4 replies to this topic

[Jiten_process]

hello...

I have got the problem of kettle vaporizer. I am designing kettle type vaporizer (AKT) on HTRI. I have got the licenser's spec. sheet which shows flow rate, temp. condition, inlet pressures, fouling etc. PZ note that i have no data for any bundle dia. or entrainment ratio specified in datasheet. So i have started with the normal procedure of designing H.E. on HTRI. I input hot fluid properties and cold fluid properties as below.

hot fluid tube side :
Inlet/oult : Diesel (423600kg/hr)
Temp : 195°C to 171°C @ 12kg/cm2g inlt pressure

Cold fluid shell side
Inlet: Water (11871kg/hr) @ 120°C & @5kg/cm2g pressure
Oultet : vapor (11277kg/hr) @ 155°C
Liquid (594kg/hr) @155°C

(It is to be noted that initially it is 120 to 155°C sensible heat transfer. )

After inputing above properties data with heat release and basic minimum required geometry data (pitch, tube length, tube thck etc) on HTRI input, i encountered with fail design run with 49% undersize. HTRI takes entrainment ratio 0.01 by default if unspecified.

Why my design run got failed with such a hugh error???

Now my questions are
1) what am i supposed to input more on above while designing kettle type vaporizer on HTRI when i have just a spec. sheet, (it is not the distillation column bottom reboiler. )
2) i know the entrainment ratio governs the kettle drum diameter size, but what is the guidlines to specify entrainment ratio; as such HTRI allows to run design without inputting of all these data.
3) What is the critical points one should take care while designing kettle reboiler?
4) i would like to be suggested some resources or literature for general guidlines of kettle type reboiler on HTRI?

Plz give ur valuable inputs and tell me if more data is required to comment...
awaiting responses...???

[Zauberberg]

Jiten,

The first thing in designing kettle-type reboilers (and every other pool boiling service) would be to define maximum allowable heat flux for a given application, and these data can be found in almost every heat transfer textbook. I am always referring to D. Kern's excellent publication, "Process Heat Transfer" which should be readilly available for everyone who is planning to work on heat transfer equipment design or rating. The higher is the shell/bundle diameter, the lower is the maximum allowable heat flux. Make sure that (Q/A) and (U*MTD) values are below critical heat flux - otherwise there will be serious problems in the operation of such reboiler. Your particular application is not a vaporizer, but rather (re)boiler since you are evaporating only a small portion of exchanger feed. Also, the amount of de-subcooling duty is quite high: exchanger feed is deeply subcooled at given process conditions, so this exchanger will act as heater/partial vaporizer, very similar to kettle reboilers.

Based on the heat flux you have used in your application, kettle-to-shell diameter ratio can be selected. I also found good references in standard ChE textbooks, and again, the higher the heat flux - the higher is the ratio between kettle and bundle/shell diameter. These ratios usually go from 1.2 to 1.7. As a final check, there is a formula for calculating maximum vapor disengagement velocity, and the velocity you get from your design should be always lower than the maximum value.

I haven't used HTRI, but I believe it is quite similar to Aspen HETRAN which I am very familiar with.
To address your questions:

1. You should have these parameters fed to the software: calculated heat duty, maximum allowable heat flux, physical parameters of both fluids within the process temperature/pressure intervals, % overdesing required (or fouling factors - but be reasonable and do not specify too high fouling factors), tube pattern (square) and tube pitch (1.5-2.0 times tube OD).

2. Entrainment is defined by maximum vapor disengagement velocity, which I have explained above.

3. The most important thing is critical heat flux. Also put your attention on minimum/maximum tubeside velocities, pressure drop available on the tube side, tube geometry.

4. Kern's book is what you need. I believe HTRI tutorials should also give you more information about the design procedure.

I had similar application in one of my projects, and I was strongly advised by our dear friend and mentor, Art Montemayor, not to use floating head design in kettle reboilers - due to high probability of leakage during operation. Therefore, additional suggestion is to switch to conventional U-tube design, or maybe you want to explore the possibilities of thermosyphon or forced circulation types.

Best regards,

[Jiten_process]

hi zauberberg

firstly, thanks a lot for your focused reply, i already read your reply the next day, but was waiting for some more reply.

neways, i got your point completely, i have gone through ludwig and found those max. allw. flux data too. But the d thing is HTRI takes care these data and accordingly selects the shell id and kettle id. What additional we can see in output is boiling mechanism. As per guidlines, kettle reboiler should be operated in flow regime (combination of nucleate boiling and convective heat transfer) which lies in the left of critical heat flux region. However there is nothing wrong to operate kettle in completely stable film boiling regime. what is your comment on it?

I found that in order to get flow boiling regime, the temp. difference between heating medium and boiling temp. should be 60°F. Further the too high difference in delta T at the both end also not recommneded, and perhaps that is where i was goofing. But finally i have converged the design with 5% overdesign and 1450mm shell id.

Then what i did is, trying to study more on HTRI, in my below problem, i have changed the heating medium and instead of diesel and i input saturated steam as a heating medium. And considering above guidlines i have used steam pressure of 16bar and succeeded in converging design with 16% overdesign and 650mm shell id, 1065mm kettled id. Then to study the effect i kept on reducing just the pressure (no any other input data) and converged for 15bar pressure too...but just at 14bar, i found my design run fails suddenly. May be i guess it is the temp. difference but still it is anyway higher than the 60°F.

I still remember mr. art's statement, these softwares is stupid computer programme however we cant blame it for mistake, coz it is we who operate it and we should be thorough in fundamentals. So i am just exercising on it.

[jcazenave]

Hi

an alternative is to use the latest offering from Aspentech, which replaces HETRAN mentioned by Zauberberg.

http://www.aspentech...e/aspen-edr.cfm

Based on your post I try and it seems to work even with lower pressure steam. However I dont know if I have all the correct data.

Kind Regards

[Zauberberg]

Jiten,

If the design of exchanger completely fails, it's usually because of contradictory input data or inconsistent design logic. It can happen, for example, that you are changing supply steam pressure and not allowing HETRAN to accomodate these changes with any degree of freedom - such are steam flow, inlet/outlet temperature etc. Check all the input data you have provided for the "failed case" and give us more information, or upload HETRAN file. They are usually very small in size.

Best regards,