Optimize Thermal & Mechanical Design for Shell & Tube Heat Exchangers

Transcription

1 Optimize Thermal & Mechanical Design for Shell & Tube Heat Exchangers Webinar Q&A This document summarizes the responses to questions posed before and during the webinar on general Heat Exchanger Design topics. The topics have been arranged into four different categories: 1. General Heat Exchanger Design 2. Exchanger Design & Rating (EDR) Capabilities 3. Input/UI 4. Calculation Results Additional questions should be directed to AspenTech Support. General Heat Exchanger Design 1. Is there a database or recommendations for the fouling factors? Yes, we provide the recommended values for TEMA Design Fouling Resistances for Industrial Fluids, Chemical Processing, Natural Gas Processing, Oil Refining, and Water. For links to the data, open the Help menu and choose Search for help on. Type TEMA Design Fouling Resistances. 2. Are there any recommendations for hydraulic aspects, optimum velocity in tubes, etc.? EDR Shell & Tube provides a full Flow Analysis including RhOV2 calculations at the Shell &Tube entrance and exit points and nozzles. Warnings will alert the user when calculated values exceed the TEMA limit. Full documentation of our flow calculations as well as a design guide for choosing economic velocities for the tubeside and shellside streams can be accessed with a subscription to the HTFS Research Network. Under Input > Program Options > Design Options >Process Limits the user can constrain the design search by minimum and maximum velocities. 3. Can EDR handle a thermosyphon reboiler? Is there an example file of it? Aspen Shell & Tube Exchanger can design, rate, and simulate operation of themosyphon reboilers. The program allows you to specify the interconnecting pipework in detail (if you have that information). Simulation mode is important, as the circulation rate and the thermal performance are highly interdependent. The program can also calculate the circulation rate that will be achieved. We provide two example files for thermosiphon reboilers. These examples are installed with Aspen EDR and you can find them and other EDR Shell & Tube sample files at the following location in Windows Explorer: C:\Program Files (x86)\aspentech\aspen Exchanger Design and Rating V8.0\Examples\Shell&Tube. 4. How do you optimize a thermosiphon reboiler in an ethylene oxide process? Our research shows that the program does a very good job of predicting actual circulation rate for both vertical and horizontal thermosyphon reboilers. Two-phase flow patterns are calculated while the program checks for potential instability. For a particular thermosyphon reboiler case, please contact us.

2 5. How do you anticipate and compensate for fouling pressure drop increases? If the user specifies a fouling layer thickness explicitly under Input > Program Options > Fouling the program will calculate the impact of this on hot-side and cold-side pressure drops. On the shellside, the fouling will be assumed to also block the tube-to-baffle clearance and the shell-to-baffle clearance. If you specify a process fouling, resistance, and thermal conductivity of the fouling material, the layer thickness can also be calculated. 6. Is the complex design, Triple Segmental, supported by TEMA? TEMA recognizes single-segmental, double-segmental, and triple-segmental baffle arrangements. If you have a large shell diameter and a large number of tubes, this can give a practical design that has a feasible baffle overlap (usually 2 or more tube rows overlap). Several of our fabricators who have been using our arrangement of triple-segmental baffles for years have had good results. Exchanger Design & Rating (EDR) Capabilities Supported Types 1. Does Aspen EDR carry out twisted tube design? Aspen Shell & Tube has links to the Koch Heat Transfer twisted tube modeling via a special dii module, which can be obtained from the Koch Heat transfer. When present, it links into the local shell-side and tube-side calculations of Shell & Tube to fully model a twisted tube unit. 2. How effective is Aspen EDR in evaluating/designing vertically oriented exchangers? The program can design, rate, or simulate vertically orientate units with the same ease and accuracy as horizontal units. 3. Is there an option for hairpin HX in EDR? Yes, under Input > Exchanger Geometry > Shells/Heads/Flanges > Shells/Heads. Here, you can select the shell type as Hairpin multi-tubular (we designate M-type ). 4. Can this design be applied to a solid-liquid heat exchanger? We do not currently model solid-liquid heat exchangers. 5. What would be the recommended approach to modeling plate and frame heat exchangers if EDR does not have built-in plate designs from vendors such as Alfa Laval? You can use Aspen Plate Exchanger to design, rate, and simulate plate and frame, brazed plate, or welded plate type exchangers. Aspen Plate Exchanger uses our own proprietary plate heat transfer and pressure drop models and correlations. It is used by many customers as an independent evaluation of vendor designs. This can be especially critical in two-phase applications. 6. Does EDR perform special code calculation, such as those required for graphite heat exchangers? ASME has new code calculations for graphite exchangers. If the market requirement is large enough, we may add these to our mechanical products in the future. 7. Can you compare a spiral exchanger design with EDR? The Alfa Laval spiral type is not currently modeled by EDR. 8. How accurate are the calculated costs vs. actual costs in EDR? This will depend on the specific cost factors that you use and how closely they match to the actual fabrication cost. We do have fabricators who customize the labor rates, material costs, and other database entries to get accurate cost figures. We have several standardized lists to choose from and you can enter your own cost factors by opening the Tools menu and selecting Costing Database (Figure 1).

