs of the finned, double-pipe heat

exchanger. The energy balances for the runs in the other exchangers are similarly

satisfied, as seen in Tables 4.1 and 4.2.

The fact that the energy balances are satisfied allows confidence to be placed in

the data obtained throughout the experiment. This, in turn, allows confidence to be

placed in the scale-up.

Analysis of the Wilson Plots

The Wilson Plot method makes use of the fact that when the velocity of the hot

fluid in a heat exchanger is varied, the only resistance to heat transfer that changes is

the fluid coefficient, h. There are empirical correlations to determine the Nusselt number

as a function of the Reynolds number and the Prandtl number. From these correlations,

it can be shown that h v0.8 for sufficiently high fluid velocity. As such, it follows that a

plot of 1/U versus 1/v0.8 should be linear.

As seen in Figures 4.1, 4.2 and 4.3, the data obtained in this experiment are

consistent with this theory. Plots of 1/U versus 1/v0.8 for all exchangers tested in this

experiment were remarkably linear. This is evident from the coefficient of determination

(R2) values of the best fit lines. The R2 value for the high cold flow rate data in the plate

heat exchanger, for example, was 0.998. This means that 99.8 percent of the variation

in the data is accounted for in the line. As seen in Figures 4.1, 4.2 and 4.3, the other

best fit lines also have R2 values close to unity. Because the R2 values are so close to

one, confidence can be placed in the best fit lines obtained.

The exchanger with least linear Wilson Plot was the finned, double-pipe heat

exchanger. The R2 values for the high and low cold flow rate data in this exchanger

were 0.962 and 0.966 respectively. While these values are still fairly close to unity, it

may be possible to obtain better results by modifying the power to which the velocity is

raised from 0.8. However, this is outside the scope of this study.

Comparison of Heat Exchanger Types

Three different types of heat exchangers, shell and tube, plate, and finned,

double-pipe, were studied in this experiment for their feasibility for scale up. At the same

range of volumetric flow rates, the overall heat transfer coefficient of each exchanger

was determined. As seen in Tables 4.1, 4.2 and 4.3, the overall heat transfer

coefficients obtained in the plate exchanger were the highest, followed by those obtained

in the shell and tube exchanger. The overall heat transfer coefficients obtained in the

finned, double-pipe exchanger were the lowest of the exchangers...

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