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 AspenPlus. The following example will be used.
Problem statement : Saturated steam at 1atm and 101° C needs to be condensed so that it may be used as a stripping fluid in a column downstream. Once again, Ethylene Glycol is available at 340 K and 1 atm. All of the steam needs to be condensed. The plant manager recommends using a vertical countercurrent heat exchanger with the steam in the tubes. Pressure drop is not a concern. Schematic:
Sat. steam 64 kmol/hr Ti = 374 K Pi = 1 atm
Ethylene Glycol To = 365 K
Ethylene Glycol 657 kmol/hr Ti = 340 K Pi = 1 atm Condensed steam To = Tsat
Condenser Design Procedure
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Once again, hand calculations will be needed for the condenser design. Before beginning the actual design on Aspen, make sure to read the following selections to become familiar with the mechanisms of condensation. Recommended readings: Perry’s 7th edition; pg. 5-20 through 5-22 and 11-11 through 11-12 Incropera and DeWitt; pg. 554 through 568. Geankoplis; pg. 263 through 266 General Design considerations When 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 dropwise condensation. The latter gives higher heat transfer coefficients; however, generally you need surface coatings to achieve the dropwise mechanism. From this aspect, design of condensers is usually done with the assumption of film condensation; as was done with this example. Just as in boiling design, the condensation heat transfer coefficients are on the 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 condensation can occur on the outside or inside of the tubes. Each setup requires different considerations 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 Ethylene glycol will move countercurrently in the shell area. Design on Aspen is very similar to that of boiling design. (See reference two) Hand calculations will be needed again since Aspen has difficulty estimating condensation heat transfer coefficients accurately. On the other hand, the hand calculations can become very tedious. Generally, a system of equations from energy balances has to be solved and possible iterations are needed.
Condenser Design Procedure
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Start by creating a flowsheet of a block from the HeatX icons. (See reference one or two for help) After the flowsheet is complete (shown above), give Aspen the other required information: 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 the previous examples, this step will be skipped in this manual)
Now at the Setup page for the heat exchanger (shown at left), run a shortcut calculation based on the “Hot stream outlet vapor fraction.” Set the specification to 0.0 so that the steam will leave the exchanger as saturated water. The flow will be countercurrent.
Click Next and run the simulation.
Condenser Design Procedure
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Here are the results from the shortcut calculations. Make sure and check the outlet conditions of both streams. The steam has completely condensed and the Ethylene glycol has risen to 365 K, the design outlet temperature. Also notice the saturation temperature of the steam.
Return to the Setup page and change the calculations to “Detailed.” The exchanger specification will remain the same.
Now click on the U-methods page and specify that the overall heat transfer coefficient will be calculated using “Film coefficients.” We will specify the calculation method for the individual heat transfer coefficients on the next page. (Note*: this is just one way to calculate heat transfer coefficients for condensation, a Fortran subroutine can be implemented to calculate the coefficients, see reference one)
Condenser Design Procedure
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Shown above is the Film Coefficients input page. Just as in the previous example we will need to enter in the heat transfer coefficients manually. (See reference two) Also remember that this page can only be reached if the overall heat transfer coefficient is calculated from “Film coefficients.” Here for the hot stream, enter in the values obtained 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 hand calculations in the “cold stream” spaces. The glycol stream has only the liquid phase so you just need a value for the “liquid.” (Shown below)
Condenser Design Procedure
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Now to set the geometry of the exchanger. Shown at left is the input page for the shell. As done before, enter in the shell type, number of tube passes, shell diameter and the shell clearance. Also remember to specify the exchanger orientation. The condenser in this example has vertical orientation with the tubeside fluid flowing down.
Click on the Tubes page.
The tubes are entered just as before. (See reference one and two) Note*., the length of tubes in condensers are typically around 16 ft. (~5 m)
Click on the Baffles page.
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Baffles are needed in condensers for effective heat transfer. Enter the results from hand calculations. If design values are unknown, then start with the number of baffles equal to twice the length of the tubes in meters. In this case, the length of the tubes is 5 meters, so 10 baffles is a good place to start. Again, do the same thing with the baffle spacing. If the baffle spacing is unknown, input the distances between the first baffles and the tubesheet. (See reference one for help) Click on the Nozzles page.
Shown at left is the nozzles input page. Input the nozzles diameters. (See reference one or two for recommendations)
Also remember that the steam is changing from a gas to a liquid within the tubes, so the tube side inlet diameter will be greater than the outlet nozzle diameter.
Click Next and run the simulation.
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The Summary page of the results section is shown here.
Check the outlet conditions of both streams; make sure the steam condensed completely.
Here is the Exchanger Details page. Compare the actual and required areas for the exchanger. Remember it is safe to over-design by about 10%. As you can see, Aspen has “calculated” an average heat transfer coefficient. (1467 W/m2 K) Actually, Aspen just found an average value from the numbers that you entered earlier.
The other results can be seen in the Detailed Results section. All of the results pages should be looked at to ensure the design is accurate and to make sure the exchanger is within recommended limits. Of course, more optimization may be needed.
Condenser Design Procedure
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The simulation should be rerun with a different exchanger specification, perhaps with the geometry. 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 change the 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, as said before, always question the results from Aspen. Compare the results with hand calculations. Furthermore, this manual gives just an introduction to heat transfer design on Aspen. More complex and detailed design calculations can be done. It just takes time to understand how Aspen works and understanding what calculation methods give the best results for each specific situation.
References 1. Lang, Jim. “Design Procedure for Heat Exchangers on AspenPlus Software” Design manual. 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 Mass Transfer. Volume 1, 5th ed., Butterworth and Heinemann, 1996. 5. Geankoplis, Christie J. Transport Processes and Unit Operations, 3rd ed., Prentice Hall, 1993. 6. Incropera and DeWitt. Fundamentals of Heat and Mass Transfer. 4th ed., John Wiley and Sons, 1996. 7. Perry, P.H. and Green, D. Perry’s Chemical Engineering Handbook. 7th ed., McGraw-Hill Book Co., 1997.
