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 lin...
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...ndations for Future Study
There are a few areas for improvements for future experiments. In future
experiments, the piping system should be better studied so that valves can be set up
more quickly. This would allow for more runs to be accomplished.
In addition, the cold tank should not be filled to capacity in future experiments
because the cold flow rates were not stable when the tank was full. Instead, filling the
tank to about half capacity would produce more stable cold flows, and reduce time until
steady state is reached.
Overall, this experiment successfully examined three different types of heat
exchangers for their feasibility for a scaled up process. Based on the overall heat
transfer coefficients determined experimentally, the plate heat exchanger provides the
most efficient heat transfer and thus should be selected for scale up.
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