Steam condensation in the presence of air

In summary, the presence of non-condensable gases, such as air, in a condenser causes an increase in air concentration at the condensation interface, leading to a decrease in steam pressure and temperature. This is due to the overall decrease in thermal resistance, causing a decrease in heat transfer rate. However, this also results in a decrease in the condensation heat transfer coefficient and heat transfer. This phenomenon is explained in detail in the book "Liquid-Vapor Phase Change Phenomena" by Van P. Carey. Further research has shown that the heat transfer coefficient may increase due to the decrease in thermal resistance, but this is not always the case.
  • #1
sanka
52
3
I've been reading about this topic in a book recently and it states that when steam (water vapor) condenses on a cold surface in the presence of air, the increase in air concentration at the condensation interface causes a reduction in the steam pressure and hence, temperature. I am wondering why this is the case? Why does the overall system pressure not just increase? For example, in a condenser operating with non-condensables (such as air), I understand that the partial pressure of air will increase at the condensation interface due to the increase in concentration there but would that increase in partial pressure not just increase the overall condenser pressure instead of depressing the steam pressure & temperature?

Please feel free to show me the error of my ways.
 
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  • #2
Care to share a reference to what you've been reading?
 
  • #3
Its in a book called "Liquid-Vapor Phase Change Phenomena" by Van P. Carey.

I have also read some papers which show that both the condensation heat transfer coefficient and heat transfer decrease as non-condensable concentration increases. It makes sense that if the steam temperature decreses due to the presence of air (non-condensable gas), then the heat transfer rate must accordingly decrease. However, if the steam temperature decreases, it implies that the thermal resistance to heat flow decreases and the heat transfer coefficient must then be increasing.

Any ideas?
 

What is steam condensation?

Steam condensation is the process by which water vapor, or steam, changes from a gaseous state to a liquid state due to a decrease in temperature.

Why does steam condense in the presence of air?

Steam condenses in the presence of air because air contains cooler molecules that come into contact with the steam, causing it to lose heat and change state from a gas to a liquid.

What factors affect steam condensation in the presence of air?

The rate of steam condensation is affected by factors such as the temperature and pressure of the surrounding air, the surface area of the steam, and the humidity of the air.

What are the applications of steam condensation in the presence of air?

Steam condensation plays a crucial role in many industrial processes, such as power generation, refrigeration, and distillation. It is also used in everyday appliances like kettles and air conditioners.

How can we control steam condensation in the presence of air?

Steam condensation can be controlled by adjusting the temperature and pressure of the surrounding air, increasing or decreasing the surface area of the steam, and controlling the humidity of the air. Additionally, insulation can be used to reduce heat transfer and prevent condensation.

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