Condensing in a heat exchanger

[mielke]

Hypothetically if we have a condenser that has 100% saturated steam in the tubes with an inlet condition of 14.7psia and 250F and an outlet condition of 200F, in which the temperature drop is a result of only cold air on the outside of the tubes.

Once the steam is cooled down to 212F it will start to condense since the operating pressure, 14.7, and the saturation pressure of steam is the same.

Im a little confused about how exactly its condensing. If it is cooled down to 200F will the pressure of the steam vapor decrease about 11.5psia (the saturation temperature of steam at 200F) leading to a condensation of about 21.7% of the initial volume of steam? Or will the pressure stay the same (neglicting pressure drop due to friction or other minor losses) as the vapor volume flow rate goes to 0. leaving 100% of the steam condensed at 200F?

which case will happen? thank you.

 

[ione]

Saturated steam @ 14.7 psia and 250 F?

 

[mielke]

*superheated

 

[hacksaw]

may be related to

thread404-294488

 

[rmw]

To be clear, what comes out at 200F? Condensate or steam (and condensate)? First you have to desuperheat to 212F and then the condensation occurs and after that the condensate subcools on its way out of the Hx.

This simple analogy assumes no pressure losses through the system.

If you have any non-condensables at all, then all the numbers above are skewed quite a little bit.

rmw

 

[ione]

First step : desuperheating from starting temperature to saturation temperature (sensible heat exploited for heat transfer)

Second step: condensation at constant saturation temperature (latent heat exploited for heat transfer). For possible condensation mechanisms I suggest you the following links

http://www.wlv.com/products/databook/ch1_1.pdf

http://www.wlv.com/products/databook/db3/data/db3ch8.pdf

Third step: subcooling of the liquid phase (sensible heat exploited for heat transfer).

As pointed out above (rmw) no pressure losses through the system, but those due to friction losses + entrance losses + exit losses. It is understood that friction losses for steam or condensate change according to the nature of the fluid as its properties change.