Effect of air on condensing steam heat tranfer coefficient

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SUMMARY

The discussion centers on predicting the steam side heat transfer coefficient in a tube and shell heat exchanger when air is evacuated, leaving steam as the dominant gas. The presence of air significantly lowers the heat transfer coefficient, and the user seeks an empirical correlation to estimate U values based on non-condensable gas fractions. Current resources do not provide a suitable correlation for this specific scenario, leading to the conclusion that experimental testing may be necessary to derive an accurate approximation for the system.

PREREQUISITES
  • Understanding of heat transfer principles, specifically condensation processes.
  • Familiarity with tube and shell heat exchanger design and operation.
  • Knowledge of empirical correlations related to heat transfer coefficients.
  • Proficiency in using spreadsheet software for mathematical modeling and goal-seeking techniques.
NEXT STEPS
  • Research empirical correlations for steam-air mixtures in heat exchangers.
  • Learn about the effects of non-condensable gases on heat transfer coefficients.
  • Investigate experimental methods for measuring heat transfer in condensation processes.
  • Explore advanced modeling techniques for predicting heat transfer in evaporative cooling systems.
USEFUL FOR

Engineers, thermal system designers, and researchers involved in heat exchanger design and optimization, particularly those focusing on condensation processes and the impact of non-condensable gases.

betadave
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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?
 
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?
 
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
 
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.
 

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