Question 5.3: Use HEXTRAN to rate the final configuration obtained for the...
Use HEXTRAN to rate the final configuration obtained for the kerosene/crude-oil exchanger in Example 5.1, and compare the results with those obtained previously by hand.
The procedure for problem setup and data entry is similar to that discussed in Example 4.4. Under Units of Measure, the viscosity unit is changed from cp to lb/ft . h for convenience, and the flowsheet is then constructed in the usual manner. The two feed streams are defined as bulk property streams, and the physical properties are entered on the appropriate forms. Note that fluid density (49.008 lbm/ft³ for kerosene and 53.066 lbm/ft³ for crude oil), not specific gravity, must be entered. Since stream pressures were not given in Example 5.1 (they are not needed for the calculations, but must be specified in HEXTRAN), a value of 50 psia is arbitrarily assigned for each stream. Flow rates and temperatures of the feed streams are entered as given in Example 5.1.
The physical parameters of the heat exchanger are entered on the appropriate forms exactly as obtained from Example 5.1. In addition, on the Baffles form the baffle thickness is specified as 0.1875 in. (the default value) and the total tubesheet thickness as 4.0 in. (2.0 in. for each tubesheet), in accordance with the design guidelines given in Section 5.7. Note that the total thickness of both tubesheets must be entered, not the individual tubesheet thickness. Only the central baffle spacing (3.85 in.) is specified; no value is entered for the inlet or outlet spacing. One pair of sealing strips is also specified on this form. Finally, under Pressure Drop
Options, the Two-Phase Film/DP Method is set to HEXTRAN 5.0x Method, and the Shell-side DP Method is set to Bell. (These settings are translated to TWOPHASE ¼ Old and DPSMETHOD = Bell in the input file under UNIT OPERATIONS/STE/CALC.) With these settings, the program uses the Delaware method for both shell-side heat transfer and pressure drop calculations.
The input file generated by the HEXTRAN GUI is given below, followed by the Exchanger Data Sheet and Extended Data Sheet, which were extracted from the output file. Information obtained from the data sheets was used to prepare the following comparison between computer and hand calculations.
| Item | Hand Calculation |
HEXTRAN |
| Re_i | 10,189 | 10,189 |
| Re_0 | 37,158 | 45,148 |
| h_i (Btu/h · ft² · ºF) | 156 | 156.2 |
| h_0 (Btu/h · ft² · ºF) | 122 | 191.2 |
| U_D (Btu/h · ft² · ºF) | 46 | 53.3 |
| ΔP_i (psi) | 10.2 | 10.06 |
| ΔP_0 (psi) | 2.2 | 2.10 |
| Over-design, % | 4.5 | 21 |
The tube-side Reynolds number, heat-transfer coefficient, and pressure drop computed by HEXTRAN agree almost exactly with the hand calculations. There is also excellent agreement between the shell-side pressure drops. However, there are significant differences in the shell-side Reynolds numbers and heat-transfer coefficients calculated by the two methods. The difference in Reynolds numbers is due to differences in the way Re is defined in the two methods. As expected, the Simplified Delaware method gives a smaller value for the heat-transfer coefficient than does the full Delaware method. However, the difference is somewhat greater than expected, amounting to about 36% of the HEXTRAN value. This difference is reflected in the over-design values and in the kerosene outlet temperature computed by HEXTRAN, 236ºF versus the target temperature of 250º F. Thus, according to HEXTRAN,
