heat recovery 1

[rgrokkam]

Dear All,
I've a two heat exchgrs A, B in series. Desalted crude is being heated on tube side with gas oil on shell side. Over a 3month time I observed that
1. outlet temp of crude from exchgr B has dropped
2. pressure drop across B is also high
both when compared to A.
(Directionally, B is more fouled than A)
Gas oil flow to A and B can be altered. I was trying to calculate any improvement in crude outlet temp by routing more gas oil through A.
After fiddling with mCpdT, enthalpy of streams and temperatures.. I arrived at a 2degC improvement in crude outlet.
I feel that this is not the right approach.
Can anyone correct me?

Thanks!

 

[25362]

One suggestion: read about the "effectiveness- NTU method", for example, in J.P.Holman's Heat Transfer McGraw-Hill.

BTW, in the conventional formulas for convection heat transfer the coefficients in turbulent flow are generally obtained from formulas such as:

Nu = k Re^m Nuⁿ​

 

[EmmanuelTop]

If you want to maximize thermal performance of existing heat exchangers, I'd suggest you to look for specialized software (Hetran, for example).

If one of the exchangers is fouled, reducing flowrate of hot fluid cannot be of much help - simply the turbulence will be reduced, and therefore the overall heat transfer coefficient/rate. Extra amount of duty you get from cleaner exchanger will hardly overweight progressive fouling and duty losses in another exchanger.

It's always better to find the cause of problem, instead of treating the consequences. It's cheaper, and definitely more reliable, and it targets long-term solution. In many occasions this is the most difficult, but it is the only proven way.

Best regards,

http://antwrp.gsfc.nasa.gov/apod/astropix.html

 

[alcoengineer]

have you considered getting the heat exchanger pulled down in scheduled maintanance and inspected for fouling?

I used to run a distillery process where we would pull the condensers down every year and inspect them (for fouling and corrosion)

It does seem odd that the outlet temperature has dropped, could this be because you have increased the pressure drop across the unit and the flow has decreased? have you looked at all of the variables and how they've changed? Input and output on the cooling fluid side etc?

To me if there's fouling it would suggest that there would be an increase in temperature.

Does the pressure drop come from an increase in viscosity due to a lower discharge temperature?

I don't know if that's any help, but there seems to be some information missing to me.

 

[rgrokkam]

Thanks for your replies.
Emmanuel Top-"If one of the exchangers is fouled, reducing flowrate of hot fluid cannot be of much help - simply the turbulence will be reduced, and therefore the overall heat transfer coefficient/rate. Extra amount of duty you get from cleaner exchanger will hardly overweight progressive fouling and duty losses in another exchanger."

Is there any way to quantify it? I have many unknowns- fouling factor, viscosity, thermal conductivity and outlet temp.

 

[EmmanuelTop]

If you know the flowrates and exchanger geometry, you can easilly calculate and compare film coefficients, and quantify the effect of reduced turbulence - both on shellside and tubeside.

Actually, reduced flowrates in fouled exchangers is exactly what happens in huge banks of parallel air-coolers (or condensers). Once when fouling begins - usually in the most distant tube banks - flowrate is reduced due to accumulated deposits, and reduced turbulence promotes further fouling. The same applies for shell & tube exchangers.

http://antwrp.gsfc.nasa.gov/apod/astropix.html

 

[RogerH]

rgrokkam
In one of the refineries I worked have during the lasst odd 25 years injected water condensate on the crude side when fouling have been encountered.
After a few hours of injection most of the foulinglayer is removed. this can be done at reduced rate on the CDU, on-line. Simple and only cost a few hours of reduced throughput.
Nozzles before the heat exchanger are usually available so it more of a question to get a pump that can handle the amounts.
Keep in mind to not overdose water and keeping it in liquid state.

 

[rgrokkam]

Dear RogerH
How does dosing condensate remove fouling layer? What's the funda behind it?

 

[RogerH]

rgokkam
I guess erosion/velocity increase only removes the fouling. Perhaps combined with some thermal chock as well.
Haven't checked the flow regimes myself and now I dont have the DS available.

Know that it works, BP "developed" this method in the mid 80's and on my previous refinery this method was std practice when pressure drop (or heat transfer) restriction started to apply.
Like the method since it is simple, quick and gives good results. One cannot restore 100% but 80% is fine by me.
RH

 

[nsuhas]

rgrokkam

Water washing.
The exchangers which you are talking abt are before or after the Desalter. I think this exchanger fouled due to salts depositions from crude and at high temp these salts get precipitates out. So if you put water into it helps in removal of salts.

Find Fouling factor
From day today run you can calculate Heat transfer coeff (U) for bothe exchangers and the compare with design values. Using design and actual U you can calculate fouling factor. Use this fouling factor in some exchanger simulation package.