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[enal]

Hi,

I have given more details in screenshots in the attachment, but essentially I have been handed over a HYSYS simulation of a High Pressure steam reboiler embedded in a Heat Recovery and Steam Generation unit.

The exchanger shows a temperature cross and I would like some input on how to relieve this constraint.

I asked my line manager and I have been told not to worry about it because, and I quote, "It is indeed true that the exchanger seems to have temperature cross but there would be no temperature cross in a real operation. As you know, it is not recommended to design a heat exchanger where it has two phase flow in either tube or shell side (if it is not a kettle-type reboiler) because in general a two-phase flow has relatively lower heat transfer coefficient than a liquid flow and also it is not easy to control the process when phase change occurs inside the heat exchanger. Therefore, in the design of evaporator of the steam cycle, it is usual to avoid generating steam inside the heat exchanger. The actual evaporation takes place in a drum which is located on the top of heat exchangers due to static head. The hot water is heated inside the heat exchanger at a higher pressure than a desired pressure and then slightly depressurised in a drum for its evaporation."

I am not entirely convinced the analysis above is correct. Any opinions from experienced designers of this type of units?

Thank you in advance.

Ennio (Enal)  HRSG-HPREBOILER.xls   151KB   34 downloads

[xavio]

Hi,

You say that the exchanger "has two phase flow in either tube or shell side", I guess it is a *FU type with hot gas located in shell side.
With *FU you certainly can handle a temperature cross because it operates as an ideal counter-current heat exchanger.

As for the tube side vaporization, there is also nothing weird about it. Vaporization HAPPENS inside the tube but the two phases are not separated until they come out to the drum.
I think the problem with your design lies in the vaporization fraction. It is impossible to design 100% vaporization at outlet with tube-side vaporizer. You will have film boiling and mist flow problems when you go beyond 30 or 40% vaporization.

I would suggest you to increase the HPW flow rate and design your heat exchanger for low vaporization fraction (5-15%) as these will minimize problems such as low heat transfer efficiency and premature fouling.

xavio