Question

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

Short answer

Answer: Noncondensable gases negatively affect the condensation heat transfer process by creating a resistance to heat transfer, reducing direct contact between the vapor and cold surfaces, and decreasing the overall heat transfer efficiency and rate of condensation. Additionally, the accumulation of noncondensable gas molecules at the condensation interface further hinders heat transfer by reducing the effective area for vapor condensation.

Step-by-step solution

Understand noncondensable gases and heat transfer

Noncondensable gases are gases that do not easily change from gas to liquid phase at the temperature and pressure conditions of interest, meaning they remain as gases even when subjected to certain conditions that would cause a condensable gas to turn into a liquid. Examples of noncondensable gases are nitrogen, oxygen, and carbon dioxide. Heat transfer refers to the movement of heat from one object or substance to another, which generally occurs when a difference in temperature exists. Condensation is a phase change where a vapor turns into a liquid. Condensation heat transfer is the transfer of heat that occurs between the vapor phase and the liquid phase as the vapor condenses.

Understand the condensation process without noncondensable gases

Before discussing the presence of noncondensable gases, let's briefly discuss the condensation process in the absence of any noncondensable gases. When a pure vapor goes through the condensation process, it transfers heat to the surroundings as it changes phase. The heat transfer is generally very efficient in this case, as the vapor has a high thermal conductivity and heat is transferred effectively between the liquid and vapor phases.

Explain the effect of noncondensable gases on condensation heat transfer

Now, let's discuss the influence of a noncondensable gas on the condensation heat transfer. When a noncondensable gas is present in a vapor mixture, it can inhibit the direct contact between the vapor and the cold surface (where the heat is being transferred), thus creating a resistance to heat transfer. This resistance reduces the overall heat transfer efficiency, thereby reducing the rate of condensation. The noncondensable gas molecules are relatively inert compared to the vapor molecules and do not easily condense, so they tend to accumulate at the condensation interface. As more and more noncondensable gas molecules build up at the interface, they reduce the effective area for vapor condensation, further hindering the heat transfer.

Summarize the effect of noncondensable gases on condensation heat transfer

In conclusion, the presence of a noncondensable gas in a vapor mixture negatively affects the condensation heat transfer process. The noncondensable gas molecules create a resistance to heat transfer by reducing the direct contact between the vapor and cold surfaces, thereby reducing the overall heat transfer efficiency and the rate of condensation. The accumulation of noncondensable gas molecules at the condensation interface further hinders heat transfer by reducing the effective area for vapor condensation.