Q & A > Question Details
We are using Superheated HP steam (P-38 kg/cm2 g, T- 380 oC) to preheat our stripper feed. I want to replace this Superheated Steam with depressurized saturated HP steam (P- 25 kg/cm2 g, T-250 oC). But I am finding it difficult to calculate how much steam I will save. Can anyone help me in finding out the amount of steam for the same rate of heat transfer of 2 MMKcal/Hr. Also I need to know the heat transfer coefficient for both type of steams.
Note: The steam at above given saturated steam parameters is not actually saturated but we need these parameters.

Thank you all for your replies....We are not going to apply this change right now...currently it is just in study phase.
Actually what I believe is:
1. superheated steam should never be used for heating purpose as it has very low heat transfer coefficient (similar to air) as compared to that of saturated steam.
2. so the heat transfer in the convective film formed by superheated steam over the tubes will be very slow.
3.therefore to make use of the heat available in the degree of superheat, we have to increase the area for heat transfer.
4.Also the latent is higher in case of saturated steam (which is the main heat available for heat transfer in case of steams).
5.Also the latent heat increases as pressure of steam decreases.
That is why we thought of this change.
But acc to your answers, I think my knowledge about steam heating is not accurate and I am missing out something somewhere. Kindly help me in this issue and kindly correct me if I am wrong somewhere.
 
Answers
04/01/2017 A: Eric Hennings, Technip Stone & Webster, EHennings@technip.com
General comments on Gas Con:
1. Process objective is to remove light ends (C2-) from bottoms stream (naphtha/LPG mix). This is achieved by stripping vapor generated low in the Stripper tower (such as by reboiling), particularly below bottom tray.
2. Feed to Stripper is usually to top tray (no rectifying zone). Excessive preheat temperature results in excessive "flashing" at top tray and additional "recycle". This heat does not improve bottoms product.
3. There may be a case of a constraint in equipment duty in reboiler or vapor flooding in lower section of the tower. In this case, preheat of Stripper feed may help.
4. Corrosion by cyanides is possible in FCC or coker stripper preheat HX.
5. Superheated steam can be "desuperheated" easily using BFW injection.
28/09/2016 A: Ganesh Maturu, Self, maturu.ganesh@gmail.com
Sorry... 2000000/669= 3000kg/hr of steam. U can get it from steam tables...
28/09/2016 A: Ganesh Maturu, Self, maturu.ganesh@gmail.com
Interesting. What is the reason for this change? Any coking issues because of high temperature steam?
Ok to calculate saturated steam flow to get 2 Gcal, simply divide duty and latent heat. Here 2000000/229=8.7tph steam.
I am still not clear how you can save steam with this. Another important factor to consider is heat exch may not be adequate as u r reducing LMTD by reducing steam pressure.
27/09/2016 A: Leonardo Leite Garcia de Souza, Petrobras SA / Duque de Caxias Refinery, llgs394041@yahoo.com.br
You will have to check the performance of your pre-heater before trying to implement this change. Notice that your new heating utility has a much lower temperature than the superheated hp steam. Therefore, you are going to need a higher heat transfer area to deliver the same amount of heat load. If your pre-heater has temperature and flow instruments in both sides you can estimate the amount of heat being transferred and the "UA" (global heat transfer coefficient x heat transfer area). With both of this values in hand you should calculate the required UA for the new heating utility (UA=Q/FdTml). Considering that the heat transfer coefficient will not change significantly from one case to the other you can estimate the increase of the required heat transfer area for the new heating utility. If the increase in heat transfer area is lower than 20 % you are safe, but I don' believe that is the case, since the depressurized steam has a much lower temperature.