3 Figure 1: How to enter your own cost factors 9. Can I specify heat loss in Aspen EDR? EDR programs do not consider external heat loss in the analysis. We assume there is a highly effective insulation in place. 10. What steps do you take to reduce vibration and resonance issues in exchanger designs? EDR does a full vibration and resonance analysis on tubes susceptible to vibration. On the Tubesheet Layout diagram in the Mechanical Summary you can view which tubes were identified by the program as possible vibration tubes. The Thermal/Hydraulic Summary shows the report of the Fluid Elastic Instability and Resonance Analysis for each of these tubes. EDR vibration calculations are based on HTFS methods or the TEMA Analysis can also be run. If any vibration indicators are detected, this will be reported in the Warnings section as an Operation Warning. Users can easily compare double-segmental baffles with single segmental; they can explore notubes in window segmental baffles and specify additional supports. It may also be effective in some cases to explore alternative shell configurations. You may like to view solution number: , Avoid Vibration Risks When Design Shell & Tube Exchangers (14:21). This shows a typical approach to eliminating vibration risk in the design of an exchanger. 11. What is the purpose of the following calculation modes: Design, Rating, Find Fouling, and Simulation? Design Mode: Performs an extensive design search to find the best exchanger geometry to achieve the specified duty. Requires a set duty and allowable pressure drops. Rating/Checking Mode: The geometry and process conditions are specified by the user. The program compares the area required to perform the duty with that specified in the input. The main results are the area ratio (>1 the exchanger is over-surface, <1 the exchanger is over surface). Some engineers use repeated rating calculations to manually find their choice of design. If the area ratio is far removed from 1.0 then Simulation will provide more reliable results. Simulation: Considers the exchanger geometry to be fully specified and calculates the outlet stream conditions that can be achieved in the exchanger from the specified inlet conditions.

4 Find Fouling: This calculation mode is similar to rating but adjusts the fouling resistance to determine the maximum values which gives an area ratio of How do you generate plots of S&T heat exchanger performance in EDR? Can you generate an overall schematic of several exchangers in series or parallel? In the new Shell & Tube Console V8 onwards temperature profile plots for shellside and tubeside are shown after each run. You can also view detailed output from the Aspen Shell & Tube incremental calculations under Results > Calculation Details > Analysis along the shell/ Analysis along the tubes. There is a tab available where you can view plats of temperature, pressure, heat transfer coefficients, vapor fraction etc. Currently there is no schematic of shells in series and parallel. 13. If I have a two-phase case (two-phase on both sides), how can I check the Heat of Vaporization in the simulation? Can the EDR plot this? Under the Overall summary, you will find a tabulation of the breakdown of heat load for a two-phase case (Figure 2): Figure 2: Breakdown of heat load Under Results > Calculation Details > Plots > Analysis Along Shell/Tubes you can view plots of specific enthalpy vapor fraction or heat load of the streams. 14. Is dynamic modeling possible with EDR? Dynamic modeling is not supported in Aspen Exchanger Design & Rating. However, using the results of EDR sizing can help to achieve a more accurate dynamic model in Aspen Plus and Aspen HYSYS. 15. Does the exchanger rating also take into account seasonal variations in temperature of the environment, as well as climatic variations of different countries? Within EDR, you can set up thermal models in Rating or Simulation modes for different operating conditions and track the results under Results > Results Summary > Recap of Designs. You may also find it very useful to use Aspen Simulation Workbook with EDR to conduct case studies of this nature with EDR working within Microsoft Excel. Visit the Support site and view Knowledge Base Item number : Exploring multiple operating scenarios using Aspen Simulation Workbook and Aspen Shell & Tube Exchanger. With your EDR models embedded in an Aspen HYSYS or Aspen Plus simulation, you have a much more effective tool to study how an exchanger and the overall process will react to seasonal variations. 16. Can the bend of the U-bundle be supported? Yes, you can specify a full baffle support and additional supports around the U-bend region itself. These can be very important in combatting flow induced vibration.

