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Chapter 10: Problem 54

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

Short Answer

Expert verified
Answer: The presence of a noncondensable gas in a vapor affects the condensation heat transfer process by decreasing the effective heat transfer area between the vapor and the cold surface, and by acting as an additional barrier to heat transfer. This leads to a decrease in the overall condensation rate and the heat transfer coefficient. The influence of noncondensable gas concentration on the condensation rate can vary up to a saturation point, beyond which the effect becomes negligible.

Step by step solution

01

Understand the basics of condensation heat transfer

Condensation heat transfer occurs when a vapor in contact with a colder surface condenses back to the liquid phase, and heat is released. This heat is transferred between the vapor and the cold surface, resulting in an exchange of thermal energy between them.
02

Learn about noncondensable gases

A noncondensable gas is a gas that does not change its phase under the given temperature and pressure conditions. In the context of vapor condensation, it means that the noncondensable gas will remain in the gas phase while the vapor undergoes a phase change. Examples of noncondensable gases are air, nitrogen, and helium.
03

Interactions of noncondensable gas and vapor in the condensation process

When a noncondensable gas is present in a vapor, it can affect the rate of condensation and the overall heat transfer process. The noncondensable gas can form a thin layer between the vapor and the cold surface, acting as a barrier to heat transfer. This barrier can reduce the rate of heat transfer between the vapor and the cold surface and, consequently, decrease the condensation rate.
04

Mechanism of the influence of noncondensable gas on the condensation heat transfer

The presence of a noncondensable gas decreases the effective heat transfer area between the vapor and the cold surface. As the noncondensable gas accumulates near the cold surface, it displaces the vapor, reducing the direct contact between vapor and the cold surface. This results in an increased thermal resistance and, ultimately, reduces the condensation heat transfer coefficient.
05

Effects of noncondensable gas concentration on the condensation rate

As the concentration of the noncondensable gas increases, the layer of noncondensable gas near the cold surface becomes thicker, leading to a further decrease in the condensation rate. This phenomenon occurs up to a certain saturation point, beyond which increasing the concentration of the noncondensable gas has negligible effect on the condensation rate.
06

Conclusion

The presence of a noncondensable gas in a vapor influences the condensation heat transfer by decreasing the effective heat transfer area between the vapor and the cold surface, and by acting as an additional barrier to heat transfer. This leads to a decrease in the overall condensation rate and the heat transfer coefficient. The influence of noncondensable gas concentration on the condensation rate can vary up to a saturation point, beyond which the effect becomes negligible.

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Most popular questions from this chapter

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

How does film boiling differ from nucleate boiling? Is the boiling heat flux necessarily higher in the stable film boiling regime than it is in the nucleate boiling regime?

Saturated water vapor at \(40^{\circ} \mathrm{C}\) is to be condensed as it flows through a tube at a rate of \(0.2 \mathrm{~kg} / \mathrm{s}\). The condensate leaves the tube as a saturated liquid at \(40^{\circ} \mathrm{C}\). The rate of heat transfer from the tube is (a) \(34 \mathrm{~kJ} / \mathrm{s}\) (b) \(268 \mathrm{~kJ} / \mathrm{s}\) (c) \(453 \mathrm{~kJ} / \mathrm{s}\) (d) \(481 \mathrm{~kJ} / \mathrm{s}\) (e) \(515 \mathrm{~kJ} / \mathrm{s}\)

A 65 -cm-long, 2-cm-diameter brass heating element is to be used to boil water at \(120^{\circ} \mathrm{C}\). If the surface temperature of the heating element is not to exceed \(125^{\circ} \mathrm{C}\), determine the highest rate of steam production in the boiler, in \(\mathrm{kg} / \mathrm{h}\).

An air conditioner condenser in an automobile consists of \(2 \mathrm{~m}^{2}\) of tubular heat exchange area whose surface temperature is \(30^{\circ} \mathrm{C}\). Saturated refrigerant-134a vapor at \(50^{\circ} \mathrm{C}\) \(\left(h_{f g}=152 \mathrm{~kJ} / \mathrm{kg}\right)\) condenses on these tubes. What heat transfer coefficent must exist between the tube surface and condensing vapor to produce \(1.5 \mathrm{~kg} / \mathrm{min}\) of condensate? (a) \(95 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\) (b) \(640 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\) (c) \(727 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\) (d) \(799 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\) (e) \(960 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\)
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