condenser design in aspen plus

Sat. steam64 kmol/hrTi = 374 KPi = 1 atm Ethylene Glycol

To = 365 K

Condensed steamTo = Tsat

Condenser Design on Aspen-Plus Software(Heat Exchanger design with a phase change)

Author: Jim Lang (©SDSM&T, 2000)

This is a continued look into the process of condensation and condenser design on Aspen-Plus. The following example will be used.

Problem statement : Saturated steam at 1atm and 101° C needs to be condensed so thatit may be used as a stripping fluid in a column downstream. Once again, Ethylene Glycolis available at 340 K and 1 atm. All of the steam needs to be condensed. The plantmanager recommends using a vertical countercurrent heat exchanger with the steam inthe tubes. Pressure drop is not a concern.

Schematic:

Ethylene Glycol657 kmol/hrTi = 340 KPi = 1 atm

Condenser Design Procedure © SDSM&T 2/9

Once again, hand calculations will be needed for the condenser design. Before beginningthe actual design on Aspen, make sure to read the following selections to become familiarwith the mechanisms of condensation.

Recommended readings:

Perry’s 7th edition; pg. 5-20 through 5-22 and 11-11 through 11-12Incropera and DeWitt; pg. 554 through 568.Geankoplis; pg. 263 through 266

General Design considerationsWhen a surface temperature of a solid is lower than the saturation temperature of a gas,condensation occurs. There are two forms of condensation: film and dropwisecondensation. The latter gives higher heat transfer coefficients; however, generally youneed surface coatings to achieve the dropwise mechanism. From this aspect, design ofcondensers is usually done with the assumption of film condensation; as was done withthis example. Just as in boiling design, the condensation heat transfer coefficients are onthe scale of 103 W/ m K.

An example of a condenser can be seen in Coulson and Richardson. Physically,condensers are very similar to normal shell-and-tube heat exchangers. The condensationcan occur on the outside or inside of the tubes. Each setup requires differentconsiderations as well as different heat transfer correlations. (See recommended readings)This example will use a vertical condenser with the condensation inside the tubes.Physically, the steam will flow from top to bottom inside the tubes while the Ethyleneglycol will move countercurrently in the shell area.

Design on Aspen is very similar to that of boiling design. (See reference two) Handcalculations will be needed again since Aspen has difficulty estimating condensation heattransfer coefficients accurately. On the other hand, the hand calculations can becomevery tedious. Generally, a system of equations from energy balances has to be solved andpossible iterations are needed.


Start by creating a flowsheet of a block from the HeatX icons. (See reference one or twofor help)After the flowsheet is complete (shown above), give Aspen the other requiredinformation: title, property methods, and the stream data given in the problem statement.(Note* you may want to run a simulation using the Heater block as was done in theprevious examples, this step will be skipped in this manual)

Now at the Setup page forthe heat exchanger (shownat left), run a shortcutcalculation based on the“Hot stream outlet vaporfraction.” Set thespecification to 0.0 so thatthe steam will leave theexchanger as saturatedwater.The flow will becountercurrent.

Click Next and run thesimulation.


Here are the results fromthe shortcut calculations.Make sure and check theoutlet conditions of bothstreams. The steam hascompletely condensed andthe Ethylene glycol hasrisen to 365 K, the designoutlet temperature.

Also notice the saturationtemperature of the steam.

Return to the Setup pageand change the calculationsto “Detailed.” Theexchanger specificationwill remain the same.

Now click on the U-methods page and specify that the overall heat transfer coefficientwill be calculated using “Film coefficients.” We will specify the calculation method forthe individual heat transfer coefficients on the next page. (Note*: this is just one way tocalculate heat transfer coefficients for condensation, a Fortran subroutine can beimplemented to calculate the coefficients, see reference one)


Shown above is the Film Coefficients input page. Just as in the previous example wewill need to enter in the heat transfer coefficients manually. (See reference two) Alsoremember that this page can only be reached if the overall heat transfer coefficient iscalculated from “Film coefficients.” Here for the hot stream, enter in the valuesobtained from hand calculations. The steam will have two phases so both the “vapor”and the “condensing” spaces need to be filled. Remember the correct units.