5 17. Does EDR have the ability to model condensers using differential heat curves? With the old B-JAC properties and VLE methods, you can specify differential condensation curves. It is normal to use the integral condensation method which has been proven to be accurate for a wide range of process conditions. Stream data transferred from the simulators is provided in integral VLE curves. 18. Is there a separate package to the HTRI program that is already supplied by AspenTech? AspenTech does not supply HTRI software products; HTRI is a competitor. 19. What are the major differences between EDR and HTRI? How does the thermal design compare with HTRI models? AspenTech is unable to make direct comparisons of our offering to HTRIs; however, there are many benefits to using Aspen EDR. The webinar associated with this Q&A document focuses on several of these benefits, including: Integration with Simulation: Aspen EDR models can be embedded in flowsheets from Aspen Plus & Aspen HYSYS industry leading process simulators for both Chemicals, Oil, and Gas applications improving flowsheets for revamp studies, troubleshooting operations, and debottlenecking processes. See the Petrofac Case Study for an example of how a customer benefitted from rigorous Aspen EDR models in Aspen HYSYS. Physical Properties: Aspen EDR can import properties from Aspen Plus & Aspen HYSYS cases (as shown in the webinar) or properties can be specified using one of the included databanks: BJAC, Aspen Properties, or COMThermo. The integration and the breadth of the property databanks significantly reduce the time required to input process data. Thermal Modeling: EDR thermal modeling is based on our HTFS research heritage from the late 1960s and continues with our current programs. This provides state of the art correlations and methods documented in the Aspen HTFS Research Network. The Research Network is licensed by many of our leading customers. We are regularly involved in analyzing comparisons between our results and those of other software suppliers. When called upon, we are always happy to demonstrate the validity of our methods with reference to sound theory and extensive validation data covering a wide range of exchanger configurations and process conditions. Thermal Design Optimization: Aspen Shell & Tube typically evaluates hundreds of exchanger configurations considering the number of shells required, as well as details of shell geometry. The optimum design is selected on a rigorous algorithm for cost of labor and materials of construction. Similar optimization capabilities are also available in the Aspen Air Cooled Exchanger program. Mechanical Design Optimization: Aspen Shell & Tube Mechanical is the only pressure vessel design product on the market that was developed exclusively for heat exchangers. Drawing on the long heritage of the B-JAC Teams program, Aspen Shell & Tube Mechanical provides powerful, optimized mechanical design that recognizes the interaction of key components. The designs produced minimize material thicknesses and reduce costs while still maintaining design integrity and conforming to internationally accepted code standards. Thermal & Mechanical Integration: The integration between the thermal software and the mechanical software significantly reduces the time to hand off designs between thermal designers and mechanical engineers. If you re interested in learning more about how Aspen EDR can benefit your business, please contact your Sales Account Manager or visit

