3 replies to this topic

[newtech]

Hello,

Why high pressure steam at about 44kg and temperature at 300+ degrees celcius enters a thermosiphon reboiler and exit as a high pressure condensate whereas its temperature is definitely more than 100 degress celcius, should'nt it be in steam instead of condensate ? This condensate is then enters a flash tank and flashes to medium pressure steam, how does this flash tank works ?

[saeed.s]

Hi,
to answer to your qustion please clarify flash drum pressure?

[newtech]

Hi,
to answer to your qustion please clarify flash drum pressure?

Hi,

I do not know the flash drum pressure, but the pressure of the flash steam is about 14kg or 11kg, actually what i want is to clarify is basic some stuff. Water boils at 100 degrees celcius @ atmospheric pressure(which means its saturation point is 100degrees celcius?),liquid would boil when its vapor pressure is greater than the external pressure right ?So a higher external pressure(greater than atm pressure) is acting on water, to boil the water it would require higher temperature right(>100degrees)? and the steam temperature would be greater than 100 degrees. At higher pressure saturation point of steam would be higher?

So when a high pressure steam(about 44kg per cm square) enters the reboiler to heat up the thermosiphon tubes, the steam loses some of its heat and condense, but the pressure and temperature of the condensate from the reboiler is still very high,Is the pressure of the condensate and the inlet steam to the reboiler the same ?at a higher pressure steam condenses at a high temperature ?! or am i right to say that after the steam loses heat,the pressure is high that restrict the condensate from maintaining its vapour form as it does not have enough heat energy?

When the high pressure condensate enters the flash tank, the condensate is able to vaporize to steam because flash tank is area of lower pressure,and at the certain lower pressure the condensate is able to vaporize as liquid has enough heat energy to flash to a medium pressure steam in that area?

thanks,

[Art Montemayor]

Newtech:

You are laboring with the phenomena of phase change. The difficulty that I believe you are having is due to you not being specific in the actual way steam is employed as a heating medium within a normal shell and tube exchanger – such as a reboiler. Let’s get down to the specific basics:

• A high pressure (say 10 kg/cm² G) steam source is used to heat the shell side of a thermosyphon reboiler. In order to heat the reboiler at a pre-determined rate of flow (and consequently at a given heat flow rate), it has to be regulated or controlled with respect to its flow rate. The flow is normally controlled on demand by use of a control valve on the steam inlet to the reboiler.
• Because of the required throttling of the steam (in order to control the flow rate), the resultant pressure in the shell side of the reboiler is LESS than the original pressure of the source (say 7 kg/cm² G). The resultant steam pressure in the shell side is the pressure of saturated steam with a corresponding saturated temperature that yields the required driving force for the needed heat transfer to take place within the fixed effective heat transfer area of the reboiler. It is at this saturated pressure that the latent heat content of that steam is given up in heating the reboiler. Upon giving up the latent heat, the equivalent amount of saturated steam is condensed and gravitates to the bottom of the reboiler shell side.
• The resultant condensate that is formed due to losing its latent heat of vaporization is drained automatically from the reboiler shell as fast as it is formed. This is done by using a steam trap or a conventional level controller and a drain control valve.
• The drained condensate is saturated at the pressure condition existing in the reboiler shell and once it exits the steam trap orifice or the drain valve trim, it flashes due to adiabatic expansion as it goes to a lower pressure. This can take place in the drain line downstream of the steam trap or the drain control valve – it all depends on the piping configuration and whether you use a flash drum or expand directly into a condensate header. Regardless of how you collect or gather the steam condensate, you will produce flash steam vapor as an adiabatic flash product, together with condensate.

All of the above can be neatly traced and identified with respect to thermodynamic properties on a Mollier Diagram or a Temperature – Entropy Diagram. I strongly suggest you do so.

Now, allow me to clear up a simplification that I took in the above description in order to be factual and truthful. If your source steam is saturated, what REALLY happens when you regulate it with a control valve is that it may expand adiabatically into slightly superheated steam. You can see this effect when you trace out the cycle on the Mollier Diagram. Therefore, in reality, you will have a desuperheated section inside your reboiler shell. This desuperheated section is not very efficient in transferring heat, but normally this is a small and not significant portion of the total heat transferred. That is why it is safe to assume saturated heat transfer conditions inside the shell side.

Note that I’ve described one method used to supply steam to a reboiler. Another method is to allow a constant, full-pressure steam source to heat the shell side and control the level of condensate in the reboiler and, therefore, the available area exposed to latent heat transfer. In this method, you have no adiabatic expansion of the source steam. But you still have to expand the formed condensate as it exits the reboiler.

I hope this clears up your understanding of how heating with steam functions.