Effect of air on condensing steam heat tranfer coefficient

In summary, the conversation discusses the challenges of predicting cooling in a system that evacuates a vessel and uses water evaporation and condensation for cooling. The main concern is estimating the heat transfer coefficient on the steam side as the air is removed and the steam becomes the dominant gas. Despite searching for an empirical correlation, no solution has been found and it seems that the experiments will have to be run to generate an accurate prediction.
  • #1
betadave
13
9
I am working on a model for predicting cooling in a system that evacuates a vessel and cools by evaporation of water and the water vapor is condensed in a tube and shell heat exchanger. My concern is the prediction of the steam side heat transfer coefficient as the air is removed from the system and the steam becomes the dominant gas. The heat transfer coefficient is lowered dramatically by air but I do know have an empirical correlation predicting steam side U values as a function of non-condensable gas fraction. Does anybody know of a correlation for this situation?
 
  • #3
Welcome to PF, sorry I didn't see this sooner...

Since all of the energy transferred is manifest from the condensation of water, can't you ignore the heat transfer coefficient and just measure the condensate flow rate?
 
  • #4
I wish I could. The mathematical model needs to calculate a water out temperature from the equation that says the Q calculated by the U value, area, and log mean temperature difference is equal to the Q gained by the water from flow rate, heat capacity and delta T. Since T out is inside a log function, I must use goal seek in the spreadsheet to find the number. I have to have an estimate of the U value and it starts essentially nothing with it is all air to whatever U value I would have with the vapors carrying the air that arrives from leakage into the vessel. I am hoping that someone knows of an empirical correlation that might allow the adjustment of the U value over the time it takes to draw out the air and reach the final temperature. It may end up that I have to run the actual tests and generate my own correlation as so far the request has drawn a blank.

Dave
 
  • #6
Thanks for the additional links. The third one was informative as it offers the effect is exponential. There are a variety of articles found on google scholar that deal with the topic but none give an empirical correlation that should apply to a horizontal tube and shell heat exchanger. I am afraid that I am just going to have to run the experiments to get an approximation for the equipment to be used.
 

1. What is the effect of air on condensing steam heat transfer coefficient?

The presence of air can significantly affect the heat transfer coefficient of condensing steam. When air is present, it creates a layer of insulation on the surface of the condensing steam, reducing the overall heat transfer coefficient.

2. How does the concentration of air affect the heat transfer coefficient?

The concentration of air has a direct impact on the heat transfer coefficient. A higher concentration of air will lead to a thicker layer of insulation, resulting in a lower heat transfer coefficient. Conversely, a lower concentration of air will allow for better heat transfer.

3. Can air pressure impact the condensing steam heat transfer coefficient?

Air pressure can have a significant impact on the condensing steam heat transfer coefficient. Higher air pressure will lead to a thicker layer of insulation, resulting in a lower heat transfer coefficient. Lower air pressure, on the other hand, will allow for better heat transfer.

4. How does the velocity of air affect the heat transfer coefficient?

The velocity of air can greatly influence the heat transfer coefficient. Higher air velocities will result in a thinner layer of insulation, allowing for better heat transfer. Lower air velocities will lead to a thicker layer of insulation, resulting in a lower heat transfer coefficient.

5. What factors can affect the effectiveness of air on condensing steam heat transfer coefficient?

Aside from the concentration, pressure, and velocity of air, other factors can also affect the effectiveness of air on condensing steam heat transfer coefficient. These include the surface roughness of the condensing steam, the temperature difference between the steam and the surrounding air, and the presence of other gases or impurities in the air.

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