Comparison between HE efficiencies 1

[MedicineEng]

Dear All:
I have a situation where I should choose between 2 different HE.
The data is the following:
HE1: material: graphite, type: annular, area: 6.4m2;
HE2: material: Hastelloy C, type: tube and shell, area: 2.3m2.
Taking into consideration that the supply of utilities doesn't change in any case (flow/temp), how do I check which one is more efficient in terms of heat transfer?
I there any rule of thumb to compare graphite HEs with metal (namely Hasteloy), like for instance graphite has to have X times more area for the same heat transfer?

Thanks a lot for your help.

 

[insult2injury]

Hard to compare the two when they are different geometries, as well as the operating conditions and application is unknown. I think that you are going to need to run through a full rating on each of them. Don't forget to look at maintenence concerns as well.

I2I

 

[25362]

The answer evidently depends on the service (film coefficients). From table 10-20 of Ludwig's volume 3: Applied Process Design for Chemical and Petrochemical Plants, titled Effect of tube wall material and film condition on overall coefficient when comparing SS 304-16 BWG (A), graphite 3/16" thk. wall (B), and film coefficients (h_o+ h_i), (C), in %

(A) (B) (C)

Heating water with steam 92.5 100 150
Condensing organic vapor with water 96.5 100 122
Cooling as above 100 100 116
As above viscous organic liquid 100 100 106

The values of (C) in kcal/(hr.m².K) are, respectively, 1500, 500, 250, 100.

From the table we learn that graphite is superior to SS 304 (assumed similar to Hastelloy) when the film coefficients are high ~1500 kcal/(hr.m².K), and their performances are similar when the film coefficients are low ~100 kcal/(hr.m².K).

 

[MedicineEng]

25632:
Thanks a lot for your answer. That's what I wanted to know. But as frequently happnes, an answer to aquestion raises another question.
I confess my little knowledge on this issue, so can you please elaborate more what are where can I find anything regarding film coefficients? I believe that this is dependent on the type of fluid, isn't it?
How can I know in my case? Do I need to run some lab tests?
Is there any table for let's say common organic solvents?

 

[25362]

HT convection film coefficients for fluids, including organic solvents, depend on their thermophysical properties, affected by the running temperatures, their flow r[é]gime sometimes affected by the surface asperities and geometry, and in the case of change of phases, as in condensation or boiling, sometimes also affected by the presence of non-condensables and deposits.

Ranges of HTC can be found in Perry's ChE manual (McGraw-Hill), or you may find of interest Fig 3-22 in Donald R. Woods' Process Design and Engineering Practice (Prentice Hall) ISBN 0-13-805755-9.

Forced convection HTC in pipes or ducts where the movement is produced by a pump, a stirrer, or some outside means, depend on dimensionless groups (called numbers herebelow):

Nu=C(Re)^a(Pr)^b(L/D)^c([μ]/[μ]_wall)^d​

where
Nu = Nusselt number
C = empirical constant
Re = Reynolds number
Pr = Prandtl number
L/D = length to diameter ratio
[μ] = viscosity
a,b,c,d = experimental exponents

For most cases ([μ]/[μ]_wall)^d ~1. For flow outside tube banks, D is replaced by a "hydraulic" diameter.

If you have all the relevant data I suggest you look for the Sieder and Tate typical graph in any book dealing with heat transfer.

HT = heat tranfer, HTC = heat transfer coefficients

 

[insult2injury]

The performance is also largely dependent on the geometry, so be sure to take a look at the pros and cons and what's recommended for your application. Will there be a phase change? A heat exchanger design book may be of use. Heat Exchangers: Selection, Rating, and Thermal Design by Kakac and Liu is a decent book for a beginner.

I2I

 

[MedicineEng]

Thank you all for your input.
Just to elaborate a little bit more in my application:
This will be a vent condenser for a reactor where we distil a mixture of organic solvents.
In fact I just wanted to see if there was any rule of thumb comparing heat exchangers of different materials, in this case metallic HE with graphite ones, but apparently there isn't.
Either way I will spend some time trying to figure out which is best for our application, but apparently there are may variables that I do not know what makes my life more difficult.