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Condensation / Re-evaporation in stack/silencer

Condensation / Re-evaporation in stack/silencer

Condensation / Re-evaporation in stack/silencer

I am struggling getting my arms around what I thought would be a simple, if not tedious, heat transfer problem.

During startup of our process steam is letdown through a control valve and vented through a long line and vent stack with silencer located at the top. I have computed overall heat transfer coefficients across the system. We are assuming worst case T = 0°F outside air temperature. I am trying to estimate how much condensate might be formed - we don't want the stack or silencer to fill up with liquid as this would cause a weight problem.

The issue is in the large diameter silencer, where I calculate that the skin temperature will be well below 212°F. This provides an opportunity for condensation on the walls of the silencer. I am confused about how I might determine how much would be condensed and/or "re-evaporated" by the steam flowing upward through the silencer. I have computed the maximum amount condensed assuming that all of the calculated heat duty converts to condensation, which is a conservative assumption.

While the inside wall temperature is computed to be <100°F, the bulk temperature of the steam (calculated from an energy balance is about 278°). So it makes sense that we would not generate that much condensate, but I would use some ideas on how to calculate.


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