Condensing Exchangers 4

[watwarrior]

I need to calculate the U factor of a tube and shell heat exchanger which has a condensing process on the shell side.

 

[epoisses]

42

 

[StoneCold]

If you give us both fluids and which one is on the shell I can give you a range of possible heat transfer rates. It will be a ball park number. If you need a more exact number then you will have to break out your heat transfer books and do the work. Or give the process conditions to a vender and have the exchanger sized that way. They like that but I don't. I like to have my own answer to back check them.

Regards
StoneCold

Regards

 

[MJCronin]

wartwarrior...

The structure and phrasing of your question indicates that you have no background in sizing exchangers for condsensing service.

While overall "U" factors work for liquid to liquid service, sizing HXs for condensing service depends a lot on:

- Specific heat of condensing fluids
- Fluid film coefficents
- Tube sizing and spacing
- Ability of the HX to remove non-condensibles
and etc....

Pick up the Kerns classic HX text or go find Perry's Handbook of Chemical Engineering and start to read up

Try Chapter 3 of this book.....

http://www.wlv.com/products/databook/databook.pdf

.....and gimmie a star


-MJC

 

[watwarrior]

Your right i have no experiance sizing exchangers. And im not trying to size the exchangers, im trying to track the fouling of these exchangers using the U factor. But i have 4 out of the 20 exchangers which end up being condensers on the shell side (although this is not the original design or purpose for the exchangers).

Both process flow are petroleum based - tube side is heavy crude oil and shell side is naptha, a lighter derivative of the crude oil.

 

[sshep]

watwarrior,

If you know all four inlet and outlet temps, and the flow and heat capacity of your crude, then you can calculate:

Q=Cp*(To-Ti); shell side energy balance
U=Q/(A*LMTD); overall heat transfer equation

where:
U=overall heat transfer coef
Q=duty
A=area (a constant)
Cp=heat capacity of your crude
To,Ti=crude outlet and inlet temp
LMTD=log mean temp difference (see referenced sources for definition and calculation using your four temperatures)

Of course an overall heat transfer coefficient may or may not track well over time depending on how constant it is through the exchanger. Condensing equipments are often conceptualized by zone analysis because the heat transfer in the condensing and subcooling portions of the exchanger may be vastly different- i.e. using different U's to calculate the required area in different zones of the exchanger which are then summed for the total area requirement. Still, the ease of calculating an overall U makes your calculation efforts worth a try.

best wishes,
sshep

 

[watwarrior]

My program has been able to predict the run length for the first series of exchangers in this pre-heat train thus far, using only the U factor ... so just for that it is worth it.

 

[rmw]

MJCronin makes an important point regarding the removal of non condensable gasses. Their presence can wreck even the best of "U" values.

rmw

 

[watwarrior]

there are no none condensable gasses in this stream. So that is not a problem. All of the vapour in the stream comes from the crude feed, which is already purged of all non-condensables before it is feed into the unit.

 

[Yuman]

He has to know the composition of each streams to calculate the Cp ,then for duty, he can calculate it by using enthalpies( knowing the flow).
or you just do it with HYSYS.