Ethanol is to be condensed around cool 10ºC tubes of 1 cm outside diameter and at a 50 saturation temperature of 78.5ºC. Plot the heat transfer per unit length on horizontal tubes arranged as shown in Figure 10-17b and as predicted by Equation 10-49 as the number of tubes is increased. How does this result compare to the heat transfer predicted for a single tube or for a dripping, in-line tube arrangement as shown in Figure 10-17c? Ethanol has a heat of vaporization hnfg of 842 J/kg at atmospheric pressure.

Question

Answer 1

Condensation would intuitively increase with the number of tubes in a horizontal tube

bank. The use of Equation 10-49 to predict the convective heat transfer coefficient for

various numbers of tubes, N, and then calculating the heat transfer per unit length by

Newton’s law of cooling, gives a perspective of how the number of tubes changes the

heat transfer. Using Mathcad, this can be done.



With the results of the heat transfer per unit length. In the column, the first entry of

720.234 is the heat transfer per unit length of one tube. Continuing, the last entry is for 20 horizontal tubes.