the exchanger is somewhat over-sized. Also, notice that HEXTRAN adjusts the number of baffles and the end spacing to account for the thickness of the tubesheets. The baffle spacing is interpreted as baffle pitch since the tube length satisfies:
L=(n_b-1)B+B_{in}+B_{out}+ \text{total tubesheet thickness}
HEXTRAN Input File for Example 5.3
| $ GENERATED FROM HEXTRAN KEYWORD EXPORTER $ $ General Data Section $ TITLE PROJECT=EX5-3, PROBLEM=KEROIL, SITE= $ DIME English, AREA=FT2, CONDUCTIVITY=BTUH, DENSITY=LB/FT3, * ENERGY=BTU, FILM=BTUH, LIQVOLUME=FT3, POWER=HP, * PRESSURE=PSIA, SURFACE=DYNE, TIME=HR, TEMPERATURE=F, * UVALUE=BTUH, VAPVOLUME=FT3, VISCOSITY=LBFH, WT=LB, * XDENSITY=API, STDVAPOR=379.490 $ PRINT ALL, * RATE=M $ CALC PGEN=New, WATER=Saturated $ $ Component Data Section $ $ Thermodynamic Data Section $ $Stream Data Section $ STREAM DATA $ PROP STRM=KEROSENE, NAME=KEROSENE, TEMP=390.00, PRES=50.000, * LIQUID(W)=45000.000, LCP(AVG)=0.59, Lcond(AVG)=0.079, * Lvis(AVG)=0.97, Lden(AVG)=49.008 $ PROP STRM=4, NAME=4 $ PROP STRM=OIL, NAME=OIL, TEMP=100.00, PRES=50.000, * LIQUID(W)=150000.000, LCP(AVG)=0.49, Lcond(AVG)=0.077, * Lvis(AVG)=8.7, Lden(AVG)=53.066 $ PROP STRM=3, NAME=3 $ $ Calculation Type Section $ SIMULATION $ TOLERANCE TTRIAL=0.01 $ LIMITS AREA=200.00, 6000.00, SERIES=1, 10, PDAMP=0.00, * TTRIAL=30 $ CALC TWOPHASE=Old, DPSMETHOD=Bell, MINFT=0.80 $ PRINT UNITS, ECONOMICS, STREAM, STANDARD, * EXTENDED, ZONES $ ECONOMICS DAYS=350, EXCHANGERATE=1.00, CURRENCY=USDOLLAR $ UTCOST OIL=3.50, GAS=3.50, ELECTRICITY=0.10, * WATER=0.03, HPSTEAM=4.10, MPSTEAM=3.90, * LPSTEAM=3.60, REFRIGERANT=0.00, HEATINGMEDIUM=0.00 $ HXCOST BSIZE=1000.00, BCOST=0.00, LINEAR=50.00, * EXPONENT=0.60, CONSTANT=0.00, UNIT $ $ Unit Operations Data $ UNIT OPERATIONS $ STE UID=STE1 TYPE Old, TEMA=AES, HOTSIDE=Shellside, ORIENTATION=Horizontal, * FLOW=Countercurrent, * UESTIMATE=50.00, USCALER=1.00 TUBE FEED=OIL, PRODUCT=3, * LENGTH=14.00, OD=1.000, * BWG=14, NUMBER=124, PASS=4, PATTERN=90, * PITCH=1.2500, MATERIAL=1, * FOUL=0.003, LAYER=0, * DPSCALER=1.00 $ SHELL FEED=KEROSENE, PRODUCT=4, * ID=19.25, SERIES=1, PARALLEL=1, * SEALS=1, MATERIAL=1, * FOUL=0.002, LAYER=0, * DPSCALER=1.00 $ BAFF Segmental=Single, * CUT=0.20, * SPACING=3.850, * THICKNESS=0.1875, SHEETS=4.000 $ TNOZZ TYPE=Conventional, ID=4.026, 4.026, NUMB=1, 1 $ SNOZZ TYPE=Conventional , ID=3.068, 3.068, NUMB=1, 1 $ CALC TWOPHASE=Old, * DPSMETHOD=Bell, * MINFT=0.80 $ PRINT STANDARD, * EXTENDED, * ZONES $ COST BSIZE=1000.00, BCOST=0.00, LINEAR=50.00, * CONSTANT=0.00, EXPONENT=0.60, Unit $ $ End of keyword file… |
HEXTRAN Output Data for Example 5.3
| ============================================================================= SHELL AND TUBE EXCHANGER DATA SHEET I—————————————————————————I I EXCHANGER NAME UNIT ID STE1 I I SIZE 19x 168 TYPE AES, HORIZONTAL CONNECTED 1 PARALLEL 1 SERIES I I AREA/UNIT 444. FT2 ( 444. FT2 REQUIRED) AREA/SHELL 444. FT2 I I—————————————————————————I I PERFORMANCE OF ONE UNIT SHELL-SIDE TUBE-SIDE I I—————————————————————————I I FEED STREAM NUMBER KEROSENE OIL I I FEED STREAM NUMBER KEROSENE OIL I I TOTAL FLUID LB /HR 45000. 150000. I I VAPOR (IN/OUT) LB /HR 0./ 0. 0./ 0. I I LIQUID LB /HR 45000./ 45000. 150000./ 150000. I I STEAM LB /HR 0./ 0. 0./ 0. I I WATER LB /HR 0./ 0. 0./ 0. I I NON CONDENSIBLE LB /HR 0. 