6 Input/UI 1. When rating an existing exchanger, how do you change the way baffle cut is specified (% area vs. % diameter)? In the EDR Shell & Tube program, the baffle cut is expressed as a percentage of the shell inside the diameter. You will need to use a manual calculation from the figures expressed in the percent shell side net free area (taking account of the area occupied by the tubes) to convert to a % shell diameter basis. 2. Where does EDR put in supports (I'm talking about the extra supports option)? If you specify additional supports between tubes in window baffles, these are equally spaced between baffles. They will appear illustrated on the setting plan. Similarly, additional supports in inlet and outlet regions will be equally spaced between the first baffle and the tube sheet. 3. How do you export heat exchanger process conditions from Aspen Plus to EDR? On a HYSYS Exchanger, go to Design > Parameters and select Rigorous Shell & Tube as your Exchanger Model. Then, go to the Rigorous Shell & Tube tab and click the Export button, which will then open a file. Save the file, choose a folder location, and name it as.edr. 4. How can you have two outlet streams (flashed vapor and liquid) on one side of a Heat Exchanger? In Shell & Tube under Input > Exchanger Geometry > Nozzles you can select the option to have a separate liquid and vapor nozzle. You may also specify dimensions or have the program size these for you. 5. How can I send the calculated geometry back to Aspen HYSYS and/or Aspen Plus to be used in the simulation? When you have an EDR model, you can import it into a HYSYS Exchanger. Go to Design > Parameters and select Rigorous Shell & Tube as your Exchanger Model. Then, go to the Rigorous Shell & Tube tab and click the Import button. A file will appear. Open the dialogue box which allows you to browse a folder location where you select your case file.edr. 6. Where can I find the internal volume of a heat exchanger, Shell and Tube side, to calculate the necessary mass of the fluid? The Shell and Tube Mechanical program outputs the shell side or tube side volumes, as well as the Operating Weight (with the process fluids present). See Figure 3 below. Figure 3: Internal volume of a heat exchanger

7 7. Where is the actual area vs. the required area reported on when rating an existing exchanger? You can find the area ratio on the Results Summary > Overall Summary form, and on the Thermal Hydraulic Summary > Performance form (Figure 4). Figure 4: Actual area vs. required area 8. Can you describe the manual input for stream properties without using the HYSYS property input? If you refer to the Aspen Support Knowledge Base, Solution number : Learn how to specify properties for Aspen Exchanger Design & Rating (EDR) family of products for process heat exchanger modeling, it shows how to manually specify properties for single phase, two-phase, and Vapor Liquid-Liquid systems. You will also find tutorials on using the physical property packages included in Shell & Tube and the other Aspen EDR products. 9. What is the physical property file extension saved as? Case files are saved as.edr files which contain all input and results for the case, including physical properties data. Calculation Results 1. Can you obtain a results report with the thermal and mechanical design combined? Currently, our Shell & Tube Thermal and Mechanical programs produce individual reports which you may tailor to your needs. 2. How would you design and double pipe an exchanger? Under Input > Exchanger Geometry > Shells/Heads/Flanges > Shells/Heads you can select the shell type as Double Pipe (designated as D-type ). 3. Before we begin, does it apply to both Division 1 & 2 of the ASME code? The mechanical calculations apply to ASME Division 1. ASME is progressively bringing together mechanical calculations so that more advanced Division 2 methods can be added to our program with each release.

8 4. In X shell design, what is the accuracy level in the mechanical design when shell side nozzles are more than 2? Generally, there shouldn t be problems with the mechanical aspects of multi-nozzle X-shell designs unless the nozzles are in very close proximity. Please contact AspenTech Support if you have a case you would like us to look at. 5. Do multi-nozzle X-shell designs consider heat loss, based on the environment, to insulation thickness? No, it is assumed that exchangers are well insulated. 6. How does Aspen EDR calculate the mixed oil and water viscosities? In our research program, we have recently looked closely at modeling in this area and provided new recommendations on handling the modeling of streams with two liquid phases. If you provide property data for two liquid phases, the program will default to the new HTFS Method handling condensation somewhat differently from heating or cooling with no phase change. You may also over-ride the program selection under Input > Program Options > Methods/Correlations > General and select your preferred method for viscosity of two liquid phases. You can also look on our support site at the Aspen Support Knowledge Base Solution White Paper - Heat Exchanger Methods for Effective Viscosities of Two Immiscible Liquid Phases. 7. EDR does not consider high pressure when calculating baffle thickness and number of baffles. Why can t you fit the same number as calculated in EDR design? Generally, there shouldn t be an issue with high pressure units in terms of number of baffles. What might be more important here is the thickness of the tube sheets, positioning of nozzles, and the interaction of these with the baffle spacing. Please contact AspenTech Support if you have a case you would like us to look at.