Outlet Temperature of Cold Fluid lower because of phase changes

[Iomcube]

Heat / Composite Curves attached are for an S&T exchanger I am modelling...

This is a vertical 1-1 heat exchanger (rising film) with steam @9barg at shell side & a brine solution in tubes @3barg.

Heat curves indicate that at shell side steam condenses so temperature changes are minimum (check vapour fraction curve)

At tube side however the vapour fraction changes from 0 (pure liquid) to 0.5 starting at ~200kw; at the same time rise in tube side T also decrements ~10C. Is this T loss attributed to tube side evaporation? Is this T decrement at the onset of evaporation a real-world happening?

Another way of saying it is that Cold fluid outlet T is lower than the T' happening somewhere along the length of tube...

 

[georgeverghese]

Suspect you may get a clue to why this is so by asking for a plot of operating pressure vs heat flow for both sides.

 

[Iomcube]

georgeverghese, if you r referring that high deltaP occurs at tube side; you are correct & this is normal for rising evaporators which are attached to a separator where the latter operates at much low pressure.

One end (bottom) is the pump's discharge @3barg; the other end (top) is open to a separator @1.2barg. So yes high deltaP.

I was merely asking is this temperature decrement (because of evaporation in tube side) is realistic?

 

[georgeverghese]

As you would know, phase change temp decreases with pressure, so high boiling point at the higher pressure end, and lower temperature at the low pressure end. This effect is probably some what attenuated by the increasing liquid phase brine concentration as you go up the tube side.

Though I cannot vouch for the absolute values here in these plots, the decreasing temp trend seems reasonable.

 

[Iomcube]

georgeverghese Thanks for the making things clear.

This effect is probably some what attenuated by the increasing liquid phase brine concentration as you go up the tube side.

Lets suppose that the separator is operating at vacuum (-0.8barg) ...attenuation will be rather high. Even then will it be reasonable? In the next post I will attach a similar graph; don't have access to simulation now

 

[georgeverghese]

Yes, would guess the sep at part vac pressure would be even lower in temperature (<135 degC), since the dominating effect is from the phase change temp variation with operating pressure.