0. I I TEMPERATURE (IN/OUT) DEG F 390.0 / 236.2 100.0 / 155.6 I I PRESSURE (IN/OUT) PSIA 50.00 / 47.90 50.00 / 39.94 I I—————————————————————————I I SP. GR., LIQ (60F / 60F H2O) 0.785 / 0.785 0.850 / 0.850 I I VAP (60F / 60F AIR) 0.000 / 0.000 0.000 / 0.000 I I DENSITY, LIQUID LB/FT3 49.008 / 49.008 53.066 / 53.066 I I VAPOR LB/FT3 0.000 / 0.000 0.000 / 0.000 I I VISCOSITY, LIQUID LB/FT-HR 0.970 / 0.970 8.700 / 8.700 I I VAPOR LB/FT-HR 0.000 / 0.000 0.000 / 0.000 I I THRML COND,LIQ BTU/HR-FT-F 0.0790 / 0.0790 0.0770 / 0.0770 I I VAP BTU/HR-FT-F 0.0000 / 0.0000 0.0000 / 0.0000 I I SPEC.HEAT,LIQUID BTU /LB F 0.5900 / 0.5900 0.4900 / 0.4900 I I VAPOR BTU /LB F 0.0000 / 0.0000 0.0000 / 0.0000 I I LATENT HEAT BTU /LB 0.00 0.00 I I VELOCITY FT/SEC 1.81 6.68 I I DP/SHELL(DES/CALC) PSI 0.00 / 2.10 0.00 / 10.06 I I FOULING RESIST FT2-HR-F/BTU 0.00200 (0.00200 REQD) 0.00300 I I—————————————————————————I I TRANSFER RATE BTU/HR-FT2-F SERVICE 53.32 ( 53.32 REQD), CLEAN 76.01 I I HEAT EXCHANGED MMBTU /HR 4.083, MTD(CORRECTED) 172.6, FT 0.954 I I—————————————————————————I I CONSTRUCTION OF ONE SHELL SHELL-SIDE TUBE-SIDE I I—————————————————————————I I DESIGN PRESSURE/TEMP PSIA /F 125./ 400. 125./ 400. I I NUMBER OF PASSES 1 4 I I MATERIAL CARB STL CARB STL I I INLET NOZZLE ID/NO IN 3.1/ 1 4.0/ 1 I I OUTLET NOZZLE ID/NO IN 3.1/ 1 4.0/ 1 I I—————————————————————————I I TUBE: NUMBER 124, OD 1.000 IN , BWG 14 , LENGTH 14.0 FT I I TYPE BARE, PITCH 1.2500 IN, PATTERN 90 DEGREES I I SHELL: ID 19.25 IN, SEALING STRIPS 1 PAIRS I I BAFFLE: CUT .200, SPACING(IN): IN 5.00, CENT 3.85, OUT 5.00,SING I I RHO-V2: INLET NOZZLE 1210.3 LB/FT-SEC2 I I TOTAL WEIGHT/SHELL,LB 3205.5 FULL OF WATER 0.738E+04 BUNDLE 2590.1 I I—————————————————————————I ============================================================================= SHELL AND TUBE EXTENDED DATA SHEET I—————————————————————————I I EXCHANGER NAME UNIT ID STE1 I I SIZE 19x 168 TYPE AES, HORIZONTAL CONNECTED 1 PARALLEL 1 SERIES I I AREA/UNIT 444. FT2 ( 444. FT2 REQUIRED) I I—————————————————————————I I PERFORMANCE OF ONE UNIT SHELL-SIDE TUBE-SIDE I I—————————————————————————I I FEED STREAM NUMBER KEROSENE OIL I I FEED STREAM NAME KEROSENE OIL I I WT FRACTION LIQUID (IN/OUT) 1.00 / 1.00 1.00 / 1.00 I I REYNOLDS NUMBER 45148. 10189. I I PRANDTL NUMBER 7.244 55.364 I I UOPK,LIQUID 0.000 / 0.000 0.000 / 0.000 I I VAPOR 0.000 / 0.000 0.000 / 0.000 I I SURFACE TENSION DYNES/CM 0.000 / 0.000 0.000 / 0.000 I I FILM COEF(SCL) BTU/HR-FT2-F 191.2 (1.000) 156.2 (1.000) I I FOULING LAYER THICKNESS IN 0.000 0.000 I I—————————————————————————I I THERMAL RESISTANCE I I UNITS: (FT2-HR-F/BTU) (PERCENT) (ABSOLUTE) I I SHELL FILM 27.88 0.00523 I I TUBE FILM 40.93 0.00767 I I TUBE METAL 1.34 0.00025 I I TOTAL FOULING 29.85 0.00560 I I ADJUSTMENT 0.00 0.00000 I I—————————————————————————I I PRESSURE DROP SHELL-SIDE TUBE-SIDE I I UNITS: (PSIA ) PERCENT) (ABSOLUTE) (PERCENT) (ABSOLUTE)I I WITHOUT NOZZLES 90.07 1.89 92.82 9.34 I I INLET NOZZLES 6.20 0.13 4.48 0.45 I I OUTLET NOZZLES 3.72 0.08 2.69 0.27 I I TOTAL /SHELL 2.10 10.06 I I TOTAL /UNIT 2.10 10.06 I I DP SCALER 1.00 1.00 I I—————————————————————————I I CONSTRUCTION OF ONE SHELL I I—————————————————————————I I TUBE:OVERALL LENGTH 14.0 FT EFFECTIVE LENGTH 13.67 FT I I TOTAL TUBESHEET THK 4.0 IN AREA RATIO (OUT/IN) 1.199 I I THERMAL COND. 30.0BTU/HR-FT-F DENSITY 490.80 LB/FT3I I—————————————————————————I I BAFFLE: THICKNESS 0.188 IN NUMBER 41 I I—————————————————————————I I BUNDLE: DIAMETER 17.0 IN TUBES IN CROSSFLOW 70 I I CROSSFLOW AREA 0.086 FT2 WINDOW AREA 0.142 FT2 I I TUBE-BFL LEAK AREA 0.033 FT2 SHELL-BFL LEAK AREA 0.022 FT2 I I—————————————————————————I |