Question 5.4: Use HEXTRAN to design a shell-and-tube heat exchanger for th...

The STE module in HEXTRAN is configured in design mode by right-clicking on the unit and selecting Change Configuration from the pop-up menu. Exchanger parameters are set as in Example 5.3, except for the following items that are left unspecified to be calculated by the program: number of tubes, tubesheet thickness, baffle spacing, number of sealing strips, and nozzle sizes. Design constraints are set as shown below, and the kerosene outlet temperature (250ºF) is given as the design specification on the Specifications panel.

The HEXTRAN input file and Exchanger Data Sheets for this run (Run 1) are given below, and the results are compared with the hand calculations in the following table. In all cases, the tubes are 1-in. OD, 14 BWG, on 1.25-in. square pitch. Notice that the nozzle diameters are rounded to one decimal place in the  EXTRAN output. The exact nozzle sizes determined by the program are the same as used in the hand calculations, namely, 4-in. schedule 40 on the tube side and 3-in. schedule 40 on the shell side.

All design criteria are met, but with a length of 26 ft and a diameter of only 13.25 in., the configuration is awkward. In an attempt to obtain a more compact design, the maximum tube length was changed from 30 to 20 ft. The results for this run (Run 2) are given in the above table. As can be seen, the tube-side Reynolds number is in the transition region, which is undesirable.

A third run was made with the minimum number of tube passes set at four and the maximum tube length set at 20 ft. The Exchanger Data Sheet for this run is given below and the results are compared with those from the other runs in the table above. It can be seen that this run produced a configuration very similar to the one obtained by hand, albeit with less heat-transfer area due to the higher shell-side coefficient computed by HEXTRAN. Although the tube-side velocity is on the high side at 8.36 ft/s, it is acceptable with carbon steel tubes. Checking the appropriate tube-count table (Table C.5) shows that a 17.25 in. shell should be adequate for a bundle containing 99 tubes. Since HEXTRAN specifies a 19 in. shell for this bundle, it is apparent that it uses a different set of tube-count data. Notice that in the HEXTRAN designs the kerosene outlet temperature exactly matches the design specification of 250ºF; thus, the over-design for these units is effectively zero.

HEXTRAN Input File for Example 5.4, Run 1

$
GENERATED FROM HEXTRAN KEYWORD EXPORTER $
$
$ General Data Section
$
TITLE PROJECT= Ex5-4, 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= PS2, NAME= PS2
$
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= PS4, NAME= PS4
$
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
$
$ Calculation Type Section
$
SIMULATION
$
TOLERANCE TTRIAL= 0.01
$
LIMITS AREA= 200.00, 6000.00, SERIES= 1, 10, PDAMP= 0.00, *
TTRIAL= 30
$
CALC TWOPHASE= New, DPSMETHOD= Stream, 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 New, TEMA= AES, HOTSIDE= Shellside, ORIENTATION= Horizontal, *
FLOW= Countercurrent, AREA= 200.00, 6000.00, *
UESTIMATE= 50.00, USCALER= 1.00
TUBE FEED= OIL, PRODUCT= PS4, *
LENGTH= 8.00, 30.00, 1.00, OD= 1.000, *
BWG= 14, PASS= 2, 16, 2, PATTERN= 90, *
PITCH= 1.2500, MATERIAL= 1, *
FOUL= 0.003, LAYER= 0, *
DPSCALER= 1.00, DPSHELL= 1.000, 15.000, VELOCITY= 3.0, 10.0
$
SHELL FEED= KEROSENE, PRODUCT= PS2, *
ID= 8.00, 60.00, SERIES= 1, 10, *
MATERIAL= 1, *
FOUL= 0.002, LAYER= 0, *
DPSCALER= 1.00, DPSHELL= 1.000, 15.000, VELOCITY= 0.0, 1000.0
$
BAFF Segmental= Single, *
CUT= 0.20, 0.35, *
THICKNESS= 0.1875
$
TNOZZ NUMB= 1, 1
$
SNOZZ NUMB= 1, 1
$
CALC TWOPHASE= Old, *
DPSMETHOD= Bell, *
MINFT= 0.80
$
SPEC Shell, Temp= 250.000000
$
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 5.4, Run 1

HEXTRAN Output Data for Example 5.4, Run 3