Sizing of heat exchangers using HYSYS or UniSim
First read heat exchanger calculations.
HYSYS and UniSim are capable of calculating Q/DTavg (which they designate as ďUA,Ē even though itís actually UAF.)† They can also provide a plot of T for both streams versus the amount of heat transferred.† Unfortunately, the academic portion of HYSYS or UniSim in Weighted or End point Engineering design give a heat-transfer area A of 60.32 m2 for all heat exchangers, regardless of what is specified.† It calculates U by dividing UA by this value.† Such U and A in are meaningless and, therefore, must not be used.† If UniSimís Heat Exchangers suite is also installed, then you can do a detailed design that will calculate area and pressure drop.† Modules are available for shell & tube exchangers, crossflow exchangers, fired heaters, feed water heaters, plate & fin exchangers and plate exchangers.† These are accessed either via tabs on the Heat Exchanger setup window, or by loading these separately.† The following assumes you are using the simpler weighted method.
For economic evaluation, you have to estimate the utility requirements and A for all of your heat exchangers, including the condensers and reboilers with the distillation columns.† Initially in creating a pfd, it is simplest to use coolers and heaters rather than heat exchangers.† It is assumed here that you have moved beyond that.† You must use one of the built-in heat exchangers in order to estimate size and utilities requirements.† However, it is recommended that you do not delete heaters and coolers in your pfd and insert heat exchangers to take their place.† Instead, record all information for the process streams connected to heat exchangers, open a separate case and enter the stream information there.† Connect these with appropriate heat exchangers and select appropriate utility streams (cooling water or steam, if possible).† Below is a procedure to use HYSYS or UniSim to perform such calculations from your converged pfd.†† First read the notes at the end.
Heuristics (rules-of-thumb) are cited in several places below.† See those listed at Heat Exchangers.†
1. (This step assumes you have used heaters and coolers in your pfd.† If you are developing the pfd and want to insert a heat exchanger, skip to step 3.)† Select a cooler from your converged case-study pfd.† Copy down all of the stream information (T, P, component flows) for the process streams that enter and leave this unit.† Enter this stream information into a new case.† Make certain you have the same Basis as in your original case.† (Alternately, you can copy from your converged pfd as follows.† Click Export in the Basis Environment, open a new case, and click on Import.† For streams, create a new stream for each process stream you need and Define its properties from the existing stream, then Copy it and Paste into your new pfd.)
2. Use the palette to select a heat exchanger to replace a cooler.† Connect the two relevant process streams to its inlet and outlet.
4. Return to the Basis Environment and add an appropriate basis for the utility.† (For water or steam, for example, use the ASME Steam Property Package.)
5. Return to the Simulation Environment.† Open the Heat Exchanger Design Connections page and enter the basis you created in 4 and appropriate names for the utility inlet and outlet streams.
6. On the Design Parameters page enter the appropriate pressure drops from the heuristics listed above.† Select Exchanger Design (Weighted) for the Heat Exchanger Model.† Increase the number of intervals for the Individual Heat Curve Details calculations.
7. On the Worksheet Compositions page enter the composition information for the utility inlet or outlet stream.
8. On the Worksheet Conditions page enter reasonable inlet and outlet conditions for the utility stream (see the heuristics listed above).† Particularly make sure neither of the temperature differences at the ends of the exchanger violates the minimum temperature approach heuristic.† If everything's okay, the heat exchanger calculations will converge.† If you get a warning that a temperature cross has occurred, the most probable reason is that you have appreciable latent heat and sensible heat being exchanged in one or both of your streams.† See i below for the solution.
9. Click on Performance Plots.† Change the X-axis to Heat Flow and the Y-axis to T.† If the curves do not cross, print out this plot.† If they do cross, make appropriate changes to the utility conditions.† If they have abrupt changes in slope, see i below as this probably indicates a shift between latent heat and sensible heat transfer.† This requires you to model this heat exchanger as two or more heat exchangers.† Add the areas of these together to get the area of the real heat exchanger.
