Chapter 10

What is the difference between film and dropwise condensation? Which is a more effective mechanism of heat transfer?

How does the presence of a noncondensable gas in a vapor influence the condensation heat transfer?

What is the modified latent heat of vaporization? For what is it used? How does it differ from the ordinary latent heat of vaporization?

Consider film condensation on a vertical plate. Will the heat flux be higher at the top or at the bottom of the plate? Why?

Consider film condensation on the outer surfaces of four long tubes. For which orientation of the tubes will the condensation heat transfer coefficient be the highest: (a) vertical, (b) horizontal side by side, (c) horizontal but in a vertical tier (directly on top of each other), or $(d)$ a horizontal stack of two tubes high and two tubes wide?

In condensate flow, how is the wetted perimeter defined? How does wetted perimeter differ from ordinary perimeter?

10-59 The Reynolds number for condensate flow is defined as $\mathrm{Re}=4\dot{m}/p{\mu }_l$, where $p$ is the wetted perimeter. Obtain simplified relations for the Reynolds number by expressing $p$ and $\dot{m}$ by their equivalence for the following geometries: $(a)$ a vertical plate of height $L$ and width $w,(b)$ a tilted plate of height $L$ and width $W$ inclined at an angle $u$ from the vertical, $(c)$ a vertical cylinder of length $L$ and diameter $D,(d)$ a horizontal cylinder of length $L$ and diameter $D$, and (e) a sphere of diameter $D$.

Saturated steam at 1 atm condenses on a 3-m-high and 8 - $\mathrm{m}$-wide vertical plate that is maintained at ${90}^{\circ }\mathrm{C}$ by circulating cooling water through the other side. Determine $(a)$ the rate of heat transfer by condensation to the plate, and ( $b$ ) the rate at which the condensate drips off the plate at the bottom. Assume wavy-laminar flow. Is this a good assumption?

Saturated steam at ${100}^{\circ }\mathrm{C}$ condenses on a 2-m $\times 2-\mathrm{m}$ plate that is tilted ${40}^{\circ }$ from the vertical. The plate is maintained at ${80}^{\circ }\mathrm{C}$ by cooling it from the other side. Determine (a) the average heat transfer coefficient over the entire plate and $(b)$ the rate at which the condensate drips off the plate at the bottom. Assume wavy-laminar flow. Is this a good assumption?

Saturated steam at ${30}^{\circ }\mathrm{C}$ condenses on the outside of a 4-cm- outer-diameter, 2-m-long vertical tube. The temperature of the tube is maintained at ${20}^{\circ }\mathrm{C}$ by the cooling water. Determine $(a)$ the rate of heat transfer from the steam to the cooling water, $(b)$ the rate of condensation of steam, and $(c)$ the approximate thickness of the liquid film at the bottom of the tube. Assume wavy-laminar flow and that the tube diameter is large, relative to the thickness of the liquid film at the bottom of the tube. Are these good assumptions?