4 replies to this topic

[theoneajit]

Dear all, I posted the below querry a week beforw as i didnt get the useful inputs from many, once again i am posting the same with more details,

In an application for heating slurry the dry saturated steam from Boiler has been used with STHE. The pressure is reduced with PRS from 10 kg/cm2 to 3 kg/cm2,I would like know the need of using the desuperheater for this system.The heat exchanger is a STHE.As, I understand the desuperheater is mainly used for superheated steam but as we reduce the pressure there may be a need of taking superheat from steam even for dry saturated steam. I discussed with many people and they are recommending not to use desuperheater.

Kindly comment on the above topic. I would like to know the reason for having or not having desuperheater for this application.

Thanks,
ASB

[S.Chittibabu]

Hai,

When the pressure is reduced with PRS from 10 kg/cm2 to 3 kg/cm2, find out the degree of superheat in the downstream. And you have to reduce the temperature by sub cooled condensate to make it saturated with the desired wetness.

[ankur2061]

ajit,

There will be definitely a small amount of superheat added to the 3 kg/cm2g steam (always mention whether gage or absolute when mentioning pressure) when pressure is reduced from 10 kg/cm2g. This will cause the temperature of the 3 kg/cm2g steam to be some what more than its saturation temperature. Considering an ideal adiabatic expansion of steam you can calculate the temperature rise due to superheat. However, actual systems are never ideal and normally temperature of superheated steam requires actual measurement to arrive at actual desuperheating water requirement.

Try measuring the temperature at the PRS outlet to make a judgement whether the temperature rise due to pressure reduction can be tolerated for your downstream process. If not, then you require desuperheating.

Hope this helps.

Regards,
Ankur.

[ankur2061]

Dear All,

As I had mentioned in my post about being able to calculate the temperature rise or degree of superheat considering an ideal adiabatic expansion, please find attached herewith the calculations for degree of superheat.

Regards,
Ankur.

Attached Files

•  Deg_of_Superheat_cheresources_query_example.xls   21KB   211 downloads

[katmar]

This is not a question to which one can give a definite answer that you either do, or do not, need a desuperheater. If I had to generalize, I would say you most likely do not need one but you need to consider the details.

Firstly, you must consider the condition of your 10 barg steam. It is very unlikely that you would ever have perfectly dry saturated steam. Where did it come from? If it came from a high pressure main and was let down to 10 barg it probably went through a desuperheater where its temperature was dropped to leave about 3 to 5 degrees of superheat. If this is the case then you can be sure that the steam will be superheated after it is dropped further to 3 barg.

However, if the 10 barg steam is at the saturation temperature and if the steam traps on the main are discharging any condensate at all, it means that there is moisture in the 10 barg steam. At normal steam main velocities this moisture will be partially picked up and entrained by the steam. Ankur has shown that each kg of 10 barg steam would need to lose 9.9 Kcal in order for it to become saturated at 3 barg. The heat of vaporization at 3 barg is 509.62 Kcal/kg. Thus, if the incoming steam contains 9.9/509.62 = 0.02 kg/kg of moisture it will be enough to cool the steam to saturation. A steam quality of 98% in a saturated line would be very good, and it is likely that you have that or worse quality. In this case the 3 barg steam would not be superheated at all.

The second thing to consider is how much heat is in the super heat and how much is in the latent heat. Even if your steam were superheated as in Ankur's calculation only 2% (same calculation as above) of the heat needs to be transferred from superheated steam compared with that from condensing steam. Even if your overall HTC in the desuperheating zone is only 20% of that in the condensing zone (and it is probably much more than this) it means 10% of your exchanger area is used to transfer the superheat. How tightly designed is your exchanger? Also, how sensitive is the fluid being heated to a bit of a higher temperature?

Lastly, imagine what is happening inside your exchanger. You will have steam coming in at a high velocity, and swirling around your tubes and blowing condensate everywhere. There will be a high degree of mixing and it is hard to imagine a well defined zone of superheat transfer. I would imagine that apart from the very first row or two of tubes, you will have saturated steam everywhere.

On balance, you would need good reasons to justify the cost of a desuperheater for this type of application. It might be cheaper just to add a few percent to the exchanger area.