Now specify the outside heat transfer coefficient by entering results from handcalculations in the “cold stream” spaces. The glycol stream has only the liquid phase soyou just need a value for the “liquid.” (Shown below)


Now to set the geometry ofthe exchanger. Shown atleft is the input page for theshell. As done before, enterin the shell type, number oftube passes, shell diameterand the shell clearance.Also remember to specifythe exchanger orientation.The condenser in thisexample has verticalorientation with thetubeside fluid flowingdown.

Click on the Tubes page.

The tubes are entered just asbefore. (See reference oneand two)

Note*., the length of tubesin condensers are typicallyaround 16 ft. (~5 m)

Click on the Baffles page.


Baffles are needed incondensers for effectiveheat transfer. Enter theresults from handcalculations.If design values areunknown, then start withthe number of bafflesequal to twice the lengthof the tubes in meters.In this case, the length ofthe tubes is 5 meters, so10 baffles is a goodplace to start.

Again, do the same thing with the baffle spacing. If the baffle spacing is unknown, inputthe distances between the first baffles and the tubesheet. (See reference one for help)

Click on the Nozzles page.

Shown at left is the nozzlesinput page. Input thenozzles diameters. (Seereference one or two forrecommendations)

Also remember that thesteam is changing from agas to a liquid within thetubes, so the tube side inletdiameter will be greaterthan the outlet nozzlediameter.

Click Next and run the simulation.


The Summary page of theresults section is shownhere.

Check the outlet conditionsof both streams; make surethe steam condensedcompletely.

Here is the ExchangerDetails page.

Compare the actual andrequired areas for theexchanger. Remember it issafe to over-design byabout 10%.

As you can see, Aspen has“calculated” an averageheat transfer coefficient.(1467 W/m2 K) Actually,Aspen just found anaverage value from thenumbers that you entered

earlier.

The other results can be seen in the Detailed Results section. All of the results pagesshould be looked at to ensure the design is accurate and to make sure the exchanger iswithin recommended limits. Of course, more optimization may be needed.


The simulation should be rerun with a different exchanger specification, perhaps with thegeometry. This will give you a good idea if the exchanger layout is designed properly.Once the design is completed print out the input page as well as the results.

Closing comments

This example showed one setup for condenser design. Some situations will be different,perhaps with the condensation occuring on the outside of the tubes. Remember to changethe input accordingly to comply with the new condenser.

You can apply this manual to other general forms of heat transfer unit operations.Fortunately, one can manipulate Aspen to achieve accurate design results. However, assaid before, always question the results from Aspen. Compare the results with handcalculations. Furthermore, this manual gives just an introduction to heat transfer designon Aspen. More complex and detailed design calculations can be done. It just takes timeto understand how Aspen works and understanding what calculation methods give thebest results for each specific situation.

References

1. Lang, Jim. “Design Procedure for Heat Exchangers on AspenPlus Software” Designmanual. June 1999.

2. Lang, Jim. “Boiling Design on Aspen-Plus.” Design manual. July 1999.

3. Aspen Plus Simulator 10.0-1. User Interface (1998).

4. Coulson and Richardson. Chemical Engineering Fluid Flow, Heat Transfer and MassTransfer. Volume 1, 5th ed., Butterworth and Heinemann, 1996.

5. Geankoplis, Christie J. Transport Processes and Unit Operations, 3rd ed., PrenticeHall, 1993.

6. Incropera and DeWitt. Fundamentals of Heat and Mass Transfer. 4th ed., John Wileyand Sons, 1996.

7. Perry, P.H. and Green, D. Perry’s Chemical Engineering Handbook. 7th ed.,McGraw-Hill Book Co., 1997.

condenser design in aspen plus

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