10. Print out the Performance Details and the Worksheet Properties.
11. Calculate A by dividing UA (actually q/DTlm) from by a value of U found in the heuristics listed above and a value of F.† Give source(s) of U and F values (see g below).
a. Utilities must be above 1 atm pressure, both entering and leaving the heat exchangers.
b. When steam is used for heating a process stream, the entering steam should be specified as all vapor (vapor fraction 1), and either P or T but not both (unless itís superheated hps).† The exiting water should be specified as all liquid (vapor fraction 0) at a P only slightly lower than the entering P (see heuristics listed above).† The flow rate should not be specified, as HYSYS/UniSim will calculate it from the heat required by the process stream and the change in specific enthalpy of the steam/water.† Note that such heat exchangers normally allow only water to exit, so they are self regulating (just like a steam radiator for heating a room).† See utility heuristics to select the steam pressure or temperature.† You are not free to choose any old value you want.† Since the maximum T for high pressure steam is 254oC, you cannot use steam to heat above 244oC. (CAPCOST accepts steam temperatures of 160oC, 184oC and 254oC.) For higher T, CAPCOST accepts "Thermal Systems" with Ts of 330oC, 400oC and 60oC.
c. When a process stream is cooled by water going to steam, the entering water should be specified as all liquid (vapor fraction 0) at a specified† P or T, but not both.† The exiting steam should be specified as all vapor at a slightly lower P.†
d. When a process stream is cooled by a refrigerant, such as ethane or ammonia, the entering refrigerant should be specified as all liquid at a high pressure and the exiting refrigerant as all vapor at a much lower pressure.† (See Perry's 7th page 11-77.)
e. When cooling water is used, specify T and P, but not vapor fraction.† If you specify a vapor fraction of 0, you are telling HYSYS/UniSim that the water is saturated (would be in equilibrium with steam) and it will give you a ridiculously low P.† Note that the heuristics for CW indicate it should enter the condenser at 30oC and exit no higher than 45oC.† With a minimum approach temperature of 10oC, this means that you cannot use cooling water to cool a stream below 40oC.† If you need a lower temperture, youíll have to use another coolant.† For lower temperatures, CAPCOST gives you the choices of refrigerated water and refrigeration to 5oC, -20oC and -50oC.†
f. Heat and cooling fluids included in the HYSYS/UniSim database are given at http://people.clarkson.edu/~wilcox/Design/therfld.xls.†† If a Fluid Package Basis has not already been created for a desired fluid, go to the Basis Environment and create it now.†
g. We have assumed that the flow is truly countercurrent (which is not true for shell-and-tube exchangers). To be more accurate, you need to divide by the correction factor F for non-countercurrent flow, which can be obtained from graphs given in Perry's, Walas and Seider et al. †Since F<1 and sometimes even <<1, this will give you a larger value for A. †
h. We have assumed that typical values of U are okay.† For more accurate results, you need to calculate U from the inside and outside heat transfer coefficients h determined using the methods described in Perry's, Walas and Seider et al.† Choose whatever method you want for your case study, but be certain to describe that method in your final case study report.† Complete heat exchanger design software is available on-line, and in the full professional versions of HYSYS and Aspen as well as in the academic version of UniSim (see above).
i. If both latent heat and sensible heat are responsible for a substantial portion of the heat exchanged in one or both streams, you need to substitute two or more fictitious heat exchangers in your modeling because this may lead to a temperature cross and because the heat transfer coefficients are very different for these two processes.† In each fictitious heat exchanger each stream must have only a latent heat change or a sensible heat change, and not both.† In your final pfd, show these as a single exchanger.† See the example.† Note that in HYSYS both the process stream and the utility stream must flow from one of the fictitious heat exchangers to the next, since these represent a single heat exchanger.†
Last revised January 29, 2009.† Please email† questions, comments and suggestions to W.R. Wilcox